if Code.ensure_loaded?(Decorator.Define) do
defmodule Nebulex.Caching do
@moduledoc """
Declarative annotation-based caching via function
[decorators](https://github.com/arjan/decorator).
For caching declaration, the abstraction provides three Elixir function
decorators: `cacheable `, `cache_evict`, and `cache_put`, which allow
functions to trigger cache population or cache eviction.
Let us take a closer look at each annotation.
> Inspired by [Spring Cache Abstraction](https://docs.spring.io/spring/docs/3.2.x/spring-framework-reference/html/cache.html).
## `cacheable` decorator
As the name implies, `cacheable` is used to demarcate functions that are
cacheable - that is, functions for whom the result is stored into the cache
so, on subsequent invocations (with the same arguments), the value in the
cache is returned without having to actually execute the function. In its
simplest form, the decorator/annotation declaration requires the name of
the cache associated with the annotated function:
@decorate cacheable(cache: Cache)
def get_account(id) do
# the logic for retrieving the account ...
end
In the snippet above, the function `get_account/1` is associated with the
cache named `Cache`. Each time the function is called, the cache is checked
to see whether the invocation has been already executed and does not have
to be repeated.
### Default Key Generation
Since caches are essentially key-value stores, each invocation of a cached
function needs to be translated into a suitable key for cache access.
Out of the box, the caching abstraction uses a simple key-generator
based on the following algorithm:
* If no params are given, return `0`.
* If only one param is given, return that param as key.
* If more than one param is given, return a key computed from the hashes
of all parameters (`:erlang.phash2(args)`).
> **IMPORTANT:** Since Nebulex v2.1.0, the default key generation implements
the algorithm described above, breaking backward compatibility with older
versions. Therefore, you may need to change your code in case of using the
default key generation.
The default key generator is provided by the cache via the callback
`c:Nebulex.Cache.__default_key_generator__/0` and it is applied only
if the option `key:` or `keys:` is not configured. By default it is
`Nebulex.Caching.SimpleKeyGenerator`. But you can change the default
key generator at compile time with the option `:default_key_generator`.
For example, one can define a cache with a default key generator like so:
defmodule MyApp.Cache do
use Nebulex.Cache,
otp_app: :my_app,
adapter: Nebulex.Adapters.Local,
default_key_generator: __MODULE__
@behaviour Nebulex.Caching.KeyGenerator
@impl true
def generate(mod, fun, args), do: :erlang.phash2({mod, fun, args})
end
The given key generator must implement the `Nebulex.Caching.KeyGenerator`
behaviour.
Also, you can provide a different key generator at any time
(overriding the default one) when using any caching annotation
through the option `:key_generator`. For example:
@decorate cache_put(cache: Cache, key_generator: MyApp.Cache.AnotherKeyGenerator)
def update_account(account) do
# the logic for updating the account ...
end
> The `:key_generator` option is available for all caching annotations.
### Custom Key Generation Declaration
Since caching is generic, it is quite likely the target functions have
various signatures that cannot be simply mapped on top of the cache
structure. This tends to become obvious when the target function has
multiple arguments out of which only some are suitable for caching
(while the rest are used only by the function logic). For example:
@decorate cacheable(cache: Cache)
def get_account(email, include_users?) do
# the logic for retrieving the account ...
end
At first glance, while the boolean argument influences the way the account
is found, it is no use for the cache.
For such cases, the `cacheable` decorator allows the user to specify the
key explicitly based on the function attributes.
@decorate cacheable(cache: Cache, key: {Account, email})
def get_account(email, include_users?) do
# the logic for retrieving the account ...
end
@decorate cacheable(cache: Cache, key: {Account, user.account_id})
def get_user_account(%User{} = user) do
# the logic for retrieving the account ...
end
It is also possible passing options to the cache, like so:
@decorate cacheable(
cache: Cache,
key: {Account, email},
opts: [ttl: 300_000]
)
def get_account(email, include_users?) do
# the logic for retrieving the account ...
end
See the **"Shared Options"** section below.
### Functions with multiple clauses
Since [decorator lib](https://github.com/arjan/decorator#functions-with-multiple-clauses)
is used, it is important to be aware of the recommendations, warns,
limitations, and so on. In this case, for functions with multiple clauses
the general advice is to create an empty function head, and call the
decorator on that head, like so:
@decorate cacheable(cache: Cache, key: email)
def get_account(email \\\\ nil)
def get_account(nil), do: nil
def get_account(email) do
# the logic for retrieving the account ...
end
## `cache_put` decorator
For cases where the cache needs to be updated without interfering with the
function execution, one can use the `cache_put` decorator. That is, the
method will always be executed and its result placed into the cache
(according to the `cache_put` options). It supports the same options as
`cacheable`.
@decorate cache_put(cache: Cache, key: {Account, acct.email})
def update_account(%Account{} = acct, attrs) do
# the logic for updating the account ...
end
Note that using `cache_put` and `cacheable` annotations on the same function
is generally discouraged because they have different behaviors. While the
latter causes the method execution to be skipped by using the cache, the
former forces the execution in order to execute a cache update. This leads
to unexpected behavior and with the exception of specific corner-cases
(such as decorators having conditions that exclude them from each other),
such declarations should be avoided.
## `cache_evict` decorator
The cache abstraction allows not just the population of a cache store but
also eviction. This process is useful for removing stale or unused data from
the cache. Opposed to `cacheable`, the decorator `cache_evict` demarcates
functions that perform cache eviction, which are functions that act as
triggers for removing data from the cache. The `cache_evict` decorator not
only allows a key to be specified, but also a set of keys. Besides, extra
options like`all_entries` which indicates whether a cache-wide eviction
needs to be performed rather than just an entry one (based on the key or
keys):
@decorate cache_evict(cache: Cache, key: {Account, email})
def delete_account_by_email(email) do
# the logic for deleting the account ...
end
@decorate cacheable(
cache: Cache,
keys: [{Account, acct.id}, {Account, acct.email}]
)
def delete_account(%Account{} = acct) do
# the logic for deleting the account ...
end
@decorate cacheable(cache: Cache, all_entries: true)
def delete_all_accounts do
# the logic for deleting all the accounts ...
end
The option `all_entries:` comes in handy when an entire cache region needs
to be cleared out - rather than evicting each entry (which would take a
long time since it is inefficient), all the entries are removed in one
operation as shown above.
## Shared Options
All three cache annotations explained previously accept the following
options:
* `:cache` - Defines what cache to use (required). Raises `ArgumentError`
if the option is not present.
* `:key` - Defines the cache access key (optional). It overrides the
`:key_generator` option. If this option is not present, a default
key is generated by the configured or default key generator.
* `:opts` - Defines the cache options that will be passed as argument
to the invoked cache function (optional).
* `:match` - Match function `(term -> boolean | {true, term})` (optional).
This function is for matching and decide whether the code-block
evaluation result is cached or not. If `true` the code-block evaluation
result is cached as it is (the default). If `{true, value}` is returned,
then the `value` is what is cached (useful to control what is meant to
be cached). Returning `false` will cause that nothing is stored in the
cache.
* `:key_generator` - The custom key generator module implementing the
`Nebulex.Caching.KeyGenerator` behaviour. If present, this option
overrides the default key generator provided by the cache, and it is
applied only if the option `key:` or `keys:` is not configured.
In other words, the option `key:` or `keys:` overrides this option.
## Putting all together
Supposing we are using `Ecto` and we want to define some cacheable functions
within the context `MyApp.Accounts`:
# The config
config :my_app, MyApp.Cache,
gc_interval: 86_400_000, #=> 1 day
backend: :shards
# The Cache
defmodule MyApp.Cache do
use Nebulex.Cache,
otp_app: :my_app,
adapter: Nebulex.Adapters.Local
end
# Some Ecto schema
defmodule MyApp.Accounts.User do
use Ecto.Schema
schema "users" do
field(:username, :string)
field(:password, :string)
field(:role, :string)
end
def changeset(user, attrs) do
user
|> cast(attrs, [:username, :password, :role])
|> validate_required([:username, :password, :role])
end
end
# Accounts context
defmodule MyApp.Accounts do
use Nebulex.Caching
alias MyApp.Accounts.User
alias MyApp.{Cache, Repo}
@ttl :timer.hours(1)
@decorate cacheable(cache: Cache, key: {User, id}, opts: [ttl: @ttl])
def get_user!(id) do
Repo.get!(User, id)
end
@decorate cacheable(
cache: Cache,
key: {User, username},
opts: [ttl: @ttl]
)
def get_user_by_username(username) do
Repo.get_by(User, [username: username])
end
@decorate cache_put(
cache: Cache,
keys: [{User, usr.id}, {User, usr.username}],
match: &match_update/1
)
def update_user(%User{} = usr, attrs) do
usr
|> User.changeset(attrs)
|> Repo.update()
end
defp match_update({:ok, usr}), do: {true, usr}
defp match_update({:error, _}), do: false
@decorate cache_evict(
cache: Cache,
keys: [{User, usr.id}, {User, usr.username}]
)
def delete_user(%User{} = usr) do
Repo.delete(usr)
end
def create_user(attrs \\\\ %{}) do
%User{}
|> User.changeset(attrs)
|> Repo.insert()
end
end
See [Cache Usage Patters Guide](http://hexdocs.pm/nebulex/cache-usage-patterns.html).
"""
use Decorator.Define, cacheable: 1, cache_evict: 1, cache_put: 1
import Nebulex.Helpers
alias Nebulex.Caching
@doc """
Provides a way of annotating functions to be cached (cacheable aspect).
The returned value by the code block is cached if it doesn't exist already
in cache, otherwise, it is returned directly from cache and the code block
is not executed.
## Options
See the "Shared options" section at the module documentation.
## Examples
defmodule MyApp.Example do
use Nebulex.Caching
alias MyApp.Cache
@ttl :timer.hours(1)
@decorate cacheable(cache: Cache, key: name)
def get_by_name(name, age) do
# your logic (maybe the loader to retrieve the value from the SoR)
end
@decorate cacheable(cache: Cache, key: age, opts: [ttl: @ttl])
def get_by_age(age) do
# your logic (maybe the loader to retrieve the value from the SoR)
end
@decorate cacheable(cache: Cache, key: clauses, match: &match_fun/1)
def all(clauses) do
# your logic (maybe the loader to retrieve the value from the SoR)
end
defp match_fun([]), do: false
defp match_fun(_), do: true
end
The **Read-through** pattern is supported by this decorator. The loader to
retrieve the value from the system-of-record (SoR) is your function's logic
and the rest is provided by the macro under-the-hood.
"""
def cacheable(attrs, block, context) do
caching_action(:cacheable, attrs, block, context)
end
@doc """
Provides a way of annotating functions to be evicted; but updating the
cached key instead of deleting it.
The content of the cache is updated without interfering with the function
execution. That is, the method would always be executed and the result
cached.
The difference between `cacheable/3` and `cache_put/3` is that `cacheable/3`
will skip running the function if the key exists in the cache, whereas
`cache_put/3` will actually run the function and then put the result in
the cache.
## Options
* `:keys` - The set of cached keys to be updated with the returned value
on function completion. It overrides `:key` and `:key_generator`
options.
See the "Shared options" section at the module documentation.
## Examples
defmodule MyApp.Example do
use Nebulex.Caching
alias MyApp.Cache
@ttl :timer.hours(1)
@decorate cache_put(cache: Cache, key: id, opts: [ttl: @ttl])
def update!(id, attrs \\\\ %{}) do
# your logic (maybe write data to the SoR)
end
@decorate cache_put(
cache: Cache,
key: id,
match: &match_fun/1,
opts: [ttl: @ttl]
)
def update(id, attrs \\\\ %{}) do
# your logic (maybe write data to the SoR)
end
@decorate cache_put(
cache: Cache,
keys: [object.name, object.id],
match: &match_fun/1,
opts: [ttl: @ttl]
)
def update_object(object) do
# your logic (maybe write data to the SoR)
end
defp match_fun({:ok, updated}), do: {true, updated}
defp match_fun({:error, _}), do: false
end
The **Write-through** pattern is supported by this decorator. Your function
provides the logic to write data to the system-of-record (SoR) and the rest
is provided by the decorator under-the-hood.
"""
def cache_put(attrs, block, context) do
caching_action(:cache_put, attrs, block, context)
end
@doc """
Provides a way of annotating functions to be evicted (eviction aspect).
On function's completion, the given key or keys (depends on the `:key` and
`:keys` options) are deleted from the cache.
## Options
* `:keys` - Defines the set of keys to be evicted from cache on function
completion. It overrides `:key` and `:key_generator` options.
* `:all_entries` - Defines if all entries must be removed on function
completion. Defaults to `false`.
* `:before_invocation` - Boolean to indicate whether the eviction should
occur after (the default) or before the function executes. The former
provides the same semantics as the rest of the annotations; once the
function completes successfully, an action (in this case eviction)
on the cache is executed. If the function does not execute (as it might
be cached) or an exception is raised, the eviction does not occur.
The latter (`before_invocation: true`) causes the eviction to occur
always, before the function is invoked; this is useful in cases where
the eviction does not need to be tied to the function outcome.
See the "Shared options" section at the module documentation.
## Examples
defmodule MyApp.Example do
use Nebulex.Caching
alias MyApp.Cache
@decorate cache_evict(cache: Cache, key: id)
def delete(id) do
# your logic (maybe write/delete data to the SoR)
end
@decorate cache_evict(cache: Cache, keys: [object.name, object.id])
def delete_object(object) do
# your logic (maybe write/delete data to the SoR)
end
@decorate cache_evict(cache: Cache, all_entries: true)
def delete_all do
# your logic (maybe write/delete data to the SoR)
end
end
The **Write-through** pattern is supported by this decorator. Your function
provides the logic to write data to the system-of-record (SoR) and the rest
is provided by the decorator under-the-hood. But in contrast with `update`
decorator, when the data is written to the SoR, the key for that value is
deleted from cache instead of updated.
"""
def cache_evict(attrs, block, context) do
caching_action(:cache_evict, attrs, block, context)
end
## Private Functions
defp caching_action(action, attrs, block, context) do
cache = attrs[:cache] || raise ArgumentError, "expected cache: to be given as argument"
match_var = attrs[:match] || quote(do: fn _ -> true end)
opts_var = attrs[:opts] || []
keygen_block = keygen_block(attrs, context)
action_block = action_block(action, block, attrs, keygen_block)
quote do
cache = unquote(cache)
opts = unquote(opts_var)
match = unquote(match_var)
unquote(action_block)
end
end
defp keygen_block(attrs, ctx) do
args =
for arg <- ctx.args do
case arg do
{:\\, _, [var, _]} -> var
var -> var
end
end
cond do
key = Keyword.get(attrs, :key) ->
quote(do: unquote(key))
keygen = Keyword.get(attrs, :key_generator) ->
quote do
unquote(keygen).generate(unquote(ctx.module), unquote(ctx.name), unquote(args))
end
true ->
quote do
cache.__default_key_generator__().generate(
unquote(ctx.module),
unquote(ctx.name),
unquote(args)
)
end
end
end
defp action_block(:cacheable, block, _attrs, keygen) do
quote do
key = unquote(keygen)
case cache.get(key, opts) do
nil -> Caching.eval_match(unquote(block), match, cache, key, opts)
val -> val
end
end
end
defp action_block(:cache_put, block, attrs, keygen) do
keys = get_keys(attrs)
key =
if is_list(keys) and length(keys) > 0,
do: {:"$keys", keys},
else: keygen
quote do
Caching.eval_match(unquote(block), match, cache, unquote(key), opts)
end
end
defp action_block(:cache_evict, block, attrs, keygen) do
before_invocation? = attrs[:before_invocation] || false
eviction = eviction_block(attrs, keygen)
if is_boolean(before_invocation?) && before_invocation? do
quote do
unquote(eviction)
unquote(block)
end
else
quote do
result = unquote(block)
unquote(eviction)
result
end
end
end
defp eviction_block(attrs, keygen) do
keys = get_keys(attrs)
all_entries? = attrs[:all_entries] || false
cond do
is_boolean(all_entries?) && all_entries? ->
quote(do: cache.delete_all())
is_list(keys) and length(keys) > 0 ->
delete_keys_block(keys)
true ->
quote(do: cache.delete(unquote(keygen)))
end
end
defp delete_keys_block(keys) do
quote do
Enum.each(unquote(keys), fn k -> if k, do: cache.delete(k) end)
end
end
defp get_keys(attrs) do
get_option(
attrs,
:keys,
"a list with at least one element",
&((is_list(&1) and length(&1) > 0) or is_nil(&1))
)
end
@doc """
This function is for internal purposes only.
**NOTE:** Workaround to avoid dialyzer warnings when using declarative
annotation-based caching via decorators.
"""
@spec eval_match(term, (term -> boolean | {true, term}), module, term, Keyword.t()) :: term
def eval_match(result, match, cache, key, opts) do
case match.(result) do
{true, value} ->
:ok = Caching.cache_put(cache, key, value, opts)
result
true ->
:ok = Caching.cache_put(cache, key, result, opts)
result
false ->
result
end
end
@doc """
Convenience function for cache_put annotation.
**NOTE:** Internal purposes only.
"""
@spec cache_put(module, {:"$keys", term} | term, term, Keyword.t()) :: :ok
def cache_put(cache, key, value, opts)
def cache_put(cache, {:"$keys", keys}, value, opts) do
entries = for k <- keys, do: {k, value}
cache.put_all(entries, opts)
end
def cache_put(cache, key, value, opts) do
cache.put(key, value, opts)
end
end
end
