Shared channels

 

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A number of networking products provide a mechanism to hide server failures from the network, or to balance inbound network requests across a set of eligible servers. These include:

The channel initiator takes advantage of these products to exploit the capabilities of shared queues.

 

Shared inbound channels

Each channel initiator in the queue-sharing group starts an additional listener task to listen on a generic port. This generic port is made available to the network through one of the technologies mentioned above. This means that an inbound network attach request for the generic port can be dispatched to any one of the listeners in the queue-sharing group that are listening on the generic port.

We can start a channel only on the channel initiator to which the inbound attach is directed if the channel initiator has access to a channel definition for a channel with that name. We can define a channel definition to be private to a queue manager or stored on the shared repository and available anywhere (a global definition). This means that we can make a channel definition available on any channel initiator in the queue-sharing group by defining it as a global definition.

There is an additional difference when starting a channel through the generic port; channel synchronization is with the queue-sharing group and not with an individual queue manager. For example, consider a client starting a channel through the generic port. When the channel first starts, it might start on queue manager QM1 and messages flow. If the channel stops and is restarted on queue manager QM2, information about the number of messages that have flowed is still correct because the synchronization is with the queue-sharing group.

We can use an inbound channel started through the generic port to put messages to any queue. The client does not know whether the target queue is shared or not. If the target queue is a shared queue, the client connects through any available channel initiator in a load-balanced fashion and the messages are put to the shared queue. If the target queue is not a shared queue, the messages might be put on any queue in the queue-sharing group with that name (the environment is one of replicated local queues), and the name of the queue determines the function, regardless of the hosting queue manager.

 

Shared outbound channels

An outbound channel is considered to be a shared channel if it is taking messages from a shared transmission queue. If it is shared, it holds synchronization information at queue-sharing group level. This means that the channel can be restarted on a different queue manager and channel initiator instance within the queue-sharing group if the communications subsystem, channel initiator, or queue manager fails. Restarting failed channels in this way is a feature of shared channels called peer channel recovery.

Workload balancing

An outbound shared channel is eligible for starting on any channel initiator within the queue-sharing group, provided that you have not specified that you want it to be started on a particular channel initiator. The channel initiator selected by WebSphere MQ is determined using the following criteria:

 

Shared channel summary

Shared channels differ from private channels in the following ways:

Private channel

Tied to a single channel initiator.

  • Outbound channel uses a local transmission queue.

  • Inbound channel started through a local port.

  • Synchronization information held in SYSTEM.CHANNEL.SYNCQ queue.

Shared Channel

Workload balanced with high availability.

  • Outbound channel uses a shared transmission queue.

  • Inbound channel started through a generic port.

  • Synchronization information held in SYSTEM.QSG.CHANNEL.SYNCQ queue.

You specify whether a channel is private or shared when you start the channel. A shared channel can be started by triggering in the same way as a private channel. However, when a shared channel is started, WebSphere MQ performs workload balancing and starts the channel on the most appropriate channel initiator within the queue-sharing group. (If required, we can specify that a shared channel is to be started on a particular channel initiator.)

 

Shared channel status

The channel initiators in a queue-sharing group maintain a shared channel-status table in DB2. This records which channels are active on which channel initiators. The shared channel-status table is used if there is a channel initiator or communications system failure. It indicates which channels need to be restarted on a different channel initiator in the queue-sharing group.