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Actor classes

Actor classes enable you to create networks of actors (canister smart contracts) programmatically. Currently, actor classes have to be defined in a separate source file. To illustrate how to define and import actor classes, the following example implements a distributed map of keys of type Nat to values of type Text. It provides simple insert and lookup functions, put(k, v) and get(k), for working with these keys and values.

To distribute the data for this example, the set of keys is partitioned into n buckets. For now, we just fix n = 8. The bucket, i, of a key, k, is determined by the remainder of k divided by n, that is, i = k % n. The ith bucket (i in [0..n)) receives a dedicated actor to store text values assigned to keys in that bucket.

The actor responsible for bucket i is obtained as an instance of the actor class Bucket(i), defined in the sample file, as follows:

import Nat "mo:base/Nat";
import Map "mo:base/RBTree";

actor class Bucket(n : Nat, i : Nat) {

type Key = Nat;
type Value = Text;

let map = Map.RBTree<Key, Value>(;

public func get(k : Key) : async ?Value {
assert((k % n) == i);

public func put(k : Key, v : Value) : async () {
assert((k % n) == i);


A bucket stores the current mapping of keys to values in a mutable map variable containing an imperative RedBlack tree, map, that is initially empty.

On get(k), the bucket actor simply returns any value stored at k, returning map.get(k).

On put(k, v), the bucket actor updates the current map to map k to ?v by calling map.put(k, v).

Both functions use the class parameters n and i to verify that the key is appropriate for the bucket by asserting ((k % n) == i).

Clients of the map can then communicate with a coordinating Map actor, implemented as follows:

import Array "mo:base/Array";
import Buckets "Buckets";

actor Map {

let n = 8; // number of buckets

type Key = Nat;
type Value = Text;

type Bucket = Buckets.Bucket;

let buckets : [var ?Bucket] = Array.init(n, null);

public func get(k : Key) : async ?Value {
switch (buckets[k % n]) {
case null null;
case (?bucket) await bucket.get(k);

public func put(k : Key, v : Value) : async () {
let i = k % n;
let bucket = switch (buckets[i]) {
case null {
let b = await Buckets.Bucket(n, i); // dynamically install a new Bucket
buckets[i] := ?b;
case (?bucket) bucket;
await bucket.put(k, v);


As this example illustrates, the Map code imports the Bucket actor class as module Buckets.

The actor maintains an array of n allocated buckets, with all entries initially null. Entries are populated with Bucket actors on demand.

On get(k, v), the Map actor:

  • uses the remainder of key k divided by n to determine the index i of the bucket responsible for that key

  • returns null if the ith bucket does not exist, or

  • delegates to that bucket by calling bucket.get(k, v) if it does.

On put(k, v), the Map actor:

  • uses the remainder of key k divided by n to determine the index i of the bucket responsible for that key

  • installs bucket i if the bucket does not exist by using an asynchronous call to the constructor, Buckets.Bucket(i), and, after awaiting the result, records it in the array buckets

  • delegates the insertion to that bucket by calling bucket.put(k, v).

While this example sets the number of buckets to 8, you can easily generalize the example by making the Map actor an actor class, adding a parameter (n : Nat) and omitting the declaration let n = 8;. For example:

actor class Map(n : Nat) {

type Key = Nat

Clients of actor class Map are now free to determine the (maximum) number of buckets in the network by passing an argument on construction.

On the Internet Computer, calls to a class constructor must be provisioned with cycles to pay for the creation of a principal. See ExperimentalCycles for instructions on how to add cycles to a call using the imperative ExperimentalCycles.add(cycles) function.

Configuring and managing actor class instances

On the Internet Computer, the primary constructor of an imported actor class always creates a new principal and installs a fresh instance of the class as the code for that principal.

To provide further control over actor class installation, Motoko endows each imported actor class with an extra, secondary constructor. This constructor takes an additional first argument that specifies the desired installation mode. The constructor is only available via special syntax that stresses its system functionality.

Using this syntax, it's possible to specify initial canister settings (such as an array of controllers), manually install, upgrade and reinstall canisters, exposing all of the lower-level facilities of the Internet Computer.

See Actor class management for more details.