68 KiB
Stream
Stability: 2 - Stable
A stream is an abstract interface for working with streaming data in Node.js.
The stream
module provides a base API that makes it easy to build objects
that implement the stream interface.
There are many stream objects provided by Node.js. For instance, a
request to an HTTP server and process.stdout
are both stream instances.
Streams can be readable, writable, or both. All streams are instances of
EventEmitter
.
The stream
module can be accessed using:
const stream = require('stream');
While it is important for all Node.js users to understand how streams work,
the stream
module itself is most useful for developers that are creating new
types of stream instances. Developer's who are primarily consuming stream
objects will rarely (if ever) have need to use the stream
module directly.
Organization of this Document
This document is divided into two primary sections with a third section for additional notes. The first section explains the elements of the stream API that are required to use streams within an application. The second section explains the elements of the API that are required to implement new types of streams.
Types of Streams
There are four fundamental stream types within Node.js:
- Readable - streams from which data can be read (for example
fs.createReadStream()
). - Writable - streams to which data can be written (for example
fs.createWriteStream()
). - Duplex - streams that are both Readable and Writable (for example
net.Socket
). - Transform - Duplex streams that can modify or transform the data as it
is written and read (for example
zlib.createDeflate()
).
Object Mode
All streams created by Node.js APIs operate exclusively on strings and Buffer
objects. It is possible, however, for stream implementations to work with other
types of JavaScript values (with the exception of null
, which serves a special
purpose within streams). Such streams are considered to operate in "object
mode".
Stream instances are switched into object mode using the objectMode
option
when the stream is created. Attempting to switch an existing stream into
object mode is not safe.
Buffering
Both Writable and Readable streams will store data in an internal
buffer that can be retrieved using writable._writableState.getBuffer()
or
readable._readableState.buffer
, respectively.
The amount of data potentially buffered depends on the highWaterMark
option
passed into the streams constructor. For normal streams, the highWaterMark
option specifies a total number of bytes. For streams operating in object mode,
the highWaterMark
specifies a total number of objects.
Data is buffered in Readable streams when the implementation calls
stream.push(chunk)
. If the consumer of the Stream does not
call stream.read()
, the data will sit in the internal
queue until it is consumed.
Once the total size of the internal read buffer reaches the threshold specified
by highWaterMark
, the stream will temporarily stop reading data from the
underlying resource until the data currently buffered can be consumed (that is,
the stream will stop calling the internal readable._read()
method that is
used to fill the read buffer).
Data is buffered in Writable streams when the
writable.write(chunk)
method is called repeatedly. While the
total size of the internal write buffer is below the threshold set by
highWaterMark
, calls to writable.write()
will return true
. Once
the size of the internal buffer reaches or exceeds the highWaterMark
, false
will be returned.
A key goal of the stream
API, particularly the stream.pipe()
method,
is to limit the buffering of data to acceptable levels such that sources and
destinations of differing speeds will not overwhelm the available memory.
Because Duplex and Transform streams are both Readable and Writable,
each maintain two separate internal buffers used for reading and writing,
allowing each side to operate independently of the other while maintaining an
appropriate and efficient flow of data. For example, net.Socket
instances
are Duplex streams whose Readable side allows consumption of data received
from the socket and whose Writable side allows writing data to the socket.
Because data may be written to the socket at a faster or slower rate than data
is received, it is important for each side to operate (and buffer) independently
of the other.
API for Stream Consumers
Almost all Node.js applications, no matter how simple, use streams in some manner. The following is an example of using streams in a Node.js application that implements an HTTP server:
const http = require('http');
const server = http.createServer( (req, res) => {
// req is an http.IncomingMessage, which is a Readable Stream
// res is an http.ServerResponse, which is a Writable Stream
let body = '';
// Get the data as utf8 strings.
// If an encoding is not set, Buffer objects will be received.
req.setEncoding('utf8');
// Readable streams emit 'data' events once a listener is added
req.on('data', (chunk) => {
body += chunk;
});
// the end event indicates that the entire body has been received
req.on('end', () => {
try {
const data = JSON.parse(body);
// write back something interesting to the user:
res.write(typeof data);
res.end();
} catch (er) {
// uh oh! bad json!
res.statusCode = 400;
return res.end(`error: ${er.message}`);
}
});
});
server.listen(1337);
// $ curl localhost:1337 -d '{}'
// object
// $ curl localhost:1337 -d '"foo"'
// string
// $ curl localhost:1337 -d 'not json'
// error: Unexpected token o
Writable streams (such as res
in the example) expose methods such as
write()
and end()
that are used to write data onto the stream.
Readable streams use the EventEmitter
API for notifying application
code when data is available to be read off the stream. That available data can
be read from the stream in multiple ways.
Both Writable and Readable streams use the EventEmitter
API in
various ways to communicate the current state of the stream.
Duplex and Transform streams are both Writable and Readable.
Applications that are either writing data to or consuming data from a stream
are not required to implement the stream interfaces directly and will generally
have no reason to call require('stream')
.
Developers wishing to implement new types of streams should refer to the section API for Stream Implementers.
Writable Streams
Writable streams are an abstraction for a destination to which data is written.
Examples of Writable streams include:
- HTTP requests, on the client
- HTTP responses, on the server
- fs write streams
- zlib streams
- crypto streams
- TCP sockets
- child process stdin
process.stdout
,process.stderr
Note: Some of these examples are actually Duplex streams that implement the Writable interface.
All Writable streams implement the interface defined by the
stream.Writable
class.
While specific instances of Writable streams may differ in various ways, all Writable streams follow the same fundamental usage pattern as illustrated in the example below:
const myStream = getWritableStreamSomehow();
myStream.write('some data');
myStream.write('some more data');
myStream.end('done writing data');
Class: stream.Writable
Event: 'close'
The 'close'
event is emitted when the stream and any of its underlying
resources (a file descriptor, for example) have been closed. The event indicates
that no more events will be emitted, and no further computation will occur.
Not all Writable streams will emit the 'close'
event.
Event: 'drain'
If a call to stream.write(chunk)
returns false
, the
'drain'
event will be emitted when it is appropriate to resume writing data
to the stream.
// Write the data to the supplied writable stream one million times.
// Be attentive to back-pressure.
function writeOneMillionTimes(writer, data, encoding, callback) {
let i = 1000000;
write();
function write() {
var ok = true;
do {
i--;
if (i === 0) {
// last time!
writer.write(data, encoding, callback);
} else {
// see if we should continue, or wait
// don't pass the callback, because we're not done yet.
ok = writer.write(data, encoding);
}
} while (i > 0 && ok);
if (i > 0) {
// had to stop early!
// write some more once it drains
writer.once('drain', write);
}
}
}
Event: 'error'
- {Error}
The 'error'
event is emitted if an error occurred while writing or piping
data. The listener callback is passed a single Error
argument when called.
Note: The stream is not closed when the 'error'
event is emitted.
Event: 'finish'
The 'finish'
event is emitted after the stream.end()
method
has been called, and all data has been flushed to the underlying system.
const writer = getWritableStreamSomehow();
for (var i = 0; i < 100; i ++) {
writer.write('hello, #${i}!\n');
}
writer.end('This is the end\n');
writer.on('finish', () => {
console.error('All writes are now complete.');
});
Event: 'pipe'
src
{stream.Readable} source stream that is piping to this writable
The 'pipe'
event is emitted when the stream.pipe()
method is called on
a readable stream, adding this writable to its set of destinations.
const writer = getWritableStreamSomehow();
const reader = getReadableStreamSomehow();
writer.on('pipe', (src) => {
console.error('something is piping into the writer');
assert.equal(src, reader);
});
reader.pipe(writer);
Event: 'unpipe'
The 'unpipe'
event is emitted when the stream.unpipe()
method is called
on a Readable stream, removing this Writable from its set of
destinations.
const writer = getWritableStreamSomehow();
const reader = getReadableStreamSomehow();
writer.on('unpipe', (src) => {
console.error('Something has stopped piping into the writer.');
assert.equal(src, reader);
});
reader.pipe(writer);
reader.unpipe(writer);
writable.cork()
The writable.cork()
method forces all written data to be buffered in memory.
The buffered data will be flushed when either the stream.uncork()
or
stream.end()
methods are called.
The primary intent of writable.cork()
is to avoid a situation where writing
many small chunks of data to a stream do not cause a backup in the internal
buffer that would have an adverse impact on performance. In such situations,
implementations that implement the writable._writev()
method can perform
buffered writes in a more optimized manner.
writable.end([chunk][, encoding][, callback])
chunk
{String|Buffer|any} Optional data to write. For streams not operating in object mode,chunk
must be a string or aBuffer
. For object mode streams,chunk
may be any JavaScript value other thannull
.encoding
{String} The encoding, ifchunk
is a Stringcallback
{Function} Optional callback for when the stream is finished
Calling the writable.end()
method signals that no more data will be written
to the Writable. The optional chunk
and encoding
arguments allow one
final additional chunk of data to be written immediately before closing the
stream. If provided, the optional callback
function is attached as a listener
for the 'finish'
event.
Calling the stream.write()
method after calling
stream.end()
will raise an error.
// write 'hello, ' and then end with 'world!'
const file = fs.createWriteStream('example.txt');
file.write('hello, ');
file.end('world!');
// writing more now is not allowed!
writable.setDefaultEncoding(encoding)
encoding
{String} The new default encoding- Returns:
this
The writable.setDefaultEncoding()
method sets the default encoding
for a
Writable stream.
writable.uncork()
The writable.uncork()
method flushes all data buffered since
stream.cork()
was called.
When using writable.cork()
and writable.uncork()
to manage the buffering
of writes to a stream, it is recommended that calls to writable.uncork()
be
deferred using process.nextTick()
. Doing so allows batching of all
writable.write()
calls that occur within a given Node.js event loop phase.
stream.cork();
stream.write('some ');
stream.write('data ');
process.nextTick(() => stream.uncork());
If the writable.cork()
method is called multiple times on a stream, the same
number of calls to writable.uncork()
must be called to flush the buffered
data.
stream.cork();
stream.write('some ');
stream.cork();
stream.write('data ');
process.nextTick(() => {
stream.uncork();
// The data will not be flushed until uncork() is called a second time.
stream.uncork();
});
writable.write(chunk[, encoding][, callback])
chunk
{String|Buffer} The data to writeencoding
{String} The encoding, ifchunk
is a Stringcallback
{Function} Callback for when this chunk of data is flushed- Returns: {Boolean}
false
if the stream wishes for the calling code to wait for the'drain'
event to be emitted before continuing to write additional data; otherwisetrue
.
The writable.write()
method writes some data to the stream, and calls the
supplied callback
once the data has been fully handled. If an error
occurs, the callback
may or may not be called with the error as its
first argument. To reliably detect write errors, add a listener for the
'error'
event.
The return value is true
if the internal buffer does not exceed
highWaterMark
configured when the stream was created after admitting chunk
.
If false
is returned, further attempts to write data to the stream should
stop until the 'drain'
event is emitted.
While a stream is not draining, calls to write()
will buffer chunk
, and
return false. Once all currently buffered chunks are drained (accepted for
delivery by the operating system), the 'drain'
event will be emitted.
It is recommended that once write() returns false, no more chunks be written
until the 'drain'
event is emitted. While calling write()
on a stream that
is not draining is allowed, Node.js will buffer all written chunks until
maximum memory usage occurs, at which point it will abort unconditionally.
Even before it aborts, high memory usage will cause poor garbage collector
performance and high RSS (which is not typically released back to the system,
even after the memory is no longer required). Since TCP sockets may never
drain if the remote peer does not read the data, writing a socket that is
not draining may lead to a remotely exploitable vulnerability.
Writing data while the stream is not draining is particularly
problematic for a Transform, because the Transform
streams are paused
by default until they are piped or an 'data'
or 'readable'
event handler
is added.
If the data to be written can be generated or fetched on demand, it is
recommended to encapsulate the logic into a Readable and use
stream.pipe()
. However, if calling write()
is preferred, it is
possible to respect backpressure and avoid memory issues using the
the 'drain'
event:
function write (data, cb) {
if (!stream.write(data)) {
stream.once('drain', cb)
} else {
process.nextTick(cb)
}
}
// Wait for cb to be called before doing any other write.
write('hello', () => {
console.log('write completed, do more writes now')
})
A Writable stream in object mode will always ignore the encoding
argument.
Readable Streams
Readable streams are an abstraction for a source from which data is consumed.
Examples of Readable streams include:
- HTTP responses, on the client
- HTTP requests, on the server
- fs read streams
- zlib streams
- crypto streams
- TCP sockets
- child process stdout and stderr
process.stdin
All Readable streams implement the interface defined by the
stream.Readable
class.
Two Modes
Readable streams effectively operate in one of two modes: flowing and paused.
When in flowing mode, data is read from the underlying system automatically
and provided to an application as quickly as possible using events via the
EventEmitter
interface.
In paused mode, the stream.read()
method must be called
explicitly to read chunks of data from the stream.
All Readable streams begin in paused mode but can be switched to flowing mode in one of the following ways:
- Adding a
'data'
event handler. - Calling the
stream.resume()
method. - Calling the
stream.pipe()
method to send the data to a Writable.
The Readable can switch back to paused mode using one of the following:
- If there are no pipe destinations, by calling the
stream.pause()
method. - If there are pipe destinations, by removing any
'data'
event handlers, and removing all pipe destinations by calling thestream.unpipe()
method.
The important concept to remember is that a Readable will not generate data until a mechanism for either consuming or ignoring that data is provided. If the consuming mechanism is disabled or taken away, the Readable will attempt to stop generating the data.
Note: For backwards compatibility reasons, removing 'data'
event
handlers will not automatically pause the stream. Also, if there are piped
destinations, then calling stream.pause()
will not guarantee
that the stream will remain paused once those destinations drain and ask for
more data.
Note: If a Readable is switched into flowing mode and there are no
consumers available handle the data, that data will be lost. This can occur,
for instance, when the readable.resume()
method is called without a listener
attached to the 'data'
event, or when a 'data'
event handler is removed
from the stream.
Three States
The "two modes" of operation for a Readable stream are a simplified abstraction for the more complicated internal state management that is happening within the Readable stream implementation.
Specifically, at any given point in time, every Readable is in one of three possible states:
readable._readableState.flowing = null
readable._readableState.flowing = false
readable._readableState.flowing = true
When readable._readableState.flowing
is null
, no mechanism for consuming the
streams data is provided so the stream will not generate its data.
Attaching a listener for the 'data'
event, calling the readable.pipe()
method, or calling the readable.resume()
method will switch
readable._readableState.flowing
to true
, causing the Readable to begin
actively emitting events as data is generated.
Calling readable.pause()
, readable.unpipe()
, or receiving "back pressure"
will cause the readable._readableState.flowing
to be set as false
,
temporarily halting the flowing of events but not halting the generation of
data.
While readable._readableState.flowing
is false
, data may be accumulating
within the streams internal buffer.
Choose One
The Readable stream API evolved across multiple Node.js versions and provides multiple methods of consuming stream data. In general, developers should choose one of the methods of consuming data and should never use multiple methods to consume data from a single stream.
Use of the readable.pipe()
method is recommended for most users as it has been
implemented to provide the easiest way of consuming stream data. Developers that
require more fine-grained control over the transfer and generation of data can
use the EventEmitter
and readable.pause()
/readable.resume()
APIs.
Class: stream.Readable
Event: 'close'
The 'close'
event is emitted when the stream and any of its underlying
resources (a file descriptor, for example) have been closed. The event indicates
that no more events will be emitted, and no further computation will occur.
Not all Readable streams will emit the 'close'
event.
Event: 'data'
chunk
{Buffer|String|any} The chunk of data. For streams that are not operating in object mode, the chunk will be either a string orBuffer
. For streams that are in object mode, the chunk can be any JavaScript value other thannull
.
The 'data'
event is emitted whenever the stream is relinquishing ownership of
a chunk of data to a consumer. This may occur whenever the stream is switched
in flowing mode by calling readable.pipe()
, readable.resume()
, or by
attaching a listener callback to the 'data'
event. The 'data'
event will
also be emitted whenever the readable.read()
method is called and a chunk of
data is available to be returned.
Attaching a 'data'
event listener to a stream that has not been explicitly
paused will switch the stream into flowing mode. Data will then be passed as
soon as it is available.
The listener callback will be passed the chunk of data as a string if a default
encoding has been specified for the stream using the
readable.setEncoding()
method; otherwise the data will be passed as a
Buffer
.
const readable = getReadableStreamSomehow();
readable.on('data', (chunk) => {
console.log(`Received ${chunk.length} bytes of data.`);
});
Event: 'end'
The 'end'
event is emitted when there is no more data to be consumed from
the stream.
Note: The 'end'
event will not be emitted unless the data is
completely consumed. This can be accomplished by switching the stream into
flowing mode, or by calling stream.read()
repeatedly until
all data has been consumed.
const readable = getReadableStreamSomehow();
readable.on('data', (chunk) => {
console.log(`Received ${chunk.length} bytes of data.`);
});
readable.on('end', () => {
console.log('There will be no more data.');
});
Event: 'error'
- {Error}
The 'error'
event may be emitted by a Readable implementation at any time.
Typically, this may occur if the underlying stream in unable to generate data
due to an underlying internal failure, or when a stream implementation attempts
to push an invalid chunk of data.
The listener callback will be passed a single Error
object.
Event: 'readable'
The 'readable'
event is emitted when there is data available to be read from
the stream. In some cases, attaching a listener for the 'readable'
event will
cause some amount of data to be read into an internal buffer.
const readable = getReadableStreamSomehow();
readable.on('readable', () => {
// there is some data to read now
});
The 'readable'
event will also be emitted once the end of the stream data
has been reached but before the 'end'
event is emitted.
Effectively, the 'readable'
event indicates that the stream has new
information: either new data is available or the end of the stream has been
reached. In the former case, stream.read()
will return the
available data. In the latter case, stream.read()
will return
null
. For instance, in the following example, foo.txt
is an empty file:
const fs = require('fs');
const rr = fs.createReadStream('foo.txt');
rr.on('readable', () => {
console.log('readable:', rr.read());
});
rr.on('end', () => {
console.log('end');
});
The output of running this script is:
$ node test.js
readable: null
end
Note: In general, the readable.pipe()
and 'data'
event mechanisms are
preferred over the use of the 'readable'
event.
readable.isPaused()
- Returns: {Boolean}
The readable.isPaused()
method returns the current operating state of the
Readable. This is used primarily by the mechanism that underlies the
readable.pipe()
method. In most typical cases, there will be no reason to
use this method directly.
const readable = new stream.Readable
readable.isPaused() // === false
readable.pause()
readable.isPaused() // === true
readable.resume()
readable.isPaused() // === false
readable.pause()
- Returns:
this
The readable.pause()
method will cause a stream in flowing mode to stop
emitting 'data'
events, switching out of flowing mode. Any data that
becomes available will remain in the internal buffer.
const readable = getReadableStreamSomehow();
readable.on('data', (chunk) => {
console.log(`Received ${chunk.length} bytes of data.`);
readable.pause();
console.log('There will be no additional data for 1 second.');
setTimeout(() => {
console.log('Now data will start flowing again.');
readable.resume();
}, 1000);
});
readable.pipe(destination[, options])
destination
{stream.Writable} The destination for writing dataoptions
{Object} Pipe optionsend
{Boolean} End the writer when the reader ends. Defaults totrue
.
The readable.pipe()
method attaches a Writable stream to the readable
,
causing it to switch automatically into flowing mode and push all of its data
to the attached Writable. The flow of data will be automatically managed so
that the destination Writable stream is not overwhelmed by a faster Readable
stream.
The following example pipes all of the data from the readable
into a file
named file.txt
:
const readable = getReadableStreamSomehow();
const writable = fs.createWriteStream('file.txt');
// All the data from readable goes into 'file.txt'
readable.pipe(writable);
It is possible to attach multiple Writable streams to a single Readable stream.
The readable.pipe()
method returns a reference to the destination stream
making it possible to set up chains of piped streams:
const r = fs.createReadStream('file.txt');
const z = zlib.createGzip();
const w = fs.createWriteStream('file.txt.gz');
r.pipe(z).pipe(w);
By default, stream.end()
is called on the destination Writable
stream when the source Readable stream emits 'end'
, so that the
destination is no longer writable. To disable this default behavior, the end
option can be passed as false
, causing the destination stream to remain open,
as illustrated in the following example:
reader.pipe(writer, { end: false });
reader.on('end', () => {
writer.end('Goodbye\n');
});
One important caveat is that if the Readable stream emits an error during processing, the Writable destination is not closed automatically. If an error occurs, it will be necessary to manually close each stream in order to prevent memory leaks.
Note: The process.stderr
and process.stdout
Writable streams are
never closed until the Node.js process exits, regardless of the specified
options.
readable.read([size])
size
{Number} Optional argument to specify how much data to read.- Return {String|Buffer|Null}
The readable.read()
method pulls some data out of the internal buffer and
returns it. If no data available to be read, null
is returned. By default,
the data will be returned as a Buffer
object unless an encoding has been
specified using the readable.setEncoding()
method or the stream is operating
in object mode.
The optional size
argument specifies a specific number of bytes to read. If
size
bytes are not available to be read, null
will be returned unless
the stream has ended, in which case all of the data remaining in the internal
buffer will be returned (even if it exceeds size
bytes).
If the size
argument is not specified, all of the data contained in the
internal buffer will be returned.
The readable.read()
method should only be called on Readable streams operating
in paused mode. In flowing mode, readable.read()
is called automatically until
the internal buffer is fully drained.
const readable = getReadableStreamSomehow();
readable.on('readable', () => {
var chunk;
while (null !== (chunk = readable.read())) {
console.log(`Received ${chunk.length} bytes of data.`);
}
});
In general, it is recommended that developers avoid the use of the 'readable'
event and the readable.read()
method in favor of using either
readable.pipe()
or the 'data'
event.
A Readable stream in object mode will always return a single item from
a call to readable.read(size)
, regardless of the value of the
size
argument.
Note: If the readable.read()
method returns a chunk of data, a 'data'
event will also be emitted.
Note: Calling stream.read([size])
after the 'end'
event has been emitted will return null
. No runtime error will be raised.
readable.resume()
- Returns:
this
The readable.resume()
method causes an explicitly paused Readable stream to
resume emitting 'data'
events, switching the stream into flowing mode.
The readable.resume()
method can be used to fully consume the data from a
stream without actually processing any of that data as illustrated in the
following example:
getReadableStreamSomehow()
.resume()
.on('end', () => {
console.log('Reached the end, but did not read anything.');
});
readable.setEncoding(encoding)
encoding
{String} The encoding to use.- Returns:
this
The readable.setEncoding()
method sets the default character encoding for
data read from the Readable stream.
Setting an encoding causes the stream data
to be returned as string of the specified encoding rather than as Buffer
objects. For instance, calling readable.setEncoding('utf8')
will cause the
output data will be interpreted as UTF-8 data, and passed as strings. Calling
readable.setEncoding('hex')
will cause the data to be encoded in hexadecimal
string format.
The Readable stream will properly handle multi-byte characters delivered through
the stream that would otherwise become improperly decoded if simply pulled from
the stream as Buffer
objects.
Encoding can be disabled by calling readable.setEncoding(null)
. This approach
is useful when working with binary data or with large multi-byte strings spread
out over multiple chunks.
const readable = getReadableStreamSomehow();
readable.setEncoding('utf8');
readable.on('data', (chunk) => {
assert.equal(typeof chunk, 'string');
console.log('got %d characters of string data', chunk.length);
});
readable.unpipe([destination])
destination
{stream.Writable} Optional specific stream to unpipe
The readable.unpipe()
method detaches a Writable stream previously attached
using the stream.pipe()
method.
If the destination
is not specified, then all pipes are detached.
If the destination
is specified, but no pipe is set up for it, then
the method does nothing.
const readable = getReadableStreamSomehow();
const writable = fs.createWriteStream('file.txt');
// All the data from readable goes into 'file.txt',
// but only for the first second
readable.pipe(writable);
setTimeout(() => {
console.log('Stop writing to file.txt');
readable.unpipe(writable);
console.log('Manually close the file stream');
writable.end();
}, 1000);
readable.unshift(chunk)
chunk
{Buffer|String} Chunk of data to unshift onto the read queue
The readable.unshift()
method pushes a chunk of data back into the internal
buffer. This is useful in certain situations where a stream is being consumed by
code that needs to "un-consume" some amount of data that it has optimistically
pulled out of the source, so that the data can be passed on to some other party.
Note: The stream.unshift(chunk)
method cannot be called after the
'end'
event has been emitted or a runtime error will be thrown.
Developers using stream.unshift()
often should consider switching to
use of a Transform stream instead. See the API for Stream Implementers
section for more information.
// Pull off a header delimited by \n\n
// use unshift() if we get too much
// Call the callback with (error, header, stream)
const StringDecoder = require('string_decoder').StringDecoder;
function parseHeader(stream, callback) {
stream.on('error', callback);
stream.on('readable', onReadable);
const decoder = new StringDecoder('utf8');
var header = '';
function onReadable() {
var chunk;
while (null !== (chunk = stream.read())) {
var str = decoder.write(chunk);
if (str.match(/\n\n/)) {
// found the header boundary
var split = str.split(/\n\n/);
header += split.shift();
const remaining = split.join('\n\n');
const buf = Buffer.from(remaining, 'utf8');
stream.removeListener('error', callback);
// set the readable listener before unshifting
stream.removeListener('readable', onReadable);
if (buf.length)
stream.unshift(buf);
// now the body of the message can be read from the stream.
callback(null, header, stream);
} else {
// still reading the header.
header += str;
}
}
}
}
Note: Unlike stream.push(chunk)
, stream.unshift(chunk)
will not end the reading process by resetting the internal reading state of the
stream. This can cause unexpected results if readable.unshift()
is called
during a read (i.e. from within a stream._read()
implementation on a custom stream). Following the call to readable.unshift()
with an immediate stream.push('')
will reset the reading state
appropriately, however it is best to simply avoid calling readable.unshift()
while in the process of performing a read.
readable.wrap(stream)
stream
{Stream} An "old style" readable stream
Versions of Node.js prior to v0.10 had streams that did not implement the
entire stream
module API as it is currently defined. (See Compatibility
for more information.)
When using an older Node.js library that emits 'data'
events and has a
stream.pause()
method that is advisory only, the
readable.wrap()
method can be used to create a Readable stream that uses
the old stream as its data source.
It will rarely be necessary to use readable.wrap()
but the method has been
provided as a convenience for interacting with older Node.js applications and
libraries.
For example:
const OldReader = require('./old-api-module.js').OldReader;
const Readable = require('stream').Readable;
const oreader = new OldReader;
const myReader = new Readable().wrap(oreader);
myReader.on('readable', () => {
myReader.read(); // etc.
});
Duplex and Transform Streams
Class: stream.Duplex
Duplex streams are streams that implement both the Readable and Writable interfaces.
Examples of Duplex streams include:
Class: stream.Transform
Transform streams are Duplex streams where the output is in some way related to the input. Like all Duplex streams, Transform streams implement both the Readable and Writable interfaces.
Examples of Transform streams include:
API for Stream Implementers
The stream
module API has been designed to make it possible to easily
implement streams using JavaScript's prototypal inheritance model.
First, a stream developer would declare a new JavaScript class that extends one
of the four basic stream classes (stream.Writable
, stream.Readable
,
stream.Duplex
, or stream.Transform
), making sure the call the appropriate
parent class constructor:
const Writable = require('stream').Writable;
class MyWritable extends Writable {
constructor(options) {
super(options);
}
}
The new stream class must then implement one or more specific methods, depending on the type of stream being created, as detailed in the chart below:
Use-case |
Class |
Method(s) to implement |
---|---|---|
Reading only |
[Readable](#stream_class_stream_readable) |
|
Writing only |
[Writable](#stream_class_stream_writable) |
|
Reading and writing |
[Duplex](#stream_class_stream_duplex) |
|
Operate on written data, then read the result |
[Transform](#stream_class_stream_transform) |
|
Note: The implementation code for a stream should never call the "public" methods of a stream that are intended for use by consumers (as described in the API for Stream Consumers section). Doing so may lead to adverse side effects in application code consuming the stream.
Simplified Construction
For many simple cases, it is possible to construct a stream without relying on
inheritance. This can be accomplished by directly creating instances of the
stream.Writable
, stream.Readable
, stream.Duplex
or stream.Transform
objects and passing appropriate methods as constructor options.
For example:
const Writable = require('stream').Writable;
const myWritable = new Writable({
write(chunk, encoding, callback) {
// ...
}
});
Implementing a Writable Stream
The stream.Writable
class is extended to implement a Writable stream.
Custom Writable streams must call the new stream.Writable([options])
constructor and implement the writable._write()
method. The
writable._writev()
method may also be implemented.
Constructor: new stream.Writable([options])
options
{Object}highWaterMark
{Number} Buffer level whenstream.write()
starts returningfalse
. Defaults to16384
(16kb), or16
forobjectMode
streams.decodeStrings
{Boolean} Whether or not to decode strings into Buffers before passing them tostream._write()
. Defaults totrue
objectMode
{Boolean} Whether or not thestream.write(anyObj)
is a valid operation. When set, it becomes possible to write JavaScript values other than string orBuffer
if supported by the stream implementation. Defaults tofalse
write
{Function} Implementation for thestream._write()
method.writev
{Function} Implementation for thestream._writev()
method.
For example:
const Writable = require('stream').Writable;
class MyWritable extends Writable {
constructor(options) {
// Calls the stream.Writable() constructor
super(options);
}
}
Or, when using pre-ES6 style constructors:
const Writable = require('stream').Writable;
const util = require('util');
function MyWritable(options) {
if (!(this instanceof MyWritable))
return new MyWritable(options);
Writable.call(this, options);
}
util.inherits(MyWritable, Writable);
Or, using the Simplified Constructor approach:
const Writable = require('stream').Writable;
const myWritable = new Writable({
write(chunk, encoding, callback) {
// ...
},
writev(chunks, callback) {
// ...
}
});
writable._write(chunk, encoding, callback)
chunk
{Buffer|String} The chunk to be written. Will always be a buffer unless thedecodeStrings
option was set tofalse
.encoding
{String} If the chunk is a string, thenencoding
is the character encoding of that string. If chunk is aBuffer
, or if the stream is operating in object mode,encoding
may be ignored.callback
{Function} Call this function (optionally with an error argument) when processing is complete for the supplied chunk.
All Writable stream implementations must provide a
writable._write()
method to send data to the underlying
resource.
Note: Transform streams provide their own implementation of the
writable._write()
.
Note: This function MUST NOT be called by application code directly. It should be implemented by child classes, and called only by the internal Writable class methods only.
The callback
method must be called to signal either that the write completed
successfully or failed with an error. The first argument passed to the
callback
must be the Error
object if the call failed or null
if the
write succeeded.
It is important to note that all calls to writable.write()
that occur between
the time writable._write()
is called and the callback
is called will cause
the written data to be buffered. Once the callback
is invoked, the stream will
emit a 'drain'
event. If a stream implementation is capable of processing
multiple chunks of data at once, the writable._writev()
method should be
implemented.
If the decodeStrings
property is set in the constructor options, then
chunk
may be a string rather than a Buffer, and encoding
will
indicate the character encoding of the string. This is to support
implementations that have an optimized handling for certain string
data encodings. If the decodeStrings
property is explicitly set to false
,
the encoding
argument can be safely ignored, and chunk
will remain the same
object that is passed to .write()
.
The writable._write()
method is prefixed with an underscore because it is
internal to the class that defines it, and should never be called directly by
user programs.
writable._writev(chunks, callback)
chunks
{Array} The chunks to be written. Each chunk has following format:{ chunk: ..., encoding: ... }
.callback
{Function} A callback function (optionally with an error argument) to be invoked when processing is complete for the supplied chunks.
Note: This function MUST NOT be called by application code directly. It should be implemented by child classes, and called only by the internal Writable class methods only.
The writable._writev()
method may be implemented in addition to
writable._write()
in stream implementations that are capable of processing
multiple chunks of data at once. If implemented, the method will be called with
all chunks of data currently buffered in the write queue.
The writable._writev()
method is prefixed with an underscore because it is
internal to the class that defines it, and should never be called directly by
user programs.
Errors While Writing
It is recommended that errors occurring during the processing of the
writable._write()
and writable._writev()
methods are reported by invoking
the callback and passing the error as the first argument. This will cause an
'error'
event to be emitted by the Writable. Throwing an Error from within
writable._write()
can result in unexpected and inconsistent behavior depending
on how the stream is being used. Using the callback ensures consistent and
predictable handling of errors.
const Writable = require('stream').Writable;
const myWritable = new Writable({
write(chunk, encoding, callback) {
if (chunk.toString().indexOf('a') >= 0) {
callback(new Error('chunk is invalid'));
} else {
callback();
}
}
});
An Example Writable Stream
The following illustrates a rather simplistic (and somewhat pointless) custom Writable stream implementation. While this specific Writable stream instance is not of any real particular usefulness, the example illustrates each of the required elements of a custom Writable stream instance:
const Writable = require('stream').Writable;
class MyWritable extends Writable {
constructor(options) {
super(options);
}
_write(chunk, encoding, callback) {
if (chunk.toString().indexOf('a') >= 0) {
callback(new Error('chunk is invalid'));
} else {
callback();
}
}
}
Implementing a Readable Stream
The stream.Readable
class is extended to implement a Readable stream.
Custom Readable streams must call the new stream.Readable([options])
constructor and implement the readable._read()
method.
new stream.Readable([options])
options
{Object}highWaterMark
{Number} The maximum number of bytes to store in the internal buffer before ceasing to read from the underlying resource. Defaults to16384
(16kb), or16
forobjectMode
streamsencoding
{String} If specified, then buffers will be decoded to strings using the specified encoding. Defaults tonull
objectMode
{Boolean} Whether this stream should behave as a stream of objects. Meaning thatstream.read(n)
returns a single value instead of a Buffer of size n. Defaults tofalse
read
{Function} Implementation for thestream._read()
method.
For example:
const Readable = require('stream').Readable;
class MyReadable extends Readable {
constructor(options) {
// Calls the stream.Readable(options) constructor
super(options);
}
}
Or, when using pre-ES6 style constructors:
const Readable = require('stream').Readable;
const util = require('util');
function MyReadable(options) {
if (!(this instanceof MyReadable))
return new MyReadable(options);
Readable.call(this, options);
}
util.inherits(MyReadable, Readable);
Or, using the Simplified Constructor approach:
const Readable = require('stream').Readable;
const myReadable = new Readable({
read(size) {
// ...
}
});
readable._read(size)
size
{Number} Number of bytes to read asynchronously
Note: This function MUST NOT be called by application code directly. It should be implemented by child classes, and called only by the internal Readable class methods only.
All Readable stream implementations must provide an implementation of the
readable._read()
method to fetch data from the underlying resource.
When readable._read()
is called, if data is available from the resource, the
implementation should begin pushing that data into the read queue using the
this.push(dataChunk)
method. _read()
should continue reading
from the resource and pushing data until readable.push()
returns false
. Only
when _read()
is called again after it has stopped should it resume pushing
additional data onto the queue.
Note: Once the readable._read()
method has been called, it will not be
called again until the readable.push()
method is called.
The size
argument is advisory. For implementations where a "read" is a
single operation that returns data can use the size
argument to determine how
much data to fetch. Other implementations may ignore this argument and simply
provide data whenever it becomes available. There is no need to "wait" until
size
bytes are available before calling stream.push(chunk)
.
The readable._read()
method is prefixed with an underscore because it is
internal to the class that defines it, and should never be called directly by
user programs.
readable.push(chunk[, encoding])
chunk
{Buffer|Null|String} Chunk of data to push into the read queueencoding
{String} Encoding of String chunks. Must be a valid Buffer encoding, such as'utf8'
or'ascii'
- Returns {Boolean}
true
if additional chunks of data may continued to be pushed;false
otherwise.
When chunk
is a Buffer
or string
, the chunk
of data will be added to the
internal queue for users of the stream to consume. Passing chunk
as null
signals the end of the stream (EOF), after which no more data can be written.
When the Readable is operating in paused mode, the data added with
readable.push()
can be read out by calling the
readable.read()
method when the 'readable'
event is
emitted.
When the Readable is operating in flowing mode, the data added with
readable.push()
will be delivered by emitting a 'data'
event.
The readable.push()
method is designed to be as flexible as possible. For
example, when wrapping a lower-level source that provides some form of
pause/resume mechanism, and a data callback, the low-level source can be wrapped
by the custom Readable instance as illustrated in the following example:
// source is an object with readStop() and readStart() methods,
// and an `ondata` member that gets called when it has data, and
// an `onend` member that gets called when the data is over.
class SourceWrapper extends Readable {
constructor(options) {
super(options);
this._source = getLowlevelSourceObject();
// Every time there's data, push it into the internal buffer.
this._source.ondata = (chunk) => {
// if push() returns false, then stop reading from source
if (!this.push(chunk))
this._source.readStop();
};
// When the source ends, push the EOF-signaling `null` chunk
this._source.onend = () => {
this.push(null);
};
}
// _read will be called when the stream wants to pull more data in
// the advisory size argument is ignored in this case.
_read(size) {
this._source.readStart();
}
}
Note: The readable.push()
method is intended be called only by Readable
Implementers, and only from within the readable._read()
method.
Errors While Reading
It is recommended that errors occurring during the processing of the
readable._read()
method are emitted using the 'error'
event rather than
being thrown. Throwing an Error from within readable._read()
can result in
unexpected and inconsistent behavior depending on whether the stream is
operating in flowing or paused mode. Using the 'error'
event ensures
consistent and predictable handling of errors.
const Readable = require('stream').Readable;
const myReadable = new Readable({
read(size) {
if (checkSomeErrorCondition()) {
process.nextTick(() => this.emit('error', err));
return;
}
// do some work
}
});
An Example Counting Stream
The following is a basic example of a Readable stream that emits the numerals from 1 to 1,000,000 in ascending order, and then ends.
const Readable = require('stream').Readable;
class Counter extends Readable {
constructor(opt) {
super(opt);
this._max = 1000000;
this._index = 1;
}
_read() {
var i = this._index++;
if (i > this._max)
this.push(null);
else {
var str = '' + i;
var buf = Buffer.from(str, 'ascii');
this.push(buf);
}
}
}
Implementing a Duplex Stream
A Duplex stream is one that implements both Readable and Writable, such as a TCP socket connection.
Because JavaScript does not have support for multiple inheritance, the
stream.Duplex
class is extended to implement a Duplex stream (as opposed
to extending the stream.Readable
and stream.Writable
classes).
Note: The stream.Duplex
class prototypically inherits from stream.Readable
and parasitically from stream.Writable
, but instanceof
will work properly
for both base classes due to overriding Symbol.hasInstance
on stream.Writable
.
Custom Duplex streams must call the new stream.Duplex([options])
constructor and implement both the readable._read()
and
writable._write()
methods.
new stream.Duplex(options)
options
{Object} Passed to both Writable and Readable constructors. Also has the following fields:allowHalfOpen
{Boolean} Defaults totrue
. If set tofalse
, then the stream will automatically end the readable side when the writable side ends and vice versa.readableObjectMode
{Boolean} Defaults tofalse
. SetsobjectMode
for readable side of the stream. Has no effect ifobjectMode
istrue
.writableObjectMode
{Boolean} Defaults tofalse
. SetsobjectMode
for writable side of the stream. Has no effect ifobjectMode
istrue
.
For example:
const Duplex = require('stream').Duplex;
class MyDuplex extends Duplex {
constructor(options) {
super(options);
}
}
Or, when using pre-ES6 style constructors:
const Duplex = require('stream').Duplex;
const util = require('util');
function MyDuplex(options) {
if (!(this instanceof MyDuplex))
return new MyDuplex(options);
Duplex.call(this, options);
}
util.inherits(MyDuplex, Duplex);
Or, using the Simplified Constructor approach:
const Duplex = require('stream').Duplex;
const myDuplex = new Duplex({
read(size) {
// ...
},
write(chunk, encoding, callback) {
// ...
}
});
An Example Duplex Stream
The following illustrates a simple example of a Duplex stream that wraps a hypothetical lower-level source object to which data can be written, and from which data can be read, albeit using an API that is not compatible with Node.js streams. The following illustrates a simple example of a Duplex stream that buffers incoming written data via the Writable interface that is read back out via the Readable interface.
const Duplex = require('stream').Duplex;
const kSource = Symbol('source');
class MyDuplex extends Duplex {
constructor(source, options) {
super(options);
this[kSource] = source;
}
_write(chunk, encoding, callback) {
// The underlying source only deals with strings
if (Buffer.isBuffer(chunk))
chunk = chunk.toString();
this[kSource].writeSomeData(chunk);
callback();
}
_read(size) {
this[kSource].fetchSomeData(size, (data, encoding) => {
this.push(Buffer.from(data, encoding));
});
}
}
The most important aspect of a Duplex stream is that the Readable and Writable sides operate independently of one another despite co-existing within a single object instance.
Object Mode Duplex Streams
For Duplex streams, objectMode
can be set exclusively for either the Readable
or Writable side using the readableObjectMode
and writableObjectMode
options
respectively.
In the following example, for instance, a new Transform stream (which is a type of Duplex stream) is created that has an object mode Writable side that accepts JavaScript numbers that are converted to hexadecimal strings on the Readable side.
const Transform = require('stream').Transform;
// All Transform streams are also Duplex Streams
const myTransform = new Transform({
writableObjectMode: true,
transform(chunk, encoding, callback) {
// Coerce the chunk to a number if necessary
chunk |= 0;
// Transform the chunk into something else.
const data = chunk.toString(16);
// Push the data onto the readable queue.
callback(null, '0'.repeat(data.length % 2) + data);
}
});
myTransform.setEncoding('ascii');
myTransform.on('data', (chunk) => console.log(chunk));
myTransform.write(1);
// Prints: 01
myTransform.write(10);
// Prints: 0a
myTransform.write(100);
// Prints: 64
Implementing a Transform Stream
A Transform stream is a Duplex stream where the output is computed in some way from the input. Examples include zlib streams or crypto streams that compress, encrypt, or decrypt data.
Note: 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 produce output
that is either much smaller or much larger than its input.
The stream.Transform
class is extended to implement a Transform stream.
The stream.Transform
class prototypically inherits from stream.Duplex
and
implements its own versions of the writable._write()
and readable._read()
methods. Custom Transform implementations must implement the
transform._transform()
method and may also implement
the transform._flush()
method.
Note: Care must be taken when using Transform streams in that data written to the stream can cause the Writable side of the stream to become paused if the output on the Readable side is not consumed.
new stream.Transform([options])
options
{Object} Passed to both Writable and Readable constructors. Also has the following fields:transform
{Function} Implementation for thestream._transform()
method.flush
{Function} Implementation for thestream._flush()
method.
For example:
const Transform = require('stream').Transform;
class MyTransform extends Transform {
constructor(options) {
super(options);
}
}
Or, when using pre-ES6 style constructors:
const Transform = require('stream').Transform;
const util = require('util');
function MyTransform(options) {
if (!(this instanceof MyTransform))
return new MyTransform(options);
Transform.call(this, options);
}
util.inherits(MyTransform, Transform);
Or, using the Simplified Constructor approach:
const Transform = require('stream').Transform;
const myTransform = new Transform({
transform(chunk, encoding, callback) {
// ...
}
});
Events: 'finish' and 'end'
The 'finish'
and 'end'
events are from the stream.Writable
and stream.Readable
classes, respectively. The 'finish'
event is emitted
after stream.end()
is called and all chunks have been processed
by stream._transform()
. The 'end'
event is emitted
after all data has been output, which occurs after the callback in
transform._flush()
has been called.
transform._flush(callback)
callback
{Function} A callback function (optionally with an error argument and data) to be called when remaining data has been flushed.
Note: This function MUST NOT be called by application code directly. It should be implemented by child classes, and called only by the internal Readable class methods only.
In some cases, a transform operation may need to emit an additional bit of
data at the end of the stream. For example, a zlib
compression stream will
store an amount of internal state used to optimally compress the output. When
the stream ends, however, that additional data needs to be flushed so that the
compressed data will be complete.
Custom Transform implementations may implement the transform._flush()
method. This will be called when there is no more written data to be consumed,
but before the 'end'
event is emitted signaling the end of the
Readable stream.
Within the transform._flush()
implementation, the readable.push()
method
may be called zero or more times, as appropriate. The callback
function must
be called when the flush operation is complete.
The transform._flush()
method is prefixed with an underscore because it is
internal to the class that defines it, and should never be called directly by
user programs.
transform._transform(chunk, encoding, callback)
chunk
{Buffer|String} The chunk to be transformed. Will always be a buffer unless thedecodeStrings
option was set tofalse
.encoding
{String} If the chunk is a string, then this is the encoding type. If chunk is a buffer, then this is the special value - 'buffer', ignore it in this case.callback
{Function} A callback function (optionally with an error argument and data) to be called after the suppliedchunk
has been processed.
Note: This function MUST NOT be called by application code directly. It should be implemented by child classes, and called only by the internal Readable class methods only.
All Transform stream implementations must provide a _transform()
method to accept input and produce output. The transform._transform()
implementation handles the bytes being written, computes an output, then passes
that output off to the readable portion using the readable.push()
method.
The transform.push()
method may be called zero or more times to generate
output from a single input chunk, depending on how much is to be output
as a result of the chunk.
It is possible that no output is generated from any given chunk of input data.
The callback
function must be called only when the current chunk is completely
consumed. The first argument passed to the callback
must be an Error
object
if an error occurred while processing the input or null
otherwise. If a second
argument is passed to the callback
, it will be forwarded on to the
readable.push()
method. In other words the following are equivalent:
transform.prototype._transform = function (data, encoding, callback) {
this.push(data);
callback();
};
transform.prototype._transform = function (data, encoding, callback) {
callback(null, data);
};
The transform._transform()
method is prefixed with an underscore because it
is internal to the class that defines it, and should never be called directly by
user programs.
Class: stream.PassThrough
The stream.PassThrough
class is a trivial implementation of a Transform
stream that simply passes the input bytes across to the output. Its purpose is
primarily for examples and testing, but there are some use cases where
stream.PassThrough
is useful as a building block for novel sorts of streams.
Additional Notes
Compatibility with Older Node.js Versions
In versions of Node.js prior to v0.10, the Readable stream interface was simpler, but also less powerful and less useful.
- Rather than waiting for calls the
stream.read()
method,'data'
events would begin emitting immediately. Applications that would need to perform some amount of work to decide how to handle data were required to store read data into buffers so the data would not be lost. - The
stream.pause()
method was advisory, rather than guaranteed. This meant that it was still necessary to be prepared to receive'data'
events even when the stream was in a paused state.
In Node.js v0.10, the Readable class was added. For backwards compatibility
with older Node.js programs, Readable streams switch into "flowing mode" when a
'data'
event handler is added, or when the
stream.resume()
method is called. The effect is that, even
when not using the new stream.read()
method and
'readable'
event, it is no longer necessary to worry about losing
'data'
chunks.
While most applications will continue to function normally, this introduces an edge case in the following conditions:
- No
'data'
event listener is added. - The
stream.resume()
method is never called. - The stream is not piped to any writable destination.
For example, consider the following code:
// WARNING! BROKEN!
net.createServer((socket) => {
// we add an 'end' method, but never consume the data
socket.on('end', () => {
// It will never get here.
socket.end('The message was received but was not processed.\n');
});
}).listen(1337);
In versions of Node.js prior to v0.10, the incoming message data would be simply discarded. However, in Node.js v0.10 and beyond, the socket remains paused forever.
The workaround in this situation is to call the
stream.resume()
method to begin the flow of data:
// Workaround
net.createServer((socket) => {
socket.on('end', () => {
socket.end('The message was received but was not processed.\n');
});
// start the flow of data, discarding it.
socket.resume();
}).listen(1337);
In addition to new Readable streams switching into flowing mode,
pre-v0.10 style streams can be wrapped in a Readable class using the
readable.wrap()
method.
readable.read(0)
There are some cases where it is necessary to trigger a refresh of the
underlying readable stream mechanisms, without actually consuming any
data. In such cases, it is possible to call readable.read(0)
, which will
always return null
.
If the internal read buffer is below the highWaterMark
, and the
stream is not currently reading, then calling stream.read(0)
will trigger
a low-level stream._read()
call.
While most applications will almost never need to do this, there are situations within Node.js where this is done, particularly in the Readable stream class internals.
readable.push('')
Use of readable.push('')
is not recommended.
Pushing a zero-byte string or Buffer
to a stream that is not in object mode
has an interesting side effect. Because it is a call to
readable.push()
, the call will end the reading process.
However, because the argument is an empty string, no data is added to the
readable buffer so there is nothing for a user to consume.