Durable Subscription: Difference between revisions
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= What are they? = | |||
"A durable subscription saves messages for an inactive subscriber and delivers these saved messages when the subscriber reconnects. In this way, a subscriber will not lose any messages even though it disconnected. A durable subscription has no effect on the behaviour of the subscriber or the messaging system while the subscriber is active (e.g., connected). A connected subscriber acts the same whether its subscription is durable or non-durable. The difference is in how the messaging system behaves when the subscriber is disconnected." -- http://www.eaipatterns.com/DurableSubscription.html | "A durable subscription saves messages for an inactive subscriber and delivers these saved messages when the subscriber reconnects. In this way, a subscriber will not lose any messages even though it disconnected. A durable subscription has no effect on the behaviour of the subscriber or the messaging system while the subscriber is active (e.g., connected). A connected subscriber acts the same whether its subscription is durable or non-durable. The difference is in how the messaging system behaves when the subscriber is disconnected." -- http://www.eaipatterns.com/DurableSubscription.html | ||
= Why should I use one? = | |||
You may occasionally disconnect from the Datafeeds service - your client may crash, you may experience connectivity problems or you might simply need to reboot a machine. A durable subscription stops you losing messages when you're not connected: | You may occasionally disconnect from the Datafeeds service - your client may crash, you may experience connectivity problems or you might simply need to reboot a machine. A durable subscription stops you losing messages when you're not connected: | ||
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The Time-to-Live (TTL) on messages is 5 minutes. As long as you reconnect within 5 minutes from your last connection, on your next connection, you will receive all messages that you may have missed whilst your client was offline. | The Time-to-Live (TTL) on messages is 5 minutes. As long as you reconnect within 5 minutes from your last connection, on your next connection, you will receive all messages that you may have missed whilst your client was offline. | ||
= How do I request a durable subscription? = | |||
You don't need to rewrite your client! Just do these three things: | You don't need to rewrite your client! Just do these three things: | ||
* When you send a [http://stomp.github.com/stomp-specification-1.1.html#CONNECT_or_STOMP_Frame CONNECT] frame, send a '''client-id''' in the header. Set this to the email address you use to subscribe - this makes it easy to identify who the durable subscriber belongs to | * When you send a [http://stomp.github.com/stomp-specification-1.1.html#CONNECT_or_STOMP_Frame CONNECT] frame, send a '''client-id''' in the header. Set this to the email address you use to subscribe - this makes it easy to identify who the durable subscriber belongs to | ||
* When you send a [http://stomp.github.com/stomp-specification-1.1.html#SUBSCRIBE SUBSCRIBE] frame, send a '''activemq.subscriptionName''' header. Set this to unique string for each feed - for example, the hostname of your subscriber and the feed name, e.g. "prodbox-vstp" | * When you send a [http://stomp.github.com/stomp-specification-1.1.html#SUBSCRIBE SUBSCRIBE] frame, send a '''activemq.subscriptionName''' header. Set this to a unique string for each feed - for example, the hostname of your subscriber and the feed name, e.g. "prodbox-vstp" | ||
* Each time you receive a message, send an [http://stomp.github.com/stomp-specification-1.1.html#ACK ACK] so the service knows you've received it | * Each time you receive a message, send an [http://stomp.github.com/stomp-specification-1.1.html#ACK ACK] so the service knows you've received it | ||
If your client disconnects and reconnects, the service will automatically send all queued messages when you reconnect. | If your client disconnects and reconnects, the service will automatically send all queued messages when you reconnect. | ||
Revision as of 20:02, 28 April 2013
What are they?
"A durable subscription saves messages for an inactive subscriber and delivers these saved messages when the subscriber reconnects. In this way, a subscriber will not lose any messages even though it disconnected. A durable subscription has no effect on the behaviour of the subscriber or the messaging system while the subscriber is active (e.g., connected). A connected subscriber acts the same whether its subscription is durable or non-durable. The difference is in how the messaging system behaves when the subscriber is disconnected." -- http://www.eaipatterns.com/DurableSubscription.html
Why should I use one?
You may occasionally disconnect from the Datafeeds service - your client may crash, you may experience connectivity problems or you might simply need to reboot a machine. A durable subscription stops you losing messages when you're not connected:
- The TD and Train Movements feeds are high-volume, and if you disconnect for even a minute, you can lose a lot of data
- The VSTP feed contains new schedules, and if you miss one of these, you can't track that train
- The TSR feed has a handful of messages on a Friday morning (when the Weekly Operating Notice is published)
The Time-to-Live (TTL) on messages is 5 minutes. As long as you reconnect within 5 minutes from your last connection, on your next connection, you will receive all messages that you may have missed whilst your client was offline.
How do I request a durable subscription?
You don't need to rewrite your client! Just do these three things:
- When you send a CONNECT frame, send a client-id in the header. Set this to the email address you use to subscribe - this makes it easy to identify who the durable subscriber belongs to
- When you send a SUBSCRIBE frame, send a activemq.subscriptionName header. Set this to a unique string for each feed - for example, the hostname of your subscriber and the feed name, e.g. "prodbox-vstp"
- Each time you receive a message, send an ACK so the service knows you've received it
If your client disconnects and reconnects, the service will automatically send all queued messages when you reconnect.