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EventCollector

For a short summary of EventCollector, see Reventless Components Overview.

Framework Implementation

This component follows the Reventless Component Structure Pattern, using separate files for interface definitions (EventCollector.res), builder logic (EventCollector_Builder.res), and adapter interface (EventCollector_Adapter.res).

Overview​

d2 diagram

The EventCollector is the event consumption component that receives events from EventTopics . It provides a unified interface for components like ReadModels, EventMappers, and SideEffectHandlers to consume events with ordering guarantees.

Purpose and Responsibilities​

  • Responsibility: Subscribe to EventTopics; buffer events; deliver events to handlers with ordering guarantees; handle retries and dead letter processing
  • In: Events from EventTopics
  • Out: Events to ReadModel projections, EventMapper mappings, or SideEffectHandler functions

Usage Pattern​

EventCollectors are typically created as part of higher-level components (ReadModel, EventMapper, SideEffectHandler) and are not used directly by application code.

Creating an EventCollector​

  • create an EventCollector module by providing an EventCollectorChannel adapter
  • call make() on that module and provide the name and the EventTopics to subscribe to
Customer_ReadModel.res
module CustomerEventCollector = Reventless.EventCollector_Builder.Make(
  EventCollectorChannel_SQS,
)

let eventCollector = CustomerEventCollector.make(
  ~name="CustomerReadModel",
  ~eventTopics=allEventTopics,
  ~opts=pulumiOptions,
)

EventCollector Operations​

The EventCollector provides operations for event handling:

Enqueue Event Operation​

The enqueueEvent operation allows programmatic event injection:

type enqueueEvent = (
  int,      // delay in seconds
  string,   // aggregate id
  string    // message content
) => promise<unit>

Usage:

// Enqueue an event with 5 second delay
await eventCollector.enqueueEvent(5, "customer-123", eventJson)

Events Handler​

The jsonEventsHandler type defines how events are processed:

type jsonEventsHandler = array<Js.Json.t> => promise<unit>

This handler receives batches of events and processes them.

Runtime Behavior​

Event Collection Flow​

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Integration with Components​

ReadModel Integration​

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EventMapper Integration​

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SideEffectHandler Integration​

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Resource Naming:

  • Component type: reventless:EventCollector
  • Resource name pattern: {componentName}EventCollector

Dependencies:

  • EventCollector subscribes to EventTopics
  • ReadModel/EventMapper/SideEffectHandler depend on EventCollector
  • Lambda execution role needs sqs:ReceiveMessage, sqs:DeleteMessage permissions
  • EventTopic - Publishes events that EventCollector subscribes to
  • EventLog - Source of DynamoDB Stream events
  • ReadModel - Uses EventCollector for projection updates
  • EventMapper - Uses EventCollector for event-to-command mapping
  • SideEffectHandler - Uses EventCollector for side effect execution
  • CommandTopic - Similar pattern for command consumption

AWS Implementation​

For detailed implementation, see EventCollector AWS Adapter Documentation.