eventhorizon

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 Go to latest Published: Feb 9, 2021 License: Apache-2.0 Imports: 7 Imported by: 0

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Event Horizon

Event Horizon is a CQRS/ES toolkit for Go.

NOTE: Event Horizon is used in production systems but the API is not final!

CQRS stands for Command Query Responsibility Segregation and is a technique where object access (the Query part) and modification (the Command part) are separated from each other. This helps in designing complex data models where the actions can be totally independent from the data output.

ES stands for Event Sourcing and is a technique where all events that have happened in a system are recorded, and all future actions are based on the events instead of a single data model. The main benefit of adding Event Sourcing is traceability of changes which can be used for example in audit logging. Additionally, "incorrect" events that happened in the past (for example due to a bug) can be compensated for with an event which will make the current data "correct", as that is based on the events.

Read more about CQRS/ES from one of the major authors/contributors on the subject: http://codebetter.com/gregyoung/2010/02/16/cqrs-task-based-uis-event-sourcing-agh/

Other material on CQRS/ES:

Inspired by the following libraries/examples:

Suggestions are welcome!

Usage

See the example folder for a few examples to get you started.

Storage drivers

These are the drivers for storage of events and entities.

Local / in memory

There are simple in memory implementations of an event store and entity repo. These are meant for testing/experimentation.

MongoDB

Fairly mature, used in production.

AWS DynamoDB

https://github.com/seedboxtech/eh-dynamo

Messaging drivers

These are the drivers for messaging, currently only publishers.

Local / in memory

Fully synchrounos. Useful for testing/experimentation.

GCP Cloud Pub/Sub

Experimental driver.

Kafka

https://github.com/Kistler-Group/eh-kafka

NATS Streaming

https://github.com/v0id3r/eh-nats

Development

To develop Event Horizon you need to have Docker and Docker Compose installed.

To start all needed services and run all tests, simply run make:

make

To manualy run the services and stop them:

make services
make stop

When the services are running testing can be done either locally or with Docker:

make test
make test_docker
go test ./...

The difference between make test and go test ./... is that make test also prints coverage info.

Get Involved

License

Event Horizon is licensed under Apache License 2.0

http://www.apache.org/licenses/LICENSE-2.0

Documentation

Overview

Package eventhorizon is a CQRS/ES toolkit.

Package eventhorizon is a CQRS/ES toolkit.

Index

Constants

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const DefaultAggregateType = "event_default"
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const DefaultMinVersionDeadline = 10 * time.Second

DefaultMinVersionDeadline is the deadline to use when creating a min version context that waits.

View Source 
const DefaultNamespace = "default"

DefaultNamespace is the namespace to use if not set in the context.

Variables

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var ErrAggregateNotFound = errors.New("aggregate not found")

ErrAggregateNotFound is when no aggregate can be found.

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var ErrAggregateNotRegistered = errors.New("aggregate not registered")

ErrAggregateNotRegistered is when no aggregate factory was registered.

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var ErrCommandNotRegistered = errors.New("command not registered")

ErrCommandNotRegistered is when no command factory was registered.

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var ErrCouldNotSaveEntity = errors.New("could not save entity")

ErrCouldNotSaveEntity is when a entity could not be saved.

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var ErrEntityHasNoVersion = errors.New("entity has no version")

ErrEntityHasNoVersion is when an entity has no version number.

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var ErrEntityNotFound = errors.New("could not find entity")

ErrEntityNotFound is when a entity could not be found.

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var ErrEventDataNotRegistered = errors.New("event data not registered")

ErrEventDataNotRegistered is when no event data factory was registered.

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var ErrIncorrectEntityVersion = errors.New("incorrect entity version")

ErrIncorrectEntityVersion is when an entity has an incorrect version.

View Source 
var ErrIncorrectEventVersion = errors.New("mismatching event version")

ErrIncorrectEventVersion is when an event is for an other version of the aggregate.

View Source 
var ErrInvalidEvent = errors.New("invalid event")

ErrInvalidEvent is when an event does not implement the Event interface.

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var ErrMissingEntityID = errors.New("missing entity ID")

ErrMissingEntityID is when a entity has no ID.

View Source 
var ErrNoEventsToAppend = errors.New("no events to append")

ErrNoEventsToAppend is when no events are available to append.

Functions

func AggregateTypeFromContext added in v0.5.1 

func AggregateTypeFromContext(ctx context.Context) string

func CheckCommand 

func CheckCommand(cmd Command) error

CheckCommand checks a command for errors.

func MarshalContext 

func MarshalContext(ctx context.Context) map[string]interface{}

MarshalContext marshals a context into a map.

func MinVersionFromContext 

func MinVersionFromContext(ctx context.Context) (int, bool)

MinVersionFromContext returns the min version from the context.

func NamespaceFromContext 

func NamespaceFromContext(ctx context.Context) string

NamespaceFromContext returns the namespace from the context, or the default namespace.

func NewContextWithMinVersion 

func NewContextWithMinVersion(ctx context.Context, minVersion int) context.Context

NewContextWithMinVersion returns the context with min version set.

func NewContextWithMinVersionWait 

func NewContextWithMinVersionWait(ctx context.Context, minVersion int) (c context.Context, cancel func())

NewContextWithMinVersionWait returns the context with min version and a default deadline set.

func NewContextWithNamespace 

func NewContextWithNamespace(ctx context.Context, namespace string) context.Context

NewContextWithNamespace sets the namespace to use in the context. The namespace is used to determine which database.

func NewContextWithNamespaceAndType added in v0.5.1 

func NewContextWithNamespaceAndType(ctx context.Context, namespace, aggregateType string) context.Context

func RegisterAggregate 

func RegisterAggregate(factory func(string) Aggregate)

RegisterAggregate registers an aggregate factory for a type. The factory is used to create concrete aggregate types when loading from the database.

An example would be: 

RegisterAggregate(func(id UUID) Aggregate { return &MyAggregate{id} })

func RegisterCommand 

func RegisterCommand(factory func() Command)

RegisterCommand registers an command factory for a type. The factory is used to create concrete command types.

An example would be: 

RegisterCommand(func() Command { return &MyCommand{} })

func RegisterContextMarshaler 

func RegisterContextMarshaler(f ContextMarshalFunc)

RegisterContextMarshaler registers a marshaler function used by MarshalContext.

func RegisterContextUnmarshaler 

func RegisterContextUnmarshaler(f ContextUnmarshalFunc)

RegisterContextUnmarshaler registers a marshaler function used by UnmarshalContext.

func RegisterEventData 

func RegisterEventData(eventType EventType, factory func() EventData)

RegisterEventData registers an event data factory for a type. The factory is used to create concrete event data structs when loading from the database.

An example would be: 

RegisterEventData(MyEventType, func() Event { return &MyEventData{} })

func UnmarshalContext 

func UnmarshalContext(vals map[string]interface{}) context.Context

UnmarshalContext unmarshals a context from a map.

func UnregisterCommand 

func UnregisterCommand(commandType CommandType)

UnregisterCommand removes the registration of the command factory for a type. This is mainly useful in mainenance situations where the command type needs to be switched at runtime.

func UnregisterEventData 

func UnregisterEventData(eventType EventType)

UnregisterEventData removes the registration of the event data factory for a type. This is mainly useful in mainenance situations where the event data needs to be switched in a migrations.

Types

type Aggregate 

type Aggregate interface {
	// Entity provides the ID of the aggregate.
	Entity

	// AggregateType returns the type name of the aggregate.
	// AggregateType() string
	AggregateType() AggregateType

	// CommandHandler is used to handle commands.
	CommandHandler
}

Aggregate is an interface representing a versioned data entity created from events. It receives commands and generates events that are stored.

The aggregate is created/loaded and saved by the Repository inside the Dispatcher. A domain specific aggregate can either implement the full interface, or more commonly embed *AggregateBase to take care of the common methods.

func CreateAggregate 

func CreateAggregate(aggregateType AggregateType, id string) (Aggregate, error)

CreateAggregate creates an aggregate of a type with an ID using the factory registered with RegisterAggregate.

type AggregateStore 

type AggregateStore interface {
	// Load loads the most recent version of an aggregate with a type and id.
	Load(context.Context, AggregateType, string) (Aggregate, error)

	// Save saves the uncommittend events for an aggregate.
	Save(context.Context, Aggregate) error
}

AggregateStore is responsible for loading and saving aggregates.

type AggregateType 

type AggregateType string

AggregateType is the type of an aggregate.

type Command 

type Command interface {
	// AggregateID returns the ID of the aggregate that the command should be
	// handled by.
	AggregateID() string

	// AggregateType returns the type of the aggregate that the command can be
	// handled by.
	AggregateType() AggregateType

	// CommandType returns the type of the command.
	CommandType() CommandType
}

Command is a domain command that is sent to a Dispatcher.

A command name should 1) be in present tense and 2) contain the intent (MoveCustomer vs CorrectCustomerAddress).

The command should contain all the data needed when handling it as fields. These fields can take an optional "eh" tag, which adds properties. For now only "optional" is a valid tag: `eh:"optional"`.

func CreateCommand 

func CreateCommand(commandType CommandType) (Command, error)

CreateCommand creates an command of a type with an ID using the factory registered with RegisterCommand.

type CommandFieldError 

type CommandFieldError struct {
	Field string
}

CommandFieldError is returned by Dispatch when a field is incorrect.

func (CommandFieldError) Error 

func (c CommandFieldError) Error() string

type CommandHandler 

type CommandHandler interface {
	HandleCommand(context.Context, Command) error
}

CommandHandler is an interface that all handlers of commands should implement.

func UseCommandHandlerMiddleware 

func UseCommandHandlerMiddleware(h CommandHandler, middleware ...CommandHandlerMiddleware) CommandHandler

UseCommandHandlerMiddleware wraps a CommandHandler in one or more middleware.

type CommandHandlerFunc 

type CommandHandlerFunc func(context.Context, Command) error

CommandHandlerFunc is a function that can be used as a command handler.

func (CommandHandlerFunc) HandleCommand 

func (h CommandHandlerFunc) HandleCommand(ctx context.Context, cmd Command) error

HandleCommand implements the HandleCommand method of the CommandHandler.

type CommandHandlerMiddleware 

type CommandHandlerMiddleware func(CommandHandler) CommandHandler

CommandHandlerMiddleware is a function that middlewares can implement to be able to chain.

type CommandType 

type CommandType string

CommandType is the type of a command, used as its unique identifier.

type ContextMarshalFunc 

type ContextMarshalFunc func(context.Context, map[string]interface{})

ContextMarshalFunc is a function that marshalls any context values to a map, used for sending context on the wire.

type ContextUnmarshalFunc 

type ContextUnmarshalFunc func(context.Context, map[string]interface{}) context.Context

ContextUnmarshalFunc is a function that marshalls any context values to a map, used for sending context on the wire.

type Entity 

type Entity interface {
	// EntityID returns the ID of the entity.
	EntityID() string
}

Entity is an item which is identified by an ID.

From http://cqrs.nu/Faq: "Entities or reference types are characterized by having an identity that's not tied to their attribute values. All attributes in an entity can change and it's still "the same" entity. Conversely, two entities might be equivalent in all their attributes, but will still be distinct."

type Event 

type Event interface {
	ID() string
	// EventType returns the type of the event.
	EventType() EventType
	// The data attached to the event.
	Data() EventData
	// Timestamp of when the event was created.
	Timestamp() time.Time

	// AggregateType returns the type of the aggregate that the event can be
	// applied to.
	AggregateType() AggregateType
	// AggregateID returns the ID of the aggregate that the event should be
	// applied to.
	AggregateID() string
	// Version of the aggregate for this event (after it has been applied).
	Version() int

	// A string representation of the event.
	String() string
}

Event is a domain event describing a change that has happened to an aggregate.

An event struct and type name should:

  1. Be in past tense (CustomerMoved)
  2. Contain the intent (CustomerMoved vs CustomerAddressCorrected).

The event should contain all the data needed when applying/handling it.

func NewEvent 

func NewEvent(eventType EventType, data EventData, timestamp time.Time) Event

NewEvent creates a new event with a type and data, setting its timestamp.

func NewEventForAggregate 

func NewEventForAggregate(eventType EventType, data EventData, timestamp time.Time,
	aggregateType AggregateType, aggregateID string, version int) Event

NewEventForAggregate creates a new event with a type and data, setting its timestamp. It also sets the aggregate data on it.

func NewIdEventForAggregate added in v0.5.2 

func NewIdEventForAggregate(id string, eventType EventType, data EventData, timestamp time.Time,
	aggregateType AggregateType, aggregateID string, version int) Event

type EventBus 

type EventBus interface {
	// PublishEvent publishes the event on the bus.
	PublishEvent(context.Context, Event) error

	// AddHandler adds a handler for an event. Panics if either the matcher
	// or handler is nil or the handler is already added.
	AddHandler(EventMatcher, EventHandler)

	// AddObserver adds an observer. Panics if the observer is nil or the observer
	// is already added.
	AddObserver(EventMatcher, EventHandler)

	// Errors returns an error channel where async handling errors are sent.
	Errors() <-chan EventBusError
}

EventBus sends published events to one of each handler type and all observers. That means that if the same handler is registered on multiple nodes only one of them will receive the event. In contrast all observers registered on multiple nodes will receive the event. Events are not garantued to be handeled or observed in order.

type EventBusError added in v0.5.1 

type EventBusError struct {
	Err   error
	Ctx   context.Context
	Event Event
}

EventBusError is an async error containing the error returned from a handler or observer and the event that it happened on.

func (EventBusError) Error added in v0.5.1 

func (e EventBusError) Error() string

Error implements the Error method of the error interface.

type EventData 

type EventData interface{}

EventData is any additional data for an event.

func CreateEventData 

func CreateEventData(eventType EventType) (EventData, error)

CreateEventData creates an event data of a type using the factory registered with RegisterEventData.

type EventHandler 

type EventHandler interface {
	// HandlerType is the type of the handler.
	HandlerType() EventHandlerType

	// HandleEvent handles an event.
	HandleEvent(context.Context, Event) error
}

EventHandler is a handler of events. If registered on a bus as a handler only one handler of the same type will receive each event. If registered on a bus as an observer all handlers of the same type will receive each event.

func UseEventHandlerMiddleware 

func UseEventHandlerMiddleware(h EventHandler, middleware ...EventHandlerMiddleware) EventHandler

UseEventHandlerMiddleware wraps a EventHandler in one or more middleware.

type EventHandlerFunc 

type EventHandlerFunc func(context.Context, Event) error

EventHandlerFunc is a function that can be used as a event handler.

func (EventHandlerFunc) HandleEvent 

func (h EventHandlerFunc) HandleEvent(ctx context.Context, e Event) error

HandleEvent implements the HandleEvent method of the EventHandler.

func (EventHandlerFunc) HandlerType added in v0.5.1 

func (h EventHandlerFunc) HandlerType() EventHandlerType

HandlerType implements the HandlerType method of the EventHandler.

type EventHandlerMiddleware 

type EventHandlerMiddleware func(EventHandler) EventHandler

EventHandlerMiddleware is a function that middlewares can implement to be able to chain.

type EventHandlerType added in v0.5.1 

type EventHandlerType string

EventHandlerType is the type of an event handler, used as its unique identifier.

type EventMatcher 

type EventMatcher func(Event) bool

EventMatcher is a func that can match event to a criteria.

func MatchAggregate 

func MatchAggregate(t AggregateType) EventMatcher

MatchAggregate matches a specific aggregate type, nil events never match.

func MatchAny 

func MatchAny() EventMatcher

MatchAny matches any event.

func MatchAnyEventOf 

func MatchAnyEventOf(types ...EventType) EventMatcher

MatchAnyEventOf matches if any of several matchers matches.

func MatchAnyOf 

func MatchAnyOf(matchers ...EventMatcher) EventMatcher

MatchAnyOf matches if any of several matchers matches.

func MatchEvent 

func MatchEvent(t EventType) EventMatcher

MatchEvent matches a specific event type, nil events never match.

type EventStore 

type EventStore interface {
	// Save appends all events in the event stream to the store.
	Save(ctx context.Context, events []Event, originalVersion int) error

	// Load loads all events for the aggregate id from the store.
	Load(context.Context, string) ([]Event, context.Context, error)
}

EventStore is an interface for an event sourcing event store.

type EventStoreError 

type EventStoreError struct {
	// Err is the error.
	Err error
	// BaseErr is an optional underlying error, for example from the DB driver.
	BaseErr error
	// Namespace is the namespace for the error.
	Namespace string
	// AggregateType
	AggregateType string
}

EventStoreError is an error in the event store, with the namespace.

func (EventStoreError) Error 

func (e EventStoreError) Error() string

Error implements the Error method of the errors.Error interface.

type EventStoreMaintainer 

type EventStoreMaintainer interface {
	EventStore

	// Replace an event, the version must match. Useful for maintenance actions.
	// Returns ErrAggregateNotFound if there is no aggregate.
	Replace(context.Context, Event) error

	// RenameEvent renames all instances of the event type.
	RenameEvent(ctx context.Context, from, to EventType) error
}

EventStoreMaintainer is an interface for a maintainer of an EventStore. NOTE: Should not be used in apps, useful for migration tools etc.

type EventType 

type EventType string

EventType is the type of an event, used as its unique identifier.

type Iter 

type Iter interface {
	Next() bool
	Value() interface{}
	// Close must be called after the last Next() to retrieve error if any
	Close() error
}

Iter is a stateful iterator object that when called Next() readies the next value that can be retrieved from Value(). Enables incremental object retrieval from repos that support it. You must call Close() on each Iter even when results were delivered without apparent error.

type ReadRepo 

type ReadRepo interface {
	// Parent returns the parent read repository, if there is one.
	// Useful for iterating a wrapped set of repositories to get a specific one.
	Parent() ReadRepo

	// Find returns an entity for an ID.
	Find(context.Context, string) (Entity, error)

	// FindAll returns all entities in the repository.
	FindAll(context.Context) ([]Entity, error)
}

ReadRepo is a read repository for entities.

type ReadWriteRepo 

type ReadWriteRepo interface {
	ReadRepo
	WriteRepo
}

ReadWriteRepo is a combined read and write repo, mainly useful for testing.

type RepoError 

type RepoError struct {
	// Err is the error.
	Err error
	// BaseErr is an optional underlying error, for example from the DB driver.
	BaseErr error
	// Namespace is the namespace for the error.
	Namespace     string
	AggregateType string
}

RepoError is an error in the read repository, with the namespace.

func (RepoError) Error 

func (e RepoError) Error() string

Error implements the Error method of the errors.Error interface.

type Snapshot added in v0.6.0 

type Snapshot interface {
	RawDataI() interface{}
	Version() int
	AggregateType() AggregateType
	AggregateId() string
}

type SnapshotStore added in v0.6.0 

type SnapshotStore interface {
	Save(ctx context.Context, a Aggregate) error

	Load(context.Context, AggregateType, string, int) (Aggregate, error)
}

type SnapshotStoreError added in v0.6.0 

type SnapshotStoreError struct {
	// Err is the error.
	Err error
	// BaseErr is an optional underlying error, for example from the DB driver.
	BaseErr error
	// Namespace is the namespace for the error.
	Namespace string
	// AggregateType
	AggregateType string
}

func (SnapshotStoreError) Error added in v0.6.0 

func (e SnapshotStoreError) Error() string

Error implements the Error method of the errors.Error interface.

type Versionable 

type Versionable interface {
	// AggregateVersion returns the version of the item.
	AggregateVersion() int
}

Versionable is an item that has a version number, used by version.ReadRepo.FindMinVersion().

type WriteRepo 

type WriteRepo interface {
	// Save saves a entity in the storage.
	Save(context.Context, Entity) error

	// Remove removes a entity by ID from the storage.
	Remove(context.Context, string) error
}

WriteRepo is a write repository for entities.

Source Files

View all Source files

Directories

Path Synopsis
aggregatestore
events
model
commandhandler
aggregate
bus
gcp
local
eventhandler
cron
projector
saga
waiter
memory
mongodb
trace
examples
guestlist/domain
guestlist/memory
Package memory contains an example of a CQRS/ES app using memory as DB.
 Package memory contains an example of a CQRS/ES app using memory as DB.
guestlist/mongodb
Package mongodb contains an example of a CQRS/ES app using the MongoDB adapter.
 Package mongodb contains an example of a CQRS/ES app using the MongoDB adapter.
todomvc
todomvc/internal/domain
middleware
commandhandler/async
commandhandler/scheduler
commandhandler/validator
eventhandler/async
cache
memory
mongodb
version
snapshotstore
mongodb

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