A simple, fast and process-free cache. Or, a Dumb Cache.

Overview

The cache is broken into a configurable number of ETS tables (called segments, defaults to 100). The segment for a given key is :erlang.phash2(key, NUMBER_OF_SEGMENT) and all operations (get, put, del, ttl, ...) are directed to the correct segment.

Expired values are only removed from the cache when get is called on them.

While a maximum cache size must be configured, the size limit is enforced on the segment. That is, given 10 segments and a max size of 10_000, the maximum size of each segment will be 1000. When a segment is full, the segment is erased.

The cache spawns no additional process.

put may result in an O(N) operation when a segment is (N = MAX_SIZE / NUMBER_OF_SEGMENT). Creating a cache with more segment reduces this cost. Specifically, the O(N) operation is a call to :ets.delete_all_objects/1. This behavior can be customized. (This default behavior might change and might involve spawning a process)

The cache will never grow beyond the configured MAX_SIZE.

Your keys might hash to only 1 or a few segments. This would negatively impact performance. However, this would likely require a very unfortunate set of keys (e.g. integers with a fixed gaps between them).

No dependencies & a single file.

Usage 1

The preferred usage, which offers better performance, is to use the define/3 macro:

defmodule MyApp.Cache do
  require DCache

  DCache.define(Users, 100_000)
  DCache.define(Products, 1_000_000, segment: 100)
end

And then call the following on application start:

MyApp.Cache.Users.setup()
MyApp.Cache.Products.setup()

You can then use the caching methods on the created module, e.g:

MyApp.Cache.Users.get(KEY)

Usage 2

Alternatively, you can create the cache at runtime. Note that this will require 1 extra ETS lookup for every call, as well as requiring 1 extra ETS table to be allocated.

DCache.setup(:users, 100_000)
DCache.setup(:products, 1_000_000, segments: 100)

Which you can then use as:

DCache.get(:users, KEY)

Functions

Regardless of which of the above two approaches you use, the same functionality is available:

get/1 & get/2

Cache.Users.get(key)
# OR
DCache.get(:users, key)

Returns the value from the cache. Returns nil if the key isn't found or if the value is expired.

del/1 & del/2

Cache.Users.del(key)
# OR
DCache.del(:users, key)

Deletes the value from the cache. Does nothing if the key isn't found. True if the key was found and delete (even if it was expired), false if the key was not in the cache

take/1 & take/2

Cache.Users.take(key)
# OR
DCache.take(:users, key)

Deletes and returns the value from the cache

ttl/1 & ttl/2

Cache.Users.ttl(key)
# OR
DCache.ttl(:users, key)

Returns the unix time, in seconds, when the value will be considered expired. Returns nil if the key isn't found. This can return a value which is less than now.

put/3 & put/4

Cache.Users.put(key, value, ttl)
# OR
DCache.put(:users, key, value, ttl)

Stores the value in the cache. The ttl is given in seconds relative to now (e.g. 300 to have the value expire in 5 minutes).

fetch/2, fetch/3 & fetch/4

Cache.Users.fetch(key, fun)
# OR
DCache.fetch(:users, key, fun)
# OR
Cache.Users.fetch(key, fun, ttl)
# OR
DCache.fetch(:users, key, fun, ttl)

Returns the value from the cache. If the key is not found, or expired, fun/1 will be called (receiving the key as an argument).

The provided fun/1 should return one of:

• {:ok, value} - to put value into the cache with the ttl that was provided to fetch. {:ok, value} will be returned from fetch.
• {:ok, value, ttl} - to put value into the cache with the specified ttl. {:ok, value} will be returned from fetch.
• {:skip, value} - does not place value in the cache, but still returns value from fetch
• {:error, anything} - does not place value in the cache and return {:error, anything} from fetch.

fetch!/2, fetch!/3 & fetch!/4

Same as fetch/2, fetch/3 & fetch/4, but unwraps {:ok, value} into value and {:error, err} into raise err.

size/0 & size/1

Cache.Users.size()
# OR
DCache.size(:users)

Returns the total number of items in the cache, including expired items. This is an O(N) operation where N is the number of segments (which will typically be small).

Custom Purgers

By default, when a segment is full, :ets.delete_all_objects/1 is used to erase the segment. This behavior can be changed by specifying a custom purging function when creating the cache:

DCache.define(Users, 100_000, purger: &MyApp.Cache.purge_users/1)
# or
DCache.setup(:users, 100_000, purger: &MyApp.Cache.purge_users/1)

The purger receives the ETS name of the segment that is full.