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doc: improve zlib docs

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General improvements to zlib doc copy

PR-URL: https://github.com/nodejs/node/pull/6746
Reviewed-By: Anna Henningsen <anna@addaleax.net>
This commit is contained in:
James M Snell 2016-05-13 13:56:30 -07:00
parent 1ba5a56f49
commit 477d35848c
1 changed files with 58 additions and 56 deletions
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doc/api/zlib.md
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@ -2,18 +2,15 @@
Stability: 2 - Stable Stability: 2 - Stable
You can access this module with: The `zlib` module provides compression functionality implemented using Gzip and
Deflate/Inflate. It can be accessed using:
```js
const zlib = require('zlib'); const zlib = require('zlib');
```
This provides bindings to Gzip/Gunzip, Deflate/Inflate, and Compressing or decompressing a stream (such as a file) can be accomplished by
DeflateRaw/InflateRaw classes. Each class takes the same options, and piping the source stream data through a `zlib` stream into a destination stream:
is a readable/writable Stream.
## Examples
Compressing or decompressing a file can be done by piping an
fs.ReadStream into a zlib stream, then into an fs.WriteStream.
```js ```js
const gzip = zlib.createGzip(); const gzip = zlib.createGzip();
@ -24,8 +21,7 @@ const out = fs.createWriteStream('input.txt.gz');
inp.pipe(gzip).pipe(out); inp.pipe(gzip).pipe(out);
``` ```
Compressing or decompressing data in one step can be done by using It is also possible to compress or decompress data in a single step:
the convenience methods.
```js ```js
const input = '.................................'; const input = '.................................';
@ -47,25 +43,33 @@ zlib.unzip(buffer, (err, buffer) => {
}); });
``` ```
To use this module in an HTTP client or server, use the [accept-encoding][] ## Compressing HTTP requests and responses
on requests, and the [content-encoding][] header on responses.
**Note: these examples are drastically simplified to show The `zlib` module can be used to implement support for the `gzip` and `deflate`
the basic concept.** Zlib encoding can be expensive, and the results content-encoding mechanisms defined by
[HTTP](https://tools.ietf.org/html/rfc7230#section-4.2).
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.
**Note: 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 ought to be cached. See [Memory Usage Tuning][] for more information
on the speed/memory/compression tradeoffs involved in zlib usage. on the speed/memory/compression tradeoffs involved in `zlib` usage.
```js ```js
// client request example // client request example
const zlib = require('zlib'); const zlib = require('zlib');
const http = require('http'); const http = require('http');
const fs = require('fs'); const fs = require('fs');
const request = http.get({ host: 'izs.me', const request = http.get({ host: 'example.com',
path: '/', path: '/',
port: 80, port: 80,
headers: { 'accept-encoding': 'gzip,deflate' } }); headers: { 'Accept-Encoding': 'gzip,deflate' } });
request.on('response', (response) => { request.on('response', (response) => {
var output = fs.createWriteStream('izs.me_index.html'); var output = fs.createWriteStream('example.com_index.html');
switch (response.headers['content-encoding']) { switch (response.headers['content-encoding']) {
// or, just use zlib.createUnzip() to handle both cases // or, just use zlib.createUnzip() to handle both cases
@ -97,10 +101,10 @@ http.createServer((request, response) => {
// Note: this is not a conformant accept-encoding parser. // Note: this is not a conformant accept-encoding parser.
// See http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.3 // See http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.3
if (acceptEncoding.match(/\bdeflate\b/)) { if (acceptEncoding.match(/\bdeflate\b/)) {
response.writeHead(200, { 'content-encoding': 'deflate' }); response.writeHead(200, { 'Content-Encoding': 'deflate' });
raw.pipe(zlib.createDeflate()).pipe(response); raw.pipe(zlib.createDeflate()).pipe(response);
} else if (acceptEncoding.match(/\bgzip\b/)) { } else if (acceptEncoding.match(/\bgzip\b/)) {
response.writeHead(200, { 'content-encoding': 'gzip' }); response.writeHead(200, { 'Content-Encoding': 'gzip' });
raw.pipe(zlib.createGzip()).pipe(response); raw.pipe(zlib.createGzip()).pipe(response);
} else { } else {
response.writeHead(200, {}); response.writeHead(200, {});
@ -109,7 +113,7 @@ http.createServer((request, response) => {
}).listen(1337); }).listen(1337);
``` ```
By default, the zlib methods with throw an error when decompressing By default, the `zlib` methods with throw an error when decompressing
truncated data. However, if it is known that the data is incomplete, or 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 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 possible to suppress the default error handling by changing the flushing
@ -146,17 +150,17 @@ The memory requirements for deflate are (in bytes):
(1 << (windowBits+2)) + (1 << (memLevel+9)) (1 << (windowBits+2)) + (1 << (memLevel+9))
``` ```
that is: 128K for windowBits=15 + 128K for memLevel = 8 That is: 128K for windowBits=15 + 128K for memLevel = 8
(default values) plus a few kilobytes for small objects. (default values) plus a few kilobytes for small objects.
For example, if you want to reduce For example, to reduce the default memory requirements from 256K to 128K, the
the default memory requirements from 256K to 128K, set the options to: options shoud be set to:
``` ```
{ windowBits: 14, memLevel: 7 } { windowBits: 14, memLevel: 7 }
``` ```
Of course this will generally degrade compression (there's no free lunch). This will, however, generally degrade compression.
The memory requirements for inflate are (in bytes) The memory requirements for inflate are (in bytes)
@ -164,25 +168,25 @@ The memory requirements for inflate are (in bytes)
1 << windowBits 1 << windowBits
``` ```
that is, 32K for windowBits=15 (default value) plus a few kilobytes That is, 32K for windowBits=15 (default value) plus a few kilobytes
for small objects. for small objects.
This is in addition to a single internal output slab buffer of size This is in addition to a single internal output slab buffer of size
`chunkSize`, which defaults to 16K. `chunkSize`, which defaults to 16K.
The speed of zlib compression is affected most dramatically by the The speed of `zlib` compression is affected most dramatically by the
`level` setting. A higher level will result in better compression, but `level` setting. A higher level will result in better compression, but
will take longer to complete. A lower level will result in less will take longer to complete. A lower level will result in less
compression, but will be much faster. compression, but will be much faster.
In general, greater memory usage options will mean that node.js has to make In general, greater memory usage options will mean that Node.js has to make
fewer calls to zlib, since it'll be able to process more data in a fewer calls to `zlib` because it will be able to process more data on
single `write` operation. So, this is another factor that affects the each `write` operation. So, this is another factor that affects the
speed, at the cost of memory usage. speed, at the cost of memory usage.
## Flushing ## Flushing
Calling [`.flush()`][] on a compression stream will make zlib return as much 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 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. quality, but can be useful when data needs to be available as soon as possible.
@ -214,13 +218,11 @@ http.createServer((request, response) => {
<!--type=misc--> <!--type=misc-->
All of the constants defined in zlib.h are also defined on All of the constants defined in `zlib.h` are also defined on `require('zlib')`.
`require('zlib')`. In the normal course of operations, it will not be necessary to use these
In the normal course of operations, you will not need to ever set any of constants. They are documented so that their presence is not surprising. This
these. They are documented here so that their presence is not section is taken almost directly from the [zlib documentation][]. See
surprising. This section is taken almost directly from the <http://zlib.net/manual.html#Constants> for more details.
[zlib documentation][]. See <http://zlib.net/manual.html#Constants> for more
details.
Allowed flush values. Allowed flush values.
@ -280,19 +282,19 @@ For initializing zalloc, zfree, opaque.
<!--type=misc--> <!--type=misc-->
Each class takes an options object. All options are optional. Each class takes an `options` object. All options are optional.
Note that some options are only relevant when compressing, and are Note that some options are only relevant when compressing, and are
ignored by the decompression classes. ignored by the decompression classes.
* flush (default: `zlib.Z_NO_FLUSH`) * `flush` (default: `zlib.Z_NO_FLUSH`)
* finishFlush (default: `zlib.Z_FINISH`) * `finishFlush` (default: `zlib.Z_FINISH`)
* chunkSize (default: 16*1024) * `chunkSize` (default: 16*1024)
* windowBits * `windowBits`
* level (compression only) * `level` (compression only)
* memLevel (compression only) * `memLevel` (compression only)
* strategy (compression only) * `strategy` (compression only)
* dictionary (deflate/inflate only, empty dictionary by default) * `dictionary` (deflate/inflate only, empty dictionary by default)
See the description of `deflateInit2` and `inflateInit2` at See the description of `deflateInit2` and `inflateInit2` at
<http://zlib.net/manual.html#Advanced> for more information on these. <http://zlib.net/manual.html#Advanced> for more information on these.
@ -303,7 +305,7 @@ Compress data using deflate.
## Class: zlib.DeflateRaw ## Class: zlib.DeflateRaw
Compress data using deflate, and do not append a zlib header. Compress data using deflate, and do not append a `zlib` header.
## Class: zlib.Gunzip ## Class: zlib.Gunzip
@ -338,7 +340,7 @@ class of the compressor/decompressor classes.
Flush pending data. Don't call this frivolously, premature flushes negatively Flush pending data. Don't call this frivolously, premature flushes negatively
impact the effectiveness of the compression algorithm. impact the effectiveness of the compression algorithm.
Calling this only flushes data from the internal zlib state, and does not 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 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 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. writes and will only produce output when data is being read from the stream.
@ -385,9 +387,9 @@ Returns a new [Unzip][] object with an [options][].
<!--type=misc--> <!--type=misc-->
All of these take a [Buffer][] or string as the first argument, an optional second All of these take a [Buffer][] or string as the first argument, an optional
argument to supply options to the zlib classes and will call the supplied second argument to supply options to the `zlib` classes and will call the
callback with `callback(error, result)`. supplied callback with `callback(error, result)`.
Every method has a `*Sync` counterpart, which accept the same arguments, but Every method has a `*Sync` counterpart, which accept the same arguments, but
without a callback. without a callback.
@ -427,8 +429,8 @@ Decompress a Buffer or string with InflateRaw.
Decompress a Buffer or string with Unzip. Decompress a Buffer or string with Unzip.
[accept-encoding]: https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.3 [`Accept-Encoding`]: https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.3
[content-encoding]: https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.11 [`Content-Encoding`]: https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.11
[Memory Usage Tuning]: #zlib_memory_usage_tuning [Memory Usage Tuning]: #zlib_memory_usage_tuning
[zlib documentation]: http://zlib.net/manual.html#Constants [zlib documentation]: http://zlib.net/manual.html#Constants
[options]: #zlib_class_options [options]: #zlib_class_options