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DurableSubscription.md

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Durable Subscription

Overview

The durable subscriptions service enables long lifetimes of subscriptions with big queue sizes and therefore zero-dataloss scenarios. If the connection to the client fails the server continues to sample values. On reconnect the client can use the transfer subscription service to gain access to the missed values.

Fix an existing server with minimal changes

  • If a custom implementation of INodeManager is used, or the Method CreateMonitoredItems is overridden, add the new parameter createDurable. As long as durable Subscriptions are not enabled in the configuration this parameter can be ignored.
  • ImplementINodeManager.RestoreMonitoredItems, this method is only called if durable subscriptions are enabled in the server configuration.
  • If a custom IMonitoredItem implementation is used, set IsDurable to false. Implement a Dispose method.

Enabling durable subscriptions on an existing server

Typically the following porting steps are necessary:

  • Provide an implementation of IMonitoredItemQueueFactory that sets SupportsDurableQueues to true. This factory returns your own implementation of IMonitoredItemQueue that persists values to storage and supports large queue sizes. As a reference see DurableMonitoredItemQueueFactory.
  • Register the IMonitoredItemQueueFactory by overriding the StandardServer method CreateMonitoredItemQueueFactory.
  • Provide an implementation of ISubscriptionStore that persists and restores subscriptions to storage to continue them after a server restart. The subscription store shall also enshure persistent queues are provided to the monitored items after a restore.
    • Register the ISubscriptionStore by overriding the StandardServer method CreateSubscriptionStore.
  • If a custom implementation of INodeManager is used, or the Method CreateMonitoredItems is overridden, add the new parameter createDurable. Pass the value of this parameter to the MonitoredItem constructor. If a custom queue length check if performed for durable subscriptions check against the durable queue length from SererConfiguration.
  • If a custom IMonitoredItem implementation is used, implement the IsDurable property. Implement a Dispose method. Add a createDurable paramter to the constructor.
  • Make the custom MonitoredItem use the IMonitoredItemQueueFactory from IServerInternal.MonitoredItemQueueFactory to get the registerd durable queues instead of using internal queues for events & value changes.
  • To test custom queues use the unit tests in Server Test Project in the file monitoredItemTests and adapt to use your own queue by providing your IMonitoredItemQueueFactory in the constructor.

Extend the ServerConfiguration

  • Set DurableSubscriptionsEnabled to true
  • Set MaxDurableNotificationQueueSize to the desired value
  • Set MaxDurableEventQueueSize to the desired value
  • Set MaxDurableSubscriptionLifetime to the desired value

Known limitations and issues

  • Subscriptions are only persistet on a gracefuls shutdown. If the server crashes or needs to be shut down forcefully all Subscriptions / MonitoredItems are lost.
  • Breaking change: The Interfaces for INodeManager & IMonitoredItem were extended to support durable subscriptions.