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Re: [xmlblaster] proposed xmlrpc plugin

Póka Balázs wrote:

I'd like to have some advice on a small project I need to get done as soon as possible.
Until now we have been using the plain SOCKET protocol to communicate with clients due to the fact that it's the only option if the server is unable to connect to the client (NAT, firewall, etc.). I need to use the update() method to receive messages. The problem with this is that clients still need full internet capability. However, we are gaining some customers who [will not/can not] change their firewall configuration because of this, and only let HTTP traffic pass through their proxies.

I've already started rewriting the XMLRPC plugin to support the apache xml-rpc library version 3, as the old one (v2) is quite outdated. The normal (non-callback) calls would be done exactly like now.

My idea about emulating callbacks _without the server needing to connect to the client_ (but with a better response time and less overhead) would be the following:
1) client issues XMLRPC request to server (can be considered something like polling)
2) server does not return from call before:
a) at least one message is waiting to be sent to the client
b) some "timeout" seconds elapses (defaults in the order of 10-30, could be adjusted)
Exactly this feature is implemented already on server side.
The java and C++ client libs support it via XmlBlasterAccess.receive() (see below).
All other clients can use it as well with the plain XML Qos markup, see

* This method synchronously accesses maxEntries messages from any xmlBlaster server side queue.
* <p>
* This is a convenience method which uses get() with a specific Qos.
* <p>Important note:<br />
* Currently you shouldn't use unlimited timeout==-1 as this could
* lead to a server side thread leak on client disconnect.
* As a workaround please use a loop and a timeout of for example 60000
* and just ignore returned arrays of length 0.
* </p>
* at param oid The identifier like
* "topic/hello" to access a history queue,
* "client/joe" to access a subject queue or
* "client/joe/session/1"
* to access a callback queue.
* The string must follow the formatting rule of ContextNode.java
* at param maxEntries The maximum number of entries to retrieve
* at param timeout The time to wait until return.
* If you choose a negative value it will block until the maxEntries
* has been reached.
* If the value is '0' (i.e. zero) it will not wait and will correspond to a non-blocking get.
* If the value is positive it will block until the specified amount in milliseconds
* has elapsed or when the maxEntries has been reached (whichever comes first).
* at param consumable Expressed with 'true' or 'false'.
* If true the entries returned are deleted from the queue
* at return An array of messages, is never null but may be an array of length=0 if no message is delivered
* at see org.xmlBlaster.util.context.ContextNode
* at see <a href="http://www.xmlblaster.org/xmlBlaster/doc/requirements/engine.qos.queryspec.QueueQuery.html";>engine.qos.queryspec.QueueQuery requirement</a>
* at see javax.jms.MessageConsumer#receive
MsgUnit[] receive(String oid, int maxEntries, long timeout, boolean consumable) throws XmlBlasterException;

regards Marcel

This way, the client only needs to poll every "timeout" seconds if there is no traffic. The average latency of messages is reduced as the server can return the message(s) as the return value of the current poll() call almost immediately when some message(s) are to be sent to the client.

My question is how I could implement this polling mechanism in the plugin (so that I don't need to rewrite anything in the application), meaning that I'd like to keep it working this way:

xmlBlasterConnection.connect (qos, listener);

and getting all messages as updates through "listener", just like with SOCKET.

thanks in advance,
Balázs Póka

Marcel Ruff