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<p>Selectors are just filters. Message Groups are a way of grouping messages together to the same consumer to partition your application or insure ordering is maintained.</p>


<p>Now you could implement message groups by hand, by having each consumer use its own selector for a specific header. e.g. if you had consumers A, B, C; you could use the values "A", "B", "C" as values of the JMSXGroupID header (or any other header name you like).</p>

<p>Then if consumer A used "JMXGroupID = 'A'" as a selector it would be the only consumer getting the A messages; ditto for B and C.</p>

<p>So that kinda simulates message groups a little. The downside is</p>

<ul><li>you have to know the set of consumers that are running to know what values of A, B or C you can put on the header. So if you start up a new consumer D, you have to change the producer to now be aware of D. Wtih message groups you can use any string whatsoever - typically the string will come from your business such as a product code, a customer ID, a stock ticker name or something (or some combination of data such as IBM and NASDAQ and Tuesday).</li></ul>


<ul><li>if for whatever reason consumer B stops running, noone consumes any messages for B - you have to know to restart B manually. With message groups things auto-partition and load balance for you with immediate failover.</li></ul>


<ul><li>there's nothing to stop you accidentally creating 2 threads with the same selector - or 2 processes on the network starting up with the same selector and accidentally consuming from the same group of messages which completely breaks ordering; with message groups you don't have to worry about this - since it guarrentees that only 1 thread in your entire system will process messages from one group at once in order - irrespective how many consumers you start.</li></ul>


<p>So in general; you can manually partition yourself using selectors. Message Groups however are a self-partitioning and auto-failvoer mechanism which also guarrentees that a single thread will process a specific message group at once, in order.</p></div>