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Node.js v10.9.0-test2018111503b825811e Documentation
Table of Contents
-
- Threadpool Usage
- Compressing HTTP requests and responses
- Memory Usage Tuning
- Flushing
- Constants
- Class: Options
- Class: zlib.Deflate
- Class: zlib.DeflateRaw
- Class: zlib.Gunzip
- Class: zlib.Gzip
- Class: zlib.Inflate
- Class: zlib.InflateRaw
- Class: zlib.Unzip
- zlib.constants
- zlib.createDeflate([options])
- zlib.createDeflateRaw([options])
- zlib.createGunzip([options])
- zlib.createGzip([options])
- zlib.createInflate([options])
- zlib.createInflateRaw([options])
- zlib.createUnzip([options])
-
- zlib.deflate(buffer[, options], callback)
- zlib.deflateSync(buffer[, options])
- zlib.deflateRaw(buffer[, options], callback)
- zlib.deflateRawSync(buffer[, options])
- zlib.gunzip(buffer[, options], callback)
- zlib.gunzipSync(buffer[, options])
- zlib.gzip(buffer[, options], callback)
- zlib.gzipSync(buffer[, options])
- zlib.inflate(buffer[, options], callback)
- zlib.inflateSync(buffer[, options])
- zlib.inflateRaw(buffer[, options], callback)
- zlib.inflateRawSync(buffer[, options])
- zlib.unzip(buffer[, options], callback)
- zlib.unzipSync(buffer[, options])
Zlib#
The zlib
module provides compression functionality implemented using Gzip and
Deflate/Inflate. It can be accessed using:
const zlib = require('zlib');
Compressing or decompressing a stream (such as a file) can be accomplished by
piping the source stream data through a zlib
stream into a destination stream:
const gzip = zlib.createGzip();
const fs = require('fs');
const inp = fs.createReadStream('input.txt');
const out = fs.createWriteStream('input.txt.gz');
inp.pipe(gzip).pipe(out);
It is also possible to compress or decompress data in a single step:
const input = '.................................';
zlib.deflate(input, (err, buffer) => {
if (!err) {
console.log(buffer.toString('base64'));
} else {
// handle error
}
});
const buffer = Buffer.from('eJzT0yMAAGTvBe8=', 'base64');
zlib.unzip(buffer, (err, buffer) => {
if (!err) {
console.log(buffer.toString());
} else {
// handle error
}
});
Threadpool Usage#
Note that all zlib APIs except those that are explicitly synchronous use libuv's threadpool. This can lead to surprising effects in some applications, such as subpar performance (which can be mitigated by adjusting the pool size) and/or unrecoverable and catastrophic memory fragmentation.
Compressing HTTP requests and responses#
The zlib
module can be used to implement support for the gzip
and deflate
content-encoding mechanisms defined by
HTTP.
The HTTP Accept-Encoding
header is used within an http request to identify
the compression encodings accepted by the client. The Content-Encoding
header is used to identify the compression encodings actually applied to a
message.
The examples given below are drastically simplified to show the basic concept.
Using zlib
encoding can be expensive, and the results ought to be cached.
See Memory Usage Tuning for more information on the speed/memory/compression
tradeoffs involved in zlib
usage.
// client request example
const zlib = require('zlib');
const http = require('http');
const fs = require('fs');
const request = http.get({ host: 'example.com',
path: '/',
port: 80,
headers: { 'Accept-Encoding': 'gzip,deflate' } });
request.on('response', (response) => {
const output = fs.createWriteStream('example.com_index.html');
switch (response.headers['content-encoding']) {
// or, just use zlib.createUnzip() to handle both cases
case 'gzip':
response.pipe(zlib.createGunzip()).pipe(output);
break;
case 'deflate':
response.pipe(zlib.createInflate()).pipe(output);
break;
default:
response.pipe(output);
break;
}
});
// server example
// Running a gzip operation on every request is quite expensive.
// It would be much more efficient to cache the compressed buffer.
const zlib = require('zlib');
const http = require('http');
const fs = require('fs');
http.createServer((request, response) => {
const raw = fs.createReadStream('index.html');
let acceptEncoding = request.headers['accept-encoding'];
if (!acceptEncoding) {
acceptEncoding = '';
}
// Note: This is not a conformant accept-encoding parser.
// See https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.3
if (/\bdeflate\b/.test(acceptEncoding)) {
response.writeHead(200, { 'Content-Encoding': 'deflate' });
raw.pipe(zlib.createDeflate()).pipe(response);
} else if (/\bgzip\b/.test(acceptEncoding)) {
response.writeHead(200, { 'Content-Encoding': 'gzip' });
raw.pipe(zlib.createGzip()).pipe(response);
} else {
response.writeHead(200, {});
raw.pipe(response);
}
}).listen(1337);
By default, the zlib
methods will throw an error when decompressing
truncated data. However, if it is known that the data is incomplete, or
the desire is to inspect only the beginning of a compressed file, it is
possible to suppress the default error handling by changing the flushing
method that is used to decompress the last chunk of input data:
// This is a truncated version of the buffer from the above examples
const buffer = Buffer.from('eJzT0yMA', 'base64');
zlib.unzip(
buffer,
{ finishFlush: zlib.constants.Z_SYNC_FLUSH },
(err, buffer) => {
if (!err) {
console.log(buffer.toString());
} else {
// handle error
}
});
This will not change the behavior in other error-throwing situations, e.g. when the input data has an invalid format. Using this method, it will not be possible to determine whether the input ended prematurely or lacks the integrity checks, making it necessary to manually check that the decompressed result is valid.
Memory Usage Tuning#
From zlib/zconf.h
, modified to Node.js's usage:
The memory requirements for deflate are (in bytes):
(1 << (windowBits + 2)) + (1 << (memLevel + 9))
That is: 128K for windowBits
= 15 + 128K for memLevel
= 8
(default values) plus a few kilobytes for small objects.
For example, to reduce the default memory requirements from 256K to 128K, the options should be set to:
const options = { windowBits: 14, memLevel: 7 };
This will, however, generally degrade compression.
The memory requirements for inflate are (in bytes) 1 << windowBits
.
That is, 32K for windowBits
= 15 (default value) plus a few kilobytes
for small objects.
This is in addition to a single internal output slab buffer of size
chunkSize
, which defaults to 16K.
The speed of zlib
compression is affected most dramatically by the
level
setting. A higher level will result in better compression, but
will take longer to complete. A lower level will result in less
compression, but will be much faster.
In general, greater memory usage options will mean that Node.js has to make
fewer calls to zlib
because it will be able to process more data on
each write
operation. So, this is another factor that affects the
speed, at the cost of memory usage.
Flushing#
Calling .flush()
on a compression stream will make zlib
return as much
output as currently possible. This may come at the cost of degraded compression
quality, but can be useful when data needs to be available as soon as possible.
In the following example, flush()
is used to write a compressed partial
HTTP response to the client:
const zlib = require('zlib');
const http = require('http');
http.createServer((request, response) => {
// For the sake of simplicity, the Accept-Encoding checks are omitted.
response.writeHead(200, { 'content-encoding': 'gzip' });
const output = zlib.createGzip();
output.pipe(response);
setInterval(() => {
output.write(`The current time is ${Date()}\n`, () => {
// The data has been passed to zlib, but the compression algorithm may
// have decided to buffer the data for more efficient compression.
// Calling .flush() will make the data available as soon as the client
// is ready to receive it.
output.flush();
});
}, 1000);
}).listen(1337);
Constants#
All of the constants defined in zlib.h
are also defined on
require('zlib').constants
. In the normal course of operations, it will not be
necessary to use these constants. They are documented so that their presence is
not surprising. This section is taken almost directly from the
zlib documentation. See https://zlib.net/manual.html#Constants for more
details.
Previously, the constants were available directly from require('zlib')
, for
instance zlib.Z_NO_FLUSH
. Accessing the constants directly from the module is
currently still possible but is deprecated.
Allowed flush values.
zlib.constants.Z_NO_FLUSH
zlib.constants.Z_PARTIAL_FLUSH
zlib.constants.Z_SYNC_FLUSH
zlib.constants.Z_FULL_FLUSH
zlib.constants.Z_FINISH
zlib.constants.Z_BLOCK
zlib.constants.Z_TREES
Return codes for the compression/decompression functions. Negative values are errors, positive values are used for special but normal events.
zlib.constants.Z_OK
zlib.constants.Z_STREAM_END
zlib.constants.Z_NEED_DICT
zlib.constants.Z_ERRNO
zlib.constants.Z_STREAM_ERROR
zlib.constants.Z_DATA_ERROR
zlib.constants.Z_MEM_ERROR
zlib.constants.Z_BUF_ERROR
zlib.constants.Z_VERSION_ERROR
Compression levels.
zlib.constants.Z_NO_COMPRESSION
zlib.constants.Z_BEST_SPEED
zlib.constants.Z_BEST_COMPRESSION
zlib.constants.Z_DEFAULT_COMPRESSION
Compression strategy.
zlib.constants.Z_FILTERED
zlib.constants.Z_HUFFMAN_ONLY
zlib.constants.Z_RLE
zlib.constants.Z_FIXED
zlib.constants.Z_DEFAULT_STRATEGY
Class: Options#
Each class takes an options
object. All options are optional.
Note that some options are only relevant when compressing, and are ignored by the decompression classes.
flush
<integer> Default:zlib.constants.Z_NO_FLUSH
finishFlush
<integer> Default:zlib.constants.Z_FINISH
chunkSize
<integer> Default:16 * 1024
windowBits
<integer>level
<integer> (compression only)memLevel
<integer> (compression only)strategy
<integer> (compression only)dictionary
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> (deflate/inflate only, empty dictionary by default)info
<boolean> (Iftrue
, returns an object withbuffer
andengine
.)
See the description of deflateInit2
and inflateInit2
at
https://zlib.net/manual.html#Advanced for more information on these.
Class: zlib.Deflate#
Compress data using deflate.
Class: zlib.DeflateRaw#
Compress data using deflate, and do not append a zlib
header.
Class: zlib.Gunzip#
Decompress a gzip stream.
Class: zlib.Gzip#
Compress data using gzip.
Class: zlib.Inflate#
Decompress a deflate stream.
Class: zlib.InflateRaw#
Decompress a raw deflate stream.
Class: zlib.Unzip#
Decompress either a Gzip- or Deflate-compressed stream by auto-detecting the header.
Class: zlib.Zlib#
Not exported by the zlib
module. It is documented here because it is the base
class of the compressor/decompressor classes.
zlib.bytesRead#
zlib.bytesWritten
instead.Deprecated alias for zlib.bytesWritten
. This original name was chosen
because it also made sense to interpret the value as the number of bytes
read by the engine, but is inconsistent with other streams in Node.js that
expose values under these names.
zlib.bytesWritten#
The zlib.bytesWritten
property specifies the number of bytes written to
the engine, before the bytes are processed (compressed or decompressed,
as appropriate for the derived class).
zlib.close([callback])#
callback
<Function>
Close the underlying handle.
zlib.flush([kind], callback)#
kind
Default:zlib.constants.Z_FULL_FLUSH
callback
<Function>
Flush pending data. Don't call this frivolously, premature flushes negatively impact the effectiveness of the compression algorithm.
Calling this only flushes data from the internal zlib
state, and does not
perform flushing of any kind on the streams level. Rather, it behaves like a
normal call to .write()
, i.e. it will be queued up behind other pending
writes and will only produce output when data is being read from the stream.
zlib.params(level, strategy, callback)#
level
<integer>strategy
<integer>callback
<Function>
Dynamically update the compression level and compression strategy. Only applicable to deflate algorithm.
zlib.reset()#
Reset the compressor/decompressor to factory defaults. Only applicable to the inflate and deflate algorithms.
zlib.constants#
Provides an object enumerating Zlib-related constants.
zlib.createDeflate([options])#
options
<Object>
Creates and returns a new Deflate
object.
zlib.createDeflateRaw([options])#
options
<Object>
Creates and returns a new DeflateRaw
object.
An upgrade of zlib from 1.2.8 to 1.2.11 changed behavior when windowBits
is set to 8 for raw deflate streams. zlib would automatically set windowBits
to 9 if was initially set to 8. Newer versions of zlib will throw an exception,
so Node.js restored the original behavior of upgrading a value of 8 to 9,
since passing windowBits = 9
to zlib actually results in a compressed stream
that effectively uses an 8-bit window only.
zlib.createGunzip([options])#
options
<Object>
Creates and returns a new Gunzip
object.
zlib.createGzip([options])#
options
<Object>
Creates and returns a new Gzip
object.
zlib.createInflate([options])#
options
<Object>
Creates and returns a new Inflate
object.
zlib.createInflateRaw([options])#
options
<Object>
Creates and returns a new InflateRaw
object.
zlib.createUnzip([options])#
options
<Object>
Creates and returns a new Unzip
object.
Convenience Methods#
All of these take a Buffer
, TypedArray
, DataView
,
ArrayBuffer
or string as the first argument, an optional second argument
to supply options to the zlib
classes and will call the supplied callback
with callback(error, result)
.
Every method has a *Sync
counterpart, which accept the same arguments, but
without a callback.
zlib.deflate(buffer[, options], callback)#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<Object>callback
<Function>
zlib.deflateSync(buffer[, options])#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<Object>
Compress a chunk of data with Deflate
.
zlib.deflateRaw(buffer[, options], callback)#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<Object>callback
<Function>
zlib.deflateRawSync(buffer[, options])#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<Object>
Compress a chunk of data with DeflateRaw
.
zlib.gunzip(buffer[, options], callback)#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<Object>callback
<Function>
zlib.gunzipSync(buffer[, options])#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<Object>
Decompress a chunk of data with Gunzip
.
zlib.gzip(buffer[, options], callback)#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<Object>callback
<Function>
zlib.gzipSync(buffer[, options])#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<Object>
Compress a chunk of data with Gzip
.
zlib.inflate(buffer[, options], callback)#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<Object>callback
<Function>
zlib.inflateSync(buffer[, options])#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<Object>
Decompress a chunk of data with Inflate
.
zlib.inflateRaw(buffer[, options], callback)#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<Object>callback
<Function>
zlib.inflateRawSync(buffer[, options])#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<Object>
Decompress a chunk of data with InflateRaw
.
zlib.unzip(buffer[, options], callback)#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<Object>callback
<Function>
zlib.unzipSync(buffer[, options])#
buffer
<Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <string>options
<Object>
Decompress a chunk of data with Unzip
.