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Diffstat (limited to 'guides/source/caching_with_rails.md')
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1 files changed, 126 insertions, 17 deletions
diff --git a/guides/source/caching_with_rails.md b/guides/source/caching_with_rails.md index 745f09f523..780e69c146 100644 --- a/guides/source/caching_with_rails.md +++ b/guides/source/caching_with_rails.md @@ -32,7 +32,7 @@ Basic Caching This is an introduction to three types of caching techniques: page, action and fragment caching. By default Rails provides fragment caching. In order to use page and action caching you will need to add `actionpack-page_caching` and -`actionpack-action_caching` to your Gemfile. +`actionpack-action_caching` to your `Gemfile`. By default, caching is only enabled in your production environment. To play around with caching locally you'll want to enable caching in your local @@ -100,8 +100,8 @@ called key-based expiration. Cache fragments will also be expired when the view fragment changes (e.g., the HTML in the view changes). The string of characters at the end of the key is a -template tree digest. It is an md5 hash computed based on the contents of the -view fragment you are caching. If you change the view fragment, the md5 hash +template tree digest. It is an MD5 hash computed based on the contents of the +view fragment you are caching. If you change the view fragment, the MD5 hash will change, expiring the existing file. TIP: Cache stores like Memcached will automatically delete old cache files. @@ -175,10 +175,28 @@ class Game < ApplicationRecord end ``` -With `touch` set to true, any action which changes `updated_at` for a game +With `touch` set to `true`, any action which changes `updated_at` for a game record will also change it for the associated product, thereby expiring the cache. +### Shared Partial Caching + +It is possible to share partials and associated caching between files with different mime types. For example shared partial caching allows template writers to share a partial between HTML and JavaScript files. When templates are collected in the template resolver file paths they only include the template language extension and not the mime type. Because of this templates can be used for multiple mime types. Both HTML and JavaScript requests will respond to the following code: + +```ruby +render(partial: 'hotels/hotel', collection: @hotels, cached: true) +``` + +Will load a file named `hotels/hotel.erb`. + +Another option is to include the full filename of the partial to render. + +```ruby +render(partial: 'hotels/hotel.html.erb', collection: @hotels, cached: true) +``` + +Will load a file named `hotels/hotel.html.erb` in any file mime type, for example you could include this partial in a JavaScript file. + ### Managing dependencies In order to correctly invalidate the cache, you need to properly define the @@ -198,11 +216,11 @@ render "comments/comments" render 'comments/comments' render('comments/comments') -render "header" => render("comments/header") +render "header" translates to render("comments/header") -render(@topic) => render("topics/topic") -render(topics) => render("topics/topic") -render(message.topics) => render("topics/topic") +render(@topic) translates to render("topics/topic") +render(topics) translates to render("topics/topic") +render(message.topics) translates to render("topics/topic") ``` On the other hand, some calls need to be changed to make caching work properly. @@ -258,7 +276,7 @@ comment format anywhere in the template, like: If you use a helper method, for example, inside a cached block and you then update that helper, you'll have to bump the cache as well. It doesn't really matter how -you do it, but the md5 of the template file must change. One recommendation is to +you do it, but the MD5 of the template file must change. One recommendation is to simply be explicit in a comment, like: ```html+erb @@ -270,9 +288,9 @@ simply be explicit in a comment, like: Sometimes you need to cache a particular value or query result instead of caching view fragments. Rails' caching mechanism works great for storing __any__ kind of information. -The most efficient way to implement low-level caching is using the `Rails.cache.fetch` method. This method does both reading and writing to the cache. When passed only a single argument, the key is fetched and value from the cache is returned. If a block is passed, the result of the block will be cached to the given key and the result is returned. +The most efficient way to implement low-level caching is using the `Rails.cache.fetch` method. This method does both reading and writing to the cache. When passed only a single argument, the key is fetched and value from the cache is returned. If a block is passed, that block will be executed in the event of a cache miss. The return value of the block will be written to the cache under the given cache key, and that return value will be returned. In case of cache hit, the cached value will be returned without executing the block. -Consider the following example. An application has a `Product` model with an instance method that looks up the product’s price on a competing website. The data returned by this method would be perfect for low-level caching: +Consider the following example. An application has a `Product` model with an instance method that looks up the product's price on a competing website. The data returned by this method would be perfect for low-level caching: ```ruby class Product < ApplicationRecord @@ -284,7 +302,7 @@ class Product < ApplicationRecord end ``` -NOTE: Notice that in this example we used the `cache_key` method, so the resulting cache-key will be something like `products/233-20140225082222765838000/competing_price`. `cache_key` generates a string based on the model’s `id` and `updated_at` attributes. This is a common convention and has the benefit of invalidating the cache whenever the product is updated. In general, when you use low-level caching for instance level information, you need to generate a cache key. +NOTE: Notice that in this example we used the `cache_key` method, so the resulting cache key will be something like `products/233-20140225082222765838000/competing_price`. `cache_key` generates a string based on the model's `id` and `updated_at` attributes. This is a common convention and has the benefit of invalidating the cache whenever the product is updated. In general, when you use low-level caching for instance level information, you need to generate a cache key. ### SQL Caching @@ -348,9 +366,9 @@ There are some common options used by all cache implementations. These can be pa * `:namespace` - This option can be used to create a namespace within the cache store. It is especially useful if your application shares a cache with other applications. -* `:compress` - This option can be used to indicate that compression should be used in the cache. This can be useful for transferring large cache entries over a slow network. +* `:compress` - Enabled by default. Compresses cache entries so more data can be stored in the same memory footprint, leading to fewer cache evictions and higher hit rates. -* `:compress_threshold` - This option is used in conjunction with the `:compress` option to indicate a threshold under which cache entries should not be compressed. This defaults to 16 kilobytes. +* `:compress_threshold` - Defaults to 1kB. Cache entries larger than this threshold, specified in bytes, are compressed. * `:expires_in` - This option sets an expiration time in seconds for the cache entry when it will be automatically removed from the cache. @@ -381,12 +399,17 @@ config.cache_store = :memory_store, { size: 64.megabytes } ``` If you're running multiple Ruby on Rails server processes (which is the case -if you're using mongrel_cluster or Phusion Passenger), then your Rails server +if you're using Phusion Passenger or puma clustered mode), then your Rails server process instances won't be able to share cache data with each other. This cache store is not appropriate for large application deployments. However, it can work well for small, low traffic sites with only a couple of server processes, as well as development and test environments. +New Rails projects are configured to use this implementation in development environment by default. + +NOTE: Since processes will not share cache data when using `:memory_store`, +it will not be possible to manually read, write or expire the cache via the Rails console. + ### ActiveSupport::Cache::FileStore This cache store uses the file system to store entries. The path to the directory where the store files will be stored must be specified when initializing the cache. @@ -396,14 +419,15 @@ config.cache_store = :file_store, "/path/to/cache/directory" ``` With this cache store, multiple server processes on the same host can share a -cache. The cache store is appropriate for low to medium traffic sites that are +cache. This cache store is appropriate for low to medium traffic sites that are served off one or two hosts. Server processes running on different hosts could share a cache by using a shared file system, but that setup is not recommended. As the cache will grow until the disk is full, it is recommended to periodically clear out old entries. -This is the default cache store implementation. +This is the default cache store implementation (at `"#{root}/tmp/cache/"`) if +no explicit `config.cache_store` is supplied. ### ActiveSupport::Cache::MemCacheStore @@ -420,6 +444,53 @@ The `write` and `fetch` methods on this cache accept two additional options that config.cache_store = :mem_cache_store, "cache-1.example.com", "cache-2.example.com" ``` +### ActiveSupport::Cache::RedisCacheStore + +The Redis cache store takes advantage of Redis support for least-recently-used +and least-frequently-used key eviction when it reaches max memory, allowing it +to behave much like a Memcached cache server. + +Deployment note: Redis doesn't expire keys by default, so take care to use a +dedicated Redis cache server. Don't fill up your persistent-Redis server with +volatile cache data! Read the +[Redis cache server setup guide](https://redis.io/topics/lru-cache) in detail. + +For an all-cache Redis server, set `maxmemory-policy` to an `allkeys` policy. +Redis 4+ support least-frequently-used (`allkeys-lfu`) eviction, an excellent +default choice. Redis 3 and earlier should use `allkeys-lru` for +least-recently-used eviction. + +Set cache read and write timeouts relatively low. Regenerating a cached value +is often faster than waiting more than a second to retrieve it. Both read and +write timeouts default to 1 second, but may be set lower if your network is +consistently low latency. + +Cache reads and writes never raise exceptions. They just return `nil` instead, +behaving as if there was nothing in the cache. To gauge whether your cache is +hitting exceptions, you may provide an `error_handler` to report to an +exception gathering service. It must accept three keyword arguments: `method`, +the cache store method that was originally called; `returning`, the value that +was returned to the user, typically `nil`; and `exception`, the exception that +was rescued. + +Putting it all together, a production Redis cache store may look something +like this: + +```ruby +cache_servers = %w[ "redis://cache-01:6379/0", "redis://cache-02:6379/0", … ], +config.cache_store = :redis_cache_store, url: cache_servers, + + connect_timeout: 30, # Defaults to 20 seconds + read_timeout: 0.2, # Defaults to 1 second + write_timeout: 0.2, # Defaults to 1 second + + error_handler: -> (method:, returning:, exception:) { + # Report errors to Sentry as warnings + Raven.capture_exception exception, level: 'warning", + tags: { method: method, returning: returning } + } +``` + ### ActiveSupport::Cache::NullStore This cache store implementation is meant to be used only in development or test environments and it never stores anything. This can be very useful in development when you have code that interacts directly with `Rails.cache` but caching may interfere with being able to see the results of code changes. With this cache store, all `fetch` and `read` operations will result in a miss. @@ -512,6 +583,30 @@ class ProductsController < ApplicationController end ``` +Sometimes we want to cache response, for example a static page, that never gets +expired. To achieve this, we can use `http_cache_forever` helper and by doing +so browser and proxies will cache it indefinitely. + +By default cached responses will be private, cached only on the user's web +browser. To allow proxies to cache the response, set `public: true` to indicate +that they can serve the cached response to all users. + +Using this helper, `last_modified` header is set to `Time.new(2011, 1, 1).utc` +and `expires` header is set to a 100 years. + +WARNING: Use this method carefully as browser/proxy won't be able to invalidate +the cached response unless browser cache is forcefully cleared. + +```ruby +class HomeController < ApplicationController + def index + http_cache_forever(public: true) do + render + end + end +end +``` + ### Strong v/s Weak ETags Rails generates weak ETags by default. Weak ETags allow semantically equivalent @@ -546,6 +641,20 @@ You can also set the strong ETag directly on the response. response.strong_etag = response.body # => "618bbc92e2d35ea1945008b42799b0e7" ``` +Caching in Development +---------------------- + +It's common to want to test the caching strategy of your application +in development mode. Rails provides the rake task `dev:cache` to +easily toggle caching on/off. + +```bash +$ bin/rails dev:cache +Development mode is now being cached. +$ bin/rails dev:cache +Development mode is no longer being cached. +``` + References ---------- |