This is primarily because we are used to thinking connection and session are the same and can be used interchangeably to represent a unique single-threaded conversation between a server and a client. This is indeed the case for DB servers and clients.
The basic idea of JMS API's connection and session is to first create a top level heavyweight physical connection, then several lightweight sessions / logical connections under the only physical connection.
Because of the connection's heavyweight nature, we usually pool them just like DB connection pooling. We hope the connection can create lightweight sessions very fast and service its session operations (sending, receiving, tx etc) efficient by multiplexing them through its only physical connection.
Add to this, the following Spring configures supports caching a single JMS connection and multiple sessions:
<bean id="cachedConnectionFactory" class="org.springframework.jms.connection.CachingConnectionFactory"
However the JMS specification doesn't say how the MOM vendors should implement connections and sessions. For earlier WebSphere MQ versions before 7, each JMS session actually also represents a brand new physical connection / channel instance just like a JMS connection.
MQ version 7 introduces the so called "sharing conversations" which allows a configurable number of JMS connections and sessions (both represent conversations in MQ terms) to share a single physical connection / channel instance.
The number of conversations that can be shared across a single physical connection / channel instance is determined by the WebSphere MQ channel property SHARECNV. The default value of this property for Server Connection Channels is 10. A value of 0 or 1 basically disables the sharing.
In order to use this feature, your client side JMS connection factory also need to enable property
For example, assuming SHARECNV is 10, you created 5 connections and 15 sessions (it doesn't matter which connections created which sessions from sharing perspective), the total physical connections needed is (5+15)/10=2
For more information, you refer to this page.
If you are developing a low latency and high performance application, you must know how to tune JMS connections and sessions on your MQM vendor.
- How to Create Sessions quickly?
If you use the previous Spring config, you should estimate the maximum sessions you needs and then pre-populate some of them based on the SHARECNV value. Otherwise some session creations will be as slow as creating a new physical connection.
- How Many Connections to Create?
Due to the multiplexing nature, if most of the multiplexed sessions under a connection are busy most of the time, you should move those busy sessions to new connections. Of course, you should manage connections using pooling such as retrieving them from an application servers.