Consumer has feelings too.

[mikita-brodau]

Consumer-Driven Event Projection

Concept

Producers publish events in a canonical schema and works as they usually do.
Consumers receive only the fields they actually use instead of entire payload.
Payload reduction is handled by the broker.

Design Constraints

• Canonical schemas are immutable
• Consumer projections are normalized and reusable

Key Properties

• Producers are unchanged
• Consumers are bandwidth-efficient
• Projections are resolved once per application version

Benefits

• Reduced network bandwidth
• Stable latency characteristics
• Scales with number of consumers without producer coupling
• No requirement of dirty hands from consumer application team.

How It Works

1. Producer
   • Publishes canonical events
   • Works as it is
2. Consumer
   • On first message receives entire payload
   • Pass data about Application name and Version
   • Declare which event fields it uses
   • Declaration is sent once to the Schema Registry per Application version
   • No per-message negotiation
3. Schema Registry
   • Stores consumer projections
   • Maps application + version to a projection
   • Provide mapping to the broker
4. Broker
   • Receives canonical events
   • Applies the registered projection per consumer
   • Delivers a reduced payload to the consumer endpoint