Component Overview

Plugin with Aggregates

The following diagram shows how all components work together in a plugin using Aggregates:

• Command Flow: Client → API → CommandGenerator → CommandTopic → Aggregate
• Event Flow: Aggregate → EventLog → EventTopic → EventCollectors → Processing Components
• Read Side: EventCollector → ReadModel → QueryDb ← API ← Client
• Event Processing: EventMapper and SideEffectHandler consuming events
• Plugin System: Cross-plugin communication via ExtensionPoints and Extensions
• Scheduling: Time-based command generation and health monitoring

Plugin with DCB Slices

The following diagram shows how the components work together in a plugin using DCB Slices (Dynamic Consistency Boundary). The key differences from the Aggregate approach are the shared DcbEventLog across all StateChangeSlices and the StateViewSlice that combines the EventCollector and ReadModel roles.

• Command Flow: Client → API → CommandTopic → StateChangeSlices (routed by command type)
• Write Side: Each StateChangeSlice reads from and appends to the shared DcbEventLog with optimistic concurrency
• Event Flow: DcbEventLog → EventTopic → StateViewSlice → QueryDb
• Automation: AutomationSlice consumes events, maintains a TODO list, and issues commands back to CommandTopic
• Outbound Translation: OutboundTranslationSlice consumes events, calls external systems, and optionally publishes commands back
• Inbound Translation: InboundTranslationSlice receives external input (webhooks, API calls), validates and translates it into domain commands
• Read Side: StateViewSlice projects events directly into QueryDb, replacing the EventCollector + ReadModel pair
• Plugin System: Cross-plugin communication via ExtensionPoints and Extensions
• Scheduling: Time-based command generation and health monitoring

Live updates. A QueryDb table can stream its changes to subscribed clients: the QueryDb stream feeds a StateTopic Lambda that publishes to the AppSync Events API. See AppSync Events live updates for the wire path.

Component reference

Each component is documented in detail on its own page. The table below is a one-line map; follow the link for types, operations, and examples.

Write side

Component	Role	Details
Aggregate	Transactional unit: handles a command against replayed state, emits events to its own EventLog	aggregate
StateChangeSlice	DCB equivalent of an Aggregate: decides against a tag-scoped read of the shared DcbEventLog and appends with optimistic concurrency	statechangeslice
EventLog	Append-only per-aggregate event storage with replay; publishes to EventTopic	eventlog
DcbEventLog	Shared, tag-queryable event store for DCB slices, with optimistic concurrency	dcbeventlog
CommandTopic	FIFO command queue with exactly-once, in-order delivery to handlers	commandtopic
CommandGenerator	Turns GraphQL mutations into commands published to a CommandTopic	commandgenerator

Read side

Component	Role	Details
ReadModel	Projects events from one or more Aggregates into persisted, queryable state	readmodel
StateViewSlice	DCB equivalent of a ReadModel: projects DcbEventLog events directly into a QueryDb (no separate EventCollector)	stateviewslice
QueryDb	Stores denormalized read-model state; serves API queries; supports indexes and TTL	querydb
EventTopic	Fans events out from an EventLog to multiple EventCollectors	eventtopic
EventCollector	Subscribes to EventTopics and delivers ordered events to ReadModels, EventMappers, and SideEffectHandlers	eventcollector

Reactions, automation & translation

Component	Role	Details
EventMapper	Maps events (from one or more Aggregates) to commands for a target Aggregate	eventmapper
AutomationSlice	DCB Automation (TODO-list) pattern: collects work from events, issues commands once, tracks resolution	automationslice
InboundTranslationSlice	Anti-corruption layer: validates external input (webhooks/APIs) and translates it into domain commands	inboundtranslationslice
OutboundTranslationSlice	Calls external services per event with per-item retry; optionally publishes commands back	outboundtranslationslice
SideEffectHandler	Like an EventMapper but targets Tasks/functions outside the command/event paradigm	sideeffecthandler
Task	Escape hatch for logic loosely coupled to events — interval jobs, uploads, foreign API calls	task
Counter	Atomic counts and dedup, used by EventMappers to avoid duplicate command generation	counter

Cross-plugin & platform

Component	Role	Details
Plugin	A bounded context and deployment unit grouping the components above	plugin
ExtensionPoint	A plugin's outbound contract: maps internal events to a stable public event vocabulary	extensionpoint
Extension	A plugin's inbound subscription: maps a remote ExtensionPoint's events to local commands	extension
Scheduler	Time-based command publishing for scheduled/periodic workflows	scheduler
Heartbeat	Periodic health/keepalive signals into the Platform Admin's Plugin ExtensionPoint	heartbeat