View source for Durable Subscription

== 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|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 [http://stomp.github.io/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.io/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.io/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.

{{Navtable-DataFeeds}}

[[Category:Network Rail Data Feeds]]