RIDDL Documentation
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  5. Messages
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Messages

Messages are a foundational concept in RIDDL because a RIDDL model implies an implementation that is a message-driven system per the Reactive Manifesto. Messages in RIDDL are a special case of an aggregate type and the lingua franca of many RIDDL definitions. They define the API for all the processors:

Differences Between Kinds of Messages

RIDDL follows Bertrand Meyer’s notion of command/query separation which states, in the context of object-oriented programming in Eiffel that:

every method should either be a command that performs an action, or a query that returns data to the caller, but not both. In other words, asking a question should not change the answer.1

Consequently, RIDDL adheres to this principal and employs the notion in message definitions since RIDDL is message-oriented not object-oriented. However, RIDDL also includes message types for the responses from commands and queries, events and results, respectively. The following subsections provide definitions of these four things

Command

A directive to perform an action, likely resulting in a change to some information.

Event

A recordable fact resulting from a change to some information.

Query

A request for information from something

Result

A response to a query containing the information sought.

Summary

The various characteristics of the four kinds of messages are shown in the table below.

Kind Request? Response? Cancellable? Relationship
Command Yes No Yes Independent
Event Maybe Yes No Consequent Of Command
Query Yes No Yes Independent
Result No Yes No Consequent Of Query

The truth table above helps you understand the relationship between the kind of message and how it is handled by a model component. The sections below get even more specific.

How Messages Are Handled By Adaptors

Kind In Regard To Handling By Adaptor
Command Intent To Translate For Context
Event Intent To Translate For Context
Query Intent to Translate For Context
Result Intent to Translate For Context

How Messages Are Handled By Applications

Kind In Regard To Handling By Application
Command Data given from user to application
Event Not Applicable
Query Not Applicable
Result Data provided to user from application

How Messages Are Handled By Contexts

Kind In Regard To Handling By Context
Command Intent To Take Some Stateless Action
Event Notification That a Command Completed
Query Intent to Read From The Context
Result Result Of Reading From Context

How Messages Are Handled By Entities

Kind In Regard To Handling By Entity
Command Intent To Modify Entity State
Event Entity State Was Changed
Query Intent To Read Entity state
Result Consequent Of Query

How Messages Are Handled By Processors

Kind In Regard To Handling By Processor
Command Intent To Update Projection State
Event The Projection’s State Was Modified
Query Intent to Read Projection State
Result Result Of Reading Projection State

How Messages Are Handled By Projections

Kind In Regard To Handling By Projection
Command Intent To Update Projection State
Event The Projection’s State Was Modified
Query Intent to Read Projection State
Result Result Of Reading Projection State

Occurs In

All Vital Definitions

Contains

Fields

  1. Meyer, Bertrand. “Eiffel: a language for software engineering”. p. 22 ↩︎