Download images from s3 bucket using Promise in no

问题:

I want to download some image files from s3 bucket on my local system using Promises in node.js.

var params = {
    Bucket: bucket_name',
    Key: 'key'
};

var fileStream = fs.createWriteStream('path/to/file.jpg');

I tried this which is working

s3.getObject(params).createReadStream.pipe(fileStream);

But I want my code look like this

return s3.getObject(params).promise()
    .then(function(data) {
        //console.log(data.Body);
        // No idea about this section
    })
    .catch(function(err) {
        throw err;
    });

I have to use Promise to ensure all images should be downloaded.

回答1:

One possible solution is to use bluebird and create a function that returns a promise on the end of the stream:

const B = require('bluebird');

function downloadFromS3 (object) {
    var p = B.Promise.defer();

    var stream = s3.getObject(params).createReadStream()

    stream.pipe(fileStream);

    stream.on('error', (e) => p.reject(e))

    stream.on('end', () => p.resolve())

    return p.promise;
}

downloadFromS3(params)
    .then(() => console.log('finished'))
    .catch(() => console.log('failed'))

Not sure if this code specifically would work, but it may give you a direction to look into.

回答2:

streamToPromise = require('stream-to-promise');

var fileStream = fs.createWriteStream('path/to/file.jpg');
streamToPromise(fileStream).then(function () {
  console.log('Image saved to file.');
});
s3.getObject(params).createReadStream.pipe(fileStream);

回答3:

Here's a native promise solution with error detection on the read stream and on the write stream.

function streamPromise(stream) {
    return new Promise((resolve, reject) => {
        stream.on('end', () => {
            resolve('end');
        });
        stream.on('finish', () => {
            resolve('finish');
        });
        stream.on('error', (error) => {
            reject(error);
        });
    });
}

async function s3Download(srcBucket, srcKey, outputPath) {
    var objReq = s3.getObject({
        Bucket: srcBucket,
        Key: srcKey
    });
    let outStream = fs.createWriteStream(outputPath);
    let readStream = objReq.createReadStream();
    readStream.on('error', (err) => {
        console.warn('s3download error', err);
        outStream.emit("error", err);
    });
    readStream.pipe(outStream);
    return streamPromise(outStream);
}

回答4:

Here is a snippet to use async/await with NodeJS 8:

const AWS = require('aws-sdk');
const fs = require('fs-extra');
const decompress = require('decompress');

const s3 = new AWS.S3();
const s3Params = {
  Bucket: s3Location.bucketName,
  Key: s3Location.objectKey,
};
const s3Object = await s3.getObject(s3Params).promise();
await fs.writeFile('myfile.zip', s3Object.Body);
await decompress('myfile.zip', 'myFileDir');

/* The compressed file is retrieved as "myfile.zip".
   Content will be extracted in myFileDir directory */