How to handle race conditions with promises

26 Oct 2023

references

Race conditions occur when multiple asynchronous operations compete for the same resources, leading to unpredictable and erroneous behavior in your application. Promises are a powerful tool in JavaScript for handling asynchronous operations, but they can also be susceptible to race conditions.

In this blog post, we will explore how race conditions can occur with promises and discuss various approaches to handle them effectively.

Understanding Race Conditions with Promises

To understand race conditions with promises, consider a scenario where two or more asynchronous tasks are executed concurrently, and they update a shared resource. The order in which these tasks complete can impact the final state of the resource.

Let’s say we have two promises, promiseA and promiseB, both modifying a shared variable counter. These promises are executed concurrently, and the order of their completion determines the final value of counter.

let counter = 0;

const promiseA = new Promise((resolve) => {
  setTimeout(() => {
    counter += 1;
    resolve();
  }, 1000);
});

const promiseB = new Promise((resolve) => {
  setTimeout(() => {
    counter += 1;
    resolve();
  }, 500);
});

Promise.all([promiseA, promiseB]).then(() => {
  console.log(counter); // The output may vary
});

In the above example, promiseB completes before promiseA, leading to a value of counter that depends on the order of completion.

Handling Race Conditions

To handle race conditions with promises, we can use various techniques and patterns. Here are a few commonly used approaches:

1. Sequential Execution

One way to handle race conditions is to ensure that promises are executed sequentially. This can be achieved by chaining the promises using the then method, where each promise waits for the previous one to complete before executing.

const promiseA = new Promise((resolve) => {
  setTimeout(() => {
    counter += 1;
    resolve();
  }, 1000);
});

const promiseB = new Promise((resolve) => {
  setTimeout(() => {
    counter += 1;
    resolve();
  }, 500);
});

promiseA.then(() => promiseB).then(() => {
  console.log(counter); // Output: 2
});

In this approach, promiseB is chained to promiseA, ensuring that promiseB waits for promiseA to complete before executing. This guarantees that the final value of counter will be consistent.

2. Using Locks or Mutexes

Another way to handle race conditions is by using locks or mutexes to synchronize access to shared resources. Mutexes ensure that only one promise can access the resource at any given time, preventing race conditions.

const mutex = new Promise((resolve) => resolve()); // Initial resolved promise acts as a lock

const promiseA = mutex.then(() => {
  return new Promise((resolve) => {
    setTimeout(() => {
      counter += 1;
      resolve();
    }, 1000);
  });
});

const promiseB = mutex.then(() => {
  return new Promise((resolve) => {
    setTimeout(() => {
      counter += 1;
      resolve();
    }, 500);
  });
});

Promise.all([promiseA, promiseB]).then(() => {
  console.log(counter); // Output: 2
});

In this approach, the promises are wrapped within a mutex, ensuring that only one promise is executed at a time, thus preventing race conditions.

3. Using `Promise.race`

The Promise.race method can be used to handle race conditions by executing multiple promises concurrently and returning the result of the first resolved or rejected promise.

const promiseA = new Promise((resolve) => {
  setTimeout(() => {
    counter += 1;
    resolve();
  }, 1000);
});

const promiseB = new Promise((resolve) => {
  setTimeout(() => {
    counter += 1;
    resolve();
  }, 500);
});

Promise.race([promiseA, promiseB]).then(() => {
  console.log(counter); // The output may vary
});

In this approach, the first promise to resolve will determine the value of counter. However, Promise.race does not guarantee consistent results and may still lead to race conditions if the order of execution is critical.

Conclusion

Race conditions can lead to unpredictable behavior in asynchronous programming. By understanding the concept of race conditions with promises and applying appropriate techniques, such as sequential execution, using locks or mutexes, or Promise.race, we can effectively handle race conditions and ensure the integrity of shared resources in our JavaScript applications.

Remember to carefully analyze your code and determine which approach is most suitable for your specific use case to prevent race conditions effectively.

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