What is the use of Sequencing Key in TIBCO BW and How to configure it ?

About Sequencing Key:

• The sequencing Key allows BW to read more than one message at the same time but keep the order for messages which have the same Key.
• Process instances can have sequencing keys that evaluate to different values, but only process instances with the same value for the sequencing key are executed sequentially. 
• Process instances with different sequencing key values can be executed concurrently.
Example:

Imagine the following messages in your queue:

message1 / key: client1

message2 / key: client2

message3 / key: client1

message4 / key: client1 

With three session configured (i.e. Max Job = 3), BW will pull the first three messages (message 1 to 3).

The message 1 and 2 will be execute concurrently while the message 3 will wait the end of the session of the message 1 (they have the same key). In other term, until you confirm the message 1 (or loose its session), the message 3 is in a wait status.

How to set Sequencing Key in TIBCO BW:

Sequencing Key can be specified on the “misc” tab of a process starter. For example, in below screenshot, a file poller activity is specified to sequence files based on their extensions (i.e. all ‘.txt’ will be processed sequentially; all ‘.pdf’ will be sequential etc.)