How to chain write stream, immediately with a read

问题:

The following line will download an image file from a specified url variable:

var filename = path.join(__dirname, url.replace(/^.*[\\\/]/, ''));
request(url).pipe(fs.createWriteStream(filename));

And these lines will take that image and save to MongoDB GridFS:

 var gfs = Grid(mongoose.connection.db, mongoose.mongo);
 var writestream = gfs.createWriteStream({ filename: filename });
 fs.createReadStream(filename).pipe(writestream);

Chaining pipe like this throws Error: 500 Cannot Pipe. Not Pipeable.

request(url).pipe(fs.createWriteStream(filename)).pipe(writestream);

This happens because the image file is not ready to be read yet, right? What should I do to get around this problem?Error: 500 Cannot Pipe. Not Pipeable.

Using the following: Node.js 0.10.10, mongoose, request and gridfs-stream libraries.

回答1:

request(url).pipe(fs.createWriteStream(filename)).pipe(writestream);

is the same as this:

var fileStream = fs.createWriteStream(filename);
request(url).pipe(fileStream);
fileStream.pipe(writestream);

So the issue is that you are attempting to .pipe one WriteStream into another WriteStream.

回答2:

// create 'fs' module variable
var fs = require("fs");

// open the streams
var readerStream = fs.createReadStream('inputfile.txt');
var writerStream = fs.createWriteStream('outputfile.txt');

// pipe the read and write operations
// read input file and write data to output file
readerStream.pipe(writerStream);

回答3:

I think the confusion in chaining the pipes is caused by the fact that the pipe method implicitly "makes choices" on it's own on what to return. That is:

readableStream.pipe(writableStream) // Returns writable stream
readableStream.pipe(duplexStream) // Returns readable stream

But the general rule says that "You can only pipe a Writable Stream to a Readable Stream." In other words only Readable Streams have the pipe() method.