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112 lines
4.3 KiB
Markdown
112 lines
4.3 KiB
Markdown
<!--
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tagline: How to reduce the performance impact of the autoloader
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-->
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# Autoloader optimization
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By default, the Composer autoloader runs relatively fast. However, due to the way
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PSR-4 and PSR-0 autoloading rules are set up, it needs to check the filesystem
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before resolving a classname conclusively. This slows things down quite a bit,
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but it is convenient in development environments because when you add a new class
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it can immediately be discovered/used without having to rebuild the autoloader
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configuration.
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The problem however is in production you generally want things to happen as fast
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as possible, as you can rebuild the configuration every time you deploy and
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new classes do not appear at random between deploys.
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For this reason, Composer offers a few strategies to optimize the autoloader.
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> **Note:** You **should not** enable any of these optimizations in **development** as
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> they all will cause various problems when adding/removing classes. The performance
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> gains are not worth the trouble in a development setting.
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## Optimization Level 1: Class map generation
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### How to run it?
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There are a few options to enable this:
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- Set `"optimize-autoloader": true` inside the config key of composer.json
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- Call `install` or `update` with `-o` / `--optimize-autoloader`
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- Call `dump-autoload` with `-o` / `--optimize`
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### What does it do?
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Class map generation essentially converts PSR-4/PSR-0 rules into classmap rules.
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This makes everything quite a bit faster as for known classes the class map
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returns instantly the path, and Composer can guarantee the class is in there so
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there is no filesystem check needed.
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On PHP 5.6+, the class map is also cached in opcache which improves the initialization
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time greatly. If you make sure opcache is enabled, then the class map should load
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almost instantly and then class loading is fast.
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### Trade-offs
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There are no real trade-offs with this method. It should always be enabled in
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production.
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The only issue is it does not keep track of autoload misses (i.e. when
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it cannot find a given class), so those fallback to PSR-4 rules and can still
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result in slow filesystem checks. To solve this issue two Level 2 optimization
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options exist, and you can decide to enable either if you have a lot of
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class_exists checks that are done for classes that do not exist in your project.
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## Optimization Level 2/A: Authoritative class maps
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### How to run it?
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There are a few options to enable this:
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- Set `"classmap-authoritative": true` inside the config key of composer.json
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- Call `install` or `update` with `-a` / `--classmap-authoritative`
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- Call `dump-autoload` with `-a` / `--classmap-authoritative`
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### What does it do?
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Enabling this automatically enables Level 1 class map optimizations.
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This option says that if something is not found in the classmap,
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then it does not exist and the autoloader should not attempt to look on the
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filesystem according to PSR-4 rules.
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### Trade-offs
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This option makes the autoloader always return very quickly. On the flipside it
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also means that in case a class is generated at runtime for some reason, it will
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not be allowed to be autoloaded. If your project or any of your dependencies does that
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then you might experience "class not found" issues in production. Enable this with care.
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> Note: This cannot be combined with Level 2/B optimizations. You have to choose one as
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> they address the same issue in different ways.
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## Optimization Level 2/B: APCu cache
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### How to run it?
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There are a few options to enable this:
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- Set `"apcu-autoloader": true` inside the config key of composer.json
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- Call `install` or `update` with `--apcu-autoloader`
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- Call `dump-autoload` with `--apcu`
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### What does it do?
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This option adds an APCu cache as a fallback for the class map. It will not
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automatically generate the class map though, so you should still enable Level 1
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optimizations manually if you so desire.
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Whether a class is found or not, that fact is always cached in APCu, so it can be
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returned quickly on the next request.
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### Trade-offs
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This option requires APCu which may or may not be available to you. It also
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uses APCu memory for autoloading purposes, but it is safe to use and cannot
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result in classes not being found like the authoritative class map
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optimization above.
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> Note: This cannot be combined with Level 2/A optimizations. You have to choose one as
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> they address the same issue in different ways.
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