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Subscription Filters

Event-based systems are always nuanced. Generic message design for future flexibility or narrow scope to prioritise known requirements now.

With subscription filters, you can get a bit of both.

What are subscription filters?

Subscription filters are a mechanism in sailhouse to only care about specific events for a given subscription. When it comes to sending events, Sailhouse will check the shape of the event sent to see if it should be included for that subscription.

slug = "new-pro-user-notification"
topic = "new-users"
path = "type"
value = "pro"

Effectively, they’re a declarative filter for your events. It cuts down on the potential noise from a subscription, and you can keep a generic topic scope if you need while fulfilling a basic requirement now.

Why not just filter in the application?

It saves on execution. A push subscription here will only be called when there is a match, reducing the number of HTTP calls to a given endpoint. For pull subscriptions, it reduces the number of events you’re pulling down. Saving bandwith, outbound acknowledgement for no-op events and removing the need for you to add boilerplate checks for events you’re recieving.

This lowers cost, allowing you to architect in a way where you get the benefits of a narrow-scope and the flexibility of generic events.

Use cases

Oh where to begin, some highlights:

  • High-volume events where you have a large number of subscriptions (e.g. new-users)
  • Allows implementation of small, isolated features without big up-stream changes to optimise cost/performance
  • A/B Testing and gradual roll out

The opportunity to filter in the event-layer gives huge technical freedom to teams of any size. A small solo engineer doesn’t have to think about the overheads of generic messaging while larger teams benefit from the autonomy given to each team by listening to what they need.

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