RIDDL Documentation
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A handler is a very important definition in RIDDL because it is RIDDL’s way of expressing both business logic and relationships between the various components in a model. Handlers do that by specifying what should be done whenever a message of a particular type is received by its parent definition. Handlers are composed as a set of on clauses that connect a message with an example. The example specifies the business logic that should be executed upon receipt of the message. Because that business logic can send and publish further messages to other components, a relationship can be inferred between the component receiving the message sent and the component that contains the handler.

There are several kinds of handlers depending on the handler’s containing definition, as shown in this table:

Occurs InHandler Focus
AdaptorTranslate messages between contexts
ApplicationHandling the messages received from the user
ContextImplements API of the stateless context
EntityHandler to use on new entity before any morph action
ProcessorProvide ETL logic for moving inputs to outputs
ProjectionHandle updates and queries on the projection
RepositoryHandle updates and queries on the repository
StateHandle messages while entity is in that state

The types of definition in which Handlers occur are known as the “active” definitions. More details are provided in the sections below.

Adaptor Handlers

Adaptor handlers provide the translation of messages from context to another.

Application Handlers

Application handlers process the events generated by the Application’s user to invoke business logic that typically results in sending further messages to the “back end” (typically a gateway context). This allows the application to be connected to the rest of the model.

Context Handlers

Context handlers imply a stateless API for the context, perhaps encapsulating the other things defined within the context.

Entity Handlers

Entity handlers specify the default or “catch all” handler for an entity.
When the entity is new, or is not in a specific state, this default handler is used to process each message. If the message is not processed by the handler, then the entity’s processing for that message is null (message ignored). When an entity is in a specified state, it processes messages defined by the handler within that state (see below).

Projection Handler

Projections provide

State Handler

State handlers process messages while an entity is in that specific state, presumably with the intent of updating that state by generating events from commands.

Occurs In


  • On Clauses - a specification of how to handle an event