IMS in the Parallel Sysplex Volume I: Reviewing the IMSplex Technology

104 IMS in the Parallel Sysplex, Volume I: Reviewing the IMSplex Technology

5.3.6 Lock table

The CQS uses the lock table to serialize access to the CQS control queue. The control queue

is currently the only queue that is locked.

The control queue is a CQS private queue that has list entries for functions such as overflow

processing, structure checkpoint, and rebuild processing. When a CQS wants to perform one

of these functions, it must first obtain the lock for the control queue. When it obtains the lock,

it proceeds to look at the list entry for the function that it needs to perform. If the list entry

already contains the name of a CQS, the function is already being performed. Otherwise, this

CQS adds its name, releases the lock on the control queue, and proceeds to perform the

function. When a CQS has its name in the list entry for a particular function, it is referred to as

the

master CQS

.

5.3.7 Event monitor controls

The event queue is located in the area of the EMCs and is used to post events of interest to

the connected users. The event queue is maintained by XES in the list structure. There is one

entry in the event queue for each connected user. When a monitored event occurs, an event

monitor control (EMC) is queued to the connector's event queue. EMCs are used to inform

the connector when an “event” occurs, for example, the arrival of a message on a registered

queue.

In this case, the IMS subsystems, represented by CQSs, are the connected users, and the

events they are interested in are the arrival and existence of messages on certain queues.

When IMS registers interest in a queue name (for example, TRANX), TRANX is referred to as

a “monitored event”. When there is work on the TRANX queue, event monitor control blocks

(EMCs) representing the queue are posted to the event queue for the registered user(s), and

the user is then informed there is work to be done.

The EMCs are 64-byte control blocks which represent a particular user's interest in a

particular queue name. For example, when IMSA registers interest in TRANX, an EMC will be

created for this relationship (IMSA-TRANX). If IMSB also registers interest in TRANX, then

another EMC for IMSB-TRANX will be created.

When an EMC is first posted to the event queue, that client is informed that the queue has

gone from “empty to non-empty”. The EMC will stay on the event queue for that user until he

either deregisters interest (at which time the EMC is deleted) or the queue becomes empty (at

which time the EMC is just removed from the event queue).

An EMC entry is a control block in the Coupling Facility list structure that is used to represent

a CQS that has a registered interest in a queue. Each EMC entry occupies 64 bytes of

storage. An EMC control block is created for each CQS that has an interest in a particular

queue. If you have three CQSs that register an interest in queue TRANA, three EMCs are

created. Care should be taken when sizing the shared queue structures to ensure that there

is enough space to store the number of EMCs that are required.

CQS reserves 20% of the total structure size for EMCs. If CFCC Level 4 or greater is being

used, the area can be dynamically increased in 5% increments up to a maximum of 50% if

required. Use the

SETXCF ALTER command to make this change. Available data elements are

used to create additional space for EMCs. If the EMC area is increased, the amount of space

available for list entries is reduced. If the EMC area runs out of space, any attempt by an IMS

system to register an interest in a queue fails.

Here’s an example of how the EMC looks. When there are multiple entries with the same

queue name (for example, multiple TRANXs) on the queue, they form a “sublist”. These