January 30, 2011
Copyright © 2011 JBoss by Red Hat .
Table of Contents
JBoss Remoting is a project that provides a general purpose distributed invocation framework for other JBoss projects and products, including the community Application Server (AS) and the Enterprise Application Platforms (EAP). Remoting is roughly similar to Java RMI, but instead of using stubs, it identifies servers by URL. Other distributed frameworks that appear in the same context are JGroups (http://www.jgroups.org/) and Netty (http://www.jboss.org/netty). The technologies served by Remoting, in particular, are EJB2, EJB3, and JBoss Messaging (http://labs.jboss.com/jbossmessaging). JBoss Web Services also uses Remoting on its client side, but the use is hidden, so to speak: there are no configuration files.
Remoting is quite flexible, with multiple, pluggable transports, marshallers, serializers, etc., and more than anyone would ever want to know is described in the Remoting Guide, but the current document, instead, focuses on those aspects of Remoting that are useful in the context of EJB2, EJB3, and JBoss Messaging. The Users Guide is not meant to introduce the reader to writing applications based on Remoting, but rather is meant to provide
The information in this guide applies to AS 5, EAP 5, and AS 6.
Remoting is based on the client server model, and communication between a client and a server is handled by one of Remoting's several transports. A transport is characterized by a pair of classes, one for the client side and one for the server side, which communicate by a shared protocol. Most of the core concepts discussed below are related to one side or the other. However, the URL, or the internal object used to represent the URL, of a server is relevant to both sides.
socket://localhost:1234/?timeout=10000&maxPoolSize=200&clientMaxPoolSize=50The protocol element, "socket", indicates that the socket transport is to be used. It will be described below (The socket transport chapter). The values of the parameters "timeout", "maxPoolSize", and "clientMaxPoolSize" are available on both the client and server sides, but they may be ignored on one side or the other. They are further described below (The socket transport chapter).
is a server side object that incorporates application logic. A ServerInvocationHandler may be associated with a particular subsystem, specified by an arbitrary string. For example, JBoss Messaging uses "JMS".
is the server side object that fields invocations and passes them to the appropriate ServerInvocationHandler. Each of EJB2, EJB3, and JBoss Messaging have their own ServerInvocationHandlers. ServerInvoker is subclassed for each transport; e.g., org.jboss.remoting.transport.socket.SocketServerInvoker, org.jboss.remoting.transport.bisocket.BisocketServerInvoker, etc.
is the external face of the ServerInvoker, by which the ServerInvoker is configured declaratively. The configuration includes a designated transport, ServerInvoker parameters, and one or more ServerInvocationHandler classes.
is a client side object corresponding to a particular ServerInvoker. It is responsible for marshalling an invocation to the wire and unmarshalling the response. Each RemoteClientInvoker is subclassed for each transport; e.g., org.jboss.remoting.transport.socket.SocketClientInvoker, org.jboss.remoting.transport.bisocket.BisocketClientInvoker, etc.
is the conduit between application code and the RemoteClientInvoker. Multiple Clients can share a single RemoteClientInvoker if they have the same InvokerLocator and the same set of configuration parameters. In a pure Remoting application, the application code would be responsible for creating the Client(s). However, in the context of the Application Server, Clients and RemoteClientInvokers are hidden below the surface. For example, a JBoss Messaging producer or consumer creates one or more Clients. In EJB2 and EJB3, Clients are create by an interceptor embedded in an object which is retrieved from the server side. In these cases, JBoss Messaging, EJB2, and EJB3 are the applications from the Remoting perspective. A Client may be created with a subsystem string which associates it with a particular ServerInvocationHandler on the server side. Note that if a ServerInvoker is configured with only a single ServerInvocationHandler, the use of a subsystem string is optional.
One more concept is necessary to make sense of the bisocket transport, discussed below in the The bisocket transport chapter. An ordinary invocation leads to a synchronous response from the server, but asynchronous communication is also possible from the server to the client. When a call is make to one of the overloaded variants of the Client method
addListener(InvokerCallbackHandler callbackHandler) throws Throwable;
the passed instance of InvokerCallbackHandler is registered as a listener for asynchronous communication, and, on the server side, the ServerInvocationHandler associated with the Client is given a "proxy" for the InvokerCallbackHandler which it can use to send asynchronous org.jboss.remoting.callback.Callback objects, or, more simply, callbacks.
There are two kinds of callbacks. For push callbacks, a dedicated ServerInvoker is created on the client side and the server side ServerInvocationHandler makes invocations, by way of the "proxy", on that ServerInvoker to send callback objects to the client side. Under the surface, the "proxy" creates a Client to handle those invocations. On the other hand, pull callbacks generated by the ServerInvocationHandler are stored on the server side, and a poller is created on the client side which makes invocations on the server side ServerInvoker to retrieve any stored callbacks.
When used in a standalone manner, Remoting clients and servers can be configured either by adding parameters to the InvokerLocator or by directly passing configuration maps to their constructors. In the context of the Application Server, however, Remoting objects are configured in xml files. There are two variations. In versions 4 and earlier of the Application Server, the various components are held together by an MBeanServer, and, in the course of initializing the Application Server, deployers create MBeans from xml description files. In version 5, the primacy of the MBeanServer has been replaced by the JBoss Microcontainer, which creates POJOs from xml description files. Both MBean and POJO descriptors are available in version 5.
An MBean descriptor of a Remoting Connector looks like the following abbreviated version derived from a JBoss Messaging example:
<mbean code="org.jboss.remoting.transport.Connector"
name="jboss.messaging:service=Connector,transport=bisocket"
display-name="Bisocket Transport Connector">
<attribute name="Configuration">
<config>
<invoker transport="bisocket">
...
<attribute name="serverBindAddress">localhost</attribute>
<attribute name="serverBindPort">4457</attribute>
<attribute name="marshaller" isParam="true">org.jboss.jms.wireformat.JMSWireFormat</attribute>
<attribute name="unmarshaller" isParam="true">org.jboss.jms.wireformat.JMSWireFormat</attribute>
<attribute name="timeout" isParam="true">300000</attribute>
<attribute name="callbackTimeout">10000</attribute>
...
</invoker>
<handlers>
<handler subsystem="JMS">org.jboss.jms.server.remoting.JMSServerInvocationHandler</handler>
</handlers>
</config>
</attribute>
</mbean>
In this case, the Connector reads the <config> attribute and creates the InvokerLocator
bisocket://localhost:4457?marshaller=org.jboss.jms.wireformat.JMSWireFormat&unmarshaller=org.jboss.jms.wireformat.JMSWireFormat&timeout=300000
which is passed into the BisocketServerInvoker that it creates (see The bisocket transport chapter).
Notes.
When an MBean descriptor like the one in the previous section is given, the Connector explicitly parses the <config> element. In Application Server 5 and EAP 5, the microcontainer automatically parses POJO descriptors written in its descriptor language, creates the described POJOs, and can inject them into other POJOs. Remoting uses a POJO descriptor of a org.jboss.remoting.ServerConfiguration object to inject configuration information into a Connector. For example, the following POJO descriptors are abbreviated and modified versions of those used by the EJB2 subsystem:
<bean name="UnifiedInvokerConnector" class="org.jboss.remoting.transport.Connector">
<property name="serverConfiguration"><inject bean="UnifiedInvokerConfiguration"/></property>
</bean>
<bean name="UnifiedInvokerConfiguration" class="org.jboss.remoting.ServerConfiguration">
<constructor>
<parameter>socket</parameter>
</constructor>
<!-- Parameters visible to both client and server -->
<property name="invokerLocatorParameters">
<map keyClass="java.lang.String" valueClass="java.lang.String">
<entry>
<key>serverBindAddress</key>
<value>localhost</value>
</entry>
<entry>
<key>serverBindPort</key>
<value>4446</value>
</entry>
<entry><key>enableTcpNoDelay</key> <value>true</value></entry>
</map>
</property>
<!-- Parameters visible only to server -->
<property name="serverParameters">
<map keyClass="java.lang.String" valueClass="java.lang.String">
<entry><key>maxPoolSize</key><value>500</value></entry>
</map>
</property>
<property name="invocationHandlers">
<map keyClass="java.lang.String" valueClass="java.lang.String">
<entry><key>JSR88</key> <value>org.jboss.deployment.remoting.DeployHandler</value></entry>
</map>
</property>
</bean>
Here, the ServerConfiguration is created and injected into the Connector object, which creates the InvokerLocator
socket://localhost:4446?enableTcpNoDelay=true
Notes.
The actual ServerInvocationHandler used by EJB2, by the way, is an instance of a different class and is injected into the SocketServerInvoker programmatically. It is described by the following POJO descriptor:
<bean name="UnifiedInvoker" class="org.jboss.invocation.unified.server.UnifiedInvoker">
<annotation>@org.jboss.aop.microcontainer.aspects.jmx.JMX(name="jboss:service=invoker,type=unified",exposedInterface=org.jboss.invocation.unified.server.UnifiedInvokerMBean.class,registerDirectly=true)</annotation>
<property name="connector"><inject bean="UnifiedInvokerConnector"/></property>
<depends>TransactionManager</depends>
</bean>
An org.jboss.invocation.unified.server.UnifiedInvoker is a ServerInvocationHandler, and it uses the "UnifiedInvokerConnector" to inject itself into the SocketServerInvoker created by the "UnifiedInvokerConnector". The <annotation> property, by the way, is used to assign an MBean name to a POJO.
There is an alternative way to configure a Connector with a ServerConfiguration. In the following modified version of two POJOs used by the EJB3 subsystem, the InvokerLocator is given explicitly to the Connector and the injected ServerConfiguration is used only to specify a ServerInvocationHandler:
<bean name="org.jboss.ejb3.RemotingConnector"
class="org.jboss.remoting.transport.Connector">
<property name="invokerLocator">
<value>socket://localhost:3873?timeout=300000</value>
</property>
<property name="serverConfiguration">
<inject bean="ServerConfiguration" />
</property>
</bean>
<!-- Remoting Server Configuration -->
<bean name="ServerConfiguration"
class="org.jboss.remoting.ServerConfiguration">
<property name="invocationHandlers">
<map keyClass="java.lang.String" valueClass="java.lang.String">
<entry>
<key>AOP</key>
<value>
org.jboss.aspects.remoting.AOPRemotingInvocationHandler
</value>
</entry>
</map>
</property>
</bean>
A Remoting transport is represented by a matching pair of classes derived from RemoteClientInvoker and SocketInvoker (see the Central concepts chapter). They are matched in the sense that they share a wire protocol. In the socket transport, the classes are org.jboss.remoting.transport.socket.SocketClientInvoker and org.jboss.remoting.transport.socket.SocketServerInvoker. SocketClientInvoker is, in fact, further subclassed by the actual client invoker class, org.jboss.remoting.transport.socket.MicroSocketClientInvoker.
The socket transport is a relatively low level transport in the sense that it interacts directly with Java sockets and server sockets. SocketServerInvoker is a classic old i/o style server, in which it listens to a server socket and, for each new socket created, it either creates a new worker thread, represented by org.jboss.remoting.transport.socket.ServerThread or reuses an idle ServerThread obtained from a pool. A ServerThread manages a single socket, waiting for an invocation to come in on the wire, passing the invocation to the ServerInvoker which passes it to the appropriate ServerInvokerHandler, and then writing the response to the wire.
On the client side, MicroSocketClientInvoker takes an invocation passed in by a Client, either creates a new connection that wraps a Java socket or reuses an idle connection from a pool, writes the invocation to the socket, and reads and returns the result. An important consideration is the determination of the viability of a pooled connection. A slow but reliable method is to write a byte to the socket and wait for the byte to come back. This method is turned off by default. Another method is for the server side to write a couple of bytes when the connection is closed, and for the client side to check for available bytes. This method is always turned on, but it is less reliable since the server side may be unable to write the bytes, or they may not have arrived yet when the test is made. A palliative for these possibilities is to configure MicroSocketClientInvoker to retry a failed invocation in the event of a java.net.SocketException or certain variants of a java.io.IOException.
A number of parameters may be configured on each side, including, for example, timeout times, pool sizes, and number of retry attempts. Note that most of these are given their default values by JBoss Messaging, EJB2, and EJB3, and, in general, the configured values of the others should not be changed without a good reason.
backlog - the preferred number of unaccepted incoming connections allowed at a given time. The actual number may be greater than the specified backlog. When the queue is full, further connection requests are rejected. The default value is 200.
continueAfterTimeout - indicates what a ServerThread should do after experiencing a java.net.SocketTimeoutException. If set to true, or if JBoss Serialization is being used (which is never the case, by default, which is why JBoss Serialization is not discussed in this guide), the server thread will continue to wait for the next invocation; otherwise, it will return itself to the thread pool. For Java serialization, the default value is false.
evictabilityTimeout - indicates the number of milliseconds during which a ServerThread waiting for the next invocation will not be interruptible. The default value is 10000 milliseconds.
idleTimeout - indicates the number of seconds a pooled ServerThread can be idle, that is, waiting on the next invocation, before it should be removed from the thread pool. The value for this property must be greater than zero in order to enable idle timeouts. The default value is -1.
immediateShutdown - indicates, when set to "true", that, when Connector.stop() is called and it calls SocketServerInvoker.stop(), all ServerThreads are shut down immediately, even if they are processing an invocation. The default value is false.
maxPoolSize - the maximum number of ServerThreads that can exist at any given time. The default value is 300.
numAcceptThreads - the number of threads listening on the ServerSocket. The default value is 1.
serverSocketClass - specifies the fully qualified class name for a custom SocketWrapper implementation to use on the server. By default, org.jboss.remoting.transport.socket.ServerSocketWrapper is used. JBoss Messaging uses a custom wrapper.
socket.check_connection - indicates if a client side pooled connection should be checked by sending a single byte from the client to the server and then back to the client. This parameter needs to be set on both client and server to work. It is false by default.
timeout - the socket timeout value passed to the Socket.setSoTimeout() method. The default on the server side is 60000 milliseconds.
writeTimeout - a timeout value imposed on socket write operations. This feature is enabled by setting writeTimeout to a value, in milliseconds, greater than zero. By default, the feature is not enabled.
The following socket parameters, in addition to SO_TIMEOUT, can be configured on the server side: SO_KEEPALIVE, OOBINLINE, SO_RCVBUF, SO_REUSEADDR, SO_SNDBUF, SO_LINGER, and "traffic class". They are configured by passing the following parameter keys to SocketServerInvoker:
keepAlive - sets socket parameter SO_KEEPALIVE
oOBInline - sets socket parameter OOBINLINE
receiveBufferSize - sets socket parameter SO_RCVBUF
reuseAddress - sets socket parameter SO_REUSEADDR
sendBufferSize - sets socket parameter SO_SNDBUF
soLinger - sets socket parameter SO_LINGER
soLingerDuration - when socket parameter SO_LINGER is set to "true", sets linger duration
trafficClass - sets socket traffic class
clientMaxPoolSize - the maximum number of socket connections that can exist at any given time. The default value is 50.
clientSocketClass - specifies the fully qualified class name for a custom SocketWrapper implementation to use on the client. By default, org.jboss.remoting.transport.socket.ClientSocketWrapper is used. JBoss Messaging uses a custom wrapper.
generalizeSocketException - If set to false, a failed invocation will be retried in the case of SocketExceptions. If set to true, a failed invocation will be retried in the case of SocketExceptions and also any IOException whose message matches the regular expression ^.*(?:connection.*reset|connection.*closed|connection.*abort|broken.*pipe|connection.*shutdown).*$. See also the "numberOfCallRetries" parameter, below. The default value is false.
numberOfCallRetries - the number of times a failed invocation will be retried. For example, it is possible that the server side of a socket connection could time out, leaving the connection invalid. In that case, the socket will be discarded and another, possibly new, socket will be used. After numberOfCallRetries attempts, an InvocationFailureException, whose cause is the original exception, will be thrown. The default value is 3. See also the "generalizeSocketException" parameter, above.
socket.check_connection - indicates if a client side pooled connection should be checked by sending a single byte from the client to the server and then back to the client. This parameter needs to be set on both client and server to work. It is false by default.
timeout - The socket timeout value passed to the Socket.setSoTimeout() method. The default on the client side is 1800000 milliseconds (30 minutes).
useOnewayConnectionTimeout - indicates if, during a client side oneway invocation, MicroSocketClientInvoker should wait for a version byte from the server, which prevents the anomalous behavior described in JBREM-706 "In socket transport, prevent client side oneway invocations from artificially reducing concurrency". The default value is true.
writeTimeout - a timeout value imposed on socket write operations. This feature is enabled by setting writeTimeout to a value, in milliseconds, greater than zero. By default, the feature is not enabled.
The following socket parameters, in addition to SO_TIMEOUT, can be configured on the client side: TCP_NODELAY, SO_KEEPALIVE, OOBINLINE, SO_RCVBUF, SO_REUSEADDR, SO_SNDBUF, SO_LINGER, and "traffic class". They are configured by passing the following parameter keys to MicroSocketClientInvoker:
enableTcpNoDelay - sets socket parameter TCP_NODELAY. The default value is false.
keepAlive - sets socket parameter SO_KEEPALIVE
oOBInline - sets socket parameter OOBINLINE
receiveBufferSize - sets socket parameter SO_RCVBUF
reuseAddress - sets socket parameter SO_REUSEADDR. The default value is true.
sendBufferSize - sets socket parameter SO_SNDBUF
soLinger - sets socket parameter SO_LINGER
soLingerDuration - when socket parameter SO_LINGER is set to "true", sets linger duration
trafficClass - sets socket traffic class
The bisocket transport is derived from the socket transport and differs in one design feature. It was created for JBoss Messaging, where the design criteria were
In the socket transport, push callbacks are handled on the client side by a dedicated SocketServerInvoker, which uses a Java server socket; that is, it opens a port. When a connection is needed on the server side to send push callbacks, a socket is created by contacting the server socket on the client side.
The bisocket transport, instead, uses server sockets only on the server side, including (1) the one created by SocketServerInvoker to create connections to handle ordinary invocations, and (2) a secondary server socket. The first use of the secondary server socket is to create a control connection in response to a request from the client side. Subsequently, all connections for sending push callbacks are created by sending a message over the control connection to the client side, asking it to make a connection to the secondary server socket.
If the control connection fails, then no new calback connections can be created, so it is possible to configure the server side BisocketServerInvoker to ping the client side BisocketClientInvoker at fixed intervals. If the BisocketClientInvoker detects a missing ping, it can connect to the secondary server socket and recreate the control connection.
Originally, JBoss Messaging turned off pinging because the default ping period was small enough to cause spurious failures. Currently, the default values are accepted.
The following bisocket configuration parameters are added to those used in the socket transport:
pingFrequency - the frequency, in milliseconds, with which the server side sends a ping over the control connection. The default value is 5000 milliseconds.
maxControlConnectionRestarts - the maximum number of times the client side will attempt to recreate the control connection following a ping failure. The default value is 10.
maxRetries - the maximum number of times two processes are attempted: (1) the attempt to create a socket, either for the control connection or for a callback connection, and (2) the attempt to get the port of the secondary server socket. The default value is 10.
pingFrequency - the frequency, in milliseconds, with which the server side is expected to send a ping over the control connection. The default value is 5000 milliseconds.
pingWindowFactor - the value which, when multiplied times the pingFrequency, gives the window within which a ping is expected by the client side. The default value is 2. It follows that the default window is 10 seconds.
secondaryBindPort - the port to which the secondary server socket is to be bound. By default, an arbitrary port is selected.
secondaryConnectPort - the port clients are to use to connect to the secondary server socket. By default, the value of secondaryBindPort is used. secondaryConnectPort is useful if the server is behind a translating firewall.
The sslsocket transport is derived from the socket transport (The socket transport) and differs only in the use of javax.net.ssl.SSLSockets and javax.net.ssl.SSLServerSockets instead of the usual Java sockets and server sockets. Similarly, the sslbisocket transport is derived from the bisocket transport (The bisocket transport) and differs only in the use of SSLSockets and SSLServerSockets.
Remoting provides a configurable extension of javax.net.ssl.SSLServerSocketFactory called org.jboss.remoting.security.SSLSocketFactoryService. It depends on an instance of org.jboss.remoting.security.SSLSocketBuilder, which creates and configures an instance of javax.net.ssl.SSLContext and uses it to create an instance of javax.net.SSLServerSocketFactory. It is the SSLSocketBuilder which can be configured with keystores, etc. SSLSocketBuilder is described in more detail in the Remoting Guide.
For example, JBoss Messaging uses instances of SSLSocketFactoryService and SSLSocketBuilder as follows in its configuration of the sslbisocket transport:
<mbean code="org.jboss.remoting.security.SSLServerSocketFactoryService"
name="jboss.messaging:service=ServerSocketFactory,type=SSL"
display-name="SSL Server Socket Factory">
<depends optional-attribute-name="SSLSocketBuilder"
proxy-type="attribute">jboss.messaging:service=SocketBuilder,type=SSL</depends>
</mbean>
<mbean code="org.jboss.remoting.security.SSLSocketBuilder"
name="jboss.messaging:service=SocketBuilder,type=SSL"
display-name="SSL Server Socket Factory Builder">
<!--
IMPORTANT - If making ANY customizations, this MUST be set to false.
Otherwise, will use default settings and the following attributes will be ignored.
-->
<attribute name="UseSSLServerSocketFactory">false</attribute>
<attribute name="KeyStoreURL">${jboss.server.home.url}/deploy/messaging/messaging.keystore</attribute>
<attribute name="KeyStorePassword">secureexample</attribute>
<attribute name="KeyPassword">secureexample</attribute>
<attribute name="SecureSocketProtocol">TLS</attribute>
<attribute name="KeyStoreAlgorithm">SunX509</attribute>
<attribute name="KeyStoreType">JKS</attribute>
</mbean>
Through these two MBeans, JBoss Messaging provides itself with a suitably configured instance of an SSLServerSocketFactory on the server side.
Although a SSLSocketBuilder can be used to create a javax.net.SSLSocketFactory, the one on the server side typically will not be available on the client side, so it is the responsibility of the particular subsystem (JBoss Messaging, EJB2, EJB3) to create a RemoteClientInvoker with a suitable SSLSocketFactory. For example, JBoss Messaging takes two steps:
Map configuration = new HashMap();
String trustStoreLoc = System.getProperty("org.jboss.remoting.trustStore");
if (trustStoreLoc != null)
{
configuration.put("org.jboss.remoting.trustStore", trustStoreLoc);
String trustStorePassword = System.getProperty("org.jboss.remoting.trustStorePassword");
if (trustStorePassword != null)
{
configuration.put("org.jboss.remoting.trustStorePassword", trustStorePassword);
}
}
...
Client client = new Client(new InvokerLocator(serverLocatorURI), configuration);
...
The following parameters are applicable to both the client and server sides for the sslsocket and sslbisocket transports:
enabledCipherSuites - a String array which is passed to SSLSocket.setEnabledCipherSuites()
enabledProtocols - a String array which is passed to SSLSocket.setEnabledProtocols()
enableSessionCreation - a boolean value which is passed to SSLSocket.setEnableSessionCreation()
Unlike the socket transport and its derivatives, which interact directly with Sockets and ServerSockets, the http family of transports uses java.net.HttpURLConnections. While they do not exhibit the same performance as the socket transports, they have the advantage of using the universal http protocol.
Any of JBoss Messaging, EJB2, and EJB3 can use the http transports instead of the socket family of transports. Additional configuration information may be found in the wiki articles EJB, JMS and JNDI over HTTP with Unified Invoker and EJB, JMS and JNDI over HTTP via NAT Firewall with Unified Invoker.
On the client side, an org.jboss.remoting.transport.http.HTTPClientInvoker, a subclass of RemoteClientInvoker, creates a java.net.HttpURLConnection for each invocation. The caching of HttpURLConnections and their Socket is left to the implementation. On the server side, the org.jboss.remoting.transport.coyote.CoyoteInvoker, which is a subclass of ServerInvoker and is based on the coyote module in Tomcat, processes http requests, calls on ServerInvoker to hand invocations off to the appropriate ServerInvocationHandler, and returns a result along with a response code.
The https transport is derived from the http transport and uses javax.net.ssl.SSLSockets and javax.net.ssl.SSLServerSockets instead of Sockets and ServerSockets.
The servlet and sslservlet transports share the client side code of the http and https transports, respectively. On the server side, the difference is that the servlet and sslservlet transports use a servlet, org.jboss.remoting.transport.servlet.web.ServerInvokerServlet, to hand an invocation off to an org.jboss.remoting.transport.servlet.ServletServerInvoker. In other words, the servlet and sslservlet transports share a port with all other servlets running in the Application Server, while the http and https transports use a separate port managed by a CoyoteInvoker.
When the ServerInvokerServlet is initialized, it needs to be informed of which ServletServerInvoker to use. One way of doing that is to give it the appropriate InvokerLocator. For example, the following web.xml file is used by JBoss Messaging:
<web-app>
<servlet>
<servlet-name>JmsServerInvokerServlet</servlet-name>
<description>The JmsServerInvokerServlet receives JMS requests via HTTP
protocol from within a web container and passes it onto the
ServletServerInvoker for processing.
</description>
<servlet-class>org.jboss.remoting.transport.servlet.web.ServerInvokerServlet</servlet-class>
<init-param>
<param-name>locatorUrl</param-name>
<param-value>
<![CDATA[servlet://${jboss.bind.address}:8080/servlet-invoker/JmsServerInvokerServlet/?dataType=jms&JBM_clientMaxPoolSize=200&failureDisconnectTimeout=0&marshaller=org.jboss.jms.wireformat.JMSWireFormat&unmarshaller=org.jboss.jms.wireformat.JMSWireFormat&numberOfCallRetries=1&pingFrequency=214748364&pingWindowFactor=10&stopLeaseOnFailure=true&clientLeasePeriod=10000&validatorPingPeriod=10000&validatorPingTimeout=5000&timeout=0&blockingMode=blocking&blockingTimeout=30000&useAllParams=true&useClientConnectionIdentity=true]]>
</param-value>
<description>The servlet server invoker</description>
</init-param>
<load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>JmsServerInvokerServlet</servlet-name>
<url-pattern>/JmsServerInvokerServlet/*</url-pattern>
</servlet-mapping>
</web-app>
Unlike the bisocket transport, which was designed especially to allow push callbacks without opening a port on the client side, a server in any of the http transports necessarily opens a ServerSocket, so push callbacks are ruled out when JBoss Messaging runs on any of the http transports. Pull callbacks are used instead, and there are some configuration parameters that can be set:
blockingMode - indicates whether to use blocking or nonblocking mode when doing pull callbacks. In nonblocking mode a poller periodically polls for waiting callbacks, and, if there are none, returns. In blocking mode, the poller periodically polls for waiting callbacks, and, if there are none, the call blocks on the server side until a callback is made available, at which point the poller immediately retrieves the callback to the client side. Blocking mode, then, is more responsive, and it is used by JBoss Messaging. By default, nonblocking mode is used.
blockingTimeout - when pull callbacks are used in blocking mode, indicates the amount of time the callback poller should block on the server side waiting for a callback. The default value is 5000 milliseconds, but JBoss Messaging uses 30000 milliseconds.
Remoting has two mechanisms for monitoring the health of established connections, which inform listeners on the client and server sides when a possible connection failure has been detected. Currently, only JBoss Messaging uses these facilities. Note that JBoss Messaging establishes connections programmatically, so some unused declarative configuration options are omitted from this discussion.
A remoting server has the capability to detect when a client is no longer available. This is done by establishing an org.jboss.remoting.Lease on the server side, managed by a ServerInvoker. On the client side, an org.jboss.remoting.LeasePinger periodically sends PING messages to the server, and on the server side an org.jboss.remoting.Lease informs registered listeners if the PING doesn't arrive within the specified timeout period. A LeasePinger is created by a RemoteClientInvoker and it sends PINGS to a particular Lease. That LeasePinger/Lease pair defines, for purposes of connection monitoring in Remoting, the abstract concept of a connection.
The following parameter is relevant to leasing configuration on the server side:
clientLeasePeriod - specifies the timeout period used by the server to determine if a PING is late. The default value is "5000". This is also the suggested lease period returned by the server when the client inquires if leasing is activated.
The following parameters are relevant to leasing configuration on the client side:
lease_period - if set to a value greater than 0 and less than the suggested lease period returned by the server, will be used to determine the time between PING messages sent by LeasePinger. This parameter is not used by JBoss Messaging.
leasePingerTimeout - specifies the per invocation timeout value use by LeasePinger when it sends PING messages. This parameter is not used by JBoss Messaging.
The actual lease window established on the server side is initially set to twice the clientLeasePeriod value, but it can expand dynamically to adjust to actual network conditions. As long as PINGs arrive within 75% of the lease window, the window will remain unchanged. However, if a PING arrives at between 75% and 100% of the lease window, the lease window will be expanded to twice the time since the previous PING. For example, if the current lease window is 20 seconds, and if a PING arrives 17 seconds after the previous PING, the lease window will be set to 34 seconds.
On the client side, an org.jboss.remoting.ConnectionValidator periodically sends a PING message to the server and reports a failure if the response does not arrive within a specified timeout period. The PING is sent on one thread, and another thread determines if the response arrives in time. Separating these two activities allows Remoting to detect a failure regardless of the cause of the failure.
A ConnectionValidator is created by a call to one of the overloaded Client.addConnectionListener() methods, and since multiple Clients may share a RemoteClientInvoker, multiple ConnectionValidators may be associated with a particular Remoting connection.
The following parameters are supported by ConnectionValidator:
validatorPingPeriod - specifies the time, in milliseconds, that elapses between the sending of PING messages to the server. The default value is 2000.
validatorPingTimeout - specifies the time, in milliseconds, allowed for arrival of a response to a PING message. The default value is 1000.
failureDisconnectTimeout - if the parameter "stopLeaseOnFailure" (see Interactions between client side and server side connection monitoring) is set to "true", then "failureDisconnectTimeout" determines the disconnect timeout value to be used by org.jboss.remoting.LeasePinger in shutting down. In particular, if "failureDisconnectTimeout" is set to "0", then LeasePinger will avoid any network i/o.
NOTE. The default values of "validatorPingPeriod" and "validatorPingTimeout" have often been found in practice to be too small, increasing the likelihood of spurious connection failures.
NOTE. It is important to set "validatorPingPeriod" to a value greater than the value of "validatorPingTimeout". Doing so gives the ConnectionValidator a chance to notify all ConnectionListeners, which might result in shutting down the connection, before the next PING is sent.
As of Remoting version 2.4, the client side and server side connection monitoring mechanisms can be, and by default are, more closely related, in two ways.
If the parameter stopLeaseOnFailure is set to true, then, upon detecting a connection failure, ConnectionValidator will stop the LeasePinger, if any, pinging a lease on the same connection. The default value is "true".
Note that by default, a LeasePinger has no identity, so if it is replaced by another LeasePinger that pings the same Lease, the connection remains unchanged. Suppose that leasing is enabled and that a RemoteClientInvoker stops and is replaced by a new RemoteClientInvoker with a new LeasePinger. If the replacement occurs quickly, the server side Lease may never miss a PING, in which case there is no evidence that anything changed on the client side. That is, the connection is still alive, as far as the server is concerned. That semantics might be perfectly acceptable for some applications, but other applications might interpret the same events as a connection failure followed by a new connection. In particular, JBoss Messaging needs the latter semantics.
As of release 2.5.2, an important concept related to connection monitoring, client connection identity, is available. Remoting can be configured to treat a connection as being defined by a LeasePinger/Lease pair in which the LeasePinger has an identity. More specifically, when configured to do so by setting the parameter useClientConnectionIdentity to "true", Remoting identifies a connection with a LeasePinger/Lease pair in which the Lease expects PINGs to arrive from a particular LeasePinger.
A Client participates in a connection when it is connected by way of the new method
public void connect(ConnectionListener listener, Map metadata) throws Exception;
This method serves to connect the Client to the server by way of a new or existing RemoteClientInvoker, and it also registers the new ConnectionValidator with the RemoteClientInvoker's LeasePinger. Subsequently, if any ConnectionValidator registered with that LeasePinger detects a connection failure, it will (if "stopLeaseOnFailure" is "true") stop the LeasePinger, and the LeasePinger will cause each registered ConnectionValidator to notify each of its registered ConnectionListeners of the connection failure. Once the LeasePinger has been shut down and all of the notifications have been made, the connection anchored by the LeasePinger is defunct, and the associated Clients should be disconnected by a call to Client.disconnect(). If such a Client is reconnected by a call to Client.connect(), it will be associated with a new LeasePinger and, therefore, a new connection.
As of release 2.5.3.SP2, Remoting also supports the concept of server connection identity. Suppose that a ServerInvoker managing a Lease stops and is replaced by a new ServerInvoker and Lease. If the replacement occurs between PINGs from a ConnectionValidator, there is no evidence that the server has been replaced. Again, that semantics might be appropriate for some applications, but JBoss Messaging needs a semantics in which the original connection is considered to have been destroyed and replaced. If the parameter useServerConnectionIdentity is set to "true", then, when a ServerInvoker responds to a PING from a ConnectionValidator, it returns a token of its unique identity. If the identity has changed, then ConnectionValidator considers the connection to have been broken and it notifies all registered listeners.
In the following, $JBOSS_HOME is the root directory of the Application Server installation, and $CONFIG is a server configuration, e.g., default, all, etc.