#
View Source Registry (Elixir v1.15.7)

A local, decentralized and scalable key-value process storage.

It allows developers to lookup one or more processes with a given key. If the registry has `:unique` keys, a key points to 0 or 1 process. If the registry allows `:duplicate` keys, a single key may point to any number of processes. In both cases, different keys could identify the same process.

Each entry in the registry is associated to the process that has registered the key. If the process crashes, the keys associated to that process are automatically removed. All key comparisons in the registry are done using the match operation (`===/2`).

The registry can be used for different purposes, such as name lookups (using the `:via` option), storing properties, custom dispatching rules, or a pubsub implementation. We explore some of those use cases below.

The registry may also be transparently partitioned, which provides more scalable behaviour for running registries on highly concurrent environments with thousands or millions of entries.

## Using in `:via`

Once the registry is started with a given name using `Registry.start_link/1`, it can be used to register and access named processes using the `{:via, Registry, {registry, key}}` tuple:

    {:ok, _} = Registry.start_link(keys: :unique, name: Registry.ViaTest)
    name = {:via, Registry, {Registry.ViaTest, "agent"}}
    {:ok, _} = Agent.start_link(fn -> 0 end, name: name)
    Agent.get(name, & &1)
    #=> 0
    Agent.update(name, &(&1 + 1))
    Agent.get(name, & &1)
    #=> 1

In the previous example, we were not interested in associating a value to the process:

    Registry.lookup(Registry.ViaTest, "agent")
    #=> [{self(), nil}]

However, in some cases it may be desired to associate a value to the process using the alternate `{:via, Registry, {registry, key, value}}` tuple:

    {:ok, _} = Registry.start_link(keys: :unique, name: Registry.ViaTest)
    name = {:via, Registry, {Registry.ViaTest, "agent", :hello}}
    {:ok, agent_pid} = Agent.start_link(fn -> 0 end, name: name)
    Registry.lookup(Registry.ViaTest, "agent")
    #=> [{agent_pid, :hello}]

To this point, we have been starting `Registry` using `start_link/1`. Typically the registry is started as part of a supervision tree though:

    {Registry, keys: :unique, name: Registry.ViaTest}

Only registries with unique keys can be used in `:via`. If the name is already taken, the case-specific `start_link` function (`Agent.start_link/2` in the example above) will return `{:error, {:already_started, current_pid}}`.

## Using as a dispatcher

`Registry` has a dispatch mechanism that allows developers to implement custom dispatch logic triggered from the caller. For example, let's say we have a duplicate registry started as so:

    {:ok, _} = Registry.start_link(keys: :duplicate, name: Registry.DispatcherTest)

By calling `register/3`, different processes can register under a given key and associate any value under that key. In this case, let's register the current process under the key `"hello"` and attach the `{IO, :inspect}` tuple to it:

    {:ok, _} = Registry.register(Registry.DispatcherTest, "hello", {IO, :inspect})

Now, an entity interested in dispatching events for a given key may call `dispatch/3` passing in the key and a callback. This callback will be invoked with a list of all the values registered under the requested key, alongside the PID of the process that registered each value, in the form of `{pid, value}` tuples. In our example, `value` will be the `{module, function}` tuple in the code above:

    Registry.dispatch(Registry.DispatcherTest, "hello", fn entries ->
      for {pid, {module, function}} <- entries, do: apply(module, function, [pid])
    end)
    # Prints #PID<...> where the PID is for the process that called register/3 above
    #=> :ok

Dispatching happens in the process that calls `dispatch/3` either serially or concurrently in case of multiple partitions (via spawned tasks). The registered processes are not involved in dispatching unless involving them is done explicitly (for example, by sending them a message in the callback).

Furthermore, if there is a failure when dispatching, due to a bad registration, dispatching will always fail and the registered process will not be notified. Therefore let's make sure we at least wrap and report those errors:

    require Logger

    Registry.dispatch(Registry.DispatcherTest, "hello", fn entries ->
      for {pid, {module, function}} <- entries do
        try do
          apply(module, function, [pid])
        catch
          kind, reason ->
            formatted = Exception.format(kind, reason, __STACKTRACE__)
            Logger.error("Registry.dispatch/3 failed with #{formatted}")
        end
      end
    end)
    # Prints #PID<...>
    #=> :ok

You could also replace the whole `apply` system by explicitly sending messages. That's the example we will see next.

## Using as a PubSub

Registries can also be used to implement a local, non-distributed, scalable PubSub by relying on the `dispatch/3` function, similarly to the previous section: in this case, however, we will send messages to each associated process, instead of invoking a given module-function.

In this example, we will also set the number of partitions to the number of schedulers online, which will make the registry more performant on highly concurrent environments:

    {:ok, _} =
      Registry.start_link(
        keys: :duplicate,
        name: Registry.PubSubTest,
        partitions: System.schedulers_online()
      )

    {:ok, _} = Registry.register(Registry.PubSubTest, "hello", [])

    Registry.dispatch(Registry.PubSubTest, "hello", fn entries ->
      for {pid, _} <- entries, do: send(pid, {:broadcast, "world"})
    end)
    #=> :ok

The example above broadcasted the message `{:broadcast, "world"}` to all processes registered under the "topic" (or "key" as we called it until now) `"hello"`.

The third argument given to `register/3` is a value associated to the current process. While in the previous section we used it when dispatching, in this particular example we are not interested in it, so we have set it to an empty list. You could store a more meaningful value if necessary.

## Registrations

Looking up, dispatching and registering are efficient and immediate at the cost of delayed unsubscription. For example, if a process crashes, its keys are automatically removed from the registry but the change may not propagate immediately. This means certain operations may return processes that are already dead. When such may happen, it will be explicitly stated in the function documentation.

However, keep in mind those cases are typically not an issue. After all, a process referenced by a PID may crash at any time, including between getting the value from the registry and sending it a message. Many parts of the standard library are designed to cope with that, such as `Process.monitor/1` which will deliver the `:DOWN` message immediately if the monitored process is already dead and `send/2` which acts as a no-op for dead processes.

## ETS

Note that the registry uses one ETS table plus two ETS tables per partition.

# Summary

## Types

body()

A pattern used to representing the output format part of a match spec

guard()

A guard to be evaluated when matching on objects in a registry

guards()

A list of guards to be evaluated when matching on objects in a registry

key()

The type of keys allowed on registration

keys()

The type of the registry

listener_message()

The message that the registry sends to listeners when a process registers or unregisters.

match_pattern()

A pattern to match on objects in a registry

meta_key()

The type of registry metadata keys

meta_value()

The type of registry metadata values

registry()

The registry identifier

spec()

A full match spec used when selecting objects in the registry

start_option()

Options used for `child_spec/1` and `start_link/1`

value()

The type of values allowed on registration

## Functions

child_spec(options)

Returns a specification to start a registry under a supervisor.

count(registry)

Returns the number of registered keys in a registry. It runs in constant time.

count_match(registry, key, pattern, guards \\ [])

Returns the number of `{pid, value}` pairs under the given `key` in `registry` that match `pattern`.

count_select(registry, spec)

Works like `select/2`, but only returns the number of matching records.

delete_meta(registry, key)

Deletes registry metadata for the given `key` in `registry`.

dispatch(registry, key, mfa_or_fun, opts \\ [])

Invokes the callback with all entries under `key` in each partition for the given `registry`.

keys(registry, pid)

Returns the known keys for the given `pid` in `registry` in no particular order.

lookup(registry, key)

Finds the `{pid, value}` pair for the given `key` in `registry` in no particular order.

match(registry, key, pattern, guards \\ [])

Returns `{pid, value}` pairs under the given `key` in `registry` that match `pattern`.

meta(registry, key)

Reads registry metadata given on `start_link/1`.

put_meta(registry, key, value)

Stores registry metadata.

register(registry, key, value)

Registers the current process under the given `key` in `registry`.

select(registry, spec)

Select key, pid, and values registered using full match specs.

start_link(options)

Starts the registry as a supervisor process.

unregister(registry, key)

Unregisters all entries for the given `key` associated to the current process in `registry`.

unregister_match(registry, key, pattern, guards \\ [])

Unregisters entries for keys matching a pattern associated to the current process in `registry`.

update_value(registry, key, callback)

Updates the value for `key` for the current process in the unique `registry`.

values(registry, key, pid)

Reads the values for the given `key` for `pid` in `registry`.

# Types

Link to this type

# body()

View Source

    @type body() :: [term()]

A pattern used to representing the output format part of a match spec

Link to this type

# guard()

View Source

    @type guard() :: atom() | tuple()

A guard to be evaluated when matching on objects in a registry

Link to this type

# guards()

View Source

    @type guards() :: [guard()]

A list of guards to be evaluated when matching on objects in a registry

Link to this type

# key()

View Source

    @type key() :: term()

The type of keys allowed on registration

Link to this type

# keys()

View Source

    @type keys() :: :unique | :duplicate

The type of the registry

Link to this type

# listener_message()

View Source (since 1.15.0)

    @type listener_message() ::
      {:register, registry(), key(), registry_partition :: pid(), value()}
      | {:unregister, registry(), key(), registry_partition :: pid()}

The message that the registry sends to listeners when a process registers or unregisters.

See the `:listeners` option in `start_link/1`.

Link to this type

# match_pattern()

View Source

    @type match_pattern() :: atom() | term()

A pattern to match on objects in a registry

Link to this type

# meta_key()

View Source

    @type meta_key() :: atom() | tuple()

The type of registry metadata keys

Link to this type

# meta_value()

View Source

    @type meta_value() :: term()

The type of registry metadata values

Link to this type

# registry()

View Source

    @type registry() :: atom()

The registry identifier

Link to this type

# spec()

View Source

    @type spec() :: [{match_pattern(), guards(), body()}]

A full match spec used when selecting objects in the registry

Link to this type

# start_option()

View Source

    @type start_option() ::
      {:keys, keys()}
      | {:name, registry()}
      | {:partitions, pos_integer()}
      | {:listeners, [atom()]}
      | {:meta, [{meta_key(), meta_value()}]}

Options used for `child_spec/1` and `start_link/1`

Link to this type

# value()

View Source

    @type value() :: term()

The type of values allowed on registration

# Functions

Link to this function

# child_spec(options)

View Source (since 1.5.0)

    @spec child_spec([start_option()]) :: Supervisor.child_spec()

Returns a specification to start a registry under a supervisor.

See `Supervisor`.

Link to this function

# count(registry)

View Source (since 1.7.0)

    @spec count(registry()) :: non_neg_integer()

Returns the number of registered keys in a registry. It runs in constant time.

## Examples

In the example below we register the current process and ask for the number of keys in the registry:

    iex> Registry.start_link(keys: :unique, name: Registry.UniqueCountTest)
    iex> Registry.count(Registry.UniqueCountTest)
    0
    iex> {:ok, _} = Registry.register(Registry.UniqueCountTest, "hello", :world)
    iex> {:ok, _} = Registry.register(Registry.UniqueCountTest, "world", :world)
    iex> Registry.count(Registry.UniqueCountTest)
    2

The same applies to duplicate registries:

    iex> Registry.start_link(keys: :duplicate, name: Registry.DuplicateCountTest)
    iex> Registry.count(Registry.DuplicateCountTest)
    0
    iex> {:ok, _} = Registry.register(Registry.DuplicateCountTest, "hello", :world)
    iex> {:ok, _} = Registry.register(Registry.DuplicateCountTest, "hello", :world)
    iex> Registry.count(Registry.DuplicateCountTest)
    2

Link to this function

# count_match(registry, key, pattern, guards \\ [])

View Source (since 1.7.0)

    @spec count_match(registry(), key(), match_pattern(), guards()) :: non_neg_integer()

Returns the number of `{pid, value}` pairs under the given `key` in `registry` that match `pattern`.

Pattern must be an atom or a tuple that will match the structure of the value stored in the registry. The atom `:_` can be used to ignore a given value or tuple element, while the atom `:"$1"` can be used to temporarily assign part of pattern to a variable for a subsequent comparison.

Optionally, it is possible to pass a list of guard conditions for more precise matching. Each guard is a tuple, which describes checks that should be passed by assigned part of pattern. For example the `$1 > 1` guard condition would be expressed as the `{:>, :"$1", 1}` tuple. Please note that guard conditions will work only for assigned variables like `:"$1"`, `:"$2"`, and so forth. Avoid usage of special match variables `:"$_"` and `:"$$"`, because it might not work as expected.

Zero will be returned if there is no match.

For unique registries, a single partition lookup is necessary. For duplicate registries, all partitions must be looked up.

## Examples

In the example below we register the current process under the same key in a duplicate registry but with different values:

    iex> Registry.start_link(keys: :duplicate, name: Registry.CountMatchTest)
    iex> {:ok, _} = Registry.register(Registry.CountMatchTest, "hello", {1, :atom, 1})
    iex> {:ok, _} = Registry.register(Registry.CountMatchTest, "hello", {2, :atom, 2})
    iex> Registry.count_match(Registry.CountMatchTest, "hello", {1, :_, :_})
    1
    iex> Registry.count_match(Registry.CountMatchTest, "hello", {2, :_, :_})
    1
    iex> Registry.count_match(Registry.CountMatchTest, "hello", {:_, :atom, :_})
    2
    iex> Registry.count_match(Registry.CountMatchTest, "hello", {:"$1", :_, :"$1"})
    2
    iex> Registry.count_match(Registry.CountMatchTest, "hello", {:_, :_, :"$1"}, [{:>, :"$1", 1}])
    1
    iex> Registry.count_match(Registry.CountMatchTest, "hello", {:_, :"$1", :_}, [{:is_atom, :"$1"}])
    2

Link to this function

# count_select(registry, spec)

View Source (since 1.14.0)

    @spec count_select(registry(), spec()) :: non_neg_integer()

Works like `select/2`, but only returns the number of matching records.

## Examples

In the example below we register the current process under different keys in a unique registry but with the same value:

    iex> Registry.start_link(keys: :unique, name: Registry.CountSelectTest)
    iex> {:ok, _} = Registry.register(Registry.CountSelectTest, "hello", :value)
    iex> {:ok, _} = Registry.register(Registry.CountSelectTest, "world", :value)
    iex> Registry.count_select(Registry.CountSelectTest, [{{:_, :_, :value}, [], [true]}])
    2

Link to this function

# delete_meta(registry, key)

View Source (since 1.11.0)

    @spec delete_meta(registry(), meta_key()) :: :ok

Deletes registry metadata for the given `key` in `registry`.

## Examples

    iex> Registry.start_link(keys: :unique, name: Registry.DeleteMetaTest)
    iex> Registry.put_meta(Registry.DeleteMetaTest, :custom_key, "custom_value")
    :ok
    iex> Registry.meta(Registry.DeleteMetaTest, :custom_key)
    {:ok, "custom_value"}
    iex> Registry.delete_meta(Registry.DeleteMetaTest, :custom_key)
    :ok
    iex> Registry.meta(Registry.DeleteMetaTest, :custom_key)
    :error

Link to this function

# dispatch(registry, key, mfa_or_fun, opts \\ [])

View Source (since 1.4.0)

    @spec dispatch(registry(), key(), dispatcher, keyword()) :: :ok
    when dispatcher:
           (entries :: [{pid(), value()}] -> term()) | {module(), atom(), [any()]}

Invokes the callback with all entries under `key` in each partition for the given `registry`.

The list of `entries` is a non-empty list of two-element tuples where the first element is the PID and the second element is the value associated to the PID. If there are no entries for the given key, the callback is never invoked.

If the registry is partitioned, the callback is invoked multiple times per partition. If the registry is partitioned and `parallel: true` is given as an option, the dispatching happens in parallel. In both cases, the callback is only invoked if there are entries for that partition.

See the module documentation for examples of using the `dispatch/3` function for building custom dispatching or a pubsub system.

Link to this function

# keys(registry, pid)

View Source (since 1.4.0)

    @spec keys(registry(), pid()) :: [key()]

Returns the known keys for the given `pid` in `registry` in no particular order.

If the registry is unique, the keys are unique. Otherwise they may contain duplicates if the process was registered under the same key multiple times. The list will be empty if the process is dead or it has no keys in this registry.

## Examples

Registering under a unique registry does not allow multiple entries:

    iex> Registry.start_link(keys: :unique, name: Registry.UniqueKeysTest)
    iex> Registry.keys(Registry.UniqueKeysTest, self())
    []
    iex> {:ok, _} = Registry.register(Registry.UniqueKeysTest, "hello", :world)
    iex> Registry.register(Registry.UniqueKeysTest, "hello", :later) # registry is :unique
    {:error, {:already_registered, self()}}
    iex> Registry.keys(Registry.UniqueKeysTest, self())
    ["hello"]

Such is possible for duplicate registries though:

    iex> Registry.start_link(keys: :duplicate, name: Registry.DuplicateKeysTest)
    iex> Registry.keys(Registry.DuplicateKeysTest, self())
    []
    iex> {:ok, _} = Registry.register(Registry.DuplicateKeysTest, "hello", :world)
    iex> {:ok, _} = Registry.register(Registry.DuplicateKeysTest, "hello", :world)
    iex> Registry.keys(Registry.DuplicateKeysTest, self())
    ["hello", "hello"]

Link to this function

# lookup(registry, key)

View Source (since 1.4.0)

    @spec lookup(registry(), key()) :: [{pid(), value()}]

Finds the `{pid, value}` pair for the given `key` in `registry` in no particular order.

An empty list if there is no match.

For unique registries, a single partition lookup is necessary. For duplicate registries, all partitions must be looked up.

## Examples

In the example below we register the current process and look it up both from itself and other processes:

    iex> Registry.start_link(keys: :unique, name: Registry.UniqueLookupTest)
    iex> Registry.lookup(Registry.UniqueLookupTest, "hello")
    []
    iex> {:ok, _} = Registry.register(Registry.UniqueLookupTest, "hello", :world)
    iex> Registry.lookup(Registry.UniqueLookupTest, "hello")
    [{self(), :world}]
    iex> Task.async(fn -> Registry.lookup(Registry.UniqueLookupTest, "hello") end) |> Task.await()
    [{self(), :world}]

The same applies to duplicate registries:

    iex> Registry.start_link(keys: :duplicate, name: Registry.DuplicateLookupTest)
    iex> Registry.lookup(Registry.DuplicateLookupTest, "hello")
    []
    iex> {:ok, _} = Registry.register(Registry.DuplicateLookupTest, "hello", :world)
    iex> Registry.lookup(Registry.DuplicateLookupTest, "hello")
    [{self(), :world}]
    iex> {:ok, _} = Registry.register(Registry.DuplicateLookupTest, "hello", :another)
    iex> Enum.sort(Registry.lookup(Registry.DuplicateLookupTest, "hello"))
    [{self(), :another}, {self(), :world}]

Link to this function

# match(registry, key, pattern, guards \\ [])

View Source (since 1.4.0)

    @spec match(registry(), key(), match_pattern(), guards()) :: [{pid(), term()}]

Returns `{pid, value}` pairs under the given `key` in `registry` that match `pattern`.

Pattern must be an atom or a tuple that will match the structure of the value stored in the registry. The atom `:_` can be used to ignore a given value or tuple element, while the atom `:"$1"` can be used to temporarily assign part of pattern to a variable for a subsequent comparison.

Optionally, it is possible to pass a list of guard conditions for more precise matching. Each guard is a tuple, which describes checks that should be passed by assigned part of pattern. For example the `$1 > 1` guard condition would be expressed as the `{:>, :"$1", 1}` tuple. Please note that guard conditions will work only for assigned variables like `:"$1"`, `:"$2"`, and so forth. Avoid usage of special match variables `:"$_"` and `:"$$"`, because it might not work as expected.

An empty list will be returned if there is no match.

For unique registries, a single partition lookup is necessary. For duplicate registries, all partitions must be looked up.

## Examples

In the example below we register the current process under the same key in a duplicate registry but with different values:

    iex> Registry.start_link(keys: :duplicate, name: Registry.MatchTest)
    iex> {:ok, _} = Registry.register(Registry.MatchTest, "hello", {1, :atom, 1})
    iex> {:ok, _} = Registry.register(Registry.MatchTest, "hello", {2, :atom, 2})
    iex> Registry.match(Registry.MatchTest, "hello", {1, :_, :_})
    [{self(), {1, :atom, 1}}]
    iex> Registry.match(Registry.MatchTest, "hello", {2, :_, :_})
    [{self(), {2, :atom, 2}}]
    iex> Registry.match(Registry.MatchTest, "hello", {:_, :atom, :_}) |> Enum.sort()
    [{self(), {1, :atom, 1}}, {self(), {2, :atom, 2}}]
    iex> Registry.match(Registry.MatchTest, "hello", {:"$1", :_, :"$1"}) |> Enum.sort()
    [{self(), {1, :atom, 1}}, {self(), {2, :atom, 2}}]
    iex> guards = [{:>, :"$1", 1}]
    iex> Registry.match(Registry.MatchTest, "hello", {:_, :_, :"$1"}, guards)
    [{self(), {2, :atom, 2}}]
    iex> guards = [{:is_atom, :"$1"}]
    iex> Registry.match(Registry.MatchTest, "hello", {:_, :"$1", :_}, guards) |> Enum.sort()
    [{self(), {1, :atom, 1}}, {self(), {2, :atom, 2}}]

Link to this function

# meta(registry, key)

View Source (since 1.4.0)

    @spec meta(registry(), meta_key()) :: {:ok, meta_value()} | :error

Reads registry metadata given on `start_link/1`.

Atoms and tuples are allowed as keys.

## Examples

    iex> Registry.start_link(keys: :unique, name: Registry.MetaTest, meta: [custom_key: "custom_value"])
    iex> Registry.meta(Registry.MetaTest, :custom_key)
    {:ok, "custom_value"}
    iex> Registry.meta(Registry.MetaTest, :unknown_key)
    :error

Link to this function

# put_meta(registry, key, value)

View Source (since 1.4.0)

    @spec put_meta(registry(), meta_key(), meta_value()) :: :ok

Stores registry metadata.

Atoms and tuples are allowed as keys.

## Examples

    iex> Registry.start_link(keys: :unique, name: Registry.PutMetaTest)
    iex> Registry.put_meta(Registry.PutMetaTest, :custom_key, "custom_value")
    :ok
    iex> Registry.meta(Registry.PutMetaTest, :custom_key)
    {:ok, "custom_value"}
    iex> Registry.put_meta(Registry.PutMetaTest, {:tuple, :key}, "tuple_value")
    :ok
    iex> Registry.meta(Registry.PutMetaTest, {:tuple, :key})
    {:ok, "tuple_value"}

Link to this function

# register(registry, key, value)

View Source (since 1.4.0)

    @spec register(registry(), key(), value()) ::
      {:ok, pid()} | {:error, {:already_registered, pid()}}

Registers the current process under the given `key` in `registry`.

A value to be associated with this registration must also be given. This value will be retrieved whenever dispatching or doing a key lookup.

This function returns `{:ok, owner}` or `{:error, reason}`. The `owner` is the PID in the registry partition responsible for the PID. The owner is automatically linked to the caller.

If the registry has unique keys, it will return `{:ok, owner}` unless the key is already associated to a PID, in which case it returns `{:error, {:already_registered, pid}}`.

If the registry has duplicate keys, multiple registrations from the current process under the same key are allowed.

If the registry has listeners specified via the `:listeners` option in `start_link/1`, those listeners will be notified of the registration and will receive a message of type `listener_message/0`.

## Examples

Registering under a unique registry does not allow multiple entries:

    iex> Registry.start_link(keys: :unique, name: Registry.UniqueRegisterTest)
    iex> {:ok, _} = Registry.register(Registry.UniqueRegisterTest, "hello", :world)
    iex> Registry.register(Registry.UniqueRegisterTest, "hello", :later)
    {:error, {:already_registered, self()}}
    iex> Registry.keys(Registry.UniqueRegisterTest, self())
    ["hello"]

Such is possible for duplicate registries though:

    iex> Registry.start_link(keys: :duplicate, name: Registry.DuplicateRegisterTest)
    iex> {:ok, _} = Registry.register(Registry.DuplicateRegisterTest, "hello", :world)
    iex> {:ok, _} = Registry.register(Registry.DuplicateRegisterTest, "hello", :world)
    iex> Registry.keys(Registry.DuplicateRegisterTest, self())
    ["hello", "hello"]

Link to this function

# select(registry, spec)

View Source (since 1.9.0)

    @spec select(registry(), spec()) :: [term()]

Select key, pid, and values registered using full match specs.

The `spec` consists of a list of three part tuples, in the shape of `[{match_pattern, guards, body}]`.

The first part, the match pattern, must be a tuple that will match the structure of the the data stored in the registry, which is `{key, pid, value}`. The atom `:_` can be used to ignore a given value or tuple element, while the atom `:"$1"` can be used to temporarily assign part of pattern to a variable for a subsequent comparison. This can be combined like `{:"$1", :_, :_}`.

The second part, the guards, is a list of conditions that allow filtering the results. Each guard is a tuple, which describes checks that should be passed by assigned part of pattern. For example the `$1 > 1` guard condition would be expressed as the `{:>, :"$1", 1}` tuple. Please note that guard conditions will work only for assigned variables like `:"$1"`, `:"$2"`, and so forth.

The third part, the body, is a list of shapes of the returned entries. Like guards, you have access to assigned variables like `:"$1"`, which you can combine with hardcoded values to freely shape entries Note that tuples have to be wrapped in an additional tuple. To get a result format like `%{key: key, pid: pid, value: value}`, assuming you bound those variables in order in the match part, you would provide a body like `[%{key: :"$1", pid: :"$2", value: :"$3"}]`. Like guards, you can use some operations like `:element` to modify the output format.

Do not use special match variables `:"$_"` and `:"$$"`, because they might not work as expected.

Note that for large registries with many partitions this will be costly as it builds the result by concatenating all the partitions.

## Examples

This example shows how to get everything from the registry:

    iex> Registry.start_link(keys: :unique, name: Registry.SelectAllTest)
    iex> {:ok, _} = Registry.register(Registry.SelectAllTest, "hello", :value)
    iex> {:ok, _} = Registry.register(Registry.SelectAllTest, "world", :value)
    iex> Registry.select(Registry.SelectAllTest, [{{:"$1", :"$2", :"$3"}, [], [{{:"$1", :"$2", :"$3"}}]}])
    [{"world", self(), :value}, {"hello", self(), :value}]

Get all keys in the registry:

    iex> Registry.start_link(keys: :unique, name: Registry.SelectAllTest)
    iex> {:ok, _} = Registry.register(Registry.SelectAllTest, "hello", :value)
    iex> {:ok, _} = Registry.register(Registry.SelectAllTest, "world", :value)
    iex> Registry.select(Registry.SelectAllTest, [{{:"$1", :_, :_}, [], [:"$1"]}])
    ["world", "hello"]

Link to this function

# start_link(options)

View Source (since 1.5.0)

    @spec start_link([start_option()]) :: {:ok, pid()} | {:error, term()}

Starts the registry as a supervisor process.

Manually it can be started as:

    Registry.start_link(keys: :unique, name: MyApp.Registry)

In your supervisor tree, you would write:

    Supervisor.start_link([
      {Registry, keys: :unique, name: MyApp.Registry}
    ], strategy: :one_for_one)

For intensive workloads, the registry may also be partitioned (by specifying the `:partitions` option). If partitioning is required then a good default is to set the number of partitions to the number of schedulers available:

    Registry.start_link(
      keys: :unique,
      name: MyApp.Registry,
      partitions: System.schedulers_online()
    )

or:

    Supervisor.start_link([
      {Registry, keys: :unique, name: MyApp.Registry, partitions: System.schedulers_online()}
    ], strategy: :one_for_one)

## Options

The registry requires the following keys:

- `:keys` - chooses if keys are `:unique` or `:duplicate`
- `:name` - the name of the registry and its tables

The following keys are optional:

- `:partitions` - the number of partitions in the registry. Defaults to `1`.
- `:listeners` - a list of named processes which are notified of register and unregister events. The registered process must be monitored by the listener if the listener wants to be notified if the registered process crashes. Messages sent to listeners are of type `listener_message/0`.
- `:meta` - a keyword list of metadata to be attached to the registry.

Link to this function

# unregister(registry, key)

View Source (since 1.4.0)

    @spec unregister(registry(), key()) :: :ok

Unregisters all entries for the given `key` associated to the current process in `registry`.

Always returns `:ok` and automatically unlinks the current process from the owner if there are no more keys associated to the current process. See also `register/3` to read more about the "owner".

If the registry has listeners specified via the `:listeners` option in `start_link/1`, those listeners will be notified of the unregistration and will receive a message of type `listener_message/0`.

## Examples

For unique registries:

    iex> Registry.start_link(keys: :unique, name: Registry.UniqueUnregisterTest)
    iex> Registry.register(Registry.UniqueUnregisterTest, "hello", :world)
    iex> Registry.keys(Registry.UniqueUnregisterTest, self())
    ["hello"]
    iex> Registry.unregister(Registry.UniqueUnregisterTest, "hello")
    :ok
    iex> Registry.keys(Registry.UniqueUnregisterTest, self())
    []

For duplicate registries:

    iex> Registry.start_link(keys: :duplicate, name: Registry.DuplicateUnregisterTest)
    iex> Registry.register(Registry.DuplicateUnregisterTest, "hello", :world)
    iex> Registry.register(Registry.DuplicateUnregisterTest, "hello", :world)
    iex> Registry.keys(Registry.DuplicateUnregisterTest, self())
    ["hello", "hello"]
    iex> Registry.unregister(Registry.DuplicateUnregisterTest, "hello")
    :ok
    iex> Registry.keys(Registry.DuplicateUnregisterTest, self())
    []

Link to this function

# unregister_match(registry, key, pattern, guards \\ [])

View Source (since 1.5.0)

    @spec unregister_match(registry(), key(), match_pattern(), guards()) :: :ok

Unregisters entries for keys matching a pattern associated to the current process in `registry`.

## Examples

For unique registries it can be used to conditionally unregister a key on the basis of whether or not it matches a particular value.

    iex> Registry.start_link(keys: :unique, name: Registry.UniqueUnregisterMatchTest)
    iex> Registry.register(Registry.UniqueUnregisterMatchTest, "hello", :world)
    iex> Registry.keys(Registry.UniqueUnregisterMatchTest, self())
    ["hello"]
    iex> Registry.unregister_match(Registry.UniqueUnregisterMatchTest, "hello", :foo)
    :ok
    iex> Registry.keys(Registry.UniqueUnregisterMatchTest, self())
    ["hello"]
    iex> Registry.unregister_match(Registry.UniqueUnregisterMatchTest, "hello", :world)
    :ok
    iex> Registry.keys(Registry.UniqueUnregisterMatchTest, self())
    []

For duplicate registries:

    iex> Registry.start_link(keys: :duplicate, name: Registry.DuplicateUnregisterMatchTest)
    iex> Registry.register(Registry.DuplicateUnregisterMatchTest, "hello", :world_a)
    iex> Registry.register(Registry.DuplicateUnregisterMatchTest, "hello", :world_b)
    iex> Registry.register(Registry.DuplicateUnregisterMatchTest, "hello", :world_c)
    iex> Registry.keys(Registry.DuplicateUnregisterMatchTest, self())
    ["hello", "hello", "hello"]
    iex> Registry.unregister_match(Registry.DuplicateUnregisterMatchTest, "hello", :world_a)
    :ok
    iex> Registry.keys(Registry.DuplicateUnregisterMatchTest, self())
    ["hello", "hello"]
    iex> Registry.lookup(Registry.DuplicateUnregisterMatchTest, "hello")
    [{self(), :world_b}, {self(), :world_c}]

Link to this function

# update_value(registry, key, callback)

View Source (since 1.4.0)

    @spec update_value(registry(), key(), (value() -> value())) ::
      {new_value :: term(), old_value :: term()} | :error

Updates the value for `key` for the current process in the unique `registry`.

Returns a `{new_value, old_value}` tuple or `:error` if there is no such key assigned to the current process.

If a non-unique registry is given, an error is raised.

## Examples

    iex> Registry.start_link(keys: :unique, name: Registry.UpdateTest)
    iex> {:ok, _} = Registry.register(Registry.UpdateTest, "hello", 1)
    iex> Registry.lookup(Registry.UpdateTest, "hello")
    [{self(), 1}]
    iex> Registry.update_value(Registry.UpdateTest, "hello", &(&1 + 1))
    {2, 1}
    iex> Registry.lookup(Registry.UpdateTest, "hello")
    [{self(), 2}]

Link to this function

# values(registry, key, pid)

View Source (since 1.12.0)

    @spec values(registry(), key(), pid()) :: [value()]

Reads the values for the given `key` for `pid` in `registry`.

For unique registries, it is either an empty list or a list with a single element. For duplicate registries, it is a list with zero, one, or multiple elements.

## Examples

In the example below we register the current process and look it up both from itself and other processes:

    iex> Registry.start_link(keys: :unique, name: Registry.UniqueLookupTest)
    iex> Registry.values(Registry.UniqueLookupTest, "hello", self())
    []
    iex> {:ok, _} = Registry.register(Registry.UniqueLookupTest, "hello", :world)
    iex> Registry.values(Registry.UniqueLookupTest, "hello", self())
    [:world]
    iex> Task.async(fn -> Registry.values(Registry.UniqueLookupTest, "hello", self()) end) |> Task.await()
    []
    iex> parent = self()
    iex> Task.async(fn -> Registry.values(Registry.UniqueLookupTest, "hello", parent) end) |> Task.await()
    [:world]

The same applies to duplicate registries:

    iex> Registry.start_link(keys: :duplicate, name: Registry.DuplicateLookupTest)
    iex> Registry.values(Registry.DuplicateLookupTest, "hello", self())
    []
    iex> {:ok, _} = Registry.register(Registry.DuplicateLookupTest, "hello", :world)
    iex> Registry.values(Registry.DuplicateLookupTest, "hello", self())
    [:world]
    iex> {:ok, _} = Registry.register(Registry.DuplicateLookupTest, "hello", :another)
    iex> Enum.sort(Registry.values(Registry.DuplicateLookupTest, "hello", self()))
    [:another, :world]