JSAMP HUB waits for callee before returning to caller

[Hugo Buddelmeijer]

Hi all,

FWIW, some more information. The problem described below was introduced 
in JSAMP 1.2 (from February 15 2011); version 1.1 behaves similar to the 
current astropy hub.

The (very useful) Web Profile was introduced in version 1.2, which 
apparently required a large rewrite of the hub code, see the links below.

http://software.astrogrid.org/doc/p/jsamp/1.3-3/downloads.html
http://www.ivoa.net/pipermail/apps-samp/2011-February/000862.html
http://software.astrogrid.org/doc/p/jsamp/1.3-3/history.html#Version_1_2

My knowledge of Java is too limited to assess what is necessary to get 
the old behaviour into the new code base. Hopefully it is possible though!

Greetings,
Hugo


On 2014-06-23 12:29, Hugo Buddelmeijer wrote:
> Hi Mark et al.,
>
> The SAMP HUB in JSAMP (and thus TOPCAT) causes unnecessary delays with
> clients that do not immediately return the XML-RPC call from the HUB.
>
> The astropy HUB does not show this behaviour, so the delays can in
> principle be avoided. Perhaps the JSAMP HUB can be improved to mimic the
> astropy behaviour?
>
>
> ** Explanation and Solution **
>
> Before returning the XML-RPC call of a calling client, the SAMP HUB in
> JSAMP first waits for the called-upon client to return their XML-RPC call.
>
> Instead, for 'call' and 'callAll', the HUB could return the XML-RPC call
> of the caller directly, before initiating the XML-RPC call to the
> callee. For 'callAndWait', the HUB could return the XML-RPC call from
> the caller as soon as it has a SAMP 'reply' from the callee,
> irrespective of whether the callee has returned their XML-RPC call.
>
>
> ** Demonstration **
>
> The attached two files demonstrate the problem. client1.py contains a
> client that causes problems. client2.py contains a client that sends
> messages to client1 through callAndWait, call, and callAll. The essence
> of client1 is this:
>
>     def receive_call(...): # Receive a SAMP call.
>         client.reply(...)  # Immediately return a SAMP reply.
>         time.sleep(5)      # Perform desired time-consuming action.
>         return             # Return an XML-RPC reply.
>
>
> With the astropy HUB, all the calls from client2 take about 0.02
> seconds. With the JSAMP HUB, the calls take 5.02 seconds. The SAMP reply
> arrives even before the 'call' and 'callAll' XML-RPC function returns.
> Furthermore, a 'callAndWait' with a timeout of 2 seconds will also take
> 5 seconds.
>
> It would be great if JSAMP could become even more robust than it already
> is. It is currently even possible to perform a denial-of-service attack
> on TOPCAT using SAMP!
>
>
> ** Workarounds **
>
> Two workarounds for this problem are:
>
> 1) Have all clients terminate their incoming XML-RPC call immediately
> before performing the actions associated with the incoming message.
> Although this is probably the best behaviour, it seems to require quite
> some complexity for clients, like multi-threading and such. Perhaps so
> much complexity that the 'S' in SAMP is not applicable anymore. Anyone a
> suggestion on how to rewrite client1.py in such a way as cleanly as
> possible?
>
> 2) Add a try/except loop around every SAMP call with its own timeout in
> each client. This might be necessary anyway, since even if JSAMP is
> updated it might take a while before all users use the new version.
> However, this makes 'callAndWait' superfluous and also requires
> something like multithreading.
>
>
> Thanks for your insight,
>
> Hugo
>
>
>
>


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