Built-in Redis cache store

* Supports vanilla Redis, hiredis, and Redis::Distributed.
* Supports Memcached-like sharding across Redises with Redis::Distributed.
* Fault tolerant. If the Redis server is unavailable, no exceptions are
  raised. Cache fetches are treated as misses and writes are dropped.
* Local cache. Hot in-memory primary cache within block/middleware scope.
* `read_/write_multi` support for Redis mget/mset. Use Redis::Distributed
  4.0.1+ for distributed mget support.
* `delete_matched` support for Redis KEYS globs.

activesupport/CHANGELOG.md

@@ -1,3 +1,41 @@
+*   Redis cache store.
+
+      # Defaults to `redis://localhost:6379/0`. Only use for dev/test.
+      config.cache_store = :redis_cache_store
+
+      # Supports all common cache store options (:namespace, :compress,
+      # :compress_threshold, :expires_in, :race_condition_tool) and all
+      # Redis options.
+      cache_password = Rails.application.secrets.redis_cache_password
+      config.cache_store = :redis_cache_store, driver: :hiredis,
+        namespace: 'myapp-cache', compress: true, timeout: 1,
+        url: "redis://:#{cache_password}@myapp-cache-1:6379/0"
+
+      # Supports Redis::Distributed with multiple hosts
+      config.cache_store = :redis_cache_store, driver: :hiredis
+        namespace: 'myapp-cache', compress: true,
+        url: %w[
+          redis://myapp-cache-1:6379/0
+          redis://myapp-cache-1:6380/0
+          redis://myapp-cache-2:6379/0
+          redis://myapp-cache-2:6380/0
+          redis://myapp-cache-3:6379/0
+          redis://myapp-cache-3:6380/0
+        ]
+
+      # Or pass a builder block
+      config.cache_store = :redis_cache_store,
+        namespace: 'myapp-cache', compress: true,
+        redis: -> { Redis.new … }
+
+    Deployment note: Take care to use a *dedicated Redis cache* rather
+    than pointing this at your existing Redis server. It won't cope well
+    with mixed usage patterns and it won't expire cache entries by default.
+
+    Redis cache server setup guide: https://redis.io/topics/lru-cache
+
+    *Jeremy Daer*
+
 *   Allow `Range#include?` on TWZ ranges
 
     In #11474 we prevented TWZ ranges being iterated over which matched

activesupport/lib/active_support/cache.rb

@@ -12,10 +12,11 @@ require "active_support/core_ext/string/inflections"
 module ActiveSupport
   # See ActiveSupport::Cache::Store for documentation.
   module Cache
-    autoload :FileStore,     "active_support/cache/file_store"
+    autoload :FileStore,        "active_support/cache/file_store"
-    autoload :MemoryStore,   "active_support/cache/memory_store"
+    autoload :MemoryStore,      "active_support/cache/memory_store"
-    autoload :MemCacheStore, "active_support/cache/mem_cache_store"
+    autoload :MemCacheStore,    "active_support/cache/mem_cache_store"
-    autoload :NullStore,     "active_support/cache/null_store"
+    autoload :NullStore,        "active_support/cache/null_store"
+    autoload :RedisCacheStore,  "active_support/cache/redis_cache_store"
 
     # These options mean something to all cache implementations. Individual cache
     # implementations may support additional options.
@@ -567,14 +568,34 @@ module ActiveSupport
           end
         end
 
-        # Prefixes a key with the namespace. Namespace and key will be delimited
+        # Expands and namespaces the cache key. May be overridden by
-        # with a colon.
+        # cache stores to do additional normalization.
-        def normalize_key(key, options)
+        def normalize_key(key, options = nil)
-          key = Cache.expand_cache_key(key)
+          namespace_key Cache.expand_cache_key(key), options
-          namespace = options[:namespace] if options
+        end
-          prefix = namespace.is_a?(Proc) ? namespace.call : namespace
+
-          key = "#{prefix}:#{key}" if prefix
+        # Prefix the key with a namespace string:
-          key
+        #
+        #   namespace_key 'foo', namespace: 'cache'
+        #   # => 'cache:foo'
+        #
+        # With a namespace block:
+        #
+        #   namespace_key 'foo', namespace: -> { 'cache' }
+        #   # => 'cache:foo'
+        def namespace_key(key, options = nil)
+          options = merged_options(options)
+          namespace = options[:namespace]
+
+          if namespace.respond_to?(:call)
+            namespace = namespace.call
+          end
+
+          if namespace
+            "#{namespace}:#{key}"
+          else
+            key
+          end
         end
 
         def normalize_version(key, options = nil)

activesupport/lib/active_support/cache/redis_cache_store.rb

@@ -0,0 +1,404 @@
+# frozen_string_literal: true
+
+begin
+  gem "redis", ">= 4.0.1"
+  require "redis"
+  require "redis/distributed"
+rescue LoadError
+  warn "The Redis cache store requires the redis gem, version 4.0.1 or later. Please add it to your Gemfile: `gem \"redis\", \"~> 4.0\"`"
+  raise
+end
+
+# Prefer the hiredis driver but don't require it.
+begin
+  require "redis/connection/hiredis"
+rescue LoadError
+end
+
+require "digest/sha2"
+require "active_support/core_ext/marshal"
+
+module ActiveSupport
+  module Cache
+    # Redis cache store.
+    #
+    # Deployment note: Take care to use a *dedicated Redis cache* rather
+    # than pointing this at your existing Redis server. It won't cope well
+    # with mixed usage patterns and it won't expire cache entries by default.
+    #
+    # Redis cache server setup guide: https://redis.io/topics/lru-cache
+    #
+    # * Supports vanilla Redis, hiredis, and Redis::Distributed.
+    # * Supports Memcached-like sharding across Redises with Redis::Distributed.
+    # * Fault tolerant. If the Redis server is unavailable, no exceptions are
+    #   raised. Cache fetches are all misses and writes are dropped.
+    # * Local cache. Hot in-memory primary cache within block/middleware scope.
+    # * `read_/write_multi` support for Redis mget/mset. Use Redis::Distributed
+    #   4.0.1+ for distributed mget support.
+    # * `delete_matched` support for Redis KEYS globs.
+    class RedisCacheStore < Store
+      # Keys are truncated with their own SHA2 digest if they exceed 1kB
+      MAX_KEY_BYTESIZE = 1024
+
+      DEFAULT_REDIS_OPTIONS = {
+        connect_timeout:    20,
+        read_timeout:       1,
+        write_timeout:      1,
+        reconnect_attempts: 0,
+      }
+
+      DEFAULT_ERROR_HANDLER = -> (method:, returning:, exception:) {
+        logger.error { "RedisCacheStore: #{method} failed, returned #{returning.inspect}: #{e.class}: #{e.message}" } if logger
+      }
+
+      DELETE_GLOB_LUA = "for i, name in ipairs(redis.call('KEYS', ARGV[1])) do redis.call('DEL', name); end"
+      private_constant :DELETE_GLOB_LUA
+
+      # Support raw values in the local cache strategy.
+      module LocalCacheWithRaw # :nodoc:
+        private
+          def read_entry(key, options)
+            entry = super
+            if options[:raw] && local_cache && entry
+              entry = deserialize_entry(entry.value)
+            end
+            entry
+          end
+
+          def write_entry(key, entry, options)
+            if options[:raw] && local_cache
+              raw_entry = Entry.new(entry.value.to_s)
+              raw_entry.expires_at = entry.expires_at
+              super(key, raw_entry, options)
+            else
+              super
+            end
+          end
+
+          def write_multi_entries(entries, options)
+            if options[:raw] && local_cache
+              raw_entries = entries.map do |key, entry|
+                raw_entry = Entry.new(entry.value.to_s)
+                raw_entry.expires_at = entry.expires_at
+              end.to_h
+
+              super(raw_entries, options)
+            else
+              super
+            end
+          end
+      end
+
+      prepend Strategy::LocalCache
+      prepend LocalCacheWithRaw
+
+      class << self
+        # Factory method to create a new Redis instance.
+        #
+        # Handles four options: :redis block, :redis instance, single :url
+        # string, and multiple :url strings.
+        #
+        #   Option  Class       Result
+        #   :redis  Proc    ->  options[:redis].call
+        #   :redis  Object  ->  options[:redis]
+        #   :url    String  ->  Redis.new(url: …)
+        #   :url    Array   ->  Redis::Distributed.new([{ url: … }, { url: … }, …])
+        #
+        def build_redis(redis: nil, url: nil, **redis_options) #:nodoc:
+          urls = Array(url)
+
+          if redis.respond_to?(:call)
+            redis.call
+          elsif redis
+            redis
+          elsif urls.size > 1
+            build_redis_distributed_client urls: urls, **redis_options
+          else
+            build_redis_client url: urls.first, **redis_options
+          end
+        end
+
+        private
+          def build_redis_distributed_client(urls:, **redis_options)
+            ::Redis::Distributed.new([], DEFAULT_REDIS_OPTIONS.merge(redis_options)).tap do |dist|
+              urls.each { |u| dist.add_node url: u }
+            end
+          end
+
+          def build_redis_client(url:, **redis_options)
+            ::Redis.new DEFAULT_REDIS_OPTIONS.merge(redis_options.merge(url: url))
+          end
+      end
+
+      attr_reader :redis_options
+      attr_reader :max_key_bytesize
+
+      # Creates a new Redis cache store.
+      #
+      # Handles three options: block provided to instantiate, single URL
+      # provided, and multiple URLs provided.
+      #
+      #   :redis Proc   -> options[:redis].call
+      #   :url   String -> Redis.new(url: …)
+      #   :url   Array  -> Redis::Distributed.new([{ url: … }, { url: … }, …])
+      #
+      # No namespace is set by default. Provide one if the Redis cache
+      # server is shared with other apps: `namespace: 'myapp-cache'`.
+      #
+      # Compression is enabled by default with a 1kB threshold, so cached
+      # values larger than 1kB are automatically compressed. Disable by
+      # passing `cache: false` or change the threshold by passing
+      # `compress_threshold: 4.kilobytes`.
+      #
+      # No expiry is set on cache entries by default. Redis is expected to
+      # be configured with an eviction policy that automatically deletes
+      # least-recently or -frequently used keys when it reaches max memory.
+      # See https://redis.io/topics/lru-cache for cache server setup.
+      #
+      # Race condition TTL is not set by default. This can be used to avoid
+      # "thundering herd" cache writes when hot cache entries are expired.
+      # See <tt>ActiveSupport::Cache::Store#fetch</tt> for more.
+      def initialize(namespace: nil, compress: true, compress_threshold: 1.kilobyte, expires_in: nil, race_condition_ttl: nil, error_handler: DEFAULT_ERROR_HANDLER, **redis_options)
+        @redis_options = redis_options
+
+        @max_key_bytesize = MAX_KEY_BYTESIZE
+        @error_handler = error_handler
+
+        super namespace: namespace,
+          compress: compress, compress_threshold: compress_threshold,
+          expires_in: expires_in, race_condition_ttl: race_condition_ttl
+      end
+
+      def redis
+        @redis ||= self.class.build_redis(**redis_options)
+      end
+
+      def inspect
+        instance = @redis || @redis_options
+        "<##{self.class} options=#{options.inspect} redis=#{instance.inspect}>"
+      end
+
+      # Cache Store API implementation.
+      #
+      # Read multiple values at once. Returns a hash of requested keys ->
+      # fetched values.
+      def read_multi(*names)
+        if mget_capable?
+          read_multi_mget(*names)
+        else
+          super
+        end
+      end
+
+      # Cache Store API implementation.
+      #
+      # Supports Redis KEYS glob patterns:
+      #
+      #   h?llo matches hello, hallo and hxllo
+      #   h*llo matches hllo and heeeello
+      #   h[ae]llo matches hello and hallo, but not hillo
+      #   h[^e]llo matches hallo, hbllo, ... but not hello
+      #   h[a-b]llo matches hallo and hbllo
+      #
+      # Use \ to escape special characters if you want to match them verbatim.
+      #
+      # See https://redis.io/commands/KEYS for more.
+      #
+      # Failsafe: Raises errors.
+      def delete_matched(matcher, options = nil)
+        instrument :delete_matched, matcher do
+          case matcher
+          when String
+            redis.eval DELETE_GLOB_LUA, [], [namespace_key(matcher, options)]
+          else
+            raise ArgumentError, "Only Redis glob strings are supported: #{matcher.inspect}"
+          end
+        end
+      end
+
+      # Cache Store API implementation.
+      #
+      # Increment a cached value. This method uses the Redis incr atomic
+      # operator and can only be used on values written with the :raw option.
+      # Calling it on a value not stored with :raw will initialize that value
+      # to zero.
+      #
+      # Failsafe: Raises errors.
+      def increment(name, amount = 1, options = nil)
+        instrument :increment, name, amount: amount do
+          redis.incrby normalize_key(name, options), amount
+        end
+      end
+
+      # Cache Store API implementation.
+      #
+      # Decrement a cached value. This method uses the Redis decr atomic
+      # operator and can only be used on values written with the :raw option.
+      # Calling it on a value not stored with :raw will initialize that value
+      # to zero.
+      #
+      # Failsafe: Raises errors.
+      def decrement(name, amount = 1, options = nil)
+        instrument :decrement, name, amount: amount do
+          redis.decrby normalize_key(name, options), amount
+        end
+      end
+
+      # Cache Store API implementation.
+      #
+      # Removes expired entries. Handled natively by Redis least-recently-/
+      # least-frequently-used expiry, so manual cleanup is not supported.
+      def cleanup(options = nil)
+        super
+      end
+
+      # Clear the entire cache on all Redis servers. Safe to use on
+      # shared servers if the cache is namespaced.
+      #
+      # Failsafe: Raises errors.
+      def clear(options = nil)
+        failsafe :clear do
+          if namespace = merged_options(options)[namespace]
+            delete_matched "*", namespace: namespace
+          else
+            redis.flushdb
+          end
+        end
+      end
+
+      def mget_capable? #:nodoc:
+        set_redis_capabilities unless defined? @mget_capable
+        @mget_capable
+      end
+
+      def mset_capable? #:nodoc:
+        set_redis_capabilities unless defined? @mset_capable
+        @mset_capable
+      end
+
+      private
+        def set_redis_capabilities
+          case redis
+          when Redis::Distributed
+            @mget_capable = true
+            @mset_capable = false
+          else
+            @mget_capable = true
+            @mset_capable = true
+          end
+        end
+
+        # Store provider interface:
+        # Read an entry from the cache.
+        def read_entry(key, options = nil)
+          failsafe :read_entry do
+            deserialize_entry redis.get(key)
+          end
+        end
+
+        def read_multi_mget(*names)
+          options = names.extract_options!
+          options = merged_options(options)
+
+          keys_to_names = names.map { |name| [ normalize_key(name, options), name ] }.to_h
+          values = redis.mget(*keys_to_names.keys)
+
+          keys_to_names.zip(values).each_with_object({}) do |((key, name), value), results|
+            if value
+              entry = deserialize_entry(value)
+              unless entry.nil? || entry.expired? || entry.mismatched?(normalize_version(name, options))
+                results[name] = entry.value
+              end
+            end
+          end
+        end
+
+        # Write an entry to the cache.
+        #
+        # Requires Redis 2.6.12+ for extended SET options.
+        def write_entry(key, entry, unless_exist: false, raw: false, expires_in: nil, race_condition_ttl: nil, **options)
+          value = raw ? entry.value.to_s : serialize_entry(entry)
+
+          # If race condition TTL is in use, ensure that cache entries
+          # stick around a bit longer after they would have expired
+          # so we can purposefully serve stale entries.
+          if race_condition_ttl && expires_in && expires_in > 0 && !raw
+            expires_in += 5.minutes
+          end
+
+          failsafe :write_entry do
+            if unless_exist || expires_in
+              modifiers = {}
+              modifiers[:nx] = unless_exist
+              modifiers[:px] = (1000 * expires_in.to_f).ceil if expires_in
+
+              redis.set key, value, modifiers
+            else
+              redis.set key, value
+            end
+          end
+        end
+
+        # Delete an entry from the cache.
+        def delete_entry(key, options)
+          failsafe :delete_entry, returning: false do
+            redis.del key
+          end
+        end
+
+        # Nonstandard store provider API to write multiple values at once.
+        def write_multi_entries(entries, expires_in: nil, **options)
+          if entries.any?
+            if mset_capable? && expires_in.nil?
+              failsafe :write_multi_entries do
+                redis.mapped_mset(entries)
+              end
+            else
+              super
+            end
+          end
+        end
+
+        # Truncate keys that exceed 1kB.
+        def normalize_key(key, options)
+          truncate_key super
+        end
+
+        def truncate_key(key)
+          if key.bytesize > max_key_bytesize
+            suffix = ":sha2:#{Digest::SHA2.hexdigest(key)}"
+            truncate_at = max_key_bytesize - suffix.bytesize
+            "#{key.byteslice(0, truncate_at)}#{suffix}"
+          else
+            key
+          end
+        end
+
+        def deserialize_entry(raw_value)
+          if raw_value
+            entry = Marshal.load(raw_value) rescue raw_value
+            entry.is_a?(Entry) ? entry : Entry.new(entry)
+          end
+        end
+
+        def serialize_entry(entry)
+          Marshal.dump(entry)
+        end
+
+        def failsafe(method, returning: nil)
+          yield
+        rescue ::Redis::BaseConnectionError => e
+          handle_exception exception: e, method: method, returning: returning
+          returning
+        end
+
+        def handle_exception(exception:, method:, returning:)
+          if @error_handler
+            @error_handler.(method: method, exception: exception, returning: returning)
+          end
+        rescue => failsafe
+          warn "RedisCacheStore ignored exception in handle_exception: #{failsafe.class}: #{failsafe.message}\n  #{failsafe.backtrace.join("\n  ")}"
+        end
+    end
+  end
+end

activesupport/test/cache/behaviors/cache_increment_decrement_behavior.rb

@@ -8,7 +8,9 @@ module CacheIncrementDecrementBehavior
     assert_equal 2, @cache.read("foo").to_i
     assert_equal 3, @cache.increment("foo")
     assert_equal 3, @cache.read("foo").to_i
-    assert_nil @cache.increment("bar")
+
+    missing = @cache.increment("bar")
+    assert(missing.nil? || missing == 1)
   end
 
   def test_decrement
@@ -18,6 +20,8 @@ module CacheIncrementDecrementBehavior
     assert_equal 2, @cache.read("foo").to_i
     assert_equal 1, @cache.decrement("foo")
     assert_equal 1, @cache.read("foo").to_i
-    assert_nil @cache.decrement("bar")
+
+    missing = @cache.decrement("bar")
+    assert(missing.nil? || missing == -1)
   end
 end

activesupport/test/cache/behaviors/local_cache_behavior.rb

@@ -20,7 +20,11 @@ module LocalCacheBehavior
   end
 
   def test_cleanup_clears_local_cache_but_not_remote_cache
-    skip unless @cache.class.instance_methods(false).include?(:cleanup)
+    begin
+      @cache.cleanup
+    rescue NotImplementedError
+      skip
+    end
 
     @cache.with_local_cache do
       @cache.write("foo", "bar")

activesupport/test/cache/stores/redis_cache_store_test.rb

@@ -0,0 +1,151 @@
+# frozen_string_literal: true
+
+require "abstract_unit"
+require "active_support/cache"
+require "active_support/cache/redis_cache_store"
+require_relative "../behaviors"
+
+module ActiveSupport::Cache::RedisCacheStoreTests
+  class LookupTest < ActiveSupport::TestCase
+    test "may be looked up as :redis_cache_store" do
+      assert_kind_of ActiveSupport::Cache::RedisCacheStore,
+        ActiveSupport::Cache.lookup_store(:redis_cache_store)
+    end
+  end
+
+  class InitializationTest < ActiveSupport::TestCase
+    test "omitted URL uses Redis client with default settings" do
+      assert_called_with Redis, :new, [
+        url: nil,
+        connect_timeout: 20, read_timeout: 1, write_timeout: 1,
+        reconnect_attempts: 0,
+      ] do
+        build
+      end
+    end
+
+    test "no URLs uses Redis client with default settings" do
+      assert_called_with Redis, :new, [
+        url: nil,
+        connect_timeout: 20, read_timeout: 1, write_timeout: 1,
+        reconnect_attempts: 0,
+      ] do
+        build url: []
+      end
+    end
+
+    test "singular URL uses Redis client" do
+      assert_called_with Redis, :new, [
+        url: "redis://localhost:6379/0",
+        connect_timeout: 20, read_timeout: 1, write_timeout: 1,
+        reconnect_attempts: 0,
+      ] do
+        build url: "redis://localhost:6379/0"
+      end
+    end
+
+    test "one URL uses Redis client" do
+      assert_called_with Redis, :new, [
+        url: "redis://localhost:6379/0",
+        connect_timeout: 20, read_timeout: 1, write_timeout: 1,
+        reconnect_attempts: 0,
+      ] do
+        build url: %w[ redis://localhost:6379/0 ]
+      end
+    end
+
+    test "multiple URLs uses Redis::Distributed client" do
+      assert_called_with Redis, :new, [
+        [ url: "redis://localhost:6379/0",
+          connect_timeout: 20, read_timeout: 1, write_timeout: 1,
+          reconnect_attempts: 0 ],
+        [ url: "redis://localhost:6379/1",
+          connect_timeout: 20, read_timeout: 1, write_timeout: 1,
+          reconnect_attempts: 0 ],
+      ], returns: Redis.new do
+        @cache = build url: %w[ redis://localhost:6379/0 redis://localhost:6379/1 ]
+        assert_kind_of ::Redis::Distributed, @cache.redis
+      end
+    end
+
+    test "block argument uses yielded client" do
+      block = -> { :custom_redis_client }
+      assert_called block, :call do
+        build redis: block
+      end
+    end
+
+    private
+      def build(**kwargs)
+        ActiveSupport::Cache::RedisCacheStore.new(**kwargs).tap do |cache|
+          cache.redis
+        end
+      end
+  end
+
+  class StoreTest < ActiveSupport::TestCase
+    setup do
+      @namespace = "namespace"
+
+      @cache = ActiveSupport::Cache::RedisCacheStore.new(timeout: 0.1, namespace: @namespace, expires_in: 60)
+      #@cache.logger = Logger.new($stdout)  # For test debugging
+
+      # For LocalCacheBehavior tests
+      @peek = ActiveSupport::Cache::RedisCacheStore.new(timeout: 0.1, namespace: @namespace)
+    end
+
+    teardown do
+      @cache.clear
+      @cache.redis.disconnect!
+    end
+  end
+
+  class RedisCacheStoreCommonBehaviorTest < StoreTest
+    include CacheStoreBehavior
+    include CacheStoreVersionBehavior
+    include LocalCacheBehavior
+    include CacheIncrementDecrementBehavior
+    include AutoloadingCacheBehavior
+  end
+
+  # Separate test class so we can omit the namespace which causes expected,
+  # appropriate complaints about incompatible string encodings.
+  class KeyEncodingSafetyTest < StoreTest
+    include EncodedKeyCacheBehavior
+
+    setup do
+      @cache = ActiveSupport::Cache::RedisCacheStore.new(timeout: 0.1)
+      @cache.logger = nil
+    end
+  end
+
+  class StoreAPITest < StoreTest
+  end
+
+  class FailureSafetyTest < StoreTest
+    test "fetch read failure returns nil" do
+    end
+
+    test "fetch read failure does not attempt to write" do
+    end
+
+    test "write failure returns nil" do
+    end
+  end
+
+  class DeleteMatchedTest < StoreTest
+    test "deletes keys matching glob" do
+      @cache.write("foo", "bar")
+      @cache.write("fu", "baz")
+      @cache.delete_matched("foo*")
+      assert !@cache.exist?("foo")
+      assert @cache.exist?("fu")
+    end
+
+    test "fails with regexp matchers" do
+      assert_raise ArgumentError do
+        @cache.delete_matched(/OO/i)
+      end
+    end
+  end
+end

guides/source/caching_with_rails.md

@@ -444,6 +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.