Counters - Node.js SDK

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Define a Counter Property

Create and Update a Counter
Query Counter Values

Realm is now Atlas Device SDK – Learn More

New in version 12.10.0.

Atlas Device SDK for Node.js offers a Counter class the you can use as a logical counter when working with synchronized databases. Traditionally, you would store a counter value and manually read, increment, and set it. However, if multiple clients attempt to update the counter, it could result in an inaccurate underlying value across clients.

Consider a scenario where a Realm object has a count property of type int. Devices A and B both read the value as 0, then increment the count by adding 1 to the read value. Instead of reflecting both devices' increments and converging to 2, the underlying value is only 1.

The Counter class makes it possible to sync these updates so the value converges to the same underlying value across all clients.

The SDK's counter is a presentation data type with an underlying type of int. This means that no migration is required when changing an int type to a counter.

Counters cannot be used as:

Define a Counter Property

To use the Counter class, declare a property in your Realm Object as type Counter.

You can optionally declare this property as nullable by making it optional and passing both Counter and null as types. With a nullable counter, you can set the counter property within your Realm Object to null as needed.

The property is initialized by using either:

Object notation
- { type: "int", presentation: "counter" }
- { type: "int", presentation: "counter", optional: true }
Shorthand
- "counter"
- "counter?"

export class ClassWithCounter extends Realm.Object {
  static schema = {
    name: "ClassWithCounter",
    primaryKey: "_id",
    properties: {
      _id: { type: "objectId", default: () => new BSON.ObjectId() },
      myCounter: { type: "int", presentation: "counter" },
      // or myCounter: "counter"
      nullableCounter: { type: "int", presentation: "counter", optional: true },
      // or nullableCounter: "counter?"
    },
  };
}

export class ClassWithCounter extends Realm.Object<ClassWithCounter> {
  _id!: BSON.ObjectId;
  myCounter!: Counter;
  nullableCounter?: Counter | null;
  static schema: ObjectSchema = {
    name: "ClassWithCounter",
    primaryKey: "_id",
    properties: {
      _id: { type: "objectId", default: () => new BSON.ObjectId() },
      myCounter: { type: "int", presentation: "counter" },
      // or myCounter: "counter"
      nullableCounter: { type: "int", presentation: "counter", optional: true },
      // or nullableCounter: "counter?"
    },
  };
}

Create and Update a Counter

To initialize a counter, create your object using the realm.create() method. Pass in your Realm Object Schema and initial counter value, as well as initial values for any other properties the object has.

const siteVisitTracker = realm.write(() => {
  return realm.create(SiteVisitTracker, { siteVisits: 0 });
});

You can then use the following methods to modify the counter value:

increment() and decrement() update the underlying value by a specified number.
set() reassigns the counter to a specified value.

siteVisitTracker.siteVisits.increment();
siteVisitTracker.siteVisits.value; // 1
siteVisitTracker.siteVisits.increment(2);
siteVisitTracker.siteVisits.value; // 3
siteVisitTracker.siteVisits.decrement(2);
siteVisitTracker.siteVisits.value; // 1
siteVisitTracker.siteVisits.increment(-2);
siteVisitTracker.siteVisits.value; // -1
siteVisitTracker.siteVisits.set(0); // reset counter value to 0

Warning

Counter Resets

Use caution when using set(), as it overwrites any prior calls to increment() and decrement(). Depending on the order of operations, this can result in the counter converging on a different value. To avoid inaccurate counter values across clients, we recommend that you avoid mixing set() with increment() and decrement().

To update a nullable counter, either to or from a null value, you must use realm.create() with an UpdateMode specified. Instead of updating the underlying counter value, this sets the counter property either to null or to a new counter.

UpdateMode updates any existing Counter object with a matching primary key, according to the specified mode:

UpdateMode.All updates all properties provided.
UpdateMode.Modified updates only modified properties.

const siteVisitTracker = realm.write(() => {
  return realm.create(SiteVisitTracker, {
    nullableSiteVisits: 0,
    siteVisits: 1,
  });
});
const myID = siteVisitTracker._id;
realm.write(() => {
  realm.create(
    SiteVisitTracker,
    { _id: myID, nullableSiteVisits: null },
    UpdateMode.Modified
  );
});
realm.write(() => {
  realm.create(
    SiteVisitTracker,
    { _id: myID, nullableSiteVisits: 0 },
    UpdateMode.Modified
  );
});

Query Counter Values

You can query counter properties like other property types. However, to query by the underlying counter value, you must pass the counter.value in a parameterized query. In the following example, we want to find all objects with a counter value greater than or equal to that of the specified counter.

const belowThreshold = realm.write(() => {
  return realm.create(SiteVisitTracker, { siteVisits: 0 });
});
const atThreshold = realm.write(() => {
  return realm.create(SiteVisitTracker, { siteVisits: 1 });
});
const aboveThreshold = realm.write(() => {
  return realm.create(SiteVisitTracker, { siteVisits: 2 });
});
const allObjects = realm.objects("SiteVisitTracker");
let filteredObjects = allObjects.filtered(
  "siteVisits >= $0",
  atThreshold.siteVisits.value
);

For more information on querying with the SDK, refer to Realm Query Language.

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