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    <p>If the events are to be processed in a specific sequence, then why not try adding "eventID" and 'orderID' fields to the messages? This way your EventServiceImpl class can sort, order and then execute in the proper order (regardless of the order they are created and/or delivered to the handler).</p> <p>Synchronizing the <code>handler.execute()</code> block will not get the desired results, I expect. All the <code>synchronized</code> keyword does is prevent multiple threads from executing that block at the same time. It does nothing in the realm of properly ordering which thread goes next.</p> <p>If the <code>synchronized</code> block does seem to make things work, then I assert you are getting very lucky in that the messages are being created, delivered and then acted upon in the proper order. In a multithread environment, this is not assured! I'd take steps to assure you are controlling this, rather than relying on good fortune.</p> <p>Example:</p> <ol> <li>Messages are created in the order 'client01-A', 'client01-C', 'client01-B', 'client01-D'</li> <li>Messages arrive at the handler in the order 'client01-D', 'client01-B', 'client01-A', 'client01-C'</li> <li>EventHandler can distinquish messages from one client to another and starts to cache 'client01' 's messages.</li> <li>EventHandler recv's 'client01-A' message and knows it can process this and does so.</li> <li>EventHandler looks in cache for message 'client01-B', finds it and processes it.</li> <li>EventHandler cannot find 'client01-C' because it hasn't arrived yet.</li> <li>EventHandler recv's 'client01-C' and processes it.</li> <li>EventHandler looks in cache for 'client01-D' finds it, processes it, and considers the 'client01' interaction complete.</li> </ol> <p>Something along these lines would assure proper processing and would promote good use of multiple threads.</p>
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    1. POPreventing race conditions for message processing
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    1. COThank you for your comments. I don't have control over the sending system, however it is designed to send messages in the correct sequence (although I suspect that it cannot be guaranteed). One of the issues is that in my code the handler is invoked each time a message is received. Say messages are received every 2ms, and the first message (only) takes 30 ms to process. Then a second instance of the handler will process message 2 whilst the first message is being processed (corrupting the data). In the method you describe would the handler have to be a singleton to process sequentially?
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    2. COA singleton would probably be the cleanest way to implement a datastructure that multiple threads can access easily, yes. Question: Since you have no control over the creation of the messages, is each message of a different type or are there >1 of the same type in each 'set' of messages?
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    3. COThere are multiple types of messages. The factory returns a handler for the type of message received. The message is then processed by calling the execute method on the handler - esentially the command pattern. In my example above the first message might contain data that the second message operates on - so the first message type handler persists the data - the second manipulates it.
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