From 3d67b4ed9c7b0089f2e623ce6ab4302e35b9cd2f Mon Sep 17 00:00:00 2001
From: anhefti <andreas.hefti@let.ethz.ch>
Date: Wed, 26 Jun 2019 15:31:18 +0200
Subject: [PATCH] SEBSERV-62 Model, DAO and service implementation

---
 pom.xml                                       |   6 +-
 .../java/ch/ethz/seb/sebserver/SEBServer.java |   8 +-
 .../sebserver/gbl/model/exam/Indicator.java   |  22 ++-
 .../gbl/model/session/ClientConnection.java   |   2 +-
 .../model/session/ClientConnectionData.java   |  57 ++++++
 .../gbl/model/session/ClientEvent.java        |  36 +++-
 .../gbl/model/session/IndicatorValue.java     |  20 ++
 .../model/session/IndicatorValueHolder.java   |  19 ++
 .../model/session/SimpleIndicatorValue.java   |  55 ++++++
 .../servicelayer/dao/ClientEventDAO.java      |  15 ++
 .../webservice/servicelayer/dao/ExamDAO.java  |   5 +
 .../servicelayer/dao/FilterMap.java           |  32 ++++
 .../dao/impl/ClientConnectionDAOImpl.java     |   3 +-
 .../dao/impl/ClientEventDAOImpl.java          | 172 ++++++++++++++++++
 .../servicelayer/dao/impl/ExamDAOImpl.java    |  17 ++
 .../servicelayer/session/ClientIndicator.java |  39 ++++
 .../session/ClientIndicatorService.java       |  13 ++
 .../session/EventHandlingStrategy.java        |  21 +++
 .../session/ExamSessionService.java           |  29 +++
 .../session/SebClientConnectionService.java   |  25 +++
 .../session/impl/AbstractClientIndicator.java |  61 +++++++
 .../session/impl/AbstractPingIndicator.java   |  40 ++++
 .../impl/AsyncBatchEventSaveStrategy.java     | 156 ++++++++++++++++
 .../impl/ClientConnectionDataInternal.java    |  54 ++++++
 .../session/impl/ClientIndicatorFactory.java  |  79 ++++++++
 .../impl/ErrorCountClientIndicator.java       |  71 ++++++++
 .../session/impl/ExamSessionCacheService.java | 132 ++++++++++++++
 .../session/impl/ExamSessionServiceImpl.java  | 114 ++++++++++++
 .../impl/PingIntervalClientIndicator.java     |  47 +++++
 .../session/impl/SingleEventSaveStrategy.java |  43 +++++
 .../config/application-dev-ws.properties      |   1 +
 .../impl/PingIntervalClientIndicatorTest.java |  52 ++++++
 32 files changed, 1433 insertions(+), 13 deletions(-)
 create mode 100644 src/main/java/ch/ethz/seb/sebserver/gbl/model/session/ClientConnectionData.java
 create mode 100644 src/main/java/ch/ethz/seb/sebserver/gbl/model/session/IndicatorValue.java
 create mode 100644 src/main/java/ch/ethz/seb/sebserver/gbl/model/session/IndicatorValueHolder.java
 create mode 100644 src/main/java/ch/ethz/seb/sebserver/gbl/model/session/SimpleIndicatorValue.java
 create mode 100644 src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/dao/ClientEventDAO.java
 create mode 100644 src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/dao/impl/ClientEventDAOImpl.java
 create mode 100644 src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/ClientIndicator.java
 create mode 100644 src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/ClientIndicatorService.java
 create mode 100644 src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/EventHandlingStrategy.java
 create mode 100644 src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/ExamSessionService.java
 create mode 100644 src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/SebClientConnectionService.java
 create mode 100644 src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/AbstractClientIndicator.java
 create mode 100644 src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/AbstractPingIndicator.java
 create mode 100644 src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/AsyncBatchEventSaveStrategy.java
 create mode 100644 src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/ClientConnectionDataInternal.java
 create mode 100644 src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/ClientIndicatorFactory.java
 create mode 100644 src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/ErrorCountClientIndicator.java
 create mode 100644 src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/ExamSessionCacheService.java
 create mode 100644 src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/ExamSessionServiceImpl.java
 create mode 100644 src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/PingIntervalClientIndicator.java
 create mode 100644 src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/SingleEventSaveStrategy.java
 create mode 100644 src/test/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/PingIntervalClientIndicatorTest.java

diff --git a/pom.xml b/pom.xml
index a3280ec5..2003ba40 100644
--- a/pom.xml
+++ b/pom.xml
@@ -112,7 +112,7 @@
               <excludes>
                 <exclude>**/batis/mapper/*.java</exclude>
                 <exclude>**/batis/model/*.java</exclude>
-                <exclude name="UselessParentheses"/>
+                <exclude name="UselessParentheses" />
               </excludes>
             </configuration>
             <executions>
@@ -242,6 +242,10 @@
       <groupId>org.springframework.boot</groupId>
       <artifactId>spring-boot-starter-security</artifactId>
     </dependency>
+    <dependency>
+      <groupId>org.springframework.boot</groupId>
+      <artifactId>spring-boot-starter-cache</artifactId>
+    </dependency>
     <!-- NOTE since org.springframework.security.oauth is not fully migrated 
       to spring-boot-starter-security we have to declare a separate version here. 
       This refers to the latest version of spring-security-oauth2 and should be 
diff --git a/src/main/java/ch/ethz/seb/sebserver/SEBServer.java b/src/main/java/ch/ethz/seb/sebserver/SEBServer.java
index ffa57ef9..e0d32da0 100644
--- a/src/main/java/ch/ethz/seb/sebserver/SEBServer.java
+++ b/src/main/java/ch/ethz/seb/sebserver/SEBServer.java
@@ -11,10 +11,11 @@ package ch.ethz.seb.sebserver;
 import org.springframework.boot.SpringApplication;
 import org.springframework.boot.autoconfigure.SpringBootApplication;
 import org.springframework.boot.autoconfigure.security.servlet.UserDetailsServiceAutoConfiguration;
+import org.springframework.cache.annotation.EnableCaching;
 
 /** SEB-Server (Safe Exam Browser Server) is a server component to maintain and support
  * Exams running with SEB (Safe Exam Browser). TODO add link(s)
- * 
+ *
  * SEB-Server uses Spring Boot as main framework is divided into two main components,
  * a webservice component that implements the business logic, persistence management
  * and defines a REST API to expose the services over HTTP. The second component is a
@@ -25,13 +26,12 @@ import org.springframework.boot.autoconfigure.security.servlet.UserDetailsServic
  * and GUI and can be used to start the components on separate servers or within the same
  * server instance. Additional to the usual profiles like dev, prod, test there are combining
  * profiles like dev-ws, dev-gui and prod-ws, prod-gui
- * 
+ *
  * TODO documentation for presets to start all-in-one server or separated gui- and webservice- server */
 @SpringBootApplication(exclude = {
-        // OAuth2ResourceServerAutoConfiguration.class,
         UserDetailsServiceAutoConfiguration.class,
-        //DataSourceAutoConfiguration.class
 })
+@EnableCaching
 public class SEBServer {
 
     public static void main(final String[] args) {
diff --git a/src/main/java/ch/ethz/seb/sebserver/gbl/model/exam/Indicator.java b/src/main/java/ch/ethz/seb/sebserver/gbl/model/exam/Indicator.java
index 112c519d..64c9a3da 100644
--- a/src/main/java/ch/ethz/seb/sebserver/gbl/model/exam/Indicator.java
+++ b/src/main/java/ch/ethz/seb/sebserver/gbl/model/exam/Indicator.java
@@ -32,8 +32,26 @@ public final class Indicator implements Entity {
     public static final String FILTER_ATTR_EXAM_ID = "examId";
 
     public enum IndicatorType {
-        LAST_PING,
-        ERROR_COUNT
+        LAST_PING(Names.LAST_PING),
+        ERROR_COUNT(Names.ERROR_COUNT)
+
+        ;
+
+        public final String name;
+
+        private IndicatorType(final String name) {
+            this.name = name;
+        }
+
+        @Override
+        public String toString() {
+            return this.name;
+        }
+
+        public interface Names {
+            public static final String LAST_PING = "LAST_PING";
+            public static final String ERROR_COUNT = "ERROR_COUNT";
+        }
     }
 
     @JsonProperty(INDICATOR.ATTR_ID)
diff --git a/src/main/java/ch/ethz/seb/sebserver/gbl/model/session/ClientConnection.java b/src/main/java/ch/ethz/seb/sebserver/gbl/model/session/ClientConnection.java
index 8f1f3bf6..b5acd432 100644
--- a/src/main/java/ch/ethz/seb/sebserver/gbl/model/session/ClientConnection.java
+++ b/src/main/java/ch/ethz/seb/sebserver/gbl/model/session/ClientConnection.java
@@ -17,7 +17,7 @@ import ch.ethz.seb.sebserver.gbl.model.Domain;
 import ch.ethz.seb.sebserver.gbl.model.GrantEntity;
 
 @JsonIgnoreProperties(ignoreUnknown = true)
-public class ClientConnection implements GrantEntity {
+public final class ClientConnection implements GrantEntity {
 
     public static final String FILTER_ATTR_EXAM_ID = Domain.CLIENT_CONNECTION.ATTR_EXAM_ID;
     public static final String FILTER_ATTR_STATUS = Domain.CLIENT_CONNECTION.ATTR_STATUS;
diff --git a/src/main/java/ch/ethz/seb/sebserver/gbl/model/session/ClientConnectionData.java b/src/main/java/ch/ethz/seb/sebserver/gbl/model/session/ClientConnectionData.java
new file mode 100644
index 00000000..b784c713
--- /dev/null
+++ b/src/main/java/ch/ethz/seb/sebserver/gbl/model/session/ClientConnectionData.java
@@ -0,0 +1,57 @@
+/*
+ * Copyright (c) 2019 ETH Zürich, Educational Development and Technology (LET)
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+package ch.ethz.seb.sebserver.gbl.model.session;
+
+import java.util.Collection;
+
+import com.fasterxml.jackson.annotation.JsonCreator;
+import com.fasterxml.jackson.annotation.JsonIgnore;
+import com.fasterxml.jackson.annotation.JsonProperty;
+
+public class ClientConnectionData {
+
+    @JsonProperty
+    public final ClientConnection clientConnection;
+    @JsonProperty
+    public final Collection<? extends IndicatorValue> indicatorValues;
+
+    @JsonCreator
+    protected ClientConnectionData(
+            @JsonProperty final ClientConnection clientConnection,
+            @JsonProperty final Collection<? extends IndicatorValue> indicatorValues) {
+
+        this.clientConnection = clientConnection;
+        this.indicatorValues = indicatorValues;
+    }
+
+    @JsonIgnore
+    public Long getConnectionId() {
+        return this.clientConnection.id;
+    }
+
+    public ClientConnection getClientConnection() {
+        return this.clientConnection;
+    }
+
+    public Collection<? extends IndicatorValue> getIndicatorValues() {
+        return this.indicatorValues;
+    }
+
+    @Override
+    public String toString() {
+        final StringBuilder builder = new StringBuilder();
+        builder.append("ClientConnectionData [clientConnection=");
+        builder.append(this.clientConnection);
+        builder.append(", indicatorValues=");
+        builder.append(this.indicatorValues);
+        builder.append("]");
+        return builder.toString();
+    }
+
+}
diff --git a/src/main/java/ch/ethz/seb/sebserver/gbl/model/session/ClientEvent.java b/src/main/java/ch/ethz/seb/sebserver/gbl/model/session/ClientEvent.java
index f4650bf3..5fd0850e 100644
--- a/src/main/java/ch/ethz/seb/sebserver/gbl/model/session/ClientEvent.java
+++ b/src/main/java/ch/ethz/seb/sebserver/gbl/model/session/ClientEvent.java
@@ -8,6 +8,8 @@
 
 package ch.ethz.seb.sebserver.gbl.model.session;
 
+import java.math.BigDecimal;
+
 import com.fasterxml.jackson.annotation.JsonCreator;
 import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
 import com.fasterxml.jackson.annotation.JsonProperty;
@@ -15,12 +17,23 @@ import com.fasterxml.jackson.annotation.JsonProperty;
 import ch.ethz.seb.sebserver.gbl.api.EntityType;
 import ch.ethz.seb.sebserver.gbl.model.Domain;
 import ch.ethz.seb.sebserver.gbl.model.Entity;
+import ch.ethz.seb.sebserver.webservice.datalayer.batis.model.ClientEventRecord;
 
 @JsonIgnoreProperties(ignoreUnknown = true)
-public final class ClientEvent implements Entity {
+public final class ClientEvent implements Entity, IndicatorValueHolder {
+
+    public static final String FILTER_ATTR_CONECTION_ID = Domain.CLIENT_EVENT.ATTR_CONNECTION_ID;
+    public static final String FILTER_ATTR_TYPE = Domain.CLIENT_EVENT.ATTR_TYPE;
+    public static final String FILTER_ATTR_FROM_DATE = "fromDate";
 
     public static enum EventType {
-        UNKNOWN(0), LOG(1);
+        UNKNOWN(0),
+        DEBUG_LOG(1),
+        INFO_LOG(2),
+        WARN_LOG(3),
+        ERROR_LOG(4),
+
+        ;
 
         public final int id;
 
@@ -58,7 +71,7 @@ public final class ClientEvent implements Entity {
     public final String text;
 
     @JsonCreator
-    ClientEvent(
+    public ClientEvent(
             @JsonProperty(Domain.CLIENT_EVENT.ATTR_ID) final Long id,
             @JsonProperty(Domain.CLIENT_EVENT.ATTR_CONNECTION_ID) final Long connectionId,
             @JsonProperty(Domain.CLIENT_EVENT.ATTR_TYPE) final EventType eventType,
@@ -111,6 +124,13 @@ public final class ClientEvent implements Entity {
         return this.numValue;
     }
 
+    @Override
+    public double getValue() {
+        return this.numValue != null
+                ? this.numValue.doubleValue()
+                : Double.NaN;
+    }
+
     public String getText() {
         return this.text;
     }
@@ -133,4 +153,14 @@ public final class ClientEvent implements Entity {
         builder.append("]");
         return builder.toString();
     }
+
+    public static final ClientEventRecord toRecord(final ClientEvent event) {
+        return new ClientEventRecord(
+                event.id,
+                event.connectionId,
+                event.eventType.id,
+                event.timestamp,
+                (event.numValue != null) ? new BigDecimal(event.numValue) : null,
+                event.text);
+    }
 }
diff --git a/src/main/java/ch/ethz/seb/sebserver/gbl/model/session/IndicatorValue.java b/src/main/java/ch/ethz/seb/sebserver/gbl/model/session/IndicatorValue.java
new file mode 100644
index 00000000..56228abb
--- /dev/null
+++ b/src/main/java/ch/ethz/seb/sebserver/gbl/model/session/IndicatorValue.java
@@ -0,0 +1,20 @@
+/*
+ * Copyright (c) 2019 ETH Zürich, Educational Development and Technology (LET)
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+package ch.ethz.seb.sebserver.gbl.model.session;
+
+import com.fasterxml.jackson.annotation.JsonProperty;
+
+import ch.ethz.seb.sebserver.gbl.model.exam.Indicator.IndicatorType;
+
+public interface IndicatorValue extends IndicatorValueHolder {
+
+    @JsonProperty(SimpleIndicatorValue.ATTR_INDICATOR_TYPE)
+    IndicatorType getType();
+
+}
diff --git a/src/main/java/ch/ethz/seb/sebserver/gbl/model/session/IndicatorValueHolder.java b/src/main/java/ch/ethz/seb/sebserver/gbl/model/session/IndicatorValueHolder.java
new file mode 100644
index 00000000..b520ce93
--- /dev/null
+++ b/src/main/java/ch/ethz/seb/sebserver/gbl/model/session/IndicatorValueHolder.java
@@ -0,0 +1,19 @@
+/*
+ * Copyright (c) 2019 ETH Zürich, Educational Development and Technology (LET)
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+package ch.ethz.seb.sebserver.gbl.model.session;
+
+import com.fasterxml.jackson.annotation.JsonProperty;
+
+@FunctionalInterface
+public interface IndicatorValueHolder {
+
+    @JsonProperty(SimpleIndicatorValue.ATTR_INDICATOR_VALUE)
+    double getValue();
+
+}
diff --git a/src/main/java/ch/ethz/seb/sebserver/gbl/model/session/SimpleIndicatorValue.java b/src/main/java/ch/ethz/seb/sebserver/gbl/model/session/SimpleIndicatorValue.java
new file mode 100644
index 00000000..78bed046
--- /dev/null
+++ b/src/main/java/ch/ethz/seb/sebserver/gbl/model/session/SimpleIndicatorValue.java
@@ -0,0 +1,55 @@
+/*
+ * Copyright (c) 2019 ETH Zürich, Educational Development and Technology (LET)
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+package ch.ethz.seb.sebserver.gbl.model.session;
+
+import com.fasterxml.jackson.annotation.JsonCreator;
+import com.fasterxml.jackson.annotation.JsonProperty;
+
+import ch.ethz.seb.sebserver.gbl.model.exam.Indicator.IndicatorType;
+
+public final class SimpleIndicatorValue implements IndicatorValue {
+
+    public static final String ATTR_INDICATOR_VALUE = "indicatorValue";
+    public static final String ATTR_INDICATOR_TYPE = "indicatorType";
+
+    @JsonProperty(ATTR_INDICATOR_TYPE)
+    public final IndicatorType type;
+    @JsonProperty(ATTR_INDICATOR_VALUE)
+    public final double value;
+
+    @JsonCreator
+    protected SimpleIndicatorValue(
+            @JsonProperty(ATTR_INDICATOR_TYPE) final IndicatorType type,
+            @JsonProperty(ATTR_INDICATOR_VALUE) final double value) {
+
+        this.type = type;
+        this.value = value;
+    }
+
+    @Override
+    public IndicatorType getType() {
+        return this.type;
+    }
+
+    @Override
+    public double getValue() {
+        return this.value;
+    }
+
+    @Override
+    public String toString() {
+        final StringBuilder builder = new StringBuilder();
+        builder.append("IndicatorValue [type=");
+        builder.append(this.type);
+        builder.append(", value=");
+        builder.append(this.value);
+        builder.append("]");
+        return builder.toString();
+    }
+}
diff --git a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/dao/ClientEventDAO.java b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/dao/ClientEventDAO.java
new file mode 100644
index 00000000..f4942554
--- /dev/null
+++ b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/dao/ClientEventDAO.java
@@ -0,0 +1,15 @@
+/*
+ * Copyright (c) 2019 ETH Zürich, Educational Development and Technology (LET)
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+package ch.ethz.seb.sebserver.webservice.servicelayer.dao;
+
+import ch.ethz.seb.sebserver.gbl.model.session.ClientEvent;
+
+public interface ClientEventDAO extends EntityDAO<ClientEvent, ClientEvent> {
+
+}
diff --git a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/dao/ExamDAO.java b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/dao/ExamDAO.java
index c2e5cd7d..4b07421d 100644
--- a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/dao/ExamDAO.java
+++ b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/dao/ExamDAO.java
@@ -8,9 +8,14 @@
 
 package ch.ethz.seb.sebserver.webservice.servicelayer.dao;
 
+import java.util.Collection;
+
 import ch.ethz.seb.sebserver.gbl.model.exam.Exam;
+import ch.ethz.seb.sebserver.gbl.util.Result;
 import ch.ethz.seb.sebserver.webservice.servicelayer.bulkaction.BulkActionSupportDAO;
 
 /** Concrete EntityDAO interface of Exam entities */
 public interface ExamDAO extends ActivatableEntityDAO<Exam, Exam>, BulkActionSupportDAO<Exam> {
+
+    Result<Collection<Long>> allIdsOfInstituion(Long institutionId);
 }
diff --git a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/dao/FilterMap.java b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/dao/FilterMap.java
index 88020ac4..16556691 100644
--- a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/dao/FilterMap.java
+++ b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/dao/FilterMap.java
@@ -9,6 +9,7 @@
 package ch.ethz.seb.sebserver.webservice.servicelayer.dao;
 
 import org.joda.time.DateTime;
+import org.joda.time.DateTimeZone;
 import org.springframework.util.LinkedMultiValueMap;
 import org.springframework.util.MultiValueMap;
 
@@ -27,8 +28,11 @@ import ch.ethz.seb.sebserver.gbl.model.sebconfig.ConfigurationValue;
 import ch.ethz.seb.sebserver.gbl.model.sebconfig.Orientation;
 import ch.ethz.seb.sebserver.gbl.model.sebconfig.SebClientConfig;
 import ch.ethz.seb.sebserver.gbl.model.session.ClientConnection;
+import ch.ethz.seb.sebserver.gbl.model.session.ClientEvent;
+import ch.ethz.seb.sebserver.gbl.model.session.ClientEvent.EventType;
 import ch.ethz.seb.sebserver.gbl.model.user.UserInfo;
 import ch.ethz.seb.sebserver.gbl.util.Utils;
+import io.micrometer.core.instrument.util.StringUtils;
 
 /** A Map containing various filter criteria from a certain API request.
  * This is used as a data object that can be used to collect API request parameter
@@ -190,4 +194,32 @@ public class FilterMap extends POSTMapper {
         return getString(ClientConnection.FILTER_ATTR_STATUS);
     }
 
+    public Long getClientEventConnectionId() {
+        return getLong(ClientEvent.FILTER_ATTR_CONECTION_ID);
+    }
+
+    public Integer getClientEventTypeId() {
+        final String typeName = getString(ClientEvent.FILTER_ATTR_TYPE);
+        if (StringUtils.isBlank(typeName)) {
+            return null;
+        }
+
+        try {
+            return EventType.valueOf(typeName).id;
+        } catch (final Exception e) {
+            return null;
+        }
+    }
+
+    public Long getClientEventFromDate() {
+        final DateTime dateTime = Utils.toDateTime(getString(ClientEvent.FILTER_ATTR_FROM_DATE));
+        if (dateTime == null) {
+            return null;
+        }
+
+        return dateTime
+                .withZone(DateTimeZone.UTC)
+                .getMillis();
+    }
+
 }
diff --git a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/dao/impl/ClientConnectionDAOImpl.java b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/dao/impl/ClientConnectionDAOImpl.java
index c5a93850..1c10671e 100644
--- a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/dao/impl/ClientConnectionDAOImpl.java
+++ b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/dao/impl/ClientConnectionDAOImpl.java
@@ -33,7 +33,6 @@ import ch.ethz.seb.sebserver.webservice.datalayer.batis.mapper.ClientConnectionR
 import ch.ethz.seb.sebserver.webservice.datalayer.batis.mapper.ClientConnectionRecordMapper;
 import ch.ethz.seb.sebserver.webservice.datalayer.batis.mapper.ClientEventRecordDynamicSqlSupport;
 import ch.ethz.seb.sebserver.webservice.datalayer.batis.mapper.ClientEventRecordMapper;
-import ch.ethz.seb.sebserver.webservice.datalayer.batis.mapper.LmsSetupRecordDynamicSqlSupport;
 import ch.ethz.seb.sebserver.webservice.datalayer.batis.model.ClientConnectionRecord;
 import ch.ethz.seb.sebserver.webservice.servicelayer.dao.ClientConnectionDAO;
 import ch.ethz.seb.sebserver.webservice.servicelayer.dao.DAOLoggingSupport;
@@ -101,7 +100,7 @@ public class ClientConnectionDAOImpl implements ClientConnectionDAO {
     public Result<Collection<ClientConnection>> allOf(final Set<Long> pks) {
         return Result.tryCatch(() -> {
             return this.clientConnectionRecordMapper.selectByExample()
-                    .where(LmsSetupRecordDynamicSqlSupport.id, isIn(new ArrayList<>(pks)))
+                    .where(ClientConnectionRecordDynamicSqlSupport.id, isIn(new ArrayList<>(pks)))
                     .build()
                     .execute()
                     .stream()
diff --git a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/dao/impl/ClientEventDAOImpl.java b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/dao/impl/ClientEventDAOImpl.java
new file mode 100644
index 00000000..317c9835
--- /dev/null
+++ b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/dao/impl/ClientEventDAOImpl.java
@@ -0,0 +1,172 @@
+/*
+ * Copyright (c) 2019 ETH Zürich, Educational Development and Technology (LET)
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+package ch.ethz.seb.sebserver.webservice.servicelayer.dao.impl;
+
+import static org.mybatis.dynamic.sql.SqlBuilder.isEqualToWhenPresent;
+import static org.mybatis.dynamic.sql.SqlBuilder.isIn;
+
+import java.math.BigDecimal;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Set;
+import java.util.function.Predicate;
+import java.util.stream.Collectors;
+
+import org.mybatis.dynamic.sql.SqlBuilder;
+import org.springframework.context.annotation.Lazy;
+import org.springframework.stereotype.Component;
+import org.springframework.transaction.annotation.Transactional;
+
+import ch.ethz.seb.sebserver.gbl.api.EntityType;
+import ch.ethz.seb.sebserver.gbl.model.EntityKey;
+import ch.ethz.seb.sebserver.gbl.model.session.ClientEvent;
+import ch.ethz.seb.sebserver.gbl.model.session.ClientEvent.EventType;
+import ch.ethz.seb.sebserver.gbl.profile.WebServiceProfile;
+import ch.ethz.seb.sebserver.gbl.util.Result;
+import ch.ethz.seb.sebserver.webservice.datalayer.batis.mapper.ClientEventRecordDynamicSqlSupport;
+import ch.ethz.seb.sebserver.webservice.datalayer.batis.mapper.ClientEventRecordMapper;
+import ch.ethz.seb.sebserver.webservice.datalayer.batis.model.ClientEventRecord;
+import ch.ethz.seb.sebserver.webservice.servicelayer.dao.ClientEventDAO;
+import ch.ethz.seb.sebserver.webservice.servicelayer.dao.DAOLoggingSupport;
+import ch.ethz.seb.sebserver.webservice.servicelayer.dao.FilterMap;
+import ch.ethz.seb.sebserver.webservice.servicelayer.dao.ResourceNotFoundException;
+import ch.ethz.seb.sebserver.webservice.servicelayer.dao.TransactionHandler;
+
+@Lazy
+@Component
+@WebServiceProfile
+public class ClientEventDAOImpl implements ClientEventDAO {
+
+    private final ClientEventRecordMapper clientEventRecordMapper;
+
+    protected ClientEventDAOImpl(final ClientEventRecordMapper clientEventRecordMapper) {
+        this.clientEventRecordMapper = clientEventRecordMapper;
+    }
+
+    @Override
+    public EntityType entityType() {
+        return EntityType.CLIENT_EVENT;
+    }
+
+    @Override
+    @Transactional(readOnly = true)
+    public Result<ClientEvent> byPK(final Long id) {
+        return recordById(id)
+                .flatMap(ClientEventDAOImpl::toDomainModel);
+    }
+
+    @Override
+    @Transactional(readOnly = true)
+    public Result<Collection<ClientEvent>> allMatching(
+            final FilterMap filterMap,
+            final Predicate<ClientEvent> predicate) {
+
+        return Result.tryCatch(() -> this.clientEventRecordMapper
+                .selectByExample()
+                .where(
+                        ClientEventRecordDynamicSqlSupport.connectionId,
+                        isEqualToWhenPresent(filterMap.getClientEventConnectionId()))
+                .and(
+                        ClientEventRecordDynamicSqlSupport.type,
+                        isEqualToWhenPresent(filterMap.getClientEventTypeId()))
+                .and(
+                        ClientEventRecordDynamicSqlSupport.timestamp,
+                        SqlBuilder.isGreaterThanOrEqualToWhenPresent(filterMap.getClientEventFromDate()))
+                .build()
+                .execute()
+                .stream()
+                .map(ClientEventDAOImpl::toDomainModel)
+                .flatMap(DAOLoggingSupport::logAndSkipOnError)
+                .filter(predicate)
+                .collect(Collectors.toList()));
+
+    }
+
+    @Override
+    @Transactional(readOnly = true)
+    public Result<Collection<ClientEvent>> allOf(final Set<Long> pks) {
+        return Result.tryCatch(() -> {
+            return this.clientEventRecordMapper.selectByExample()
+                    .where(ClientEventRecordDynamicSqlSupport.id, isIn(new ArrayList<>(pks)))
+                    .build()
+                    .execute()
+                    .stream()
+                    .map(ClientEventDAOImpl::toDomainModel)
+                    .flatMap(DAOLoggingSupport::logAndSkipOnError)
+                    .collect(Collectors.toList());
+        });
+    }
+
+    @Override
+    @Transactional
+    public Result<ClientEvent> createNew(final ClientEvent data) {
+        return Result.tryCatch(() -> {
+
+            final EventType eventType = data.getEventType();
+
+            final ClientEventRecord newRecord = new ClientEventRecord(
+                    null,
+                    data.connectionId,
+                    (eventType != null) ? eventType.id : EventType.UNKNOWN.id,
+                    data.timestamp,
+                    (data.numValue != null) ? new BigDecimal(data.numValue) : null,
+                    data.text);
+
+            this.clientEventRecordMapper.insert(newRecord);
+            return newRecord;
+        })
+                .flatMap(ClientEventDAOImpl::toDomainModel)
+                .onError(TransactionHandler::rollback);
+    }
+
+    @Override
+    @Transactional
+    public Result<ClientEvent> save(final ClientEvent data) {
+        throw new UnsupportedOperationException("Update is not supported for client events");
+    }
+
+    @Override
+    @Transactional
+    public Result<Collection<EntityKey>> delete(final Set<EntityKey> all) {
+        throw new UnsupportedOperationException(
+                "Delete is not supported for particular client events. "
+                        + "Use delete of a client connection to delete also all client events of this connection.");
+    }
+
+    private Result<ClientEventRecord> recordById(final Long id) {
+        return Result.tryCatch(() -> {
+
+            final ClientEventRecord record = this.clientEventRecordMapper.selectByPrimaryKey(id);
+            if (record == null) {
+                throw new ResourceNotFoundException(
+                        entityType(),
+                        String.valueOf(id));
+            }
+
+            return record;
+        });
+    }
+
+    private static Result<ClientEvent> toDomainModel(final ClientEventRecord record) {
+        return Result.tryCatch(() -> {
+
+            final Integer type = record.getType();
+            final BigDecimal numericValue = record.getNumericValue();
+            return new ClientEvent(
+                    record.getId(),
+                    record.getConnectionId(),
+                    (type != null) ? EventType.byId(type) : EventType.UNKNOWN,
+                    record.getTimestamp(),
+                    (numericValue != null) ? numericValue.doubleValue() : null,
+                    record.getText());
+        });
+
+    }
+
+}
diff --git a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/dao/impl/ExamDAOImpl.java b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/dao/impl/ExamDAOImpl.java
index cfed25db..49161dae 100644
--- a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/dao/impl/ExamDAOImpl.java
+++ b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/dao/impl/ExamDAOImpl.java
@@ -303,6 +303,23 @@ public class ExamDAOImpl implements ExamDAO {
         }).flatMap(this::toDomainModel);
     }
 
+    @Override
+    @Transactional(readOnly = true)
+    public Result<Collection<Long>> allIdsOfInstituion(final Long institutionId) {
+        return Result.tryCatch(() -> {
+            return this.examRecordMapper.selectIdsByExample()
+                    .where(
+                            ExamRecordDynamicSqlSupport.institutionId,
+                            isEqualTo(institutionId))
+                    .and(
+                            ExamRecordDynamicSqlSupport.active,
+                            isEqualToWhenPresent(BooleanUtils.toIntegerObject(true)))
+                    .build()
+                    .execute();
+
+        });
+    }
+
     private Result<Collection<EntityKey>> allIdsOfInstitution(final EntityKey institutionKey) {
         return Result.tryCatch(() -> {
             return this.examRecordMapper.selectIdsByExample()
diff --git a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/ClientIndicator.java b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/ClientIndicator.java
new file mode 100644
index 00000000..b4cda4ee
--- /dev/null
+++ b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/ClientIndicator.java
@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2018 ETH Zürich, Educational Development and Technology (LET)
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+package ch.ethz.seb.sebserver.webservice.servicelayer.session;
+
+import java.util.Set;
+
+import ch.ethz.seb.sebserver.gbl.model.exam.Indicator;
+import ch.ethz.seb.sebserver.gbl.model.session.ClientEvent.EventType;
+import ch.ethz.seb.sebserver.gbl.model.session.IndicatorValue;
+import ch.ethz.seb.sebserver.gbl.model.session.IndicatorValueHolder;
+
+/** A client indicator is a indicator value holder for a specific Indicator
+ * on a running client connection.
+ * A client indicator can be used to verify a indicator value at a specific time of or
+ * a client indicator can be used for in memory caching of the current value of the
+ * indicator for a defined client connection. */
+public interface ClientIndicator extends IndicatorValue {
+
+    void init(Indicator indicatorDefinition, Long connectionId, boolean cachingEnabled);
+
+    String name();
+
+    Long examId();
+
+    Long connectionId();
+
+    double computeValueAt(long timestamp);
+
+    Set<EventType> observedEvents();
+
+    void notifyValueChange(IndicatorValueHolder indicatorValueHolder);
+
+}
diff --git a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/ClientIndicatorService.java b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/ClientIndicatorService.java
new file mode 100644
index 00000000..f88a314d
--- /dev/null
+++ b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/ClientIndicatorService.java
@@ -0,0 +1,13 @@
+/*
+ * Copyright (c) 2019 ETH Zürich, Educational Development and Technology (LET)
+ * 
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+package ch.ethz.seb.sebserver.webservice.servicelayer.session;
+
+public interface ClientIndicatorService {
+
+}
diff --git a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/EventHandlingStrategy.java b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/EventHandlingStrategy.java
new file mode 100644
index 00000000..9a2d5dce
--- /dev/null
+++ b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/EventHandlingStrategy.java
@@ -0,0 +1,21 @@
+/*
+ * Copyright (c) 2018 ETH Zürich, Educational Development and Technology (LET)
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+package ch.ethz.seb.sebserver.webservice.servicelayer.session;
+
+import java.util.function.Consumer;
+
+import ch.ethz.seb.sebserver.gbl.model.session.ClientEvent;
+
+public interface EventHandlingStrategy extends Consumer<ClientEvent> {
+
+    String EVENT_CONSUMER_STRATEGY_CONFIG_PROPERTY_KEY = "sebserver.webservice.api.exam.event-handling-strategy";
+    String EVENT_CONSUMER_STRATEGY_SINGLE_EVENT_STORE = "SINGLE_EVENT_STORE_STRATEGY";
+    String EVENT_CONSUMER_STRATEGY_ASYNC_BATCH_STORE = "ASYNC_BATCH_STORE_STRATEGY";
+
+}
diff --git a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/ExamSessionService.java b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/ExamSessionService.java
new file mode 100644
index 00000000..4545a752
--- /dev/null
+++ b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/ExamSessionService.java
@@ -0,0 +1,29 @@
+/*
+ * Copyright (c) 2019 ETH Zürich, Educational Development and Technology (LET)
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+package ch.ethz.seb.sebserver.webservice.servicelayer.session;
+
+import java.util.Collection;
+
+import ch.ethz.seb.sebserver.gbl.model.exam.Exam;
+import ch.ethz.seb.sebserver.gbl.model.session.ClientEvent;
+import ch.ethz.seb.sebserver.gbl.util.Result;
+
+public interface ExamSessionService {
+
+    boolean isExamRunning(Long examId);
+
+    Result<Exam> getRunningExam(Long examId);
+
+    Result<Collection<Exam>> getRunningExamsForInstitution(Long institutionId);
+
+    void notifyPing(Long connectionId, long timestamp, int pingNumber);
+
+    void notifyClientEvent(final ClientEvent event, Long connectionId);
+
+}
diff --git a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/SebClientConnectionService.java b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/SebClientConnectionService.java
new file mode 100644
index 00000000..fb47953b
--- /dev/null
+++ b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/SebClientConnectionService.java
@@ -0,0 +1,25 @@
+/*
+ * Copyright (c) 2019 ETH Zürich, Educational Development and Technology (LET)
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+package ch.ethz.seb.sebserver.webservice.servicelayer.session;
+
+import ch.ethz.seb.sebserver.gbl.model.session.ClientConnection;
+import ch.ethz.seb.sebserver.gbl.util.Result;
+
+public interface SebClientConnectionService {
+
+    Result<ClientConnection> createClientConnection(Long instituionId, Long examId);
+
+    Result<ClientConnection> establishClientConnection(
+            String connectionToken,
+            Long examId,
+            String userSessionId);
+
+    Result<ClientConnection> closeConnection(String connectionToken);
+
+}
diff --git a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/AbstractClientIndicator.java b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/AbstractClientIndicator.java
new file mode 100644
index 00000000..6913f891
--- /dev/null
+++ b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/AbstractClientIndicator.java
@@ -0,0 +1,61 @@
+/*
+ * Copyright (c) 2018 ETH Zürich, Educational Development and Technology (LET)
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+package ch.ethz.seb.sebserver.webservice.servicelayer.session.impl;
+
+import org.joda.time.DateTime;
+import org.joda.time.DateTimeZone;
+
+import ch.ethz.seb.sebserver.gbl.model.exam.Indicator;
+import ch.ethz.seb.sebserver.webservice.servicelayer.session.ClientIndicator;
+
+public abstract class AbstractClientIndicator implements ClientIndicator {
+
+    protected Long examId;
+    protected Long connectionId;
+    protected String name;
+    protected boolean cachingEnabled;
+
+    protected double currentValue = Double.NaN;
+
+    @Override
+    public void init(
+            final Indicator indicatorDefinition,
+            final Long connectionId,
+            final boolean cachingEnabled) {
+
+        this.examId = (indicatorDefinition != null) ? indicatorDefinition.examId : null;
+        this.connectionId = connectionId;
+        this.cachingEnabled = cachingEnabled;
+    }
+
+    @Override
+    public Long examId() {
+        return this.examId;
+    }
+
+    @Override
+    public Long connectionId() {
+        return this.connectionId;
+    }
+
+    @Override
+    public String name() {
+        return this.name;
+    }
+
+    @Override
+    public double getValue() {
+        if (this.currentValue == Double.NaN || !this.cachingEnabled) {
+            this.currentValue = computeValueAt(DateTime.now(DateTimeZone.UTC).getMillis());
+        }
+
+        return this.currentValue;
+    }
+
+}
diff --git a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/AbstractPingIndicator.java b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/AbstractPingIndicator.java
new file mode 100644
index 00000000..10d9132f
--- /dev/null
+++ b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/AbstractPingIndicator.java
@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2019 ETH Zürich, Educational Development and Technology (LET)
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+package ch.ethz.seb.sebserver.webservice.servicelayer.session.impl;
+
+import java.util.Collections;
+import java.util.EnumSet;
+import java.util.Set;
+
+import ch.ethz.seb.sebserver.gbl.model.session.ClientEvent.EventType;
+
+public abstract class AbstractPingIndicator extends AbstractClientIndicator {
+
+    private final Set<EventType> EMPTY_SET = Collections.unmodifiableSet(EnumSet.noneOf(EventType.class));
+
+    protected int pingCount = 0;
+    protected int pingNumber = 0;
+
+    public void notifyPing(final long timestamp, final int pingNumber) {
+        super.currentValue = timestamp;
+        this.pingCount++;
+        this.pingNumber = pingNumber;
+    }
+
+    @Override
+    public double computeValueAt(final long timestamp) {
+        return timestamp;
+    }
+
+    @Override
+    public Set<EventType> observedEvents() {
+        return this.EMPTY_SET;
+    }
+
+}
diff --git a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/AsyncBatchEventSaveStrategy.java b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/AsyncBatchEventSaveStrategy.java
new file mode 100644
index 00000000..2a0f821e
--- /dev/null
+++ b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/AsyncBatchEventSaveStrategy.java
@@ -0,0 +1,156 @@
+/*
+ * Copyright (c) 2018 ETH Zürich, Educational Development and Technology (LET)
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+package ch.ethz.seb.sebserver.webservice.servicelayer.session.impl;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.concurrent.BlockingDeque;
+import java.util.concurrent.Executor;
+import java.util.concurrent.LinkedBlockingDeque;
+
+import org.apache.ibatis.session.ExecutorType;
+import org.apache.ibatis.session.SqlSessionFactory;
+import org.mybatis.spring.SqlSessionTemplate;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.springframework.boot.context.event.ApplicationReadyEvent;
+import org.springframework.context.annotation.Lazy;
+import org.springframework.context.event.EventListener;
+import org.springframework.scheduling.annotation.AsyncConfigurer;
+import org.springframework.stereotype.Component;
+import org.springframework.transaction.PlatformTransactionManager;
+import org.springframework.transaction.TransactionDefinition;
+import org.springframework.transaction.support.TransactionTemplate;
+
+import ch.ethz.seb.sebserver.gbl.model.session.ClientEvent;
+import ch.ethz.seb.sebserver.gbl.profile.WebServiceProfile;
+import ch.ethz.seb.sebserver.webservice.datalayer.batis.mapper.ClientEventRecordMapper;
+import ch.ethz.seb.sebserver.webservice.servicelayer.session.EventHandlingStrategy;
+
+/** Approach 2 to handle/save client events internally
+ *
+ * This Approach uses a queue to collect ClientEvents that are stored later. The queue is shared between some
+ * worker-threads that batch gets and stores the events from the queue afterwards. this approach is less blocking from
+ * the caller perspective and also faster on store data by using bulk-insert
+ *
+ * A disadvantage is an potentially multiple event data loss on total server fail. The data in the queue is state that
+ * is not stored somewhere yet and can't be recovered on total server fail.
+ *
+ * If the performance of this approach is not enough or the potentially data loss on total server fail is a risk that
+ * not can be taken, we have to consider using a messaging system/server like rabbitMQ or Apache-Kafka that brings the
+ * ability to effectively store and recover message queues but also comes with more complexity on setup and installation
+ * side as well as for the whole server system. */
+@Lazy
+@Component(EventHandlingStrategy.EVENT_CONSUMER_STRATEGY_ASYNC_BATCH_STORE)
+@WebServiceProfile
+public class AsyncBatchEventSaveStrategy implements EventHandlingStrategy {
+
+    private static final Logger log = LoggerFactory.getLogger(AsyncBatchEventSaveStrategy.class);
+
+    private static final int NUMBER_OF_WORKER_THREADS = 4;
+    private static final int BATCH_SIZE = 100;
+
+    private final SqlSessionFactory sqlSessionFactory;
+    private final Executor executor;
+    private final TransactionTemplate transactionTemplate;
+
+    private final BlockingDeque<ClientEvent> eventQueue = new LinkedBlockingDeque<>();
+    private boolean workersRunning = false;
+
+    public AsyncBatchEventSaveStrategy(
+            final SqlSessionFactory sqlSessionFactory,
+            final AsyncConfigurer asyncConfigurer,
+            final PlatformTransactionManager transactionManager) {
+
+        this.sqlSessionFactory = sqlSessionFactory;
+        this.executor = asyncConfigurer.getAsyncExecutor();
+
+        this.transactionTemplate = new TransactionTemplate(transactionManager);
+        this.transactionTemplate.setPropagationBehavior(TransactionDefinition.PROPAGATION_REQUIRES_NEW);
+    }
+
+    @EventListener(ApplicationReadyEvent.class)
+    protected void recover() {
+        runWorkers();
+    }
+
+    @Override
+    public void accept(final ClientEvent event) {
+        if (!this.workersRunning) {
+            log.error("Received ClientEvent on none enabled AsyncBatchEventSaveStrategy. ClientEvent is ignored");
+            return;
+        }
+
+        this.eventQueue.add(event);
+    }
+
+    private void runWorkers() {
+        if (this.workersRunning) {
+            log.warn("runWorkers called when workers are running already. Ignore that");
+            return;
+        }
+
+        this.workersRunning = true;
+
+        log.info("Start {} Event-Batch-Store Worker-Threads", NUMBER_OF_WORKER_THREADS);
+        for (int i = 0; i < NUMBER_OF_WORKER_THREADS; i++) {
+            this.executor.execute(batchSave());
+        }
+    }
+
+    private Runnable batchSave() {
+        return () -> {
+
+            log.debug("Worker Thread {} running", Thread.currentThread());
+
+            final Collection<ClientEvent> events = new ArrayList<>();
+            final SqlSessionTemplate sqlSessionTemplate = new SqlSessionTemplate(
+                    this.sqlSessionFactory,
+                    ExecutorType.BATCH);
+            final ClientEventRecordMapper clientEventMapper = sqlSessionTemplate.getMapper(
+                    ClientEventRecordMapper.class);
+
+            long sleepTime = 100;
+
+            try {
+                while (this.workersRunning) {
+                    events.clear();
+                    this.eventQueue.drainTo(events, BATCH_SIZE);
+
+                    try {
+                        if (!events.isEmpty()) {
+                            sleepTime = 100;
+                            this.transactionTemplate
+                                    .execute(status -> {
+                                        events.stream()
+                                                .map(ClientEvent::toRecord)
+                                                .forEach(clientEventMapper::insert);
+                                        return null;
+                                    });
+                        } else {
+                            sleepTime += 100;
+                        }
+                    } catch (final Exception e) {
+                        log.error("unexpected Error while trying to batch store client-events: ", e);
+                    }
+
+                    try {
+                        Thread.sleep(sleepTime);
+                    } catch (final InterruptedException e) {
+                        e.printStackTrace();
+                    }
+                }
+            } finally {
+                sqlSessionTemplate.close();
+                log.debug("Worker Thread {} stopped", Thread.currentThread());
+            }
+        };
+    }
+
+}
diff --git a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/ClientConnectionDataInternal.java b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/ClientConnectionDataInternal.java
new file mode 100644
index 00000000..cc555f7d
--- /dev/null
+++ b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/ClientConnectionDataInternal.java
@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) 2019 ETH Zürich, Educational Development and Technology (LET)
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+package ch.ethz.seb.sebserver.webservice.servicelayer.session.impl;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.EnumMap;
+
+import ch.ethz.seb.sebserver.gbl.model.session.ClientConnection;
+import ch.ethz.seb.sebserver.gbl.model.session.ClientConnectionData;
+import ch.ethz.seb.sebserver.gbl.model.session.ClientEvent.EventType;
+import ch.ethz.seb.sebserver.webservice.servicelayer.session.ClientIndicator;
+
+public class ClientConnectionDataInternal extends ClientConnectionData {
+
+    final Collection<AbstractPingIndicator> pingMappings;
+    final EnumMap<EventType, Collection<ClientIndicator>> indicatorMapping;
+
+    protected ClientConnectionDataInternal(
+            final ClientConnection clientConnection,
+            final Collection<ClientIndicator> clientIndicators) {
+
+        super(clientConnection, clientIndicators);
+
+        this.indicatorMapping = new EnumMap<>(EventType.class);
+        this.pingMappings = new ArrayList<>();
+        for (final ClientIndicator clientIndicator : clientIndicators) {
+            if (clientIndicator instanceof AbstractPingIndicator) {
+                this.pingMappings.add((AbstractPingIndicator) clientIndicator);
+            }
+            for (final EventType eventType : clientIndicator.observedEvents()) {
+                this.indicatorMapping
+                        .computeIfAbsent(eventType, key -> new ArrayList<>())
+                        .add(clientIndicator);
+            }
+        }
+    }
+
+    Collection<ClientIndicator> getindicatorMapping(final EventType eventType) {
+        if (!this.indicatorMapping.containsKey(eventType)) {
+            return Collections.emptyList();
+        }
+
+        return this.indicatorMapping.get(eventType);
+    }
+
+}
diff --git a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/ClientIndicatorFactory.java b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/ClientIndicatorFactory.java
new file mode 100644
index 00000000..ea08ff5a
--- /dev/null
+++ b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/ClientIndicatorFactory.java
@@ -0,0 +1,79 @@
+/*
+ * Copyright (c) 2018 ETH Zürich, Educational Development and Technology (LET)
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+package ch.ethz.seb.sebserver.webservice.servicelayer.session.impl;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.context.ApplicationContext;
+import org.springframework.context.annotation.Lazy;
+import org.springframework.stereotype.Component;
+
+import ch.ethz.seb.sebserver.gbl.model.exam.Indicator;
+import ch.ethz.seb.sebserver.gbl.model.session.ClientConnection;
+import ch.ethz.seb.sebserver.gbl.profile.WebServiceProfile;
+import ch.ethz.seb.sebserver.webservice.servicelayer.dao.IndicatorDAO;
+import ch.ethz.seb.sebserver.webservice.servicelayer.session.ClientIndicator;
+
+@Lazy
+@Component
+@WebServiceProfile
+public class ClientIndicatorFactory {
+
+    private static final Logger log = LoggerFactory.getLogger(ClientIndicatorFactory.class);
+
+    private final ApplicationContext applicationContext;
+    private final IndicatorDAO indicatorDAO;
+    private final boolean enableCaching;
+
+    @Autowired
+    public ClientIndicatorFactory(
+            final ApplicationContext applicationContext,
+            final IndicatorDAO indicatorDAO,
+            @Value("${sebserver.indicator.caching}") final boolean enableCaching) {
+
+        this.applicationContext = applicationContext;
+        this.indicatorDAO = indicatorDAO;
+        this.enableCaching = enableCaching;
+    }
+
+    public Collection<ClientIndicator> createFor(final ClientConnection clientConnection) {
+        final List<ClientIndicator> result = new ArrayList<>();
+
+        try {
+
+            final Collection<Indicator> examIndicators = this.indicatorDAO
+                    .allForExam(clientConnection.examId)
+                    .getOrThrow();
+
+            for (final Indicator indicatorDef : examIndicators) {
+                try {
+                    final ClientIndicator indicator = this.applicationContext
+                            .getBean(indicatorDef.type.name(), ClientIndicator.class);
+                    indicator.init(indicatorDef, clientConnection.id, this.enableCaching);
+                    result.add(indicator);
+                } catch (final Exception e) {
+                    log.warn("No Indicator with type: {} found as registered bean. Ignore this one.", indicatorDef.type,
+                            e);
+                }
+            }
+        } catch (final Exception e) {
+            log.error("Failed to create ClientIndicator for ClientConnection: {}", clientConnection);
+        }
+
+        return Collections.unmodifiableList(result);
+    }
+
+}
diff --git a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/ErrorCountClientIndicator.java b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/ErrorCountClientIndicator.java
new file mode 100644
index 00000000..a650c956
--- /dev/null
+++ b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/ErrorCountClientIndicator.java
@@ -0,0 +1,71 @@
+/*
+ * Copyright (c) 2019 ETH Zürich, Educational Development and Technology (LET)
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+package ch.ethz.seb.sebserver.webservice.servicelayer.session.impl;
+
+import static org.mybatis.dynamic.sql.SqlBuilder.isEqualTo;
+import static org.mybatis.dynamic.sql.SqlBuilder.isLessThan;
+
+import java.util.Collections;
+import java.util.EnumSet;
+import java.util.Set;
+
+import org.springframework.beans.factory.config.ConfigurableBeanFactory;
+import org.springframework.context.annotation.Lazy;
+import org.springframework.context.annotation.Scope;
+import org.springframework.stereotype.Component;
+
+import ch.ethz.seb.sebserver.gbl.model.exam.Indicator.IndicatorType;
+import ch.ethz.seb.sebserver.gbl.model.session.ClientEvent;
+import ch.ethz.seb.sebserver.gbl.model.session.ClientEvent.EventType;
+import ch.ethz.seb.sebserver.gbl.model.session.IndicatorValueHolder;
+import ch.ethz.seb.sebserver.webservice.datalayer.batis.mapper.ClientEventRecordDynamicSqlSupport;
+import ch.ethz.seb.sebserver.webservice.datalayer.batis.mapper.ClientEventRecordMapper;
+
+@Lazy
+@Component(IndicatorType.Names.ERROR_COUNT)
+@Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
+public final class ErrorCountClientIndicator extends AbstractClientIndicator {
+
+    private final Set<EventType> OBSERVED_SET = Collections.unmodifiableSet(EnumSet.of(EventType.ERROR_LOG));
+
+    private final ClientEventRecordMapper clientEventRecordMapper;
+
+    protected ErrorCountClientIndicator(final ClientEventRecordMapper clientEventRecordMapper) {
+        this.clientEventRecordMapper = clientEventRecordMapper;
+    }
+
+    @Override
+    public IndicatorType getType() {
+        return IndicatorType.ERROR_COUNT;
+    }
+
+    @Override
+    public double computeValueAt(final long timestamp) {
+
+        final Long errors = this.clientEventRecordMapper.countByExample()
+                .where(ClientEventRecordDynamicSqlSupport.connectionId, isEqualTo(this.connectionId))
+                .and(ClientEventRecordDynamicSqlSupport.type, isEqualTo(ClientEvent.EventType.ERROR_LOG.id))
+                .and(ClientEventRecordDynamicSqlSupport.timestamp, isLessThan(timestamp))
+                .build()
+                .execute();
+
+        return errors.doubleValue();
+    }
+
+    @Override
+    public void notifyValueChange(final IndicatorValueHolder indicatorValueHolder) {
+        this.currentValue = getValue() + 1d;
+    }
+
+    @Override
+    public Set<EventType> observedEvents() {
+        return this.OBSERVED_SET;
+    }
+
+}
diff --git a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/ExamSessionCacheService.java b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/ExamSessionCacheService.java
new file mode 100644
index 00000000..3fa94972
--- /dev/null
+++ b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/ExamSessionCacheService.java
@@ -0,0 +1,132 @@
+/*
+ * Copyright (c) 2019 ETH Zürich, Educational Development and Technology (LET)
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+package ch.ethz.seb.sebserver.webservice.servicelayer.session.impl;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.springframework.cache.annotation.CacheEvict;
+import org.springframework.cache.annotation.Cacheable;
+import org.springframework.context.annotation.Lazy;
+import org.springframework.stereotype.Service;
+
+import ch.ethz.seb.sebserver.gbl.model.exam.Exam;
+import ch.ethz.seb.sebserver.gbl.model.session.ClientConnection;
+import ch.ethz.seb.sebserver.gbl.profile.WebServiceProfile;
+import ch.ethz.seb.sebserver.gbl.util.Result;
+import ch.ethz.seb.sebserver.webservice.servicelayer.dao.ClientConnectionDAO;
+import ch.ethz.seb.sebserver.webservice.servicelayer.dao.ExamDAO;
+
+@Lazy
+@Service
+@WebServiceProfile
+public class ExamSessionCacheService {
+
+    public static final String CACHE_NAME_RUNNING_EXAM = "RUNNING_EXAM";
+    public static final String CACHE_NAME_ACTIVE_CLIENT_CONNECTION = "ACTIVE_CLIENT_CONNECTION";
+
+    private static final Logger log = LoggerFactory.getLogger(ExamSessionCacheService.class);
+
+    private final ExamDAO examDAO;
+    private final ClientConnectionDAO clientConnectionDAO;
+    private final ClientIndicatorFactory clientIndicatorFactory;
+
+    protected ExamSessionCacheService(
+            final ExamDAO examDAO,
+            final ClientConnectionDAO clientConnectionDAO,
+            final ClientIndicatorFactory clientIndicatorFactory) {
+
+        this.examDAO = examDAO;
+        this.clientConnectionDAO = clientConnectionDAO;
+        this.clientIndicatorFactory = clientIndicatorFactory;
+    }
+
+    @Cacheable(
+            cacheNames = CACHE_NAME_RUNNING_EXAM,
+            key = "#examId",
+            unless = "#result == null")
+    Exam getRunningExam(final Long examId) {
+
+        if (log.isDebugEnabled()) {
+            log.debug("Verify running exam for id: {}" + examId);
+        }
+
+        final Result<Exam> byPK = this.examDAO.byPK(examId);
+        if (byPK.hasError()) {
+            log.error("Failed to find/load Exam with id {}", examId, byPK.getError());
+            return null;
+        }
+
+        final Exam exam = byPK.get();
+        if (!isRunning(exam)) {
+            return null;
+        }
+
+        return exam;
+    }
+
+    @CacheEvict(
+            cacheNames = CACHE_NAME_RUNNING_EXAM,
+            key = "#exam.id",
+            condition = "#target.isRunning(#result)")
+    Exam evict(final Exam exam) {
+
+        if (log.isDebugEnabled()) {
+            log.debug("Conditional eviction of running Exam from cache: {}", isRunning(exam));
+        }
+
+        return exam;
+    }
+
+    boolean isRunning(final Exam exam) {
+        if (exam == null) {
+            return false;
+        }
+        return ((exam.startTime.isEqualNow() || exam.startTime.isBeforeNow()) &&
+                exam.endTime.isAfterNow());
+    }
+
+    @Cacheable(
+            cacheNames = CACHE_NAME_ACTIVE_CLIENT_CONNECTION,
+            key = "#connectionId",
+            unless = "#result == null")
+    ClientConnectionDataInternal getActiveClientConnection(final Long connectionId) {
+
+        if (log.isDebugEnabled()) {
+            log.debug("Verify ClientConnection for running exam for caching by id: ", connectionId);
+        }
+
+        final Result<ClientConnection> byPK = this.clientConnectionDAO.byPK(connectionId);
+        if (byPK.hasError()) {
+            log.error("Failed to find/load ClientConnection with id {}", connectionId, byPK.getError());
+            return null;
+        }
+
+        final ClientConnection clientConnection = byPK.get();
+
+        // verify exam is running
+        if (getRunningExam(clientConnection.examId) == null) {
+            log.error("Exam for ClientConnection with id { is not currently running}", connectionId);
+            return null;
+        }
+
+        return new ClientConnectionDataInternal(
+                clientConnection,
+                this.clientIndicatorFactory.createFor(clientConnection));
+    }
+
+    @CacheEvict(
+            cacheNames = CACHE_NAME_ACTIVE_CLIENT_CONNECTION,
+            key = "#connectionId")
+    void evict(final Long connectionId) {
+        if (log.isDebugEnabled()) {
+            log.debug("Eviction of ClientConnectionData from cache: {}", connectionId);
+        }
+    }
+
+}
diff --git a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/ExamSessionServiceImpl.java b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/ExamSessionServiceImpl.java
new file mode 100644
index 00000000..37c94e02
--- /dev/null
+++ b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/ExamSessionServiceImpl.java
@@ -0,0 +1,114 @@
+/*
+ * Copyright (c) 2019 ETH Zürich, Educational Development and Technology (LET)
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+package ch.ethz.seb.sebserver.webservice.servicelayer.session.impl;
+
+import java.util.Collection;
+import java.util.NoSuchElementException;
+import java.util.stream.Collectors;
+
+import org.springframework.context.ApplicationContext;
+import org.springframework.context.annotation.Lazy;
+import org.springframework.core.env.Environment;
+import org.springframework.stereotype.Service;
+
+import ch.ethz.seb.sebserver.gbl.model.exam.Exam;
+import ch.ethz.seb.sebserver.gbl.model.session.ClientEvent;
+import ch.ethz.seb.sebserver.gbl.profile.WebServiceProfile;
+import ch.ethz.seb.sebserver.gbl.util.Result;
+import ch.ethz.seb.sebserver.webservice.servicelayer.dao.ExamDAO;
+import ch.ethz.seb.sebserver.webservice.servicelayer.session.EventHandlingStrategy;
+import ch.ethz.seb.sebserver.webservice.servicelayer.session.ExamSessionService;
+import io.micrometer.core.instrument.util.StringUtils;
+
+@Lazy
+@Service
+@WebServiceProfile
+public class ExamSessionServiceImpl implements ExamSessionService {
+
+    private final ExamSessionCacheService examSessionCacheService;
+    private final ExamDAO examDAO;
+    private final EventHandlingStrategy eventHandlingStrategy;
+
+    protected ExamSessionServiceImpl(
+            final ExamSessionCacheService examSessionCacheService,
+            final ExamDAO examDAO,
+            final Environment environment,
+            final ApplicationContext applicationContext) {
+
+        this.examSessionCacheService = examSessionCacheService;
+        this.examDAO = examDAO;
+
+        String eventHandlingStrategyProperty =
+                environment.getProperty(EventHandlingStrategy.EVENT_CONSUMER_STRATEGY_CONFIG_PROPERTY_KEY);
+
+        if (StringUtils.isBlank(eventHandlingStrategyProperty)) {
+            eventHandlingStrategyProperty = EventHandlingStrategy.EVENT_CONSUMER_STRATEGY_SINGLE_EVENT_STORE;
+        }
+
+        this.eventHandlingStrategy = applicationContext.getBean(
+                eventHandlingStrategyProperty,
+                EventHandlingStrategy.class);
+    }
+
+    @Override
+    public boolean isExamRunning(final Long examId) {
+        return this.examSessionCacheService
+                .isRunning(this.examSessionCacheService.getRunningExam(examId));
+    }
+
+    @Override
+    public Result<Exam> getRunningExam(final Long examId) {
+        final Exam exam = this.examSessionCacheService.getRunningExam(examId);
+        if (this.examSessionCacheService.isRunning(exam)) {
+            return Result.of(exam);
+        } else {
+            if (exam != null) {
+                this.examSessionCacheService.evict(exam);
+            }
+            return Result.ofError(new NoSuchElementException(
+                    "No currenlty running exam found for id: " + examId));
+        }
+    }
+
+    @Override
+    public Result<Collection<Exam>> getRunningExamsForInstitution(final Long institutionId) {
+        return this.examDAO.allIdsOfInstituion(institutionId)
+                .map(col -> col.stream()
+                        .map(examId -> this.examSessionCacheService.getRunningExam(examId))
+                        .filter(exam -> exam != null)
+                        .collect(Collectors.toList()));
+    }
+
+    @Override
+    public void notifyPing(final Long connectionId, final long timestamp, final int pingNumber) {
+        final ClientConnectionDataInternal activeClientConnection =
+                this.examSessionCacheService.getActiveClientConnection(connectionId);
+
+        if (activeClientConnection != null) {
+            activeClientConnection.pingMappings
+                    .stream()
+                    .forEach(pingIndicator -> pingIndicator.notifyPing(timestamp, pingNumber));
+        }
+    }
+
+    @Override
+    public void notifyClientEvent(final ClientEvent event, final Long connectionId) {
+        this.eventHandlingStrategy.accept(event);
+
+        final ClientConnectionDataInternal activeClientConnection =
+                this.examSessionCacheService.getActiveClientConnection(connectionId);
+
+        if (activeClientConnection != null) {
+            activeClientConnection.getindicatorMapping(event.eventType)
+                    .stream()
+                    .forEach(indicator -> indicator.notifyValueChange(event));
+        }
+    }
+
+}
diff --git a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/PingIntervalClientIndicator.java b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/PingIntervalClientIndicator.java
new file mode 100644
index 00000000..ff0eb815
--- /dev/null
+++ b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/PingIntervalClientIndicator.java
@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2019 ETH Zürich, Educational Development and Technology (LET)
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+package ch.ethz.seb.sebserver.webservice.servicelayer.session.impl;
+
+import org.joda.time.DateTime;
+import org.joda.time.DateTimeZone;
+import org.springframework.beans.factory.config.ConfigurableBeanFactory;
+import org.springframework.context.annotation.Lazy;
+import org.springframework.context.annotation.Scope;
+import org.springframework.stereotype.Component;
+
+import ch.ethz.seb.sebserver.gbl.model.exam.Indicator.IndicatorType;
+import ch.ethz.seb.sebserver.gbl.model.session.IndicatorValueHolder;
+
+@Lazy
+@Component(IndicatorType.Names.LAST_PING)
+@Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
+public final class PingIntervalClientIndicator extends AbstractPingIndicator {
+
+    public PingIntervalClientIndicator() {
+        this.cachingEnabled = true;
+        this.currentValue = computeValueAt(DateTime.now(DateTimeZone.UTC).getMillis());
+    }
+
+    @Override
+    public IndicatorType getType() {
+        return IndicatorType.LAST_PING;
+    }
+
+    @Override
+    public double getValue() {
+        final long now = DateTime.now(DateTimeZone.UTC).getMillis();
+        return now - super.currentValue;
+    }
+
+    @Override
+    public void notifyValueChange(final IndicatorValueHolder indicatorValueHolder) {
+
+    }
+
+}
diff --git a/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/SingleEventSaveStrategy.java b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/SingleEventSaveStrategy.java
new file mode 100644
index 00000000..b11f6e5a
--- /dev/null
+++ b/src/main/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/SingleEventSaveStrategy.java
@@ -0,0 +1,43 @@
+/*
+ * Copyright (c) 2018 ETH Zürich, Educational Development and Technology (LET)
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+package ch.ethz.seb.sebserver.webservice.servicelayer.session.impl;
+
+import org.springframework.context.annotation.Lazy;
+import org.springframework.stereotype.Component;
+
+import ch.ethz.seb.sebserver.gbl.model.session.ClientEvent;
+import ch.ethz.seb.sebserver.gbl.profile.WebServiceProfile;
+import ch.ethz.seb.sebserver.webservice.servicelayer.dao.ClientEventDAO;
+import ch.ethz.seb.sebserver.webservice.servicelayer.session.EventHandlingStrategy;
+
+/** Approach 1 to handle/save client events internally
+ *
+ * This saves one on one client event to persistence within separated transaction.
+ * NOTE: if there are a lot of clients connected, firing events at small intervals like 100ms,
+ * this is blocking to much because every event is saved within its own SQL commit and also
+ * in its own transaction.
+ *
+ * An advantage of this approach is minimal data loss on server fail. **/
+@Lazy
+@Component(EventHandlingStrategy.EVENT_CONSUMER_STRATEGY_SINGLE_EVENT_STORE)
+@WebServiceProfile
+public class SingleEventSaveStrategy implements EventHandlingStrategy {
+
+    private final ClientEventDAO clientEventDAO;
+
+    public SingleEventSaveStrategy(final ClientEventDAO clientEventDAO) {
+        this.clientEventDAO = clientEventDAO;
+    }
+
+    @Override
+    public void accept(final ClientEvent event) {
+        this.clientEventDAO.save(event);
+    }
+
+}
diff --git a/src/main/resources/config/application-dev-ws.properties b/src/main/resources/config/application-dev-ws.properties
index c42025e6..04938194 100644
--- a/src/main/resources/config/application-dev-ws.properties
+++ b/src/main/resources/config/application-dev-ws.properties
@@ -19,6 +19,7 @@ sebserver.webservice.api.exam.endpoint.discovery=${sebserver.webservice.api.exam
 sebserver.webservice.api.exam.endpoint.v1=${sebserver.webservice.api.exam.endpoint}/v1
 sebserver.webservice.api.exam.accessTokenValiditySeconds=1800
 sebserver.webservice.api.exam.refreshTokenValiditySeconds=-1
+sebserver.webservice.api.exam.event-handling-strategy=ASYNC_BATCH_STORE_STRATEGY
 sebserver.webservice.api.pagination.maxPageSize=500
 
 management.endpoints.web.base-path=/actuator
diff --git a/src/test/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/PingIntervalClientIndicatorTest.java b/src/test/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/PingIntervalClientIndicatorTest.java
new file mode 100644
index 00000000..2885f7f0
--- /dev/null
+++ b/src/test/java/ch/ethz/seb/sebserver/webservice/servicelayer/session/impl/PingIntervalClientIndicatorTest.java
@@ -0,0 +1,52 @@
+/*
+ * Copyright (c) 2019 ETH Zürich, Educational Development and Technology (LET)
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+package ch.ethz.seb.sebserver.webservice.servicelayer.session.impl;
+
+import static org.junit.Assert.assertEquals;
+
+import org.joda.time.DateTimeUtils;
+import org.junit.Test;
+
+import com.fasterxml.jackson.core.JsonProcessingException;
+
+import ch.ethz.seb.sebserver.gbl.api.JSONMapper;
+
+public class PingIntervalClientIndicatorTest {
+
+    @Test
+    public void testCreation() {
+
+        DateTimeUtils.setCurrentMillisFixed(1);
+
+        final PingIntervalClientIndicator pingIntervalClientIndicator = new PingIntervalClientIndicator();
+        assertEquals("0.0", String.valueOf(pingIntervalClientIndicator.getValue()));
+    }
+
+    @Test
+    public void testInterval() {
+        DateTimeUtils.setCurrentMillisFixed(1);
+
+        final PingIntervalClientIndicator pingIntervalClientIndicator = new PingIntervalClientIndicator();
+        assertEquals("0.0", String.valueOf(pingIntervalClientIndicator.getValue()));
+
+        DateTimeUtils.setCurrentMillisFixed(10);
+
+        assertEquals("9.0", String.valueOf(pingIntervalClientIndicator.getValue()));
+    }
+
+    @Test
+    public void testSerialization() throws JsonProcessingException {
+        DateTimeUtils.setCurrentMillisFixed(1);
+        final PingIntervalClientIndicator pingIntervalClientIndicator = new PingIntervalClientIndicator();
+        final JSONMapper jsonMapper = new JSONMapper();
+        final String json = jsonMapper.writeValueAsString(pingIntervalClientIndicator);
+        assertEquals("{\"indicatorValue\":0.0,\"indicatorType\":\"LAST_PING\"}", json);
+    }
+
+}