csa112 Event Subscription
An event service is an communications mechanism organised around the categories of data rather then senders and receivers. A "publisher" posts a message to a well-known event channel and all "subscribers" receive the message. Publishers and subscribers do not need to know who each other are; they need only know the event channel name.
As the Messaging service is similar to mail, the Event service is similar to a bulletin board.
The objective of csa112 is to learn how to use the SAFplus Platform event service to subscribe to events and to extract the relevant information from the events received by subscription.
What you will learn
You will learn:
- how to initialize the event library
- how to subscribe to events
- how to receive events
- how to extract the event data
The SAFplus Event service is SA-Forum compliant and this tutorial expects that you will refer to the relevant SA-Forum document for an in-depth description of the APIs used.
One difference to note between this application and previous ones is that this application will not have a main loop in
initializeAmf. Instead we will do our work in callback code. This means that when we return from
initializeAmf we won't exit the application.
Here we define variables to hold the payload of our event. Note the initialization of the
evtChannelName. The length is set to the length of the
EVENT_CHANNEL_NAME string WITHOUT including the
We initialize the OpenClovis Event Manager with
saEvtInitialize. We pass the address of the variable where we want the event handle stored. We pass the address of our event callback functions structure. Note that the only event callback function that we specify is
csa112Comp_appEventCallback. We do not specify the other callback which is the Asynchronous channel open callback. We specify
NULL for that field of
evtCallbacks because we don't open our event channel asynchronously.:
Within the above code we open the channel synchronously. We pass the handle we got back from
saEvtInitialize, the channel name we defined previously, flags that indicate this process is subscribing to the channel and it's a global channel, and that the channel should be created if it's not already there, a timeout value of "forever", and the address of the location where the channel handle should be stored. Then, we subscribe to a specific event stream on the event channel. The
EVENT_TYPE constant is defined in
common/common.h as 5432. This value is used by the event publisher when it sends events. The constant 1 being passed simply identifies this specific subscription on this channel. This value will be passed to our event delivery callback function.
The event delivery callback function is named
csa112Comp_appEventCallback. It checks whether our incoming data buffer needs to be deallocated. If there is a buffer, it deallocates it and then allocates enough data to receive the incoming event data. Then, we extract the data from the event into the newly allocated buffer using
clEventDataGet. Finally, we print the event data we received using
clprintf which is another member of Clovis' OS Abstraction Layer. As its name and usage suggest, it is the OSAL replacement for printf.
Before building the project,copy the common directory which can be found under
<eval_dir>/src/app to the working area
# cp -r <eval_dir>/src/app/common .
How to Run csa112 and What to Observe
- Start example csa212 in the same manner that we have started previous examples, with the SAFplus Platform Console.(Unlock csa212SGI0 instead of csa112SGI0 to run csa212).
# cd /root/asp/bin # ./asp_console cli[Test]-> setc 1 cli[Test:SCNodeI0]-> setc cpm cli[Test:SCNodeI0:CPM]-> amsLockAssignment sg csa212SGI0
Looking at the log file for this example you should see the following.
Fri Oct 1 13:14:16.935 2010 (PayloadNodeI1.10366 : csa212CompEO.---.---.00001 : INFO) Component [csa212CompI3] : PID . Initializing Fri Oct 1 13:14:16.935 2010 (PayloadNodeI1.10366 : csa212CompEO.---.---.00002 : INFO) IOC Address : 0x4 Fri Oct 1 13:14:16.935 2010 (PayloadNodeI1.10366 : csa212CompEO.---.---.00003 : INFO) IOC Port : 0x80 Fri Oct 1 13:15:31.389 2010 (PayloadNodeI1.10366 : csa212CompEO.---.---.00004 : INFO) Component [csa212CompI3] : PID . CSI Set Received
- Now unlock the application with:
cli[Test:SCNodeI0:CPM]-> amsUnlock sg csa212SGI0
At this point you should see the following in the log.
Fri Oct 1 13:15:31.389 2010 (PayloadNodeI1.10366 : csa212CompEO.---.---.00012 : INFO) csa212: ACTIVE state requested; activating service
Thats all there is to this example. What you are looking at in the log is the output of our event listener. It doesn't appear very interesting because we have not yet written an event publisher that will give the program something to output. We'll do that in the next example (csa213).
- At this point we can shut down our example in the usual fashion.
cli[Test:SCNodeI0:CPM]-> amsLockAssignment sg csa212SGI0 cli[Test:SCNodeI0:CPM]-> amsLockInstantiation sg csa212SGI0 cli[Test:SCNodeI0:CPM]-> end cli[Test:SCNodeI0]-> end cli[Test]-> bye
Summary and References
We've seen how to create a basic event subscriber application using Clovis' event manager library. For further reading check the Clovis SAFplus Platform API reference guide, specifically the section on the event manager. Also, the header file:
clEventExtApi.h should be helpful.