Concepts:
- Each processor is a named queue entry. This is
not a change from today, except that these may
be real queues in the JMS sense of the word if
the underlying queue manager uses JMS. But the
approach should NOT be JSM/MQ specific. It
should work just fine with JDBC-based queues.
- A queue entry would normally contain a JAMES Mail
object. JavaMail Message objects would be
contained, or more likely referenced, by a JAMES
Mail object. I say "normally" just in case we
want to permit other message types, e.g., JAMES
control messages, to be posted, since JAMES
processors effectively become services.
- Each processor is a transaction. What this means
is that we can try to wrap a global transaction
around behavior within the processor. This will
have some impact on Mailets, too.
- Each processor is associated with a queue manager
and, optionally, a retry schedule.
- I believe that a queue implementation independent
scheduler that provides the next time at which a
message should be processed may be sufficient.
Each queue entry would carry a timestamp before
which it should not be processed. "Restarting"
the queue would be as simple as changing that
timestamp entry.
- A new RETRY Mail state can be set to rollback the
transaction and put the Mail back into the queue.
We should decide on commit and rollback semantics.
- The processor acquires a new attribute that explicitly
sets the fall-through state. The default shall be the
new RETRY state, except for messages that exhause the
retry schedule.
There are all sorts of ways to express this in XML, one of which might be:
<processor name="<queue-name>"
[onException="..."]
[fallthrough="<message-state>" def: RETRY]
[class="<class>" def: LinearProcessor] >
<queue [queueManager="<class>"> def: DefaultQueueManager] >
<!-- queue configuration -->
</queue>
<schedule [class="<class>" def: DefaultScheduler]>
<!-- see RemoteDelivery -->
</schedule>
<mailet ...>
</processor>
Despite that this defines the queue manager as a child of the processor, we
would invert that in code, with the queue manager calling the processor. I
believe that thread management is defined squarely on the queue manager.
The concept still needs fleshing out, but I wanted to air the thoughts.
This gives a lot of flexibility in implementation. For example, one might
implement a processor as an MDB. The processor is responsible for workflow
within the transaction, and for bridging between the queue manager and the
mailet API.
For this reason, I would not define interfaces for the queue manager and
processor, but rather define semantics and responsibilities. The Mailet API
is the inviolate API that all processors would be expected to support. A
processor would accept the queue entry from the queue manager, invoke the
mailet pipeline, handle any exceptions, etc. A queue manager would handle
acquisition of appropriate entries from the queue queue, threading, calling
the processor, etc. I envision enhancements to the ToProcessor mailet, and
related calls, and am still debating how queue aliases might be handled,
which can be quite powerful.
--- Noel
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