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QMF Python Console Tutorial
QMF Python Console Tutorial has been edited by Ted Ross (Jan 29, 2009). Content:Prerequisite - Install Qpid MessagingQMF uses Qpid Messaging as its means of communication. To use QMF, Qpid messaging must be installed somewhere in the network. Qpid can be downloaded as source from Apache, is packaged with a number of Linux distributions, and can be purchased from commercial vendors that use Qpid. Please see Download for information as to where to get Qpid Messaging. Qpid Messaging includes a message broker (qpidd) which typically runs as a daemon on a system. It also includes client bindings in various programming languages. The Python-language client library includes the QMF console libraries needed for this tutorial. Please note that Qpid Messaging has two broker implementations. One is implemented in C++ and the other in Java. At press time, QMF is supported only by the C++ broker. If the goal is to get the tutorial examples up and running as quickly as possible, all of the Qpid components can be installed on a single system (even a laptop). For more realistic deployments, the broker can be deployed on a server and the client/QMF libraries installed on other systems. Synchronous Console OperationsThe Python console API for QMF can be used in a synchronous style, an asynchronous style, or a combination of both. Synchronous operations are conceptually simple and are well suited for user-interactive tasks. All operations are performed in the context of a Python function call. If communication over the message bus is required to complete an operation, the function call blocks and waits for the expected result (or timeout failure) before returning control to the caller. Creating a QMF Console Session and Attaching to a BrokerFor the purposes of this tutorial, code examples will be shown as they are entered in an interactive python session. $ python Python 2.5.2 (r252:60911, Sep 30 2008, 15:41:38) [GCC 4.3.2 20080917 (Red Hat 4.3.2-4)] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>>
Invoking Methods on an ObjectUp to this point, we have used the QMF Console API to find managed objects and view their attributes, a read-only activity. The next topic to illustrate is how to invoke a method on a managed object. Methods allow consoles to control the managed agents by either triggering a one-time action or by changing the values of attributes in an object. First, we'll cover some background information about methods. A QMF object class (of which a QMF object is an instance), may have zero or more methods. To obtain a list of methods available for an object, use the getMethods function. >>> methodList = queue.getMethods() getMethods returns an array of method descriptors (of type qmf.console.SchemaMethod). To get a summary of a method, you can simply print it. The _repr_ function returns a string that looks like a function prototype. >>> print methodList [purge(request)] >>> For the purposes of illustration, we'll use a more interesting method available on the broker object which represents the connected Qpid message broker. >>> br = sess.getObjects(_class="broker", _package="org.apache.qpid.broker")[0] >>> mlist = br.getMethods() >>> for m in mlist: ... print m ... echo(sequence, body) connect(host, port, durable, authMechanism, username, password, transport) queueMoveMessages(srcQueue, destQueue, qty) >>> We have just learned that the broker object has three methods: echo, connect, and queueMoveMessages. We'll use the echo method to "ping" the broker. >>> result = br.echo(1, "Message Body")
>>> print result
OK (0) - {'body': u'Message Body', 'sequence': 1}
>>> print result.status
0
>>> print result.text
OK
>>> print result.outArgs
{'body': u'Message Body', 'sequence': 1}
>>>
In the above example, we have invoked the echo method on the instance of the broker designated by the proxy "br" with a sequence argument of 1 and a body argument of "Message Body". The result indicates success and contains the output arguments (in this case copies of the input arguments). To be more precise... Calling echo on the proxy causes the input arguments to be marshalled and sent to the remote agent where the method is executed. Once the method execution completes, the output arguments are marshalled and sent back to the console to be stored in the method result. You are probably wondering how you are supposed to know what types the arguments are and which arguments are input, which are output, or which are both. This will be addressed later in the "Discovering what Kinds of Objects are Available" section. Asynchronous Console OperationsQMF is built on top of a middleware messaging layer (Qpid Messaging). Because of this, QMF can use some communication patterns that are difficult to implement using network transports like UDP, TCP, or SSL. One of these patterns is called the Publication and Subscription pattern (pub-sub for short). In the pub-sub pattern, data sources publish information without a particular destination in mind. Data sinks (destinations) subscribe using a set of criteria that describes what kind of data they are interested in receiving. Data published by a source may be received by zero, one, or many subscribers. QMF uses the pub-sub pattern to distribute events, object creation and deletion, and changes to properties and statistics. A console application using the QMF Console API can receive these asynchronous and unsolicited events and updates. This is useful for applications that store and analyze events and/or statistics. It is also useful for applications that react to certain events or conditions. Note that console applications may always use the synchronous mechanisms. Creating a Console Class to Receive Asynchronous DataAsynchronous API operation occurs when the console application supplies a Console object to the session manager. The Console object (which overrides the qmf.console.Console class) handles all asynchronously arriving data. The Console class has the following methods. Any number of these methods may be overridden by the console application.
Discovering what Kinds of Objects are Available |
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