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lrucache: Lua-land LRU Cache based on LuaJIT FFI

Installation

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CentOS/RHEL 7 or Amazon Linux 2

yum -y install https://extras.getpagespeed.com/release-latest.rpm
yum -y install lua-resty-lrucache

CentOS/RHEL 8+, Fedora Linux, Amazon Linux 2023

yum -y install https://extras.getpagespeed.com/release-latest.rpm
yum -y install lua5.1-resty-lrucache

To use this Lua library with NGINX, ensure that nginx-module-lua is installed.

This document describes lua-resty-lrucache v0.13 released on Jan 19 2023.

lua-resty-lrucache - Lua-land LRU cache based on the LuaJIT FFI.

Status

This library is considered production ready.

Synopsis

-- file myapp.lua: example "myapp" module

local _M = {}

-- alternatively: local lrucache = require "resty.lrucache.pureffi"
local lrucache = require "resty.lrucache"

-- we need to initialize the cache on the lua module level so that
-- it can be shared by all the requests served by each nginx worker process:
local c, err = lrucache.new(200)  -- allow up to 200 items in the cache
if not c then
    error("failed to create the cache: " .. (err or "unknown"))
end

function _M.go()
    c:set("dog", 32)
    c:set("cat", 56)
    ngx.say("dog: ", c:get("dog"))
    ngx.say("cat: ", c:get("cat"))

    c:set("dog", { age = 10 }, 0.1)  -- expire in 0.1 sec
    c:delete("dog")

    c:flush_all()  -- flush all the cached data
end

return _M

## nginx.conf

http {
    # only if not using an official OpenResty release
    server {
        listen 8080;

        location = /t {
            content_by_lua_block {
                require("myapp").go()
            }
        }
    }
}

Description

This library implements a simple LRU cache for OpenResty and the ngx_lua module.

This cache also supports expiration time.

The LRU cache resides completely in the Lua VM and is subject to Lua GC. As such, do not expect it to get shared across the OS process boundary. The upside is that you can cache arbitrary complex Lua values (such as deep nested Lua tables) without the overhead of serialization (as with ngx_lua's shared dictionary API). The downside is that your cache is always limited to the current OS process (i.e. the current Nginx worker process). It does not really make much sense to use this library in the context of init_by_lua because the cache will not get shared by any of the worker processes (unless you just want to "warm up" the cache with predefined items which will get inherited by the workers via fork()).

This library offers two different implementations in the form of two classes: resty.lrucache and resty.lrucache.pureffi. Both implement the same API. The only difference is that the latter is a pure FFI implementation that also implements an FFI-based hash table for the cache lookup, while the former uses native Lua tables.

If the cache hit rate is relatively high, you should use the resty.lrucache class which is faster than resty.lrucache.pureffi.

However, if the cache hit rate is relatively low and there can be a lot of variations of keys inserted into and removed from the cache, then you should use the resty.lrucache.pureffi instead, because Lua tables are not good at removing keys frequently. You would likely see the resizetab function call in the LuaJIT runtime being very hot in on-CPU flame graphs if you use the resty.lrucache class instead of resty.lrucache.pureffi in such a use case.

Methods

To load this library,

  1. use an official OpenResty release or follow the Installation instructions.
  2. use require to load the library into a local Lua variable:
local lrucache = require "resty.lrucache"

or

local lrucache = require "resty.lrucache.pureffi"

new

syntax: cache, err = lrucache.new(max_items [, load_factor])

Creates a new cache instance. Upon failure, returns nil and a string describing the error.

The max_items argument specifies the maximal number of items this cache can hold.

The load-factor argument designates the "load factor" of the FFI-based hash-table used internally by resty.lrucache.pureffi; the default value is 0.5 (i.e. 50%); if the load factor is specified, it will be clamped to the range of [0.1, 1] (i.e. if load factor is greater than 1, it will be saturated to 1; likewise, if load-factor is smaller than 0.1, it will be clamped to 0.1). This argument is only meaningful for resty.lrucache.pureffi.

set

syntax: cache:set(key, value, ttl?, flags?)

Sets a key with a value and an expiration time.

When the cache is full, the cache will automatically evict the least recently used item.

The optional ttl argument specifies the expiration time. The time value is in seconds, but you can also specify the fraction number part (e.g. 0.25). A nil ttl argument means the value would never expire (which is the default).

The optional flags argument specifies a user flags value associated with the item to be stored. It can be retrieved later with the item. The user flags are stored as an unsigned 32-bit integer internally, and thus must be specified as a Lua number. If not specified, flags will have a default value of 0. This argument was added in the v0.10 release.

get

syntax: data, stale_data, flags = cache:get(key)

Fetches a value with the key. If the key does not exist in the cache or has already expired, nil will be returned.

Starting from v0.03, the stale data is also returned as the second return value if available.

Starting from v0.10, the user flags value associated with the stored item is also returned as the third return value. If no user flags were given to an item, its default flags will be 0.

delete

syntax: cache:delete(key)

Removes an item specified by the key from the cache.

count

syntax: count = cache:count()

Returns the number of items currently stored in the cache including expired items if any.

The returned count value will always be greater or equal to 0 and smaller than or equal to the size argument given to cache:new.

This method was added in the v0.10 release.

capacity

syntax: size = cache:capacity()

Returns the maximum number of items the cache can hold. The return value is the same as the size argument given to cache:new when the cache was created.

This method was added in the v0.10 release.

get_keys

syntax: keys = cache:get_keys(max_count?, res?)

Fetch the list of keys currently inside the cache up to max_count. The keys will be ordered in MRU fashion (Most-Recently-Used keys first).

This function returns a Lua (array) table (with integer keys) containing the keys.

When max_count is nil or 0, all keys (if any) will be returned.

When provided with a res table argument, this function will not allocate a table and will instead insert the keys in res, along with a trailing nil value.

This method was added in the v0.10 release.

flush_all

syntax: cache:flush_all()

Flushes all the existing data (if any) in the current cache instance. This is an O(1) operation and should be much faster than creating a brand new cache instance.

Note however that the flush_all() method of resty.lrucache.pureffi is an O(n) operation.

Prerequisites

nginx.conf

http {
    # only if not using an official OpenResty release
    ...
}

and then load the library in Lua:lua local lrucache = require "resty.lrucache" ```

See Also

GitHub

You may find additional configuration tips and documentation for this module in the GitHub repository for nginx-module-lrucache.