title: Camel Aggregator completionSize vs correlationExpression?
body: <p>Given the following Camel Aggregator (via Spring XML):</p>
  
  <pre><code>&lt;aggregate strategyRef="myggregationStrategy" completionSize="3"&gt;
      &lt;correlationExpression&gt; 
          &lt;simple&gt;${header.id} == 1&lt;/simple&gt; 
      &lt;/correlationExpression&gt;
      &lt;to uri="bean:postProcessor?method=process" /&gt;
  &lt;/aggregate&gt; 
  </code></pre>
  
  <p>What is the relationship between <code>completionSize</code>, <code>correlationExpression</code> and when the <code>myAggregationStrategy#aggregate</code> method gets called? Does <code>aggregate</code> get called when 3 messages have arrived at the aggregator? Does <code>correlationExpression</code> act as some sort of filter? How do they all work with each other?</p>