Difference between revisions of "Durable Subscription"

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= What are they? =
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= What is a Durable Subscription? =
  
 
"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? =
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= 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? =
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= 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:

Revision as of 20:03, 28 April 2013

What is a Durable Subscription?

"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.