lua-resty-mlcache

Fast and automated multi-level cache for OpenResty.

This library can be can be manipulated as a key/value store caching scalar Lua types and tables, but is built on top of lua_shared_dict and lua-resty-lrucache. This combination allows for extremely performant and flexible caching.

Features:

• Caching and negative caching with TTLs.
• Built-in mutex via lua-resty-lock to prevent dog-pile effects to your database/backend on cache misses.
• Built-in inter-workers communication to propagate cache invalidations and allow workers to update their L1 (lua-resty-lrucache) caches upon changes (set(), delete()).
• Multiple isolated instances can be created to hold various types of data while relying on the same lua_shared_dict L2 cache.

Illustration of the various caching levels built into this library:

┌─────────────────────────────────────────────────┐
│ Nginx                                           │
│       ┌───────────┐ ┌───────────┐ ┌───────────┐ │
│       │worker     │ │worker     │ │worker     │ │
│ L1    │           │ │           │ │           │ │
│       │ Lua cache │ │ Lua cache │ │ Lua cache │ │
│       └───────────┘ └───────────┘ └───────────┘ │
│             │             │             │       │
│             ▼             ▼             ▼       │
│       ┌───────────────────────────────────────┐ │
│       │                                       │ │
│ L2    │           lua_shared_dict             │ │
│       │                                       │ │
│       └───────────────────────────────────────┘ │
│                           │                     │
│                           ▼                     │
│                  ┌──────────────────┐           │
│                  │     callback     │           │
│                  └────────┬─────────┘           │
└───────────────────────────┼─────────────────────┘
                            │
  L3                        │   I/O fetch
                            ▼

                   Database, API, I/O...

The cache level hierarchy is:

• L1: Least-Recently-Used Lua-land cache using lua-resty-lrucache. Provides the fastest lookup if populated, and avoids exhausting the workers' Lua VM memory.
• L2: lua_shared_dict memory zone shared by all workers. This level is only accessed if L1 was a miss, and prevents workers from requesting the L3 cache.
• L3: a custom function that will only be run by a single worker to avoid the dog-pile effect on your database/backend (via lua-resty-lock). Values fetched via L3 will be set to the L2 cache for other workers to access.

Table of Contents

• Synopsis
• Requirements
• Installation
• Methods
  • new
  • get
  • peek
  • set
  • delete
  • update
• License

Synopsis

# nginx.conf

http {
    # you do not need to configure the following line when you
    # use LuaRocks or opm.
    lua_package_path "/path/to/lua-resty-mlcache/lib/?.lua;;";

    lua_shared_dict cache_dict 1m;

    init_by_lua_block {
        local mlcache = require "resty.mlcache"

        local cache, err = mlcache.new("my_cache", "cache_dict", {
            lru_size = 500,    -- size of the L1 (Lua-land LRU) cache
            ttl      = 3600,   -- 1h ttl for hits
            neg_ttl  = 30,     -- 30s ttl for misses
        })
        if err then

        end

        -- we put our instance in the global table for brivety in
        -- this example, but prefer an upvalue to one of your modules
        -- as recommended by ngx_lua
        _G.cache = cache
    }

    server {
        listen 8080;

        location / {
            content_by_lua_block {
                local function callback(username)
                    -- this only runs *once* until the key expires, so
                    -- do expansive operations like connecting to a remote
                    -- backend here. i.e: call a MySQL server in this callback
                    return db:get_user(username) -- { name = "John Doe", email = "john@example.com" }
                end

                -- this call will respectively hit L1 and L2 before running the
                -- callback (L3). The returned value will then be stored in L2 and
                -- L1 for the next request.
                local user, err = cache:get("my_key", nil, callback, "John Doe")
                if err then

                end

                ngx.say(user.username) -- "John Doe"
            }
        }
    }
}

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Requirements

• OpenResty >= 1.11.2.2
  • ngx_lua
  • lua-resty-lrucache
  • lua-resty-lock

This library should be entirely compatible with older versions of OpenResty, but it has only been tested against OpenResty 1.11.2.2 and greater.

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Installation

With Luarocks:

$ luarocks install lua-resty-mlcache

Or via opm:

$ opm get thibaultcha/lua-resty-mlcache

Or manually:

Once you have a local copy of this module's lib/ directory, add it to your LUA_PATH (or lua_package_path directive for OpenResty):

/path/to/lib/?.lua;

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Methods

new

syntax: cache, err = mlcache.new(name, shm, opts?)

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

The first argument name is an arbitrary name of your choosing for this cache, and must be a string. Each mlcache instance namespaces the values it holds according to its name, so several instances with the same name would share the same data.

The second argument shm is the name of the lua_shared_dict shared memory zone. Several instances of mlcache can use the same shm (values will be namespaced).

The third argument opts is optional. If provided, it must be a table holding the desired options for this instance. The possible options are:

• lru_size: a number defining the size of the underlying L1 cache (lua-resty-lrucache instance). This size is the maximal number of items that the L1 LRU cache can hold. Default: 100.
• ttl: a number specifying the expiration time period of the cached values. The unit is seconds, but accepts fractional number parts, like 0.3. A ttl of 0 means the cached values will never expire. Default: 30.
• neg_ttl: a number specifying the expiration time period of the cached misses (when the L3 callback returns nil). The unit is seconds, but accepts fractional number parts, like 0.3. A neg_ttl of 0 means the cached misses will never expire. Default: 5.
• lru: a lua-resty-lrucache instance of your choice. If specified, mlcache will not instanciate an LRU. One can use this value to use the resty.lrucache.pureffi implementation of lua-resty-lrucache if desired.
• resty_lock_opts: options for lua-resty-lock instances. When mlcache runs the L3 callback, it uses lua-resty-lock to ensure that a single worker runs the provided callback.
• ipc_shm: if you wish to use set() or delete(), you must specify a second lua_shared_dict shared memory zone. This shm will be used as a pub/sub backend for invalidation events propagation across workers. Several mlcache instances can use the same ipc_shm (events will be namespaced).

Example:

local mlcache = require "resty.mlcache"

local cache, err = mlcache.new("my_cache", "cache_shared_dict", {
    lru_size = 1000, -- hold up to 1000 items in the L1 cache (Lua VM)
    ttl      = 3600, -- caches scalar types and tables for 1h
    neg_ttl  = 60    -- caches nil values for 60s,
})
if not cache then
    error("could not create mlcache: " .. err)
end

You can create several mlcache instances relying on the same underlying lua_shared_dict shared memory zone:

local mlcache = require "mlcache"

local cache_1 = mlcache.new("cache_1", "cache_shared_dict", { lru_size = 100 })
local cache_2 = mlcache.new("cache_2", "cache_shared_dict", { lru_size = 1e5 })

In the above example, cache_1 is ideal for holding a few, very large values. cache_2 can be used to hold a large number of small values. Both instances will rely on the same shm: lua_shared_dict mlcache_shm 2048m;. Even if you use identical keys in both caches, they will not conflict with each other since they each bear a different name.

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get

syntax: value, err, hit_level = cache:get(key, opts?, callback, ...)

Performs a cache lookup. This is the primary and most efficient method of this module. A typical pattern is to not call set(), and let get() perform all the work.

When it succeeds, it returns value and no error. Because nil misses from the L3 callback are cached, value can be nil, hence one must rely on the second return value err to determine if this method succeeded or not.

The third return value is a number which is set if no error was encountered. It indicated the level at which the value was fetched: 1 for L1, 2 for L2, and 3 for L3.

If an error is encountered, this method returns nil plus a string describing the error.

The first argument key is a string. Each value must be stored under a unique key.

The second argument opts is optional. If provided, it must be a table holding the desired options for this key. These options will supersede the instance's options:

• ttl: a number specifying the expiration time period of the cached values. The unit is seconds, but accepts fractional number parts, like 0.3. A ttl of 0 means the cached values will never expire. Default: inherited from the instance.
• neg_ttl: a number specifying the expiration time period of the cached misses (when the L3 callback returns nil). The unit is seconds, but accepts fractional number parts, like 0.3. A neg_ttl of 0 means the cached misses will never expire. Default: inherited from the instance.

The third argument callback must be a function. Its signature and return values are documented in the following example:

-- arg1, arg2, and arg3 are arguments forwarded to the callback from the
-- `get()` variadic arguments, like so:
-- cache:get(key, opts, callback, arg1, arg2, arg3)

local function callback(arg1, arg2, arg3)
    -- I/O lookup logic
    -- ...

    -- value: the value to cache (Lua scalar or table)
    -- nil: ignored for now - will be used in later versions of this module
    -- ttl: ttl for this value - will override `ttl` or `neg_ttl` if specified
    return value, nil, ttl
end

This function can throw Lua errors as it runs in protected mode. Such errors thrown from the callback will be logged by this module for debug purposes.

When called, get() follows the below steps:

1. query the L1 cache (lua-resty-lrucache instance). This cache lives in the Lua-land, and as such, it is the most efficient to query.
   1. if the L1 cache has the value, it returns the value.
   2. if the L1 cache does not have the value (L1 miss), it continues.
2. query the L2 cache (lua_shared_dict shared memory zone). This cache is shared by all workers, and is less efficient than the L1 cache. It also involves serialization for Lua tables.
   1. if the L2 cache has the value, it sets the value in the L1 cache, and returns it.
   2. if the L2 cache does not have the value (L2 miss), it continues.
3. creates a lua-resty-lock, and ensures that a single worker will run the callback (other workers trying to access the same value will wait).
4. a single worker runs the L3 callback.
5. the callback returns (ex: it performed a database query), and the worker sets the value in the L2 and L1 caches, and returns it.
6. other workers that were trying to access the same value but were waiting fetch the value from the L2 cache (they do not run the L3 callback) and return it.

Example:

local mlcache = require "mlcache"

local cache, err = mlcache.new("my_cache", "cache_shared_dict", {
    lru_size = 1000
})
if not cache then
    -- ...
end

local function fetch_user(db, user_id)
    local user, err = db:query_user(user_id)
    if err then
        error(err) -- safe to throw: will be logged
    end

    return user -- table or nil
end

local db      = my_db_connection -- lua-resty-mysql instance
local user_id = 3

local user, err = cache:get("users:" .. user_id, { ttl = 3600 }, fetch_user, db, user_id)
if err then
    ngx.log(ngx.ERR, "could not retrieve user: ", err)
    return
end

-- `user` could be a table, but could also be `nil` (does not exist)
-- regardless, it will be cached and subsequent calls to get() will
-- return the cached value, for up to `ttl` or `neg_ttl`.
if user then
    ngx.say("user exists: ", user.name)
else
    ngx.say("user does not exists")
end

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peek

syntax: ttl, err, value = cache:peek(key)

Peeks into the L2 (lua_shared_dict) cache.

The first and only argument key is a string, and it is the key to lookup.

This method returns nil and a string describing the error upon failure.

Upon success, but if there is no such value for the queried key, it returns nil as its first argument, and no error.

Upon success, and if there is such a value for the queried key, it returns a number indicating the remaining TTL of the cached value. The third returned value in that case will be the cached value itself, for convenience.

This method is useful if you want to know whether a value is cached or not. A value stored in the L2 cache is considered cache, regardless of whether or not it is also set in the L1 cache of the worker. That is because the L1 cache is too volatile (as its size unit is in a number of slots), and the L2 cache is still several orders of magnitude faster than the L3 callback.

As its only intent is to take a "peek" into the cache to determine its warmth for a given value, peel() does not count as a query like get(), and does not set the value in the L1 cache.

Example:

local mlcache = require "mlcache"

local cache = mlcache.new("my_cache", "cache_shared_dict")

local ttl, err, value = cache:peek("key")
if err then
    ngx.log(ngx.ERR, "could not peek cache: ", err)
    return
end

ngx.say(ttl)   -- nil because `key` has no value yet
ngx.say(value) -- nil

-- cache the value

cache:get("key", { ttl = 5 }, function() return "some value" end)

-- wait 2 seconds

ngx.sleep(2)

local ttl, err, value = cache:peek("key")
if err then
    ngx.log(ngx.ERR, "could not peek cache: ", err)
    return
end

ngx.say(ttl)   -- 3
ngx.say(value) -- "some value"

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set

syntax: ok, err = cache:set(key, opts?, value)

Unconditionally sets a value in the L2 cache and publish an event to other workers so they can evict the value from their L1 cache.

The first argument key is a string, and is the key under which to store the value.

The second argument opts is optional, and if provided, is identical to the one of get().

The third argument value is the value to cache, similar to the return value of the L3 callback. Just like the callback's return value, it must be a Lua scalar, a table, or nil.

On failure, this method returns nil and a string describing the error.

On success, the first return value will be true.

Note: methods such as set() and delete() require that other instances of mlcache (from other workers) evict the value from their L1 (LRU) cache. Since OpenResty has currently no built-in mechanism for inter-worker communication, this module relies on a polling mechanism via a lua_shared_dict shared memory zone to propagate inter-worker events. If set() or delete() are called from a single worker, other workers must call update() before their cache is requested, to make sure they evicted their L1 value, and that the L2 (fresh value) will be returned.

Note bis: It is generally considered inefficient to call set() on a hot code path (such as in a request being served by OpenResty). Instead, one should rely on get() and its built-in mutex in the L3 callback. set() is better suited when called occasionally from a single worker, upon a particular event that triggers a cached value to be updated, for example. Once set() updated the L2 cache with the fresh value, other workers will rely on update() to poll invalidation events. Calling get() on those other workers thus triggers an L1 miss, but the L2 access will hit the fresh value.

See: update()

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delete

syntax: ok, err = cache:delete(key)

Delete a value in the L2 cache and publish an event to other workers so they can evict the value from their L1 cache.

The first and only argument key is the string at which the value is stored.

On failure, this method returns nil and a string describing the error.

On success, the first return value will be true.

Note: methods such as set() and delete() require that other instances of mlcache (from other workers) evict the value from their L1 (LRU) cache. Since OpenResty has currently no built-in mechanism for inter-worker communication, this module relies on a polling mechanism via a lua_shared_dict shared memory zone to propagate inter-worker events. If set() or delete() are called from a single worker, other workers must call update() before their cache is requested, to make sure they evicted their L1 value, and that the L2 (fresh value) will be returned.

See: update()

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update

syntax: ok, err = cache:update()

Poll and execute pending cache invalidation events published to ipc_shm by other workers.

Methods such as set() and delete() require that other instances of mlcache (from other workers) evict the value from their L1 (LRU) cache. Since OpenResty has currently no built-in mechanism for inter-worker communication, this module relies on a polling mechanism via the ipc_shm shared memory zone to propagate inter-worker events. The lua_shared_dict specified in ipc_shm must not be used by other actors than mlcache itself.

This method allows a worker to update its L1 cache (by purging values considered stale due to an other worker calling set() or delete()) before processing a request.

A typical design pattern is to call update() only once on each request processing. This allows your hot code paths to perform a single shm access in the best case scenario: no invalidation events were received, all get() calls will hit in the L1 (LRU) cache. Only on a worst case scenario (n values were evicted by another worker) will get() access the L2 or L3 cache n times. Subsequent requests will then hit the best case scenario again, because get() populated the L1 cache.

For example, if your workers make use of set() or delete() anywhere in your application, call update() at the entrance of your hot code path, before using get():

http {
    listen 9000;

    location / {
        content_by_lua_block {
            local cache = ... -- retrieve mlcache instance or use your own module

            -- make sure L1 cache is evicted of stale values
            -- before calling get()
            local ok, err = cache:update()
            if not ok then
                ngx.log(ngx.ERR, "failed to poll eviciton events: ", err)
                -- /!\ we might get stale data from get()
            end

            -- L1/L2/L3 lookup (best case: L1)
            local value, err = cache:get("key_1", nil, cb1)
            if err then
                -- ...
            end

            -- L1/L2/L3 lookup (best case: L1)
            local other_value, err = cache:get(key_2", nil, cb2)
            if err then
                -- ...
            end

            -- value and other_value are up-to-date because:
            -- either they were not considered stable and came from L1 (best case scenario)
            -- either they were stale and evicted from L1, and came from L2
            -- either they were not in L1 nor L2, and came from L3 (worst case scneario)
        }
    }

    location /delete {
        content_by_lua_block {
            local cache = ... -- retrieve mlcache instance or use your own module

            -- delete some value
            local ok, err = cache:delete("key_1")
            if not ok then
                ngx.log(ngx.ERR, "failed to delete value from cache: ", err)
                return ngx.exit(500)
            end

            ngx.exit(204)
        }
    }

    location /set {
        content_by_lua_block {
            local cache = ... -- retrieve mlcache instance or use your own module

            -- update some value
            local ok, err = cache:set("key_1", nil, 123)
            if not ok then
                ngx.log(ngx.ERR, "failed to set value in cache: ", err)
                return ngx.exit(500)
            end

            ngx.exit(200)
        }
    }
}

Note: you do not need to call update() to refresh your workers if they never call set()or delete(). When workers only rely on get(), values expire naturally from the L1/L2 caches according to their TTL.

Note bis: this library was built with the intent to use a better solution for inter-worker communication as soon as one emerges. In future versions of this library, if an IPC module can avoid the polling approach, so will this library. update() is only a necessary evil due to today's Nginx/OpenResty "limitations".

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License

Work licensed under the MIT License.

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