CODEWITHSUNDEEP
javascriptnode.jshttprequestcancellation
avo
avo

Reputation: 10731

How to cancel http request properly in Node.js?

I need to implement a cancel-able client-side HTTP request in Node.js, without using external libraries. I'm giving a Promise object - cancellationPromise - which gets rejected when the cancellation is externally requested. This is how I know I may need to call request.abort().

The question is, should I be calling request.abort() only if https.request is still pending and response object is not yet available?

Or, should I be calling it even if I already got the response object and am processing the response data, like in the code below? In which case, will that stop any more response.on('data') events from coming?

  async simpleHttpRequest(url, oauthToken, cancellationPromise) {
    let cancelled = null;
    let oncancel = null;

    cancellationPromise.catch(error => { 
      cancelled = error; oncancel && oncancel(error) });

    try {
      const response = await new Promise((resolve, reject) => {
        const request = https.request(
          url.toString(), 
          { 
            method: 'GET', 
            headers: { 'Authorization': `Bearer ${oauthToken}` }        
          }, 
          resolve);

        oncancel = error => request.abort();
        request.on('error', reject);
        request.end();
      });

      if (cancelled) throw cancelled;

      // do I need "oncancel = null" here?
      // or should I still allow to call request.abort() while fetching the response's data?

      // oncancel = null;

      try {
        // read the response 
        const chunks = await new Promise((resolve, reject) => {
          response.on('error', reject);  
          const chunks = [];
          response.on('data', data => chunks.push(data));
          response.on('end', () => resolve(chunks));
        });

        if (cancelled) throw cancelled;
        const data = JSON.parse(chunks.join(''));
        return data;
      }
      finally {
        response.resume();
      }
    }
    finally {
      oncancel = null;
    }
  }

Upvotes: 21

Views: 17102

Answers (1)

Hash
Hash

Reputation: 4715

It depends what you want to achieve by aborting a request.

Just a bit of background. HTTP 1 is not able to "cancel" a request it sends it and then waits for the response. You cannot "roll back" the request you did. You need a distributed transaction to do so. (Further reading.) As the MDN developer document states:

The XMLHttpRequest.abort() method aborts the request if it has already been sent. When a request is aborted, its readyState is changed to XMLHttpRequest.UNSENT (0) and the request's status code is set to 0.

Basically you stop the response from being processed by your application. The other application will probably (if you called abort() after it was sent to it) finish its processing anyways.

From the perspective of the question:

The question is, should I be calling request.abort() only if https.request is still pending and response object is not yet available?

TL.DR.: It only matters from the point of view of your application. As I glance at your code, I think it will work fine.

Upvotes: 24

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