Projection

zio.blocks.projection.Projection

See theProjection companion object

final class Projection[A]

A declarative description of a read model A built from events: which sources to listen to, how to route their events, and what each kind of event does to A.

A Projection is immutable data and does nothing by itself. Every builder call returns a new Projection, and an engine (such as ProjectionEngine) later reads it to process events. Handlers only describe changes as ProjectionActions, so they cannot perform effects.

Create one with Projection[A](name) (one row per entity) or Projection.global[A](name) (shared totals), then chain from, a routing call, and on:

val greetings = Projection[Greeting]("greetings")
 .from("people")
 .routeToSelf
 .on[PersonGreeted]
 .insert((e, ctx) => Greeting(ctx.entityId, s"Hello, ${e.name}!"))

Further from(...) calls add more sources, and further on[...] calls add more handlers to the latest source. An on[...] with no from(...) before it creates a default source named "_default" that routes to the entity itself.

Call validate before starting an engine to catch an empty projection.

Value parameters

bindings: the sources with their routing and handlers, in the order they were added
entityPath: where the store lives and which field of A is its ID. It is None for global projections
isGlobal: whether this is a global projection
name: the projection's name, which also names its store file
schema: the schema of the projected type A

Attributes

Companion: object
Source: Projection.scala
Graph
Supertypes: class Object

trait Matchable

class Any

Members list

Value members

Concrete methods

All handlers of all sources, in binding order.

Attributes

Source: Projection.scala

Same as handle. Kept for binary compatibility.

Attributes

Source: Projection.scala

Starts a new source binding. Follow it with routeToSelf, routeToAll, routedBy or routedByField to choose how the source's events are routed.

Value parameters

sourceName: the name of the event source

Attributes

Source: Projection.scala

Finds the handler for event and runs it, returning the action to apply, or None if no handler matches the event's class.

When source is given, only the handlers of the first binding with that name are considered. Otherwise all handlers are searched in binding order, and the first match wins, even across sources. Pass source when the projection has several sources, so that an event is not handled by another source's handler.

Value parameters

ctx: the metadata of the event
event: the event to handle
source: restricts the search to this source

Attributes

Source: Projection.scala

Starts a handler for events of type E and attaches it to the most recently added source. Follow it with insert, delete, aggregate or another HandlerBuilder method to say what the event does.

If no source has been added yet, a source named "_default" that routes to the entity itself is created first.

Attributes

Source: Projection.scala

The routing key of event for the named source: ctx.entityId for routeToSelf, the extracted key for routedBy, and None for routeToAll or when there is no source with that name.

Attributes

Source: Projection.scala

The routing key of event using the most recently added source, or ctx.entityId if there are no sources. The result follows the same rules as routingKey.

Attributes

Source: Projection.scala

The projection's ProjectionScope, derived from how it was built:

ProjectionScope.Global if it was created with Projection.global
otherwise ProjectionScope.CrossEntity if any source uses routedBy, taking the key from the first such source
otherwise ProjectionScope.PerEntity

Note that routeToAll alone does not make a projection global.