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MSMQ Part 1/2: Architecture and Simple Implementation Using VB


MSMQ is a messaging solution that uses the store-forward style of distributed computing to share data across networks in terms of messages. In part one of this two part series, Liviu explains the architecture of an enterprise system in relation to MSMQ. He also shows us how to implement a simple message queuing application using Visual Basic 6 and the MSMQ COM objects.

Author Info:
By: Liviu Tudor
Rating: 5 stars5 stars5 stars5 stars5 stars / 107
March 19, 2002
TABLE OF CONTENTS:
  1. · MSMQ Part 1/2: Architecture and Simple Implementation Using VB
  2. · MSMQ Architecture
  3. · Hello world with MSMQ
  4. · Creating, looking up and deleting queues
  5. · Conclusion

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MSMQ Part 1/2: Architecture and Simple Implementation Using VB - Creating, looking up and deleting queues
(Page 4 of 5 )

In our example we've seen a quick way of locating, opening and creating a queue. On this page we will take a closer look at how to locate a queue using more than just PathName and an MSMQQueue object.

FormatName Property

If you look in MSDN under the MSMQQueueInfo object property, you will notice a property called FormatName. The "name" bit at the end of this property suggests that it might be somehow related to the PathName, and that's not far from the truth.

Using the FormatName property, we can access a certain queue more or less in the same way that we did with the PathName property. The difference is (apart from the format) that by using this approach, a client can provide disconnected messaging (i.e. not reliant on a SC).

The format of this property is as follows:

QueueInfo.FormatName = "DIRECT=Protocol:Address\Queue"

QueueInfo.FormatName = "DIRECT=OS:ComputerName\Queue"

QueueInfo.FormatName = "PUBLIC=QueueGUID"

QueueInfo.FormatName = "PRIVATE=ComputerGUID\Queue"


Where the first part denotes the way the communication is achieved: PUBLIC (locating the queue via a SC), PRIVATE (locating a private queue), or DIRECT (bypass the SC – this is normally used for disconnected messaging, when we don't have access to the SC).

Protocol determines the protocol to be used for locating the machine holding the queue. Possible values are SPX, TCP or whatever other protocols are supported by MSMQ. In most of the cases the value will be TCP, because this is the standard protocol in an enterprise network. Queue is the queue name (if we considered our previous example, Queue would be "Greeting").

To get an idea of how queues are opened, we can use the following code to inspect the FormatName property:

Private Sub TestFormatName()

Dim qinfo As MSMQQueueInfo

Dim q As MSMQQueue

Set qinfo = New MSMQQueueInfo

qinfo.PathName = ".\PRIVATE$\Greeting2"

qinfo.Label = "Second Greeting Queue"

qinfo.Create

MsgBox "Queue format name is: " + qinfo.FormatName

Set q = qinfo.Open(MQ_SEND_ACCESS, MQ_DENY_NONE)

End Sub


Of course, you will have to replace the PathName with whatever path you're using to store your message in (in this case we are creating a private queue on the local machine, but the queue could be public and created on any server in your enterprise).

Deleting a queue

Deleting a queue is just as simple as creating one, and I won't go into to much detail about it. The only thing we have to do once we have located our queue is call the delete method of an MSMQQueueInfo object.

Here's a quick example on how to delete all queues called "Greeting".

Dim query As New MSMQQuery

Dim qinfos As MSMQQueueInfos

Dim qinfo As MSMQQueueInfo

Private Sub DelQ()

Set qinfos = query.LookupQueue(Label:="Greeting")

qinfos.Reset

Set qinfo = qinfos.Next

While Not qinfo Is Nothing

qinfo.Delete

Set qinfo = qinfos.Next

Wend

End Sub


As you may have noticed, we are using two new object types here: MSMQQuery and MSMQQueueInfos. Briefly, MSMQQuery allows us to query for queues. It has one single method called LookupQueue, which allows us to query the active directory for existing public queues. This method returns an MSMQQueueInfos object, which is basically a list of MSMQQueueInfo objects. If we reconsider the relationship diagram presented earlier on, here's what it looks like:

The extended MSMQ relationship diagram

Locating a queue by name

As we’ve just seen on the last page, we have already discovered another way of locating a queue – by using the MSMQQuery object and calling its LookupQueue method. This method returns an MSMQQueueInfos object, which is a collection of MSMQQueueInfo objects. The syntax of the function includes quite a few parameters, however the most interesting (and most used) one is label, which allows us to specify the name of the queue we're after.

Furthermore, another very interesting parameter is the one called RelLabel, which allows us to define a relationship between the value we pass as the parameter Label and the actual label of the queue (i.e. equal, not equal, greater than etc.) The possible values for this parameter are:

REL_EQ Equal

REL_NEQ Not equal

REL_LT Less than

REL_GT Greater than

REL_GE Greater than or equal

REL_LE Less than or equal

For example, to retrieve all of the queues not labelled "Greeting", we could use the following piece of code:

Dim Qs As MSMQQueueInfos

Dim MQ As New MSMQQuery

Set Qs = MQ.LookupQueue( Label:="Greeting", RelLabel:=REL_NEQ)


Once we get hold of the MSMQQueueInfos object, we can iterate through its items using the Next method, as seen on the previous page. However, remember before starting to iterate through an MSMQQueueInfos object, we must call its reset method to reset the position of the pointer right back to the beginning of the list.

So, getting back to our receiver example, here's another way to locate a queue called "Greeting":

Dim MQ As New MSMQQuery

Dim QS As MSMQQueryInfos

Dim QI As MSMQQueryInfo

Dim Q As MSMQQueue

Set QS = MQ.LookupQueue( Label:="Greeting") 'default relation is REL_EQ

QS.Reset

Set QI = QS.Next

Set Q = QI.Open( MQ_RECEIVE_ACCESS, MQ_DENY_NONE )

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