Is uploading a file to S3 in NodeJS blocking? - node.js

Question

I've been seeing performance issues on our application and I'm a bit unsure if uploading a file to S3 could block NodeJS.
I'm using express, formidable and aws-sdk.
Here's a middleware using formidable. This stores the file in req.file and continues to the next middleware that performs the upload to S3.
var formidable = require("formidable");
module.exports = function() {
return function(req, res, next) {
var form = new formidable.IncomingForm({
"keepExtensions": true,
"uploadDir": config.tempDir
});
form.parse(req, function(error, fields, files) {
if (error) {
return res.sendError("Error while parsing multipart-form " + error, 500);
}
req.files = files;
req.fields = fields;
next();
});
};
};
Here's the middleware that actually makes the request to S3 using AWS SDK
const AWS = require("aws-sdk");
const fs = require("fs");
const s3 = new AWS.S3(s3config.s3options);
const logger = require("logger");
function uploadFile(req, res, next) {
const requestId = getRequestIdFromRequestHeaders(req);
const file = options.file(req);
const contentType = options.contentType && options.contentType(req) || file.type;
const destinationPath = options.destinationPath(req);
req.s3uploaded = false;
logger.debug(requestId, "invoking uploadFile", contentType, destinationPath);
req.s3FilePath = "https://" + s3config.bucket + ".s3.amazonaws.com/" + destinationPath;
if (!options.writeConcern) {
logger.debug(requestId, "write concern not expected. Calling next");
next();
}
const stream = fs.createReadStream(file.path);
s3.upload({
"Bucket": s3config.bucket,
"ContentLength": file.size,
"Key": destinationPath,
"Body": stream,
"ContentType": contentType
}, s3config.s3options.uploadOptions, function(error) {
fs.unlink(file.path, error => {
if (error) {
logger.error(requestId, "Unable to remove file", file.path);
}
});
if (error) {
return next(error);
}
if (options.writeConcern) {
if (!req.s3uploaded) {
req.s3uploaded = true;
next();
}
}
}).on("httpUploadProgress", progress => {
logger.debug(requestId, "progress", progress.loaded, "of", progress.total);
if (progress.total !== undefined && progress.loaded === progress.total) {
logger.debug(requestId, "upload done, invoking next from httpUploadProgress");
if (!req.s3uploaded) {
req.s3uploaded = true;
next();
}
}
});
};

Answer 1

The documentation for the AWS SDK for JavaScript (v2) includes this statement on their Calling Services Asynchronously page (emphasis mine):
All requests made through the SDK are asynchronous. This is important to keep in mind when writing browser scripts. JavaScript running in a web browser typically has just a single execution thread. After making an asynchronous call to an AWS service, the browser script continues running and in the process can try to execute code that depends on that asynchronous result before it returns.