Entity

An entity in RIDDL is the same as it is in DDD which defines it this way:

Entity Definitions

An object primarily defined by its identity is called an Entity.

and

Many objects are not fundamentally defined by their attributes, but rather by a thread of continuity and identity.

There are three main aspects to this definition of entity:

• Entities in the software implementation of the model are objects, containing both state and function. They can both be and do.
  This means they can represent any physical object, a concept, etc.
• Entities have an identity; they are identified by some unique value that no other entity of the same type may have.
• Entities are continuous; they have a lifecycle, evolving from creation, through their useful lifespan, to destruction.

An entity is the fundamental processor of work in a reactive system and in a RIDDL model. Entities are most often implemented in software using one of these techniques:

• using the user model where actors process messages and encapsulate state.
• object-oriented programming which also encapsulate states and defines functions to manipulate that state.
• functional programming where a collection of functions process messages or function calls using exclusive access to some data and a queue of messages; this simulates the user model.

Entities have a unique immutable persistent identifier, much like people have names except our personal names are not unique. The unique identifier is used to locate the entity in a computing system and for other computing purposes. These immutable and unique identifiers convey equivalence. That is when two values of an identifier are the same, then by definition, they must refer to the same entity. Changing the state of the entity does not break this equivalence. Type Id, which provides the means to reference the entity in its context or between contexts. An Entity’s immutable identity conveys equivalence. Individual pieces of data of an entity can change their values (mutable).

Entities hold state, whether that state is persistent or not. However, for entities, the most important state value is the unique identifier for that entity. Consider this diagram:

Two instances of the same Entity may have different attribute values, but because both instances have the same identity value, they represent the same Entity. The identifier “John Smith” is used in two entities that differ in their state. By definition, this means they refer to the same entity. However, when you compare “John Smith” with “Jane Smith”, they are not the same entity, even if all their other attributes are the same.

Entities can have various optional characteristics as shown in the sections below

Arguments: Multiple, a list of the names of technologies intended to be used in the implementation. Implication: This does not impact the behavior of the entity except in the differences provided by various implementation technologies.

Arguments: None Implication: The entity should use event sourcing to store its state as a log of change events

• Arguments: None
• Implication: The entity should store only its latest value without using event sourcing.

• Arguments: None
• Implication: The entity should not persist its state at all and is only
  available in transient memory. All entity values will be lost when the service is stopped.

• Arguments: None
• Implication: The entity is an aggregate root entity through which all commands and queries are sent on behalf of the aggregated entities.

• Arguments: None
• Implication: The entity’s implementation should favor consistency over
  availability in Erik Brewer’s CAP theorem.

• Arguments: None
• Implication: The entity’s implementation should favor availability over consistence in Erik Brewer’s CAP theorem.

• Arguments: None
• Implications: The entity is intended to implement a finite state machine.

• Arguments: None
• Implications: The entity should allow receipt of commands and queries via a message queue.

• Arguments: one string indicating the kind of entity
• Implications: The general kind of entity being defined. This option takes a single value which provides the kind of entity. Examples of useful kinds are “device”, “user”, “concept”, “machine”. This entity option may be used by downstream AST processors, especially code generators. Downstream processors may require additional entity kind values.

• Contexts

• Authors - define who the authors of the entity are
• Functions - named definitions of processing
• Handler - how to handle messages sent to an entity
• Includes - inclusion of entity content from a file
• Invariants - logical expressions that must always hold true
• Options - define translation options for the entity
• State - the data an entity holds
• Types - the definition of a type of information
• Terms - the definition of a term related to the entity