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Distributed Entity Cache
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Clear (DCC) Mechanism
Why and when use it?
The distributed cache clearing DCC mechanism is needed when you have multiple OFBiz servers in a cluster that are sharing a single database. When one entity engine has either a create, update or delete operation that goes through it, it will clear its own caches. But it can also sent out a message to the other servers in the pool to clear their caches.
This is a feature that runs through the service engine which operates on top of the entity engine. When you're doing a distributed cache clear it will result in service calls to the other OFBiz servers in the cluster. In most cases you will use Java Messaging Service, to send a message to a JMS server that can then be distributed to other servers in the cluster.
How to set it?
To keep it simple we will only set the mandatory values. There are other options which are covered by defaults.
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Because of The infamous Java serialization vulnerability the RMI container has been disabled in the default configuration of OFBiz, and hence JNDI (relies on RMI in OFBiz). So if you want to use the DCC mechanism you will first need to uncomment as explained at
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The Entity Engine
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entityengine.xml |
This is the easiest part, for a given delegator you only have to set its distributed-cache-clear-enable attribute to "true" (false by default). As an example:
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<delegator name="default" entity-model-reader="main" entity-group-reader="main" entity-eca-reader="main" distributed-cache-clear-enabled="true">
<group-map group-name="org.ofbiz" datasource-name="localderby"/>
<group-map group-name="org.ofbiz.olap" datasource-name="localderbyolap"/>
<group-map group-name="org.ofbiz.tenant" datasource-name="localderbytenant"/>
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The Service Engine
The location of the JMS definition is in the service-engineframework/service/config/serviceengine.xml file. By default you set a jms-service of name "serviceMessenger". You define there a JMS server with its name, a jndi name and a topic name. To make as less changes as possible we use "default" for the server name and set the values in the jndi.properties file. I could have also set a server name in jndiservers.xml but my catch phrase is "the less changes the better". This is the service-engine.xml setting, same on each servers
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<!-- JMS Service Active MQ Topic Configuration (set as default in jndi.properties, the less changes the better) -->
<jms-service name="serviceMessenger" send-mode="all">
<server jndi-server-name="default"
jndi-name="topicConnectionFactory"
topic-queue="OFBTopic"
type="topic"
listen="true"/>
</jms-service>
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I decided to use Apache ActiveMQ as JMS server and to simply set these properties in the jndi.properties files (commenting out the OOTB default):
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java.naming.factory.initial=org.apache.activemq.jndi.ActiveMQInitialContextFactory java.naming.provider.url=tcp://172.18.7.4<AMQ-IP-SERVER1>:61616 topic.OFBTopic=OFBTopic connectionFactoryNames=connectionFactory, queueConnectionFactory, topicConnectionFactory |
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At this stage you need to install an ActiveMQ server somewhere. Initially, I decided to install the last available release of ActiveMQ : 5.5.0. But it turned that there are some known issues in this release. So I finally took the 5.4.2 release. To test, I installed it on my XP development machine, and on the cluster. It could can also be embedded in OFBiz, but I decided to simply run it as an external broker. I don't think it's interesting to have it embedded in OFBiz: you just install, run it and forget about it (it sets /etc/ for you). For testing I used the ActiveMQ recommended default setting for that. For production you will want to run it as a Unix Daemon (or Windows Service).
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You can then monitor ActiveMQ using the Web Console by pointing your browser at http://localhost:8161/admin/ and then topics page
Single point of failure
The setting above is sufficient in a staging environment but is a single point of failure in a production environment. So we need to create a cluster of ActiveMQ brokers. Since they should not consume much resources (only 256MB of memory at max and not much CPU cycles), we can put each instance on the same machines than the OFBiz instances.
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It's fairly simple to set through JNDI. For this we only need to replace in jndi.properties files
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java.naming.provider.url=tcp://172.18.7.4<AMQ-IP-SERVER1>:61616 |
by
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java.naming.provider.url=failover:(tcp://172.18.7.4:61616<AMQ-IP-SERVER1>61616?soTimeout=60000,tcp://172.18.7.5:61616<AMQ-IP-SERVER2>61616?soTimeout=60000)?randomize=false |
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- set advisorySupport="false" for the broker (exceptif except if you want to use advisroy advisory messages)
use transport.soTimeout=60000 and set enableStatusMonitor="true" for Openwire connector
Code Block <broker xmlns="http://activemq.apache.org/schema/core" brokerName="localhost" dataDirectory="${activemq.base}/data" destroyApplicationContextOnStop="true" advisorySupport="false"> .... <transportConnector name="openwire" uri="tcp://0.0.0.0:61616?transport.soTimeout=60000" enableStatusMonitor="true"/>
A possible pitfall
This issue should be resolved if you use a recent version (say after mid 2011): we have now the proper slf4j jar files in the classpath to redirect logging to log4j.
This section can be bypassed but might help in case of troubles
I first installed ActiveMQ 5.5.0 on my developement machine on XP. When I ran OFBiz I got this non blocking error
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SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.
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This lead to this tip
*Failed to load class org.slf4j.impl.StaticLoggerBinder*
This error is reported when the org.slf4j.impl.StaticLoggerBinder class could not be loaded into memory. This happens when no appropriate SLF4J binding could be found on the class path. Placing one (and only one) of slf4j-nop.jar, slf4j-simple.jar, slf4j-log4j12.jar, slf4j-jdk14.jar or logback-classic.jar on the class path should solve the problem. As of SLF4J version 1.6, in the absence of a binding, SLF4J will default to a no-operation (NOP) logger implementation. You can download SLF4J bindings from the project download page.
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