Definitions

following is a list of useful definitions in the context of DDD/CQRS/ES.
I tried to keep them as generic as possible, but some of them are also mapped to this specific library to help better understanding.

Message

A packet of data, that can carry some information

Command message

A message that its intent is to convey a request to change system behavior
commands are named using imperative verbs, like ReceivePayment, ConfirmOrder, MarkAsDelivered.
commands may be rejected by system, according to its logic, policies, state, ...

Event message

A message that its intent is to capture a fact.
events are always named using past-participle verbs, like OrderReceived, AssetSecured, PaymentReceived.
events may not be rejected as they are facts about what happened and you can't deny them.

Document message

A message that its purpose is to transfer information, without any other intent, consumers decide on how to act and what is the intent.
documents are named using nouns, like HealthReport, InvestorWeeklyReport, ShippingStatistics, CandleData.
Most of the time, documents represent aggregated data and are not very thin, and might be even files with a few dozen megabytes in size. (but do not send documents of this size to message brokers please!)

Components

Projection

An aggregated data structure per event stream, that maintains a specific view point from historical events.

Query

A request to ask system for some data. queries are idempotent by nature, and cannot change any system behavior. they can be cached most of the time.

Event stream

History of events from beginning in chronological order.

Command handler

A component that receives command message, decides what to do based on domain logic, and acts upon it.
it is basically a function in the form of cmd => F[EitherNec[Rejection, Unit]].

Event handler

A component that receives a sub-set of events from one or more event streams, and runs side effects.
they are basically modeled as a function that transforms an event stream to a process stream.
e.g. fs2.Pipe[F, Event, Nothing]
event handlers can send commands, that makes them process managers.
event handlers should be deterministic as much as possible, for example if an event handler wants to send any messages, message IDs must be reproducible, not using a random UUID; this will help with managing workflows and delivery guaranties.

Process manager

An event handler that may or may not maintain a state, and can send messages.
Process managers are building blocks of communication, choreography and also orchestration.

Saga

A process manager that involves in a long running business process, that potentially models a transactional behavior which involves compensating actions.

Tactical design

Value object

An immutable value that its value is its purpose. modeled as simply types, e.g. enums, opaque types, primitive types.

Entity

A value that has identity, and can be changed without losing its identity. modeled as case classes that have identity. entity field are mostly value objects.

Aggregate

A collection of entities, that together form a boundary, that is the unit of transactions. they are portions of state of the state machine (automaton).

Aggregate root

the root entity in an aggregate, that can be referenced from outside. this is the state machine entire state, which has a unique id that identifies its event stream too.