I'll move this discussion over to dev@activemq.apache.org as it belongs
there...
Below I go on sketching how FileMessage might work. As you can see it is
still quite sketchy in some parts so all comments and good ideas are
welcome.
Btw. is FileMessage a good name? In some cases we are not transferring
files, but data in memory. Basically we are transferring a finite byte
sequence. Is ByteSequenceMessage too awkward? I think it is maybe...
There are a few different possible implementations...
(i) FileMessage ends up being a wrapper on top of the existing JMS streams
This would be the preferred method in our case.
Current streaming API uses a designated destination for the stream. If
that destination is used for other purposes (other streams or messages)
there has to be a way of separating current stream from the rest of the
traffic.
Streams support message selectors, which looks like the best solution.
The problem is that if there are non-FileMessages sent to that
destination also and listeners for them, they have to also use selectors
to weed out FileMessage streams.
It would be great if FileMessage would behave just like other types of
messages. In the streaming case I guess it is not possible to achieve?
(ii) FileMessage uses some out of band transfer mechanism.
(iii) direct connection. This option is similar to (ii) but rather
than putting the file on some remote file server, the file stays on
the producer until the consumer has received it;
The nice thing is I think all of these approaches can be handled
nicely by the single FileMessage API; then it can be a
configuration/policy issue as to exactly which implementation is used.
If we look at the three implementations and what they would require from
the two endpoints:
1. producer must inform what selector is to be used to receive data
through JMS stream, consumers must acknowledge when they are ready to
receive data
2. producer must place file available to external server and place URL
to the message
3. producer must open a port and place URL to the message
.. we see that streaming is maybe the trickiest one. If producer just
starts streaming there is no guarantee (in the general case) that
consumer will receive the whole stream from the beginning. Is that correct?
In case 2, there is the question about pruning the files. Are they left
to external server for ever? Or should consumer be capable of confirming
the transfer, and after confirmation producer would remove the file? I
don't think we should assume the consumer has a write access to file server.
Case 3 is actually pretty straightforward. Do we want allow also a push
option where consumer opens the port and producer delivers the file?
In terms of getting started, the simplest route is probably to add the
API in first (to start with assuming just a URL to the file which is a
no brainer) then we should be able to start adding different providers
to suit.
Yes, I think that's the best way to go forward. I'll write something
based on that JIRA issue and send it to dev@activemq.apache.org for
comments. Does that sound good?
