Subscription Manager

The WBEMSubscriptionManager class is a subscription manager that provides for creating and removing indication subscriptions, indication filters and listener destinations for multiple WBEM servers and multiple WBEM listeners and for getting information about existing indication subscriptions.

A WBEM listener that is used with this subscription manager is identified through its URL, so it may be the WBEM listener provided by pywbem (see class WBEMListener) or any external WBEM listener.

This tutorial presents a full blown example on how to set up and shut down a subscription manager and some filters, listener destinations and subscription, including error handling. The code sections shown below are meant to be concatenated into one script.

The following code fragment creates a WBEMConnection object for connecting to the WBEM server that is the target for the subscriptions, i.e. the server that lateron will emit the indications. It also creates a WBEMServer object based on that connection, that will be added to the subscription manager lateron.

The code also defines the URL of a WBEM listener that will be the receiver of the indications. The WBEM listener is not subject of this tutorial, so we assume it just exists, for the purpose of this tutorial:

In [ ]:
from __future__ import print_function
import pywbem

# The WBEM server that should emit the indications
server = 'http://myserver'
username = 'user'
password = 'password'

# The URL of the WBEM listener
listener_url = 'http://mylistener'

conn = pywbem.WBEMConnection(server, (username, password),
                             no_verification=True)
server = pywbem.WBEMServer(conn)

Next, we create the subscription manager, and add the WBEM server to it.

This causes interaction to happen with the WBEM server: The add_server() method determines whether the WBEM server has any listener destinations, indication filters, or subscriptions that are owned by this subscription manager. This is determined based upon the subscription manager ID ('fred' in this example). Such instances could exist for example if this script was used before and has been aborted or failed. Because of the interaction with the WBEM server, exceptions could be raised.

In this tutorial, we use a try-block to show where exceptions can happen, but we just re-raise them without doing any recovery.

In [ ]:
sub_mgr = pywbem.WBEMSubscriptionManager(subscription_manager_id='fred')

try:
    server_id = sub_mgr.add_server(server)
except pywbem.Error as exc:
    raise

Now, we want to create a listener destination for our WBEM listener. We use "owned" listener destinations in order to benefit from the automatic recovery, conditional creation, and cleanup of owned instances (see section WBEMSubscriptionManager for details about owned instances).

Because the subscription manager has discovered instances owned by it already, the add_listener_destinations() method creates a listener destination instance only if it does not exist yet. That makes our code easy, because we don't have to care about that. However, because the method possibly needs to create an instance in the WBEM server, we again need to be prepared for exceptions:

In [ ]:
try:
    dest_inst = sub_mgr.add_listener_destinations(server_id, listener_url, owned=True)
except pywbem.Error as exc:
    raise

In this tutorial, we create a dynamic filter. We could have used a static (pre-existing) filter as well, of course.

Suppose, the management profile we implement requires us to use a specific filter name, so we need to create a "permanent" filter (an "owned" filter requires that the subscription manager has control over the filter name).

We further assume for the sake of simplicity of this tutorial, that the subscription is active only as long as the script runs, and that we tear everything down at the end of the script. Because the filter is permanent, we don't get the benefit of the automatic cleanup for owned filters, so we need to clean up the filter explicitly. Again, in order not to overload this tutorial, we do this in a straight forward flow without protecting ourselves againt exceptions. In a more real life example, you could for example protect the flow with a try-block and clean up in its finally clause.

The next piece of code creates a permanent filter:

In [ ]:
filter_source_ns = "root/cimv2"
filter_query = "SELECT * FROM CIM_AlertIndication " \
               "WHERE OwningEntity = 'DMTF' " \
               "AND MessageID LIKE 'SVPC0123|SVPC0124|SVPC0125'"
filter_language = "DMTF:CQL"
filter_name = "DMTF:System Virtualization:Alerts"

try:
    filter_inst = sub_mgr.add_filter(server_id, filter_source_ns, filter_query,
                                     filter_language, owned=False, name=filter_name)
except pywbem.Error as exc:
    raise

We now have a filter and a listener destination at hand. We can now activate the emission of indications defined by the filter to the WBEM listener defined by the listener destination, by creating an indication subscription.

The subscription is between an owned destination and a permanent filter, so it will automatically become owned. This allows the subscription manager to deal with its lifecycle automatically when attempting to clean up the underlying owned filter.

In [ ]:
try:
    sub_inst = sub_mgr.add_subscriptions(server_id, filter_inst.path, dest_inst.path)
except pywbem.Error as exc:
    raise

Once we want to stop the indication sending, we need to remove any permanent instances explicitly, and can leave the cleanup of owned instances to the subscription manager.

However, because subscriptions must be removed before their underlying filters or destinations, we need to remove the owned subscription explicitly as well, then the filter, then we can leave the rest (the destination) to the subscription manager cleanup that happens when we remove the server:

In [ ]:
try:
    sub_mgr.remove_subscriptions(server_id, sub_inst.path)
    sub_mgr.remove_filter(server_id, filter_inst.path)
    sub_mgr.remove_server(server_id)  # will automatically remove the owned destination
except pywbem.Error as exc:
    raise