Consuming Multiple Messages In Parallel from Multiple Windows Services

Hi Remus

What if I need multiple clients to read (RECEIVE) the same message?

Would it be possible?

Thanks

No.

A message can only be received once. Normally the first RECEIVE statement removes it from the queue, so no other RECEIVE can find the same message.

Also there is no way for the clients to specify the message to be received. With a WHERE clause the RECEIVE statement at most can restrict the result set to a particular conversation, but not to a particular message.

And finally RECEIVE statement is always executing in READ COMMITED isolation level, so two clients cannot receive messages from the same conversation group in different transactions, since each RECEIVE will attempt to place an exclusive lock on the conversation group and only one transaction can have an exclusive lock at any given moment.

HTH,
~ Remus

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If you are looking at a publish/subscribe type scenario, where you want messages to be delivered to multiple services, you could implement a service that maintains a list of subscriber services and upon receiving a message, sends a copy out each of its subscribers. See the sample on Remus' blog:

http://blogs.msdn.com/remusrusanu/archive/2005/12/12/502942.aspx

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Hi Rushi/Remus

I tested that example, setting the same subscription from two different clients. Then I sent a publication, and read messages.

For what I understand in that example when a client subscribes for a particular publication, his conversationID is saved on a table.

When a publication occurs a procedure sends messages to all subscribers, using that conversationID.

BUT, in case there are two subscriptions and a single client is listening for messages, two identical messages are read by the client.

In case there are two clients listening a lot of confusion, sometimes one client gets two messages, sometimes one, sometimes nothing...

I was expecting , since the conversationID seems to address to a single endpoint, only one message...

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Assuming that on a publish/subscribe scenario each client must create a unique subscription, I realize that each client have to create its own queue and service.

The problem is sending messages then.

The initiator should send the same message to all queues, but how? The number of queues created is not defined, is there a way to do it?

Is my theory correct? Or am I on the wrong direction?

Thanks for helping

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The subscribers are individual conversations. If they are on the same queue, then you must use the RECEIVE ... FROM queue WHERE conversation_handle = ... syntax to retrieve only the notifications for a given client (subscription).

If you use the RECEIVE w/o a WHERE clause, then the clients will mix the notifications, if they are on the same queue.

In the pub/sub sample at http://blogs.msdn.com/remusrusanu/archive/2005/12/12/502942.aspx the initiator doesn't know nor need to how many clients/queues are there. It will iterate through subscriptions and send a message to each one. Clients can be on the same queue or on different queue, it doesn't matter. The subscription notifications are all reply messages (from target to initiator, since is the client that initiates the subscription), so the pub/sub service does not need to know upfront how many clients are there, it just sends replies on the existing dialogs.

HTH,
~ Remus

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Thanks for the clarification Remus.

Now it's working good!

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Hi,

I have a couple of questions to make.

How can i trigger notifications to my application ( C#)

without hanging in WaitFor Operation?

It's possible for broker service to call some remote

object that my application provide ?

Can Publish/Subscribe using Broker Service be used

for a low latency notifications (150ms ) max with milions of

messages published per second?

Thanks in advance

Srgio