From 04adf0e5a1e5f245c5b26c87bdce1648efb799f1 Mon Sep 17 00:00:00 2001
From: isaacs <i@izs.me>
Date: Tue, 18 Dec 2012 08:43:45 -0800
Subject: blog: post about streams2 feature

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+title: A New Streaming API for Node v0.10
+author: Isaac Z. Schlueter
+date: Tue Dec 18 08:44:33 PST 2012
+slug: streams2
+category: feature
+
+**tl;dr**
+
+* Node streams are great, except for all the ways in which they're
+  terrible.
+* A new Stream implementation is coming in 0.10, that has gotten the
+  nickname "streams2".
+* Readable streams have a `read()` method that returns a buffer or
+  null.
+* `'data'` events, `pause()`, and `resume()` will still work as before
+  (except that they'll actully work how you'd expect).
+* Old programs will **almost always** work without modification, but
+  streams start out in a paused state, and need to be read from to be
+  consumed.
+* **WARNING**: If you never add a `'data'` event handler, or call
+  `resume()`, then it'll sit in a paused state forever and never
+  emit `'end'`.
+
+-------
+
+Throughout the life of Node, we've been gradually iterating on the
+ideal event-based API for handling data.  Over time, this developed
+into the "Stream" interface that you see throughout Node's core
+modules and many of the modules in npm.
+
+Consistent interfaces increase the portability and reliability of our
+programs and libraries.  Overall, the move from domain-specific events
+and methods towards a unified stream interface was a huge win.
+However, there are still several problems with Node's streams as of
+v0.8.  In a nutshell:
+
+1. The `pause()` method doesn't pause.  It is advisory-only.  In
+   Node's implementation, this makes things much simpler, but it's
+   confusing to users, and doesn't do what it looks like it does.
+2. `'data'` events come right away (whether you're ready or not).
+   This makes it unreasonably difficult to do common tasks like load a
+   user's session before deciding how to handle their request.
+3. There is no way to consume a specific number of bytes, and then
+   leave the rest for some other part of the program to deal with.
+4. It's unreasonably difficult to implement streams and get all the
+   intricacies of pause, resume, write-buffering, and data events
+   correct.  The lack of shared classes mean that we all have to solve
+   the same problems repeatedly, making similar mistakes and similar
+   bugs.
+
+Common simple tasks should be easy, or we aren't doing our job.
+People often say that Node is better than most other platforms at this
+stuff, but in my opinion, that just is an indictment of the current
+state of software more than a compliment.  Being better than the next
+guy isn't enough; we have to be the best imaginable, and while it was
+a big step in the right direction, the Streams in Node up until now
+leave a lot wanting.
+
+So, just fix it, right?
+
+Well, we are now sitting on the results of several years of explosive
+growth in the Node community, so any chnages have to be made very
+carefully.  If we break all the Node programs in 0.10, then no one
+will ever want to upgrade to 0.10, and it's all pointless.  We had
+this streams conversation around 0.4, then again around 0.6, then
+again around 0.8.  Every time, the conclusion was "Too much work, too
+hard to make backwards-compatible", and we always had other more
+pressing problems to solve.
+
+So, finally, in 0.10, we cannot put it off any longer.  We've bitten
+the bullet and are making a significant change to the Stream
+implementation.  You may have seen conversations on twitter or IRC or
+the mailing list about "streams2".  I also gave [a talk in
+November](https://dl.dropbox.com/u/3685/presentations/streams2/streams2-ko.pdf)
+about this subject.  A lot of node module authors have been involved
+with the development of streams2 (and of course the node core team),
+and at this point, I'm confident that it's an improvement over what
+we've got.
+
+The first preview release with this change will be 0.9.4.  I highly
+recommend trying this release and providing feedback before it lands
+in a stable version.  Some polish is still required before 0.10, which
+will likely be released late January, but at this point, it should be
+considered feature-complete.
+
+The feature is described pretty thoroughly in the documentation, so
+I'm including it below.  Please read it, especially the section on
+"compatibility".
+
+--------
+
+# Stream
+
+    Stability: 2 - Unstable
+
+A stream is an abstract interface implemented by various objects in
+Node.  For example a request to an HTTP server is a stream, as is
+stdout. Streams are readable, writable, or both. All streams are
+instances of EventEmitter.
+
+You can load the Stream base classes by doing `require('stream')`.
+There are base classes provided for Readable streams, Writable
+streams, Duplex streams, and Transform streams.
+
+## Compatibility
+
+In earlier versions of Node, the Readable stream interface was
+simpler, but also less powerful and less useful.
+
+* Rather than waiting for you to call the `read()` method, `'data'`
+  events would start emitting immediately.  If you needed to do some
+  I/O to decide how to handle data, then you had to store the chunks
+  in some kind of buffer so that they would not be lost.
+* The `pause()` method was advisory, rather than guaranteed.  This
+  meant that you still had to be prepared to receive `'data'` events
+  even when the stream was in a paused state.
+
+In Node v0.10, the Readable class described below was added.  For
+backwards compatibility with older Node programs, Readable streams
+switch into "old mode" when a `'data'` event handler is added, or when
+the `pause()` or `resume()` methods are called.  The effect is that,
+even if you are not using the new `read()` method and `'readable'`
+event, you no longer have to worry about losing `'data'` chunks.
+
+Most programs will continue to function normally.  However, this
+introduces an edge case in the following conditions:
+
+* No `'data'` event handler is added.
+* The `pause()` and `resume()` methods are never called.
+
+For example, consider the following code:
+
+```javascript
+// WARNING!  BROKEN!
+net.createServer(function(socket) {
+
+  // we add an 'end' method, but never consume the data
+  socket.on('end', function() {
+    // It will never get here.
+    socket.end('I got your message (but didnt read it)\n');
+  });
+
+}).listen(1337);
+```
+
+In versions of node prior to v0.10, the incoming message data would be
+simply discarded.  However, in Node v0.10 and beyond, the socket will
+remain paused forever.
+
+The workaround in this situation is to call the `resume()` method to
+trigger "old mode" behavior:
+
+```javascript
+// Workaround
+net.createServer(function(socket) {
+
+  socket.on('end', function() {
+    socket.end('I got your message (but didnt read it)\n');
+  });
+
+  // start the flow of data, discarding it.
+  socket.resume();
+
+}).listen(1337);
+```
+
+In addition to new Readable streams switching into old-mode, pre-v0.10
+style streams can be wrapped in a Readable class using the `wrap()`
+method.
+
+## Class: stream.Readable
+
+<!--type=class-->
+
+A `Readable Stream` has the following methods, members, and events.
+
+Note that `stream.Readable` is an abstract class designed to be
+extended with an underlying implementation of the `_read(size, cb)`
+method. (See below.)
+
+### new stream.Readable([options])
+
+* `options` {Object}
+  * `bufferSize` {Number} The size of the chunks to consume from the
+    underlying resource. Default=16kb
+  * `lowWaterMark` {Number} The minimum number of bytes to store in
+    the internal buffer before emitting `readable`.  Default=0
+  * `highWaterMark` {Number} The maximum number of bytes to store in
+    the internal buffer before ceasing to read from the underlying
+    resource.  Default=16kb
+  * `encoding` {String} If specified, then buffers will be decoded to
+    strings using the specified encoding.  Default=null
+
+In classes that extend the Readable class, make sure to call the
+constructor so that the buffering settings can be properly
+initialized.
+
+### readable.\_read(size, callback)
+
+* `size` {Number} Number of bytes to read asynchronously
+* `callback` {Function} Called with an error or with data
+
+All Readable stream implementations must provide a `_read` method
+to fetch data from the underlying resource.
+
+**This function MUST NOT be called directly.**  It should be
+implemented by child classes, and called by the internal Readable
+class methods only.
+
+Call the callback using the standard `callback(error, data)` pattern.
+When no more data can be fetched, call `callback(null, null)` to
+signal the EOF.
+
+This method is prefixed with an underscore because it is internal to
+the class that defines it, and should not be called directly by user
+programs.  However, you **are** expected to override this method in
+your own extension classes.
+
+
+### readable.wrap(stream)
+
+* `stream` {Stream} An "old style" readable stream
+
+If you are using an older Node library that emits `'data'` events and
+has a `pause()` method that is advisory only, then you can use the
+`wrap()` method to create a Readable stream that uses the old stream
+as its data source.
+
+For example:
+
+```javascript
+var OldReader = require('./old-api-module.js').OldReader;
+var oreader = new OldReader;
+var Readable = require('stream').Readable;
+var myReader = new Readable().wrap(oreader);
+
+myReader.on('readable', function() {
+  myReader.read(); // etc.
+});
+```
+
+### Event: 'readable'
+
+When there is data ready to be consumed, this event will fire.  The
+number of bytes that are required to be considered "readable" depends
+on the `lowWaterMark` option set in the constructor.
+
+When this event emits, call the `read()` method to consume the data.
+
+### Event: 'end'
+
+Emitted when the stream has received an EOF (FIN in TCP terminology).
+Indicates that no more `'data'` events will happen. If the stream is
+also writable, it may be possible to continue writing.
+
+### Event: 'data'
+
+The `'data'` event emits either a `Buffer` (by default) or a string if
+`setEncoding()` was used.
+
+Note that adding a `'data'` event listener will switch the Readable
+stream into "old mode", where data is emitted as soon as it is
+available, rather than waiting for you to call `read()` to consume it.
+
+### Event: 'error'
+
+Emitted if there was an error receiving data.
+
+### Event: 'close'
+
+Emitted when the underlying resource (for example, the backing file
+descriptor) has been closed. Not all streams will emit this.
+
+### readable.setEncoding(encoding)
+
+Makes the `'data'` event emit a string instead of a `Buffer`. `encoding`
+can be `'utf8'`, `'utf16le'` (`'ucs2'`), `'ascii'`, or `'hex'`.
+
+The encoding can also be set by specifying an `encoding` field to the
+constructor.
+
+### readable.read([size])
+
+* `size` {Number | null} Optional number of bytes to read.
+* Return: {Buffer | String | null}
+
+Call this method to consume data once the `'readable'` event is
+emitted.
+
+The `size` argument will set a minimum number of bytes that you are
+interested in.  If not set, then the entire content of the internal
+buffer is returned.
+
+If there is no data to consume, or if there are fewer bytes in the
+internal buffer than the `size` argument, then `null` is returned, and
+a future `'readable'` event will be emitted when more is available.
+
+Note that calling `stream.read(0)` will always return `null`, and will
+trigger a refresh of the internal buffer, but otherwise be a no-op.
+
+### readable.pipe(destination, [options])
+
+* `destination` {Writable Stream}
+* `options` {Object} Optional
+  * `end` {Boolean} Default=true
+
+Connects this readable stream to `destination` WriteStream. Incoming
+data on this stream gets written to `destination`.  Properly manages
+back-pressure so that a slow destination will not be overwhelmed by a
+fast readable stream.
+
+This function returns the `destination` stream.
+
+For example, emulating the Unix `cat` command:
+
+    process.stdin.pipe(process.stdout);
+
+By default `end()` is called on the destination when the source stream
+emits `end`, so that `destination` is no longer writable. Pass `{ end:
+false }` as `options` to keep the destination stream open.
+
+This keeps `writer` open so that "Goodbye" can be written at the
+end.
+
+    reader.pipe(writer, { end: false });
+    reader.on("end", function() {
+      writer.end("Goodbye\n");
+    });
+
+Note that `process.stderr` and `process.stdout` are never closed until
+the process exits, regardless of the specified options.
+
+### readable.unpipe([destination])
+
+* `destination` {Writable Stream} Optional
+
+Undo a previously established `pipe()`.  If no destination is
+provided, then all previously established pipes are removed.
+
+### readable.pause()
+
+Switches the readable stream into "old mode", where data is emitted
+using a `'data'` event rather than being buffered for consumption via
+the `read()` method.
+
+Ceases the flow of data.  No `'data'` events are emitted while the
+stream is in a paused state.
+
+### readable.resume()
+
+Switches the readable stream into "old mode", where data is emitted
+using a `'data'` event rather than being buffered for consumption via
+the `read()` method.
+
+Resumes the incoming `'data'` events after a `pause()`.
+
+
+## Class: stream.Writable
+
+<!--type=class-->
+
+A `Writable` Stream has the following methods, members, and events.
+
+Note that `stream.Writable` is an abstract class designed to be
+extended with an underlying implementation of the `_write(chunk, cb)`
+method. (See below.)
+
+### new stream.Writable([options])
+
+* `options` {Object}
+  * `highWaterMark` {Number} Buffer level when `write()` starts
+    returning false. Default=16kb
+  * `lowWaterMark` {Number} The buffer level when `'drain'` is
+    emitted.  Default=0
+  * `decodeStrings` {Boolean} Whether or not to decode strings into
+    Buffers before passing them to `_write()`.  Default=true
+
+In classes that extend the Writable class, make sure to call the
+constructor so that the buffering settings can be properly
+initialized.
+
+### writable.\_write(chunk, callback)
+
+* `chunk` {Buffer | Array} The data to be written
+* `callback` {Function} Called with an error, or null when finished
+
+All Writable stream implementations must provide a `_write` method to
+send data to the underlying resource.
+
+**This function MUST NOT be called directly.**  It should be
+implemented by child classes, and called by the internal Writable
+class methods only.
+
+Call the callback using the standard `callback(error)` pattern to
+signal that the write completed successfully or with an error.
+
+If the `decodeStrings` flag is set in the constructor options, then
+`chunk` will be an array rather than a Buffer.  This is to support
+implementations that have an optimized handling for certain string
+data encodings.
+
+This method is prefixed with an underscore because it is internal to
+the class that defines it, and should not be called directly by user
+programs.  However, you **are** expected to override this method in
+your own extension classes.
+
+
+### writable.write(chunk, [encoding], [callback])
+
+* `chunk` {Buffer | String} Data to be written
+* `encoding` {String} Optional.  If `chunk` is a string, then encoding
+  defaults to `'utf8'`
+* `callback` {Function} Optional.  Called when this chunk is
+  successfully written.
+* Returns {Boolean}
+
+Writes `chunk` to the stream.  Returns `true` if the data has been
+flushed to the underlying resource.  Returns `false` to indicate that
+the buffer is full, and the data will be sent out in the future. The
+`'drain'` event will indicate when the buffer is empty again.
+
+The specifics of when `write()` will return false, and when a
+subsequent `'drain'` event will be emitted, are determined by the
+`highWaterMark` and `lowWaterMark` options provided to the
+constructor.
+
+### writable.end([chunk], [encoding])
+
+* `chunk` {Buffer | String} Optional final data to be written
+* `encoding` {String} Optional.  If `chunk` is a string, then encoding
+  defaults to `'utf8'`
+
+Call this method to signal the end of the data being written to the
+stream.
+
+### Event: 'drain'
+
+Emitted when the stream's write queue empties and it's safe to write
+without buffering again. Listen for it when `stream.write()` returns
+`false`.
+
+### Event: 'close'
+
+Emitted when the underlying resource (for example, the backing file
+descriptor) has been closed. Not all streams will emit this.
+
+### Event: 'pipe'
+
+* `source` {Readable Stream}
+
+Emitted when the stream is passed to a readable stream's pipe method.
+
+### Event 'unpipe'
+
+* `source` {Readable Stream}
+
+Emitted when a previously established `pipe()` is removed using the
+source Readable stream's `unpipe()` method.
+
+## Class: stream.Duplex
+
+<!--type=class-->
+
+A "duplex" stream is one that is both Readable and Writable, such as a
+TCP socket connection.
+
+Note that `stream.Duplex` is an abstract class designed to be
+extended with an underlying implementation of the `_read(size, cb)`
+and `_write(chunk, callback)` methods as you would with a Readable or
+Writable stream class.
+
+Since JavaScript doesn't have multiple prototypal inheritance, this
+class prototypally inherits from Readable, and then parasitically from
+Writable.  It is thus up to the user to implement both the lowlevel
+`_read(n,cb)` method as well as the lowlevel `_write(chunk,cb)` method
+on extension duplex classes.
+
+### new stream.Duplex(options)
+
+* `options` {Object} Passed to both Writable and Readable
+  constructors. Also has the following fields:
+  * `allowHalfOpen` {Boolean} Default=true.  If set to `false`, then
+    the stream will automatically end the readable side when the
+    writable side ends and vice versa.
+
+In classes that extend the Duplex class, make sure to call the
+constructor so that the buffering settings can be properly
+initialized.
+
+## Class: stream.Transform
+
+A "transform" stream is a duplex stream where the output is causally
+connected in some way to the input, such as a zlib stream or a crypto
+stream.
+
+There is no requirement that the output be the same size as the input,
+the same number of chunks, or arrive at the same time.  For example, a
+Hash stream will only ever have a single chunk of output which is
+provided when the input is ended.  A zlib stream will either produce
+much smaller or much larger than its input.
+
+Rather than implement the `_read()` and `_write()` methods, Transform
+classes must implement the `_transform()` method, and may optionally
+also implement the `_flush()` method.  (See below.)
+
+### new stream.Transform([options])
+
+* `options` {Object} Passed to both Writable and Readable
+  constructors.
+
+In classes that extend the Transform class, make sure to call the
+constructor so that the buffering settings can be properly
+initialized.
+
+### transform.\_transform(chunk, outputFn, callback)
+
+* `chunk` {Buffer} The chunk to be transformed.
+* `outputFn` {Function} Call this function with any output data to be
+  passed to the readable interface.
+* `callback` {Function} Call this function (optionally with an error
+  argument) when you are done processing the supplied chunk.
+
+All Transform stream implementations must provide a `_transform`
+method to accept input and produce output.
+
+**This function MUST NOT be called directly.**  It should be
+implemented by child classes, and called by the internal Transform
+class methods only.
+
+`_transform` should do whatever has to be done in this specific
+Transform class, to handle the bytes being written, and pass them off
+to the readable portion of the interface.  Do asynchronous I/O,
+process things, and so on.
+
+Call the callback function only when the current chunk is completely
+consumed.  Note that this may mean that you call the `outputFn` zero
+or more times, depending on how much data you want to output as a
+result of this chunk.
+
+This method is prefixed with an underscore because it is internal to
+the class that defines it, and should not be called directly by user
+programs.  However, you **are** expected to override this method in
+your own extension classes.
+
+### transform.\_flush(outputFn, callback)
+
+* `outputFn` {Function} Call this function with any output data to be
+  passed to the readable interface.
+* `callback` {Function} Call this function (optionally with an error
+  argument) when you are done flushing any remaining data.
+
+**This function MUST NOT be called directly.**  It MAY be implemented
+by child classes, and if so, will be called by the internal Transform
+class methods only.
+
+In some cases, your transform operation may need to emit a bit more
+data at the end of the stream.  For example, a `Zlib` compression
+stream will store up some internal state so that it can optimally
+compress the output.  At the end, however, it needs to do the best it
+can with what is left, so that the data will be complete.
+
+In those cases, you can implement a `_flush` method, which will be
+called at the very end, after all the written data is consumed, but
+before emitting `end` to signal the end of the readable side.  Just
+like with `_transform`, call `outputFn` zero or more times, as
+appropriate, and call `callback` when the flush operation is complete.
+
+This method is prefixed with an underscore because it is internal to
+the class that defines it, and should not be called directly by user
+programs.  However, you **are** expected to override this method in
+your own extension classes.
+
+
+## Class: stream.PassThrough
+
+This is a trivial implementation of a `Transform` stream that simply
+passes the input bytes across to the output.  Its purpose is mainly
+for examples and testing, but there are occasionally use cases where
+it can come in handy.
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