Class: ActiveSupport::Cache::Store

Inherits:
Object
  • Object
show all
Defined in:
activesupport/lib/active_support/cache.rb

Overview

An abstract cache store class. There are multiple cache store implementations, each having its own additional features. See the classes under the ActiveSupport::Cache module, e.g. ActiveSupport::Cache::MemCacheStore. MemCacheStore is currently the most popular cache store for large production websites.

Some implementations may not support all methods beyond the basic cache methods of fetch, write, read, exist?, and delete.

ActiveSupport::Cache::Store can store any serializable Ruby object.

cache = ActiveSupport::Cache::MemoryStore.new

cache.read('city')   # => nil
cache.write('city', "Duckburgh")
cache.read('city')   # => "Duckburgh"

Keys are always translated into Strings and are case sensitive. When an object is specified as a key and has a cache_key method defined, this method will be called to define the key. Otherwise, the to_param method will be called. Hashes and Arrays can also be used as keys. The elements will be delimited by slashes, and the elements within a Hash will be sorted by key so they are consistent.

cache.read('city') == cache.read(:city)   # => true

Nil values can be cached.

If your cache is on a shared infrastructure, you can define a namespace for your cache entries. If a namespace is defined, it will be prefixed on to every key. The namespace can be either a static value or a Proc. If it is a Proc, it will be invoked when each key is evaluated so that you can use application logic to invalidate keys.

cache.namespace = -> { @last_mod_time }  # Set the namespace to a variable
@last_mod_time = Time.now  # Invalidate the entire cache by changing namespace

Caches can also store values in a compressed format to save space and reduce time spent sending data. Since there is overhead, values must be large enough to warrant compression. To turn on compression either pass compress: true in the initializer or as an option to fetch or write. To specify the threshold at which to compress values, set the :compress_threshold option. The default threshold is 16K.

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(options = nil) ⇒ Store

Creates a new cache. The options will be passed to any write method calls except for :namespace which can be used to set the global namespace for the cache.


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# File 'activesupport/lib/active_support/cache.rb', line 164

def initialize(options = nil)
  @options = options ? options.dup : {}
end

Instance Attribute Details

#optionsObject (readonly)

Returns the value of attribute options


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# File 'activesupport/lib/active_support/cache.rb', line 158

def options
  @options
end

#silenceObject (readonly) Also known as: silence?

Returns the value of attribute silence


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# File 'activesupport/lib/active_support/cache.rb', line 158

def silence
  @silence
end

Instance Method Details

#cleanup(options = nil) ⇒ Object

Cleanups the cache by removing expired entries.

Options are passed to the underlying cache implementation.

All implementations may not support this method.

Raises:

  • (NotImplementedError)

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# File 'activesupport/lib/active_support/cache.rb', line 457

def cleanup(options = nil)
  raise NotImplementedError.new("#{self.class.name} does not support cleanup")
end

#clear(options = nil) ⇒ Object

Clears the entire cache. Be careful with this method since it could affect other processes if shared cache is being used.

The options hash is passed to the underlying cache implementation.

All implementations may not support this method.

Raises:

  • (NotImplementedError)

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# File 'activesupport/lib/active_support/cache.rb', line 467

def clear(options = nil)
  raise NotImplementedError.new("#{self.class.name} does not support clear")
end

#decrement(name, amount = 1, options = nil) ⇒ Object

Decrements an integer value in the cache.

Options are passed to the underlying cache implementation.

All implementations may not support this method.

Raises:

  • (NotImplementedError)

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# File 'activesupport/lib/active_support/cache.rb', line 448

def decrement(name, amount = 1, options = nil)
  raise NotImplementedError.new("#{self.class.name} does not support decrement")
end

#delete(name, options = nil) ⇒ Object

Deletes an entry in the cache. Returns true if an entry is deleted.

Options are passed to the underlying cache implementation.


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# File 'activesupport/lib/active_support/cache.rb', line 405

def delete(name, options = nil)
  options = merged_options(options)

  instrument(:delete, name) do
    delete_entry(normalize_key(name, options), options)
  end
end

#delete_matched(matcher, options = nil) ⇒ Object

Deletes all entries with keys matching the pattern.

Options are passed to the underlying cache implementation.

All implementations may not support this method.

Raises:

  • (NotImplementedError)

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# File 'activesupport/lib/active_support/cache.rb', line 430

def delete_matched(matcher, options = nil)
  raise NotImplementedError.new("#{self.class.name} does not support delete_matched")
end

#exist?(name, options = nil) ⇒ Boolean

Returns true if the cache contains an entry for the given key.

Options are passed to the underlying cache implementation.

Returns:

  • (Boolean)

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# File 'activesupport/lib/active_support/cache.rb', line 416

def exist?(name, options = nil)
  options = merged_options(options)

  instrument(:exist?, name) do
    entry = read_entry(normalize_key(name, options), options)
    (entry && !entry.expired?) || false
  end
end

#fetch(name, options = nil) ⇒ Object

Fetches data from the cache, using the given key. If there is data in the cache with the given key, then that data is returned.

If there is no such data in the cache (a cache miss), then nil will be returned. However, if a block has been passed, that block will be passed the key and 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.

cache.write('today', 'Monday')
cache.fetch('today')  # => "Monday"

cache.fetch('city')   # => nil
cache.fetch('city') do
  'Duckburgh'
end
cache.fetch('city')   # => "Duckburgh"

You may also specify additional options via the options argument. Setting force: true forces a cache “miss,” meaning we treat the cache value as missing even if it's present. Passing a block is required when `force` is true so this always results in a cache write.

cache.write('today', 'Monday')
cache.fetch('today', force: true) { 'Tuesday' } # => 'Tuesday'
cache.fetch('today', force: true) # => ArgumentError

The `:force` option is useful when you're calling some other method to ask whether you should force a cache write. Otherwise, it's clearer to just call `Cache#write`.

Setting :compress will store a large cache entry set by the call in a compressed format.

Setting :expires_in will set an expiration time on the cache. All caches support auto-expiring content after a specified number of seconds. This value can be specified as an option to the constructor (in which case all entries will be affected), or it can be supplied to the fetch or write method to effect just one entry.

cache = ActiveSupport::Cache::MemoryStore.new(expires_in: 5.minutes)
cache.write(key, value, expires_in: 1.minute) # Set a lower value for one entry

Setting :race_condition_ttl is very useful in situations where a cache entry is used very frequently and is under heavy load. If a cache expires and due to heavy load several different processes will try to read data natively and then they all will try to write to cache. To avoid that case the first process to find an expired cache entry will bump the cache expiration time by the value set in :race_condition_ttl. Yes, this process is extending the time for a stale value by another few seconds. Because of extended life of the previous cache, other processes will continue to use slightly stale data for a just a bit longer. In the meantime that first process will go ahead and will write into cache the new value. After that all the processes will start getting the new value. The key is to keep :race_condition_ttl small.

If the process regenerating the entry errors out, the entry will be regenerated after the specified number of seconds. Also note that the life of stale cache is extended only if it expired recently. Otherwise a new value is generated and :race_condition_ttl does not play any role.

# Set all values to expire after one minute.
cache = ActiveSupport::Cache::MemoryStore.new(expires_in: 1.minute)

cache.write('foo', 'original value')
val_1 = nil
val_2 = nil
sleep 60

Thread.new do
  val_1 = cache.fetch('foo', race_condition_ttl: 10) do
    sleep 1
    'new value 1'
  end
end

Thread.new do
  val_2 = cache.fetch('foo', race_condition_ttl: 10) do
    'new value 2'
  end
end

cache.fetch('foo') # => "original value"
sleep 10 # First thread extended the life of cache by another 10 seconds
cache.fetch('foo') # => "new value 1"
val_1 # => "new value 1"
val_2 # => "original value"

Other options will be handled by the specific cache store implementation. Internally, #fetch calls #read_entry, and calls #write_entry on a cache miss. options will be passed to the #read and #write calls.

For example, MemCacheStore's #write method supports the :raw option, which tells the memcached server to store all values as strings. We can use this option with #fetch too:

cache = ActiveSupport::Cache::MemCacheStore.new
cache.fetch("foo", force: true, raw: true) do
  :bar
end
cache.fetch('foo') # => "bar"

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# File 'activesupport/lib/active_support/cache.rb', line 284

def fetch(name, options = nil)
  if block_given?
    options = merged_options(options)
    key = normalize_key(name, options)

    entry = nil
    instrument(:read, name, options) do |payload|
      cached_entry = read_entry(key, options) unless options[:force]
      entry = handle_expired_entry(cached_entry, key, options)
      payload[:super_operation] = :fetch if payload
      payload[:hit] = !!entry if payload
    end

    if entry
      get_entry_value(entry, name, options)
    else
      save_block_result_to_cache(name, options) { |_name| yield _name }
    end
  elsif options && options[:force]
    raise ArgumentError, 'Missing block: Calling `Cache#fetch` with `force: true` requires a block.'
  else
    read(name, options)
  end
end

#fetch_multi(*names) ⇒ Object

Fetches data from the cache, using the given keys. If there is data in the cache with the given keys, then that data is returned. Otherwise, the supplied block is called for each key for which there was no data, and the result will be written to the cache and returned.

Options are passed to the underlying cache implementation.

Returns a hash with the data for each of the names. For example:

cache.write("bim", "bam")
cache.fetch_multi("bim", "unknown_key") do |key|
  "Fallback value for key: #{key}"
end
# => { "bim" => "bam",
#      "unknown_key" => "Fallback value for key: unknown_key" }

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# File 'activesupport/lib/active_support/cache.rb', line 376

def fetch_multi(*names)
  options = names.extract_options!
  options = merged_options(options)
  results = read_multi(*names, options)

  names.each_with_object({}) do |name, memo|
    memo[name] = results.fetch(name) do
      value = yield name
      write(name, value, options)
      value
    end
  end
end

#increment(name, amount = 1, options = nil) ⇒ Object

Increments an integer value in the cache.

Options are passed to the underlying cache implementation.

All implementations may not support this method.

Raises:

  • (NotImplementedError)

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# File 'activesupport/lib/active_support/cache.rb', line 439

def increment(name, amount = 1, options = nil)
  raise NotImplementedError.new("#{self.class.name} does not support increment")
end

#muteObject

Silences the logger within a block.


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# File 'activesupport/lib/active_support/cache.rb', line 175

def mute
  previous_silence, @silence = defined?(@silence) && @silence, true
  yield
ensure
  @silence = previous_silence
end

#read(name, options = nil) ⇒ Object

Fetches data from the cache, using the given key. If there is data in the cache with the given key, then that data is returned. Otherwise, nil is returned.

Options are passed to the underlying cache implementation.


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# File 'activesupport/lib/active_support/cache.rb', line 314

def read(name, options = nil)
  options = merged_options(options)
  key = normalize_key(name, options)
  instrument(:read, name, options) do |payload|
    entry = read_entry(key, options)
    if entry
      if entry.expired?
        delete_entry(key, options)
        payload[:hit] = false if payload
        nil
      else
        payload[:hit] = true if payload
        entry.value
      end
    else
      payload[:hit] = false if payload
      nil
    end
  end
end

#read_multi(*names) ⇒ Object

Reads multiple values at once from the cache. Options can be passed in the last argument.

Some cache implementation may optimize this method.

Returns a hash mapping the names provided to the values found.


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# File 'activesupport/lib/active_support/cache.rb', line 341

def read_multi(*names)
  options = names.extract_options!
  options = merged_options(options)

  results = {}
  names.each do |name|
    key = normalize_key(name, options)
    entry = read_entry(key, options)
    if entry
      if entry.expired?
        delete_entry(key, options)
      else
        results[name] = entry.value
      end
    end
  end
  results
end

#silence!Object

Silences the logger.


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# File 'activesupport/lib/active_support/cache.rb', line 169

def silence!
  @silence = true
  self
end

#write(name, value, options = nil) ⇒ Object

Writes the value to the cache, with the key.

Options are passed to the underlying cache implementation.


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# File 'activesupport/lib/active_support/cache.rb', line 393

def write(name, value, options = nil)
  options = merged_options(options)

  instrument(:write, name, options) do
    entry = Entry.new(value, options)
    write_entry(normalize_key(name, options), entry, options)
  end
end