SharedCounter

Introduction

The SharedCounter distributed data structure (DDS) is used to store an integer counter value that can be modified by multiple clients simultaneously. The data structure affords incrementing and decrementing the shared value via its increment method. Decrements are done by providing a negative value.

The SharedCounter is a specialized Optimistic DDS. It operates on communicated deltas (amounts by which the shared value should be incremented or decremented), rather than direct changes to the shared value. In this way, it avoids the pitfalls of DDSes with simpler merge strategies, in which one user’s edit may clobber another’s (see below).

Note that the SharedCounter only operates on integer values.

Why a specialized DDS?

You may be asking yourself, why not just store the shared integer value directly in another DDS like a SharedMap? Why incur the overhead of another runtime type?

The key to the answer here is that DDSes with simpler merge strategies (like SharedMap) take a somewhat brute-force approach to merging concurrent edits. For a semantic data type like a counter, this can result in undesirable behavior.

SharedMap Example

Let’s illustrate the issue with an example.

Consider a polling widget. The widget displays a list of options and allows users to click a checkbox to vote for a given option. Next to each option in the list, a live counter is displayed that shows the number of votes for that item.

Whenever a user checks an option, all users should see the counter corresponding to that option increment by 1.

In this example, the application is storing its vote counts in a SharedMap, where the map keys are strings representing the IDs of the options, and the values are numbers representing the associated vote counts.

For simplicity, we will look at a scenario in which 2 users vote for the same option at around the same time.

Specifically, User A clicks the checkbox for option Foo, which currently has 0 votes. The application then optimistically updates the vote count for that object by writing the updated counter value of 1 for option Foo to its SharedMap.

The value change operation (op) is then transmitted to the service to be sequenced and eventually sent to other users in the collaborative session.

At around the same time, User B clicks the checkbox for option Foo, which in their view currently has 0 votes. Similarly to before, the application optimistically updates the associated counter value to 1, and transmits its own update op.

The service receives the op from User A first, and sequences their op updating Foo to 1 as op 0. User B’s op is received second, and is sequenced as op 1.

Both users then receive acknowledgement of their update, and receive op 0 and op 1 to be applied in order. Both clients apply op 0 by setting Foo to 1. Then both clients apply op 1 by setting Foo to 1.

But this isn’t right. Two different users voted for option Foo, but the counter now displays 1.

SharedCounter solves this problem by expressing its operations in terms of increments and decrements rather than as direct value updates.

So for the scenario above, if the system was using SharedCounters to represent the vote counts, User A would submit an op incrementing Foo by +1, rather than updating the value of Foo from 0 to 1. At around the same time, User B would submit their own +1 op for Foo.

Assuming the same sequencing, both users first apply op 0 and increment their counter for Foo by +1 (from 0 to 1). Next, they both apply op 1 and increment their counter for Foo by +1 a second time (from 1 to 2).

Now both users see the right vote count for Foo!

Usage

The SharedCounter object provides a simple API surface for managing a shared integer whose value may be incremented and decremented by collaborators.

A new SharedCounter value will be initialized with its value set to 0. If you wish to initialize the counter to a different value, you may modify the value before attaching the container, or before storing it in another shared object like a SharedMap.

Installation

The package containing the SharedCounter library is @fluidframework/shared-counter.

To get started, run the following from a terminal in your repository:

npm install @fluidframework/shared-counter

Creation

The workflow for creating a SharedCounter is effectively the same as many of our other DDSes. For an example of how to create one, please see our workflow examples for SharedMap creation.

Incrementing / decrementing the value

Once you have created your SharedCounter, you can change its value using the increment method. This method accepts a positive or negative integer to be applied to the shared value.

sharedCounter.increment(3); // Adds 3 to the current value
sharedCounter.increment(-5); // Subtracts 5 from the current value

incremented event

The incremented event is sent when a client in the collaborative session changes the counter value via the increment method.

Signature:

(event: "incremented", listener: (incrementAmount: number, newValue: number) => void)

By listening to this event, you can receive and apply the changes coming from other collaborators. Consider the following code example for configuring a Counter widget:

const sharedCounter = container.initialObjects.sharedCounter;
let counterValue = sharedCounter.value;

const incrementButton = document.createElement('button');
button.textContent = "Increment";
const decrementButton = document.createElement('button');
button.textContent = "Decrement";

// Increment / decrement shared counter value when the corresponding button is clicked
incrementButton.addEventListener('click', () => sharedCounter.increment(1));
decrementButton.addEventListener('click', () => sharedCounter.increment(-1));

const counterValueLabel = document.createElement('label');
counterValueLabel.textContent = `${counterValue}`;

// This function will be called each time the shared counter value is incremented
// (including increments from this client).
// Update the local counter value and the corresponding label being displayed in the widget.
const updateCounterValueLabel = (delta) => {
    counterValue += delta;
    counterValueLabel.textContent = `${counterValue}`;
};

// Register to be notified when the counter is incremented
sharedCounter.on("incremented", updateCounterValueLabel);

In the code above, whenever a user presses either the Increment or Decrement button, the sharedCounter.increment is called with +/- 1. This causes the incremented event to be sent to all of the clients who have this container open.

Since updateCounterValueLabel is listening for all incremented events, the view will always refresh with the appropriate updated value any time a collaborator increments or decrements the counter value.

API documentation

For a comprehensive view of the counter package’s API documentation, see the SharedCounter API docs.