EAI Patterns with Actor Model: Resequencer

At this point we’ve covered a number of Message Routers in addition to some less complex, but essential, messaging patterns. With regard to my discussion of Request-Response, Ramesh Mandaleeka asked a few questions about Actor Model and message delivery. One of his questions was “How [do you] handle message sequencing?”

With Akka and other Actor systems, in general you do not need to worry about the sequence in which messages will be received by an Actor as a result of direct message sends from another Actor. As discussed in the Akka documentation, direct messages from one Actor to a second Actor are always received in the order in which the first Actor sent them. To repeat just a bit of the Akka documentation here, assume the following:

  • Actor A1 sends messages M1, M2, M3 to A2
  • Actor A3 sends messages M4, M5, M6 to A2

Based on these two scenarios, we arrive at these facts:

  1. If M1 is delivered it must be delivered before M2 and M3
  2. If M2 is delivered it must be delivered before M3
  3. If M4 is delivered it must be delivered before M5 and M6
  4. If M5 is delivered it must be delivered before M6
  5. A2 can see messages from A1 interleaved with messages from A3
  6. Since there is no guaranteed delivery, any of the messages may be dropped, i.e. not arrive at A2

The bottom line here is, don’t be concerned about a sequence of basic messages sent directly from one Actor to another being received out of order. It just won’t happen.

. . .

The full text is now available as part of my book Reactive Enterprise with Actor Model on Safari Books Online.

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