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Chapter 5. Clustering and High Availability

5.1. Mobicents SIP Servlets for JBoss: Clustering Support
5.1.1. SIP Servlets Server Cluster: Installing, Configuring and Running
5.1.2. SIP Sessions Passivation/Activation
5.2. Mobicents SIP Servlets for JBoss: Transparent Failover
5.2.1. Mobicents Failover Capabilities
5.2.2. MSS for JBoss Cluster: Installing, Configuring and Running
5.3. Load Balancer
5.3.1. SIP Load Balancer: Installing, Configuring and Running
5.3.2. IP Load Balancing
5.3.3. SIP Load Balancing Basics
5.3.4. HTTP Load Balancing Basics
5.3.5. Pluggable balancer algorithms
5.3.6. Distributed load balancing
5.3.7. Implementation of the Mobicents Load Balancer
5.3.8. SIP Message Flow

Mobicents supports the clustering of SIP Servlets-enabled JBoss Application Servers for performance, reliability and failover purposes. Note that only MSS for JBoss Servers can be used as cluster nodes; MSS for Tomcat Containers are not supported.

The SIP Servlets Server uses the JBoss Application Server as its servlet container, and takes advantage of its capabilities, including clustering and failover. For detailed background information about JBoss Application Server clustering refer to the JBoss Application Server Clustering Guide.

Software Prerequisites

A SIP Servlets-enabled JBoss Application Server

Before proceeding, ensure you have correctly configured your JBoss Application Server, according to SIP Servlet Server requirements:

The easiest way to set up a cluster of SIP Servlets-enabled JBoss Application Servers is to install, configure and test the binary distribution on one machine, and then copy the entire installation (directory) to the other machines in the cluster. This is the approach taken in this chapter.

Install a SIP Servlets Server with JBoss by following the instructions detailed in Section 2.1, “SIP Servlet-Enabled JBoss Application Server: Installing, Configuring and Running”.

Afer meeting the requirement you can begin to configure the server Section 5.1.1.2, “Configuring” below.

Clustering with MSS for JBoss nodes requires running all of the nodes using the "all" Server Configuration Profile, which is specified when you invoke run.sh or run.bat.

Running MSS for JBoss with the "all" Configuration Profile, on Linux

To run the server on Linux using the "all" Configuration Profile, start the server with the following command:

MSS-jboss-<version>]$ ./bin/run.sh -c all
Running MSS for JBoss with the "all" Configuration Profile, on Windows

To run the server on Windows using the "all" Configuration Profile, open the Command Prompt, change your folder to the topmost folder of your MSS for JBoss installation, and issue the following command:

C:Usersuser\<username>My DownloadsMSS-jboss-<version>>binrun.bat -c all
Distributing requests between nodes

Together with the application server nodes, it is advised to run a SIP load-balancer or an IP load-balancer. The IP load balancer will distribute the traffic evenly between the nodes. A load-balancer is a single entry-point to all nodes. All calls should be made through the load balancer if High Availability is required. For more information about load balancing, refer to ???.

By default, the servers are configured with one SIP load-balancer set to the IP address 127.0.0.1. This is specified in the balancers attribute in the server.xml configuration file as follows:


<Service name="jboss.web" 
      className="org.mobicents.servlet.sip.startup.failover.SipStandardBalancerNodeService"
  balancers="127.0.0.1"  
  sipPathName="org.mobicents.ha"  
  sipApplicationDispatcherClassName="org.mobicents.servlet.sip.core.SipApplicationDispatcherImpl"
concurrencyControlMode="None" 
  darConfigurationFileLocation="conf/dars/mobicents-dar.properties"
  sipStackPropertiesFile="conf/mss-sip-stack.properties">

Multiple load balancers can be specified and all of them will be updated on the health status of the node. The complete syntax for the balancers string is the following:


<Service name="jboss.web" 
      ...
  balancers="ipAddress1:sipPort1:rmiPort1;ipAddress2:sipPort2:rmiPort2;..3...4.."  
  ...>

If the RMI port is omitted, port 2000 is assumed, and if the SIP port is omitted, 5065 is assumed.

Warning

The SIP port specified in the balancers string for each balancer refers to the internal SIP port of the SIP balancer. That is because the internal port faces the cluster nodes directly. Requests coming through the internal port will go to the external port and vice versa. If you put the external port in the balancers string, then the SIP LB will assume that the requests comes from outside the cluster and it will route it back to some cluster node instead of routing it outside the cluster as expected. Always use the SIP internal port in the balancers string. The exception to this rule is when a single port is used for internal and external ports in the SIP load balancer. In that case, the direction analysis is done based on Via headers and the requests are routed correctly without extra settings.

When multiple SIP load balancers are specified, the outgoing requests will always go through the first one, or an IP load balancer can be used and the requests will be distributed based on the IP balancer policy. To route the outgoing requests to a particular IP address (the IP load balancer address for example) the outboundProxy property can be used:


<Service name="jboss.web" 
      ...
  balancers="127.0.0.1:5060:2000;127.0.0.1:5160:2100"  
outboundProxy="127.0.0.1:5500"
  ...>

In this example configuration, all outbound requests will go through 127.0.0.1:5500, while the node will perform the health checks against two SIP load balancers. If the 127.0.0.1:5500 machine is an IP load balancer, it should be configured to spray the SIP load balancers, and they will route the requests outside the cluster reliably.

The outboundProxy attribute overrides the default effect of specifying a SIP port for SIP load balancers in the balancers string.



                   <max-active-sessions>20</max-active-sessions>
                   <passivation-config>
                      <use-session-passivation>true</use-session-passivation>
                      <passivation-min-idle-time>60</passivation-min-idle-time>
                      <passivation-max-idle-time>600</passivation-max-idle-time>
                   </passivation-config>
                

In most cases you don't need to do anything for Step 2; the standard JBoss AS configuration for distributable session caching should suit your needs. The following is a bit more detail in case you're interested or want to change from the defaults.

SIP Session passivation relies on JBoss Cache's Cache Loader passivation for storing and retrieving the passivated sessions. Therefore the cache instance used by your sipapps clustered session manager must be configured to enable Cache Loader passivaton. The Cache Loader configuration for the standard-session-cache config is as follows:



                       <property name="cacheLoaderConfig">
                         <bean class="org.jboss.cache.config.CacheLoaderConfig">
                                <!-- Do not change these -->
                                <property name="passivation">true </property>                              
                                <property name="shared">false </property>
                               
                                <property name="individualCacheLoaderConfigs">
                                  <list>
                                     <bean class="org.jboss.cache.loader.FileCacheLoaderConfig">
                                        <!-- Where passivated sessions are stored -->
                                        <property name="location">${jboss.server.data.dir}${/}session </property>
                                        <!-- Do not change these -->
                                        <property name="async">false </property>
                                        <property name="fetchPersistentState">true </property>
                                        <property name="purgeOnStartup">true </property>
                                        <property name="ignoreModifications">false </property>
                                        <property name="checkCharacterPortability">false </property>
                                     </bean>
                                  </list>
                                </property>
                         </bean>
                      </property>
                

Refer to the JBossCache documentation for further details about Cache Loader configuration.

A Mobicents SIP Servlets Server for JBoss cluster does not employ any standby nodes. Typically, proxies and registrars must share the user location table by using a database cluster.

The Mobicents SIP Load Balancer, which is a SIP Call ID-aware load balancer, is used as the intermediary. The SIP Load Balancer forwards stateful transaction requests to cluster nodes based on its provisioning algorithm. The SIP Load Balancer acts as an entry-point to the cluster, and distributes the incoming requests between nodes. It is always advised to use a SIP load balancer or an IP load balancer in a cluster configuration.

This choice of implementation has many benefits:

The SIP Stack used by the Mobicents SIP Servlets for JBoss supports two modes:


The SIP Load Balancer is used to balance the load of SIP service requests and responses between nodes in a SIP Servlets Server cluster. Both MSS for JBoss and MSS for Tomcat servers can be used in conjunction with the SIP Load Balancer to increase the performance and availability of SIP services and applications.

In terms of functionality, the SIP Load Balancer is a simple stateless proxy server that intelligently forwards SIP session requests and responses between User Agents (UAs) on a Wide Area Network (WAN), and SIP Servlets Server nodes, which are almost always located on a Local Area Network (LAN). All SIP requests and responses pass through the SIP Load Balancer.

Configuring the SIP load balancer and the two SIP Servlets-enabled Server nodes is described in Configuring the Mobicents SIP Load Balancer and SIP Server Nodes.

Procedure 5.1. Configuring the Mobicents SIP Load Balancer and SIP Server Nodes

  1. Configure lb.properties Configuration Properties File

    Configure the SIP Load Balancer's Configuration Properties file by substituting valid values for your personal setup. Example 5.1, “Complete Sample lb.properties File” shows a sample lb.properties file, with key element descriptions provided after the example. The lines beginning with the pound sign are comments.

    Example 5.1. Complete Sample lb.properties File

    # Mobicents Load Balancer Settings
    # For an overview of the Mobicents Load Balancer visit http://docs.google.com/present/view?id=dc5jp5vx_89cxdvtxcm
    # The Load balancer will listen for both TCP and UDP connections
    
    
    
    # The binding address of the load balancer. This also specifies the 
    # default value for both internalHost and externalHost if not specified separately.
    host=127.0.0.1
    
    # The binding address of the load balancer where clients should connect (if the host property is not specified)
    #externalHost=127.0.0.1
    
    # The SIP port from where servers will receive messages
    # delete if you want to use only one port for both inbound and outbound)
    internalPort=5065
    
    # The SIP port used where clients should connect
    externalPort=5060
    
    # The binding address of the load balancer where SIP application servers should connect (if the host property is not specified)
    #internalHost=127.0.0.1
    
    # The RMI port used for heartbeat signals
    rmiRegistryPort=2000
    
    # The HTTP port for HTTP forwarding
    # if you like to activate the integrated HTTP load balancer, this is the entry point
    httpPort=2080
    #If no nodes are active the LB can redirect the traffic to the unavailableHost specified in this property,
    #otherwise, it will return 503 Service Unavailable
    #unavailableHost=google.com
    
    # If you are using IP load balancer, put the IP address and port here
    #externalIpLoadBalancerAddress=127.0.0.1
    #externalIpLoadBalancerPort=111
     
    # Requests initited from the App Servers can route to this address (if you are using 2 IP load balancers for bidirectional SIP LB)
    #internalIpLoadBalancerAddress=127.0.0.1
    #internalIpLoadBalancerPort=111
    
    # The addresses in the SIP LB Via headers can be either the real addresses or those specified in the external and internal IP LB addresses
    useIpLoadBalancerAddressInViaHeaders=false
    
    # Designate extra IP addresses as serer nodes
    #extraServerNodes=222.221.21.12:21,45.6.6.7:9003,33.5.6.7,33.9.9.2
    
    # Call-ID affinity algortihm settings. This algorithm is the default. No need to uncomment it.
    #algorithmClass=org.mobicents.tools.sip.balancer.CallIDAffinityBalancerAlgorithm
    # This property specifies how much time to keep an association before being evitcted.
    # It is needed to avoid memory leaks on dead calls. The time is in seconds.
    #callIdAffinityMaxTimeInCache=500
    #The following attribute specified the policy after failover. If set to true all calls from the failed node
    #will go to a new healthy node (all calls to the same node). If set to false the calls will go to random new nodes.
    #callIdAffinityGroupFailover=false
    
    # Uncomment to enable the consistent hash based on Call-ID algorithm.
    #algorithmClass=org.mobicents.tools.sip.balancer.HeaderConsistentHashBalancerAlgorithm
    # This property is not required, it defaults to Call-ID if not set, cna be "from.user" or "to.user" when you want the SIP URI username
    #sipHeaderAffinityKey=Call-ID
    #specify the GET HTTP parameter to be used as hash key
    #httpAffinityKey=appsession
     
    # Uncomment to enable the persistent consistent hash based on Call-ID algorithm.
    #algorithmClass=org.mobicents.tools.sip.balancer.PersistentConsistentHashBalancerAlgorithm
    # This property is not required, it defaults to Call-ID if not set
    #sipHeaderAffinityKey=Call-ID
    #specify the GET HTTP parameter to be used as hash key
    #httpAffinityKey=appsession
     
    #This is the JBoss Cache 3.1 configuration file (with jgroups), if not specified it will use default
    #persistentConsistentHashCacheConfiguration=/home/config.xml
     
    # Call-ID affinity algortihm settings. This algorithm is the default. No need to uncomment it.
    #algorithmClass=org.mobicents.tools.sip.balancer.CallIDAffinityBalancerAlgorithm
    # This property specifies how much time to keep an association before being evitcted.
    # It is needed to avoid memory leaks on dead calls. The time is in seconds.
    #callIdAffinityMaxTimeInCache=500
    
    # Uncomment to enable the consistent hash based on Call-ID algorithm.
    #algorithmClass=org.mobicents.tools.sip.balancer.HeaderConsistentHashBalancerAlgorithm
    # This property is not required, it defaults to Call-ID if not set, cna be "from.user" or "to.user" when you want the SIP URI username
    #sipHeaderAffinityKey=Call-ID
    #specify the GET HTTP parameter to be used as hash key
    #httpAffinityKey=appsession
    
    # Uncomment to enable the persistent consistent hash based on Call-ID algorithm.
    #algorithmClass=org.mobicents.tools.sip.balancer.PersistentConsistentHashBalancerAlgorithm
    # This property is not required, it defaults to Call-ID if not set
    #sipHeaderAffinityKey=Call-ID
    #specify the GET HTTP parameter to be used as hash key
    #httpAffinityKey=appsession
     
    #This is the JBoss Cache 3.1 configuration file (with jgroups), if not specified it will use default
    #persistentConsistentHashCacheConfiguration=/home/config.xml
    
    
    #If a node doesnt check in within that time (in ms), it is considered dead
    nodeTimeout=5100
    #The consistency of the above condition is checked every heartbeatInterval milliseconds
    heartbeatInterval=150
    
    
    #JSIP stack configuration.....
    javax.sip.STACK_NAME = SipBalancerForwarder
    javax.sip.AUTOMATIC_DIALOG_SUPPORT = off
    # You need 16 for logging traces. 32 for debug + traces.
    # Your code will limp at 32 but it is best for debugging.
    gov.nist.javax.sip.TRACE_LEVEL = 0
    
    // Specify if message contents should be logged.
    gov.nist.javax.sip.LOG_MESSAGE_CONTENT=false
    
    gov.nist.javax.sip.DEBUG_LOG = logs/sipbalancerforwarderdebug.txt
    gov.nist.javax.sip.SERVER_LOG = logs/sipbalancerforwarder.xml
    gov.nist.javax.sip.THREAD_POOL_SIZE = 64
    gov.nist.javax.sip.REENTRANT_LISTENER = true
    
                

    host

    Local IP address, or interface, on which the SIP load balancer will listen for incoming requests.

    externalPort

    Port on which the SIP load balancer listens for incoming requests from SIP User Agents.

    internalPort

    Port on which the SIP load balancer forwards incoming requests to available, and healthy, SIP Server cluster nodes.

    rmiRegistryPort

    Port on which the SIP load balancer will establish the RMI heartbeat connection to the application servers. When this connection fails or a disconnection instruction is received, an application server node is removed and handling of requests continues without it by redirecting the load to the lie nodes.

    httpPort

    Port on which the SIP load balancer will accept HTTP requests to be distributed across the nodes.

    internalTransport

    Transport protocol for the internal SIP connections associated with the internal SIP port of the load balancer. Possible choices are UDP, TCP and TLS.

    externalTransport

    Transport protocol for the external SIP connections associated with the external SIP port of the load balancer. Possible choices are UDP, TCP and TLS. It must match the transport of the internal port.

    externalIpLoadBalancerAddress

    Address of the IP load balancer (if any) used for incoming requests to be distributed in the direction of the application server nodes. This address may be used by the SIP load balancer to be put in SIP headers where the external address of the SIP load balancer is needed.

    externalIpLoadBalancerPort

    The port of the external IP load balancer. Any messages arriving at this port should be distributed across the external SIP ports of a set of SIP load balancers.

    internalIpLoadBalancerAddresst

    Address of the IP load balancer (if any) used for outgoing requests (requests initiated from the servers) to be distributed in the direction of the clients. This address may be used by the SIP load balancer to be put in SIP headers where the internal address of the SIP load balancer is needed.

    internalIpLoadBalancerPort

    The port of the internal IP load balancer. Any messages arriving at this port should be distributed across the internal SIP ports of a set of SIP load balancers.

    extraServerNodes

    Comma-separated list of hosts that are server nodes. You can put here alternative names of the application servers here and they will be recognized. Names are important, because they might be used for direction-analysis. Requests coming from these server will go in the direction of the clients and will not be routed back to the cluster.

    algorithmClass

    The fully-qualified Java class name of the balancing algorithm to be used. There are three algorithms to choose from and you can write your own to implement more complex routing behaviour. Refer to the sample configuration file for details about the available options for each algorithm. Each algorithm can have algorithm-specific properties for fine-grained configuration.

    nodeTimeout

    In milliseonds. Default value is 5100. If a server node doesnt check in within this time (in ms), it is considered dead.

    heartbeatInterval

    In milliseconds. Default value is 150 milliseonds. The hearbeat interval must be much smaller than the interval specified in the JAIN SIP property on the server machines - org.mobicents.ha.javax.sip.HEARTBEAT_INTERVAL

  2. Configure logging

    The SIP load balancer uses Java Logging as a logging mechanism. You can configure it through a property file and specify the property file to be used by using the following command -Djava.util.logging.config.file=./lb-logging.properties. Please refer to JDK logging for more informationon how to configure the Java logging.

  3. Configure the server.xml configuration file

    Ensure the following attributes are configured for the <service> element in server.xml.

    • The balancers attribute must contain a IP address (or list of addresses) of the SIP load balancer(s) to which heartbeat information will be sent.

    • The sipPathName attribute must contain the following value org.mobicents.ha to indicate that the server will be using the Mobicents JAIN SIP HA SIP Stack which is an extension of the JAIN SIP Stack offering transparent replication.

  4. Configure the mss-sip-stack.properties configuration file

    • The org.mobicents.ha.javax.sip.cache.MobicentsSipCache.cacheName property must contain the name of the cache that will be responsible for holding the replicated data of the SIP Stack layer (namely the established SIP dialog data). The value has to be one of the cache name present in the jboss-cache-manager-jboss-beans.xml file of the jboss-cache-manager JBoss Service of the container. The default value is standard-session-cache

    • The org.mobicents.ha.javax.sip.BALANCERS property must be configured with the list of load balancer IP address and internal ports. As an example, suppose a single SIP Load Balancer is running with IP 192.168.0.1 and internal port 5065, the property would be set with value 192.168.0.1:5065. To specify multiple balancers use ; as separator. If this property is used the balancers attribute located in server.xml should not be used as it is a replacement for it.

    • The org.mobicents.ha.javax.sip.LoadBalancerHeartBeatingServiceClassName property is optional, it defines the class name of the HeartBeating service implementation, currently the only one available is org.mobicents.ha.javax.sip.LoadBalancerHeartBeatingServiceImpl

    • The org.mobicents.ha.javax.sip.LoadBalancerElector property is optional, it defines the class of the load balancer elector from JAIN SIP HA Stack. The elector is used to define which load balancer will receive outgoing requests, which are out of dialog or in dialog with null state. Currently only one elector implementation is available, org.mobicents.ha.javax.sip.RoundRobinLoadBalancerElector, which, as the class name says, uses round robin algorythm to select the balancer.

Configuration File Locations

On MSS for Tomcat server installations, server.xml is located in <install_directory>/conf.

On MSS for JBoss server installations, the default server.xml configuration file is located in server/default/deploy/jboss-web.sar (or server/default/deploy/jboss-web.deployer for JBoss Application Server 4.x and EAP 4.x).

On MSS for JBoss installations, with JBoss clustering support enabled, the "all" server.xml file must be configured. It is located in server/all/deploy/jboss-web.deployer.

To determine what profile should be altered for each MSS for JBoss installation, refer to Section 5.1, “Mobicents SIP Servlets for JBoss: Clustering Support”.

Easy Node Configuration with JMX

Both SIP Servlet-enabled JBoss and Tomcat have JMX (Java Management Extensions) interfaces that allow for easy server configuration. The JMX Console is available once the server has been started by navigating to http://localhost:8080/jmx-console/.

Both the balancers and heartBeatInterval attribute values are available under name=Mobicents-SIP-Servlets,type=load-balancer-heartbeat-service in the JMX Console.

balancers

Host names of the SIP load balancer(s) with corresponding addBalancerAddress and removeBalancerAddress methods.

heartBeatInterval

Interval at which each heartbeat is sent to the SIP load balancer(s).

The MSS SIP Load Balancer can work in concert with HTTP load balancers such as mod_jk. Whenever an HTTP session is bound to a particular node, an instruction is sent to the SIP Load Balancer to direct the SIP calls from the same application session to the same node.

It is sufficient to configure mod_jk to work for HTTP in JBoss in order to enable cooperative load balancing. MSS will read the configuration and will use it without any extra configuration. You can read more about configuring mod_jk with JBoss in your JBoss Application Server documentation.

Alternatively you may disable this behaviour and make the HTTP load balancer follow the decisions made by the SIP load balancer with the httpFollowsSip flag. This is achieved by changing the jvmRoute part of the session ID cookie used internally by mod_jk.

The httpFollowsSip flag in the service configuration makes the application server aware of how different mod_jk and SIP load balancers have assigned request affinity for each application session. The application servers assign exactly one node to each Sip Servlets application session and this node is the node where the last SIP request associated with the application session has landed (decised by the SIP load balancer). Then the application server will actively update the session ID cookie (the jvmRoute part) of any HTTP request that arrives at the wrong node. The application server will do so with a specially composed HTTP redirect response or with a HTML refresh hint. As a backup strategy, if the request is bound to seek non-existing node forever and it will let the request be served by a new node. This avoids having a client stuck reloading the same page over and over.

One problem with this flag is that if you have two or more SIP sessions associated with the same application session and the load balancer has decided to send SIP requests to different nodes, which might happend if you use Call-ID based affinity, then the application server will have to change the jvmRoute very often for every SIP request resulting in significant overhead. It is generally not adviced to enable this flag if you have more than 1 SIP session per application session and the means to guarantee all SIP sessions from the application session will land on the same node.

This is an example how to enable the option. It is disabled by default.

<Connector port="5080" 
     ipAddress = "${jboss.bind.address}"
     ...
     httpFollowsSip="true" />

Procedure 5.2. Running the SIP Load Balancer and SIP Server Nodes

  1. Start the SIP Load Balancer

    Start the SIP load balancer, ensuring the Configuration Properties file (lb.properties in this example) is specified. In the Linux terminal, or using the Windows Command Prompt, the SIP Load Balancer is started by issuing a command similar to this one:

    java -jar sip-balancer-jar-with-dependencies.jar lb-configuration.properties

    Executing the SIP load balancer produces output similar to the following example:

    home]$ java -jar sip-balancer-jar-with-dependencies.jar lb-configuration.properties 
    Oct 21, 2008 1:10:58 AM org.mobicents.tools.sip.balancer.SIPBalancerForwarder start
    INFO: Sip Balancer started on address 127.0.0.1, external port : 5060, port : 5065
    Oct 21, 2008 1:10:59 AM org.mobicents.tools.sip.balancer.NodeRegisterImpl startServer
    INFO: Node registry starting...
    Oct 21, 2008 1:10:59 AM org.mobicents.tools.sip.balancer.NodeRegisterImpl startServer
    INFO: Node expiration task created
    Oct 21, 2008 1:10:59 AM org.mobicents.tools.sip.balancer.NodeRegisterImpl startServer
    INFO: Node registry started

    The output shows the IP address on which the SIP Load Balancer is listening, as well as the external and internal listener ports.

  2. Configure SIP Server Nodes

    SIP Servlets Server nodes can run on the JBoss Application Server, or the Tomcat Servlet Container. The SIP Servlets Server binary distributions define the type of SIP Servlets Server nodes used, and should already be installed from Software Prerequisites.

    The server.xml file specifies the nodes used. Because there is more then one client node specified, unique listener ports must be specified for each node to monitor HTTP and/or SIP connections. Example 5.2, “Changing the SIP Connector Port for Servlet Server Nodes in server.xml” describes the affected element in the server.xml file.

    Configuration File Location

    For the JBoss SIP Servlets Server binary distribution, server.xml is located in the <install_directory>/server/all/deploy/jboss-web.deployer/ directory (for JBoss Application Server 4.x or EAP 4.x <install_directory>/server/all/deploy/jboss-web.deployer/). For the Tomcat binary distribution, server.xml is located in the <install_directory>/conf/ directory.


  3. Start Load Balancer Client Nodes

    Start all SIP load balancer client nodes.

All User Agents send SIP messages, such as INVITE and MESSAGE, to the same SIP URI (the IP address and port number of the SIP Load Balancer on the WAN). The Load Balancer then parses, alters, and forwards those messages to an available node in the cluster. If the message was sent as a part of an existing SIP session, it will be forwarded to the cluster node which processed that User Agent's original transaction request.

The SIP Server that receives the message acts upon it and sends a response back to the SIP Load Balancer. The SIP Load Balancer reparses, alters and forwards the message back to the original User Agent. This entire proxying and provisioning process is carried out independent of the User Agent, which is only concerned with the SIP service or application it is using.

By using the Load Balancer, SIP traffic is balanced across a pool of available SIP Servers, increasing the overall throughput of the SIP service or application running on either individual nodes of the cluster. In the case of a MSS server with </distributed> capabilities, load balancing advantages are applied across the entire cluster.

The SIP Load Balancer is also able to failover requests mid-call from unavailable nodes to available ones, thus increasing the reliability of the SIP service or application. The Load Balancer increases throughput and reliability by dynamically provisioning SIP service requests and responses across responsive nodes in a cluster. This enables SIP applications to meet the real-time demand for SIP services.

In addition to the SIP load balancing, there are several options for coordinated or cooperative load balancing with other protocols such as HTTP.

Typically, a JBoss Application Server will use apache HTTP server with mod_jk, mod_proxy, mod_cluster or similar extension installed as an HTTP load balancer. This apache-based load balancer will parse incoming HTTP requests and will look for the session ID of those requests in order to ensure all requests from the same session arrive at the same application server.

By default, this is done by examining the jsessionid HTTP cookie or GET parameter and looking for the jvmRoute assigned to the session. The typical jsessionid value is of the form <sessionId>.<jvmRoute>. The very first request for each new HTTP session does not have a session ID assigned; the apache routes the request to a random application server node.

When the node responds it assigns a session ID and jvmRoute to the response of the request in a HTTP cookie. This response goes back to the client through apache, which keeps track of which node owns each jvmRoute. Once the very first request is served this way, the subsequent requests from this session will carry the assigned cookie, and the apache load balancer will always route the requests to the node, which advertised itself as the jvmRoute owner.

Instead of using apache, an integrated HTTP Load Balancer is also available. The SIP Load Balancer has a HTTP port where you can direct all incoming HTTP requests. The integrated HTTP load balancer behaves exactly like apache by default, but this behavior is extensible and can be overridden completely with the pluggable balancer algorithms. The integrated HTTP load balancer is much easier to configure and generally requires no effort, because it reuses most SIP settings and assumes reasonable default values.

Unlike the native apache, the integrated HTTP Load Balancer is written completely in Java, thus a performance penalty should be expected when using it. However, the integrated HTTP Balancer has an advantage when related SIP and HTTP requests must stick to the same node.

The SIP/HTTP Load Balancer exposes an interface to allow users to customize the routing decision making for special purposes. By default there are three built-in algorithms. Only one algorithm is active at any time and it is specified with the algorithmClass property in the configuration file.

It is up to the algorithm how and whether to support distributed architecture or how to store the information needed for session affinity. The algorithms will be called for every SIP and HTTP request and other significant events to make more informed decisions.

The following is a list of the built-in algorithms:

org.mobicents.tools.sip.balancer.CallIDAffinityBalancerAlgorithm

This algorithm is not distributable. It selects nodes randomly to serve a give Call-ID extracted from the requests and responses. It keeps a map with Call-ID -> nodeId associations and this map is not shared with other load balancers which will cause them to make different decisions. For HTTP it behaves like apache.

org.mobicents.tools.sip.balancer.HeaderConsistentHashBalancerAlgorithm

This algorithm is distributable and can be used in distributed load balancer configurations. It extracts the hash value of specific headers from SIP and HTTP messages to decide which application server node will handle the request. Information about the options in this algorithms is available in the balancer configuration file comments.

org.mobicents.tools.sip.balancer.PersistentConsistentHashBalancerAlgorithm

This algorithm is distributable and is similar to the previous algorithm, but it attempts to keep session affinity even when the cluster nodes are removed or added, which would normally cause hash values to point to different nodes.

org.mobicents.tools.sip.balancer.ClusterSubdomainAffinityAlgorithm

This algorithm is not distributable, but supports grouping server nodes to act as a subcluster. Any call of a node that belongs to a cluster group will be preferentially failed over to a node from the same group. To configure a group you can just add the subclusterMap property in the load balancer properties and listing the IP addresses of the nodes. The groups are enclosed in parentheses and the IP addresses are separate by commas as follows:

subclusterMap=( 192.168.1.1, 192.168.1.2 ) ( 10.10.10.10,  20.20.20.20,  30.30.30.30)

The nodes specified in a group do not have to alive and nodes that are not specified are still allowed to join the cluster. Otherwise the algorthim behaves exactly as the default Call-ID affinity algorthim.