Sharding: Data Subscriptions

To allow data subscriptions in a sharded database:

• From a user's point of view, creating a data subscription is done once, just like it is done under a non-sharded database.
• Behind the scenes, though, the orchestrator that was appointed to handle this client and received its subscription request uses the subscription's settings and creates a data subscription with each shard.
• Each shard independently organizes available data in batches and sends the batches to the orchestrator in response to data requests.
• The orchestrator unifies the data sent by shards, handles documents (e.g. by adding included documents even if the original document arrives from one shard and the included document from another), and arranges them in new consumable batches.
• The orchestrator keeps track of the worker's progress.
  When the worker consumes its current delivery and requests another, the orchestrator sends it the next available batch.

Distributing the subscriptions between all shards this way allows RavenDB to serve its workers efficiently no matter how large the overall database gets.

Subscription

1. Data Subscription Worker
2. Worker Subscription and Data Requests
   • The worker subscribes with the orchestrator.
     The worker is unaware that the destination database is sharded, no special syntax or preparation is needed.
   • The worker informs the orchestrator when all data was consumed.
3. Orchestrator
   • A subscription is created with the orchestrator.
   • The orchestrator registers which data has been consumed by the worker and which is still to be delivered.
4. Orchestrator Subscription and Data Requests
   • The orchestrator subscribes with all shards.
   • The orchestrator informs all shards when all data was consumed.
5. Shard
   • A subscription is created with the shard.
   • Relevant data is organized in consumable batches.
     The shard registers which data has been consumed and which is still to be delivered.
6. Data Delivery to Orchestrator
   When the orchestrator informs the shard that all data has been consumed and the shard has an available batch, the batch is delivered to the orchestrator.
7. Data Delivery to Worker
   When the worker informs the orchestrator that all data has been consumed and the orchestrator has an available batch, the batch is delivered to the worker.

Data subscriptions features that are not supported yet under a sharded database:

• Concurrent Subscriptions
  Allowing multiple workers to connect a common subscription simultaneously.
• Data Subscriptions Revisions Support
  Subscribing to document revisions.

• SubscriptionCreationOptions.ChangeVector
  Providing an arbitrary change vector from which the subscription would start processing is currently not supported.
  

  Setting ChangeVector to one of the following special values is supported:

  • "LastDocument" (the latest change vector on the database)
  • "BeginningOfTime" (the earliest change vector on the database)
  • "DoNotChange" (keep current subscription change vector)