Friday, October 7, 2011

Dynamically changing app-specific logging under JBoss AS: parallel deployment

01.12.2011. Update: Go read first this post. Then come back here if you want.

Last time I described one way of changing the app-specific logging configuration at runtime. The proposed solution is limited to changing log level only. This post describes what can be done to be able to change the complete logging configuration without redeploying the application.

There are several ways to do it but they share the same basic idea: instead of deploying one artifact (the application with the logging configuration) deploy the application and the logging configuration separately. Later, if some logging detail must be changed, only the separately deployed logging configuration has to be changed. JBoss detects the change and redeploys the configuration. The application keeps running. Important note: if the application has to be redeployed then both deployments must be redeployed.

First things first: in order for this scheme to work the application and the logging configuration must be specially prepared. The steps are:
  1. Configure your application to use the app-specific logging and make sure it really works.

  2. Split your logging configuration in two files. One file remains packaged in the application as jboss-logging.xml. It should look like this:
    <logging xmlns="urn:jboss:logging:6.0" context="MyWonderfulLogContext"> 
    <define-context name="MyWonderfulLogContext"/>
    Only context attribute and define-context element stay in the file. While it is possible to include some other logging configuration in the file I would not recommend it because these definitions can't be changed without redeploying the application itself.

  3. Deploy your application. You will notice that it generates no logging. It is logical: there is after all no logging definitions in jboss-logging.xml of the application.

  4. Prepare the second deployment artifact. The main part of it is the remaining logging configuration. It is the original jboss-logging.xml without define-context element.:
    <logging xmlns="urn:jboss:logging:6.0" context="MyWonderfulLogContext"> 
    <!-- Here come definitions of handlers, loggers, etc -->

  5. Deploy it. There are 3 possibilities:

    • Package it into something JBoss recognizes as a valid deployment (a jar or an ear). The packaging does not have to match the packaging of your application. For example create a file named MyWonderfulLogContext.ear with a single file META-INF/jboss-logging.xml. Deploy this file next to your application. As of this moment you should see some logging being generated. If you need to change the configuration you have to change create jboss-logging.xml, repackage MyWonderfulLogContext.ear and redeploy it.

    • You can also deploy the prepared package exploded. For example, create directory MyWonderfulLogContext.ear in the deploy directory of JBoss, create META-INF subdirectory and copy jboss-logging.xml into it. Later you can just edit this file if you need to change the configuration.

    • Rename the prepared jboss-logging.xml to something like MyWonderfulLogContext-jboss-logging.xml (the name must end with jboss-logging.xml) and copy it into the deploy directory of JBoss. Later you can just edit this file if you need to change the configuration.

      This does not work with out of the box JBoss: it recognizes only the file named 'jboss-logging.xml'. Fortunately it is not hardcoded in some java class but specified as part of the configuration. To enable JBoss to recognize more than just jboss-logging.xml you need to change file JBOSS_HOME/server/<servername>/deployers/jboss-logging.deployer/META-INF/logmanager-jboss-beans.xml. Locate bean named "JBossLoggingMetaDataSchemaResolverDeployer" and change
      <property name="name">jboss-logging.xml</property>
      <property name="suffix">jboss-logging.xml</property>

What makes the parallel deployment scheme work is define-context element in jboss-logging.xml of the main application. As soon as the application is deployed JBoss creates the context object. The loggers used by the application are retrieved from this context object. As long as the application is deployed the context stays around. (Re-)Deploying the second deployment artifact just adds or removes the logging configuration to or from the context.

You can combine this scheme with the JMX bean solution. The JMX bean belongs to the context so it is also available as soon as the context is created (at deployment/startup) and remains valid until the log context is valid.

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