Implementing a polling mechanism with promises

26 Oct 2023

In certain scenarios, it is necessary to continuously check for updates or changes in a system. This can be achieved through a polling mechanism where a request is made at regular intervals to fetch updates. One way to implement polling is by using promises in JavaScript. In this blog post, we will explore how to implement a polling mechanism with promises.

Introduction
Implementing the Polling Mechanism
Handling the Polling Results
Controlling the Polling Interval
Implementing Error Handling
Conclusion

Introduction

Promises in JavaScript provide a clean way to handle asynchronous operations. By combining promises with a polling mechanism, we can efficiently fetch updates at regular intervals without blocking the main thread.

Implementing the Polling Mechanism

To implement the polling mechanism, we can create a function that wraps the logic of making a request and resolving a promise when the desired condition is met. Here’s an example:

function poll(condition, interval) {
  return new Promise((resolve, reject) => {
    const timerId = setInterval(async () => {
      try {
        const result = await makeRequest(); // Replace with your own request logic
        if (condition(result)) {
          clearInterval(timerId);
          resolve(result);
        }
      } catch (error) {
        clearInterval(timerId);
        reject(error);
      }
    }, interval);
  });
}

In the above code, the poll function takes two arguments: condition and interval. The condition is a function that defines the desired condition for resolving the promise, and the interval specifies the time interval between each poll attempt.

Inside the poll function, a setInterval is used to repeatedly call the polling logic at the specified interval. The polling logic makes the request and checks if the condition is met. If the condition is met, the promise is resolved with the result. Otherwise, the polling continues until the condition is satisfied.

Handling the Polling Results

To handle the polling results, we can use the .then method on the promise returned by the poll function. Here’s an example of how to use it:

poll((result) => result === 'success', 1000)
  .then((result) => {
    console.log('Polling successful:', result);
    // Handle the successful result
  })
  .catch((error) => {
    console.error('Polling failed:', error);
    // Handle the error
  });

In the above code, poll((result) => result === 'success', 1000) starts the polling mechanism that checks if the result is 'success' every 1 second. If the condition is met, the .then block is executed and the successful result can be handled. If there is an error during the polling, the .catch block is executed to handle the error.

Controlling the Polling Interval

To control the polling interval dynamically, we can add an optional parameter to the poll function. Here’s an updated version of the poll function:

function poll(condition, interval, maxAttempts = Infinity) {
  return new Promise((resolve, reject) => {
    let attempts = 0;
    const timerId = setInterval(async () => {
      try {
        const result = await makeRequest(); // Replace with your own request logic
        if (condition(result)) {
          clearInterval(timerId);
          resolve(result);
        } else if (++attempts === maxAttempts) {
          clearInterval(timerId);
          reject(new Error('Max polling attempts reached'));
        }
      } catch (error) {
        clearInterval(timerId);
        reject(error);
      }
    }, interval);
  });
}

In the updated poll function, an additional maxAttempts parameter is added with a default value of Infinity. This allows us to set a maximum number of polling attempts. If the maximum number of attempts is reached before the condition is met, the promise is rejected with an appropriate error.

Implementing Error Handling

To implement error handling within the polling mechanism, we can use a try-catch block around the polling logic. Here’s an example:

const makeRequest = async () => {
  // Make the request and return the result
  // Handle any errors within this function
};

Within the makeRequest function, you can handle any errors specific to the request logic. This ensures that errors are caught within each polling attempt and appropriately handled.

Conclusion

Polling mechanisms with promises provide a powerful way to continuously fetch updates or changes in a system. By combining the flexibility of promises with the interval-based nature of polling, we can build robust and efficient solutions for various scenarios.

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