IBM WebSphere MQ Series Important Concepts

QueueManager:
QueueManager is responsible for storing and routing messages to other Queue Manager within MQ and it also communicate with outside world e.g. Java program or any other MQ client.

Channel:
Queue Manager use channel to transmit messages to other QueueManager. Channel carries one way traffic in MQ Series (i.e. channels are uni directional). You can have either sending channel or receiving channel in MQ

Dead Letter Queue:
A queue which is used by QueueManager to archive messages for a non existent queue. Also, if a local queue is full, in that case also, messages are routed to Dead Letter Queue.

CCDT file (Client Channel Definition table):
CCDT file is a binary file which contains connection details required by MQ clients e.g. Java application using JMS to connect to MQ Server. In order to connect to MQ Server, MQ clients needs MQ Server host name, MQ Server port name and server channel name. All these details are encapsulated in CCDT file named as AMQCLCHL.TAB. In order to create MQ Connection, MQ clients needs location of this file, which is provided as configuration. most of MQ errors comes either with incorrect CCDT files.

SSLPEER (SSL Peer):
SSLPEER is a String usually DN (Distinguished name) of MQ Client which connect to QueueManager securely using QueueManager. This is a mechanism WMQ uses to identify client. In case of Java or JMS client, SSLPEER is DN of client certificate stored in its keyStore and sent to server during SSL handshake.

Backout Queue:
Dead letter queue is used to store messages which is received for non existent queue. On the other hand backout queue are application specific queue.If MQ client is not able to process message and ask for redelivery, message is redelivered to client with incremented delivery count. Once this deliveryCount crossed a configured threshold, message is moved to back-out queue for later processing or error handling. In short if MQ Series not able to deliver message to client after a preconfigured attempt, WMQ moves message to backout queue.

If the application is unable to move the message to the Back Out Queue (BOQ) for some reason, then an attempt is made to move the queue to the Dead Letter Queue (DLQ).  For example, there are some permission issues on the Back Out Queue, so the message could not be written to the BOQ and it will be moved to Dead letter Queue.

Binding connection and Client Connection:
If MQ clients sits on same physical server where QueueManager is located than it can create binding connection which is relatively faster than client connection, which is usually created by MQ clients residing on same network but not same host. Most of application uses MQ client connection to connect QueueMangaer, which is easy and flexible.

Local Queue and Remote Queue:
local queues are queue on same QueueManager while remote queue refers to queue on different QueueManager.