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Chapter 8. Message Store Lockers

Abstract

Message store locks are used to elect the master broker in master/slave groups. They are also useful for ensuring that multiple brokers are not attempting to share the same message store. Red Hat JBoss A-MQ's lockers are configurable to allow for tuning.

8.1. Locker Basics
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Overview
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Red Hat JBoss A-MQ provides two default lockers that are used based on the type of message store being used:
  • shared file locker—used by KahaDB and LevelDB stores
  • database locker—used by the JDBC store
    Note
    JBoss A-MQ also provides a leased database locker that can be in cases where the brokers may periodically lose their connection to the message store.
These default lockers are configurable to optimize their performance.
For further optimization, you can implement your own locker and plug it into the message store. Doing so involves implementing a simple Java interface and adding some configuration to the persistence adapter.
Message store locks are primarily leveraged by the broker for electing masters in master/slave configurations. For more information on master/slave groups see chapter "Master/Slave" in "Fault Tolerant Messaging".

Configuring a persistence adapter's locker
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To configure the locker used by a persistence adapter you add a locker element as a child to the adapter's configuration element as shown in Example 8.1, “Configuring a Message Store Locker”.

Example 8.1. Configuring a Message Store Locker

<persistenceAdapter>
  <kahaDB directory = "target/activemq-data">
    <locker>
      ...
    </locker>
  </kahaDB>
</persistenceAdapter>
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Standard locker configuration properties
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All locker implementations are required to have the two common configuration properties described in Table 8.1, “Common Locker Properties”.
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Table 8.1. Common Locker Properties
PropertyDefault ValueDescription
lockAcquireSleepInterval 1000Specifies the delay interval, in milliseconds, between attempts to acquire a lock.
failIfLocked falseSpecifies in the broker should immediately fail if a lock cannot be obtained.
The properties are specified as attributes to the locker's XML configuration element.
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