Lua-land LRU Cache based on LuaJIT FFI
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README.markdown

Name

lua-resty-lrucache - Lua-land LRU Cache based on LuaJIT FFI

Table of Contents

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
    return 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 {
    lua_package_path "/path/to/lua-resty-lrucache/lib/?.lua;;";

    server {
        listen 8080;

        location = /t {
            content_by_lua '
                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. So do not expect it to get shared across the OS process boundary. The upside is that you can cache arbitrary complex Lua values (like 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 (like 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).

There are two different implementations included in this library, in the form of two classes: resty.lrucache and resty.lrucache.pureffi. They share exactly 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 for it.

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

But 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 by design and you would 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 this use case.

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Methods

To load this library,

  1. you need to specify this library's path in ngx_lua's lua_package_path directive. For example, lua_package_path "/path/to/lua-resty-lrucache/lib/?.lua;;";.
  2. you use require to load the library into a local Lua variable:
    local lrucache = require "resty.lrucache"

or

    local lrucache = require "resty.lrucache.pureffi"

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new

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

Creates a new cache instance. If failed, returns nil and a string describing the error.

The max_items argument specifies the maximal number of items held in the cache.

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.

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set

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

Sets a key with a value and an expiration time.

The ttl argument specifies the expiration time period. The time value is in seconds, but you can also specify the fraction number part, like 0.25. A nil ttl argument value means never expired (which is the default).

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

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get

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

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

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

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delete

syntax: cache:delete(key)

Removes an item specified by the key from the cache.

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flush_all

syntax: cache:flush_all(key)

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

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

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Prerequisites

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Installation

It is recommended to use the latest OpenResty bundle directly. At least OpenResty 1.4.2.9 is required. And you need to enable LuaJIT when building your OpenResty bundle by passing the --with-luajit option to its ./configure script. No extra Nginx configuration is required.

If you want to use this library with your own Nginx build (with ngx_lua), then you need to ensure you are using at least ngx_lua 0.8.10.

Also, You need to configure the lua_package_path directive to add the path of your lua-resty-lrucache source tree to ngx_lua's Lua module search path, as in

    # nginx.conf
    http {
        lua_package_path "/path/to/lua-resty-lrucache/lib/?.lua;;";
        ...
    }

and then load the library in Lua:

    local lrucache = require "resty.lrucache"

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TODO

  • add new method get_stale for fetching already expired items.

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Community

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English Mailing List

The openresty-en mailing list is for English speakers.

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Chinese Mailing List

The openresty mailing list is for Chinese speakers.

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Bugs and Patches

Please report bugs or submit patches by

  1. creating a ticket on the GitHub Issue Tracker,
  2. or posting to the OpenResty community.

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Author

Yichun "agentzh" Zhang (章亦春) agentzh@gmail.com, OpenResty Inc.

Shuxin Yang.

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Copyright and License

This module is licensed under the BSD license.

Copyright (C) 2014-2018, by Yichun "agentzh" Zhang, OpenResty Inc.

Copyright (C) 2014-2017, by Shuxin Yang.

All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

  • Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

  • Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

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See Also

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