On 20/02/2013 12:13, Nigel Deakin wrote:
>
> I think what is needed is a new overview of queue and topic semantics towards the beginning of the spec. I'll work on
> some text and circulate it for comments.
I've now drafted this, adding new sections 4.1.2. Queue semantics and 4.2.2. Topic semantics. These are the first
sections in the spec that describe how queues and topics behave. I've pasted the new sections below, but you may find it
easier to read the pdf version from the usual place:
http://java.net/projects/jms-spec/sources/repository/content/jms2.0/specification/word/JMS20.pdf
(Note that I plan to include this new text in the "proposed final draft" that I will be submitting shortly, since
leaving it out would make things more complicated to review. However despite the name this isn't the final version, so
please comment in the usual way)
Here are the changes I have made:
QUEUES: I've added a new section 4.1.2. "Queue semantics" as follows. This is intended to be an introductory section
which describes the basic semantics, and replaces both of the sections I mentioned in my previous email (in JMS 1.1
these were 4.4.9 "Multiple sessions" and section 5.8 "QueueReceiver").
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4.1.2. Queue semantics
When point-to-point messaging is being used, an application sends messages to a queue.
An application may consume messages from the queue by creating a consumer (a MessageConsumer, JMSConsumer or
QueueReceiver object) on that queue. A consumer may be used to consume messages either synchronously or asynchronously.
A queue may have more than one consumer. Each message in the queue is delivered to only one consumer.
A consumer may be configured to use a message selector. In this case only messages whose properties match the message
selector will be delivered to the consumer. Messages which are not selected remain on the queue or are delivered to
another consumer.
The order in which an individual consumer receives messages is described in section 6.2.9 “Message order” below.
By definition, if a consumer uses a message selector, or there are other consumers on the same queue, then a consumer
may not receive all the messages on the queue. However those messages that are delivered to the consumer will be
delivered in the order defined in section 6.2.9.
Apart from the requirements of any message selectors, JMS does not define how messages are distributed between multiple
consumers on the same queue.
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TOPICS: As for topics, I had already spent time rewriting section 4.2.2 "Topic subscriptions" to cover the basic
semantics of topics, covering shared/unshared and durable/non-durable subscriptions. I have renamed this to 4.2.2.
"Topic semantics" and added some new text to the start to match that added above for queues:
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4.2.2. Topic semantics
When pub/sub messaging is being used, an application sends messages to a topic.
An application consumes messages from a topic by creating a subscription on that topic, and creating a consumer (a
MessageConsumer, JMSConsumer or TopicSubscriber object) on that subscription.
A subscription may be thought of as an entity within the JMS provider itself whereas a consumer is a JMS object within
the application.
A subscription will receive a copy of every message that is sent to the topic after the subscription is created, except
if a message selector is specified. If a message selector is specified then only those messages whose properties match
the message selector will be added to the subscription.
Each copy of the message is treated as a completely separate message. Work done on one copy has no effect on any other;
acknowledging one does not acknowledge any other; one message may be delivered immediately, while another waits for its
consumer to process messages ahead of it.
Some subscriptions are restricted to a single consumer. In this case all the messages in the subscription are delivered
to that consumer. Some subscriptions allow multiple consumers. In this case each message in the subscription is
delivered to only one consumer. JMS does not define how messages are distributed between multiple consumers on the same
subscription
The order in which messages are delivered to a consumer is described in section 6.2.10 “Message order” below. By
definition, if a subscription uses a message selector, or there are other consumers on the same subscription,then a
consumer may not receive all the messages sent to the topic. However those messages that are delivered to the consumer
will be delivered in the order defined in section 6.2.10.
(the remainder of this section introduces shared/unshared and durable/non-durable subscriptions, and is unchanged)
Nigel