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_dictL2 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_dictmemory 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.
# 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"
}
}
}
}
- 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.
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;
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 will
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, like0.3. Attlof0means 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 returnsnil). The unit is seconds, but accepts fractional number parts, like0.3. Aneg_ttlof0means 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 theresty.lrucache.pureffiimplementation 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 secondlua_shared_dictshared memory zone. This shm will be used as a pub/sub backend for invalidation events propagation across workers. Several mlcache instances can use the sameipc_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)
endYou 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 cache_shared_dict 2048m;. Even if
you use identical keys in both caches, they will not conflict with each other
since they each bear a different name.
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, like0.3. Attlof0means 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 returnsnil). The unit is seconds, but accepts fractional number parts, like0.3. Aneg_ttlof0means 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
endThis 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:
- 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.
- if the L1 cache has the value, it returns the value.
- if the L1 cache does not have the value (L1 miss), it continues.
- query the L2 cache (
lua_shared_dictshared memory zone). This cache is shared by all workers, and is less efficient than the L1 cache. It also involves serialization for Lua tables.- if the L2 cache has the value, it sets the value in the L1 cache, and returns it.
- if the L2 cache does not have the value (L2 miss), it continues.
- 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).
- a single worker runs the L3 callback.
- the callback returns (ex: it performed a database query), and the worker sets the value in the L2 and L1 caches, and returns it.
- 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")
endsyntax: 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"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()
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()
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".
Work licensed under the MIT License.