ShardRegion

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Defined in: src/cluster/sharding/ShardRegion.ts:113

ShardRegion is the node-local router for a sharded type. It talks to the ShardCoordinator to discover the home of each shard, hosts a Shard actor for every shard that lives locally, and forwards everything else to the remote region that owns the target shard. Messages whose shard home is unknown or in handoff are buffered until the coordinator answers.

Entities are grandchildren, not children: the region routes to a shard and the shard owns the entity actors. What the region deliberately keeps is the passivation policy — the idle sweep, the maxEntities LRU, and the shard-level sweep that stops a shard once it has stood empty long enough. It sees every message on this node, so it is the only place where a node-wide entity cap can be enforced; a shard only ever sees its own slice.

Deciding here is also what makes stopping a shard loss-free. The region marks the shard and stops routing to it in the same synchronous step as it sends the stop, so nothing it routes can land in a mailbox that is already draining — a shard that timed itself out would have to tell the region afterwards, and everything in that gap would be dropped.

Extends

Actor<TMessage | ShardingMessage | Terminated | Passivate>

Type Parameters

TMessage

TMessage = unknown

Constructors

Constructor

new ShardRegion<TMessage>(config): ShardRegion<TMessage>

Defined in: src/cluster/sharding/ShardRegion.ts:159

Parameters

config

ShardRegionConfig<TMessage>

Returns

ShardRegion<TMessage>

Overrides

Actor.constructor

Properties

config

readonly config: ShardRegionConfig<TMessage>

Defined in: src/cluster/sharding/ShardRegion.ts:159

Methods

displayName()

displayName(): string

Defined in: src/Actor.ts:192

Human-readable name for this actor in log lines and in the DevTools actor tree (#891). Defaults to the full path — which is already the log source, so an actor that doesn’t override this logs exactly as it did before.

override displayName(): string { return `User(${this.entityId})`; }

Purely cosmetic. The path stays the identity everywhere that routes, correlates or aggregates — metric labels, tracing attributes, dead letters, ActorRef.toString(), every wire identifier — so a display name is free to be ambiguous, unstable, or shared between actors.

Resolved on every record, not captured once. Two consequences: keep it cheap and side-effect free, and expect it to be called before preStart (hence the optional chain — the context is attached after construction). In exchange a name may be derived from state, and it updates when that state does. Throwing, or returning anything but a non-empty string, falls back to the path and warns once: a naming hook must not be able to take a log line down with it.

ActorOptions.withDisplayName(...) outranks this, for the same reason withSupervisorStrategy(...) outranks supervisorStrategy — the spawn site is the more specific statement. It has to: every Behaviors actor is a TypedActor that inherits this default, so a method that won would silently swallow the spawn-site value for exactly the actors that have no subclass to override. For a name that only becomes known at runtime, this.context.setDisplayName(...) outranks both.

Returns

string

Inherited from

Actor.displayName

onReceive()

onReceive(message): void

Defined in: src/cluster/sharding/ShardRegion.ts:225

Main message handler. Receives each envelope dequeued from the mailbox. A thrown error (sync or async) is caught by the supervisor.

Parameters

message

Terminated | Passivate | ShardingMessage | TMessage

Returns

void

Overrides

Actor.onReceive

postRestart()

postRestart(_reason): void | Promise<void>

Defined in: src/Actor.ts:152

Called on the fresh instance after a restart. Default: call preStart().

Parameters

_reason

Error

Returns

void | Promise<void>

Inherited from

Actor.postRestart

postStop()

postStop(): void

Defined in: src/cluster/sharding/ShardRegion.ts:218

Called after the actor has been terminated. Children are already stopped.

Returns

void

Overrides

Actor.postStop

preRestart()

preRestart(_reason, _message?): void | Promise<void>

Defined in: src/Actor.ts:119

Called before a restart, on the instance about to be thrown away. The default calls postStop() and nothing else.

Override to release what the instance holds outside itself — a file handle, an open socket, a broker connection — or to do something other than drop the message that failed.

Stopping this actor’s children is not done here: the framework tears them down after this hook returns and waits for them before building the replacement, because postRestart re-runs preStart and a named child needs its name back. To keep the children instead, see Actor.stopChildrenOnRestart.

Parameters

_reason

Error

_message?

Terminated | Passivate | ShardingMessage | TMessage

Returns

void | Promise<void>

Inherited from

Actor.preRestart

preStart()

preStart(): void

Defined in: src/cluster/sharding/ShardRegion.ts:188

Called after construction and before the first message is processed.

Returns

void

Overrides

Actor.preStart

stopChildrenOnRestart()

stopChildrenOnRestart(): boolean

Defined in: src/Actor.ts:149

Whether a restart tears this actor’s children down before rebuilding it. Default: true.

A restart replaces the Actor instance while the cell — and therefore the child map — survives. Keeping the children was the old behaviour and it made an ordinary pattern impossible: postRestart re-runs preStart, so an actor that spawns a named child there hit Child name … is not unique on its first restart and never recovered (#634).

Override to false when the children are expensive to rebuild, hold state the parent cannot restore, or are supervised independently — a connection pool, say. They then outlive the restart exactly as before, and it is on you to make preStart idempotent — by adopting the survivor from the cell, this.child = this.context.child('name').toNullable() ?? this.context.spawn(Child, 'name'), or with context.spawnAnonymous. An instance field cannot do it: preStart runs on a fresh instance after every restart, so this.child ??= … is always unset and re-spawns into the name the surviving child still holds, which fails the spawn and restarts the actor again.

This is a separate hook rather than a preRestart override because the teardown has to be awaited: the new instance cannot be built until the old children are actually gone, and preRestart has no way to tell the cell that it started something worth waiting for.