View Source Volley (Volley v1.0.0)
GenStage and Broadway producers for EventStoreDB
Volley provides a GenStage producer Volley.InOrderSubscription and a
GenStage/Broadway producer Volley.PersistentSubscription. Both of these
subscription producers can read a stream from beginning to end and then
keep up-to-date as new events are published to the EventStoreDB.
These producers can be used to build a reactive, event-driven, eventually consistent system suitable for Event Sourcing. In terms of Event Sourcing, these producers can be used to build process managers, sagas, and read models.
inorder-vs-persistent-subscriptions
InOrder vs. persistent subscriptions
The Volley.InOrderSubscription producer is a simpler subscription
model which uses Spear.read_stream/3 and Spear.subscribe/4 to read an
EventStoreDB stream in order. Volley.InOrderSubscription is a client-side
subscription: the client is responsible for storing its stream revision.
Volley.PersistentSubscriptions use the Persistent Subscription feature
of EventStoreDB to store stream revisions and perform back-pressure on the
EventStoreDB server-side. Persistent subscriptions do not have strict
ordering guarantees, which allows features like competing consumers,
batch processing, and message-parking (with a built-in dead letter approach).
See the EventStoreDB documentation on persistent subscriptions and
Spear.connect_to_persistent_subscription/5 for more information.
InOrder subscriptions have less resource impact on the EventStoreDB but are less flexible than persistent subscriptions. InOrder subscriptions are subject to head-of-line blocking: failing to process an event must halt the subscription in order to keep ordering. Persistent subscriptions offer more complex subscription strategies and can avoid head-of-line blocking but handlers may be more complex or difficult to write as they need to account for events potentially arriving out of order.
Systems do not need to be limited to only one kind of event listener: a mix of in-order and persistent subscriptions may be wise.