defmodule Nebulex.Adapters.Partitioned do
@moduledoc ~S"""
Built-in adapter for partitioned cache topology.
## Overall features
* Partitioned cache topology (Sharding Distribution Model).
* Configurable primary storage adapter.
* Configurable Keyslot to distributed the keys across the cluster members.
* Support for transactions via Erlang global name registration facility.
* Stats support rely on the primary storage adapter.
## Partitioned Cache Topology
There are several key points to consider about a partitioned cache:
* _**Partitioned**_: The data in a distributed cache is spread out over
all the servers in such a way that no two servers are responsible for
the same piece of cached data. This means that the size of the cache
and the processing power associated with the management of the cache
can grow linearly with the size of the cluster. Also, it means that
operations against data in the cache can be accomplished with a
"single hop," in other words, involving at most one other server.
* _**Load-Balanced**_: Since the data is spread out evenly over the
servers, the responsibility for managing the data is automatically
load-balanced across the cluster.
* _**Ownership**_: Exactly one node in the cluster is responsible for each
piece of data in the cache.
* _**Point-To-Point**_: The communication for the partitioned cache is all
point-to-point, enabling linear scalability.
* _**Location Transparency**_: Although the data is spread out across
cluster nodes, the exact same API is used to access the data, and the
same behavior is provided by each of the API methods. This is called
location transparency, which means that the developer does not have to
code based on the topology of the cache, since the API and its behavior
will be the same with a local cache, a replicated cache, or a distributed
cache.
* _**Failover**_: Failover of a distributed cache involves promoting backup
data to be primary storage. When a cluster node fails, all remaining
cluster nodes determine what data each holds in backup that the failed
cluster node had primary responsible for when it died. Those data becomes
the responsibility of whatever cluster node was the backup for the data.
However, this adapter does not provide fault-tolerance implementation,
each piece of data is kept in a single node/machine (via sharding), then,
if a node fails, the data kept by this node won't be available for the
rest of the cluster members.
> Based on **"Distributed Caching Essential Lessons"** by **Cameron Purdy**
and [Coherence Partitioned Cache Service][oracle-pcs].
[oracle-pcs]: https://docs.oracle.com/cd/E13924_01/coh.340/e13819/partitionedcacheservice.htm
## Additional implementation notes
`:pg2` or `:pg` (>= OTP 23) is used under-the-hood by the adapter to manage
the cluster nodes. When the partitioned cache is started in a node, it creates
a group and joins it (the cache supervisor PID is joined to the group). Then,
when a function is invoked, the adapter picks a node from the group members,
and then the function is executed on that specific node. In the same way,
when a partitioned cache supervisor dies (the cache is stopped or killed for
some reason), the PID of that process is automatically removed from the PG
group; this is why it's recommended to use consistent hashing for distributing
the keys across the cluster nodes.
> **NOTE:** `pg2` will be replaced by `pg` in future, since the `pg2` module
is deprecated as of OTP 23 and scheduled for removal in OTP 24.
This adapter depends on a local cache adapter (primary storage), it adds
a thin layer on top of it in order to distribute requests across a group
of nodes, where is supposed the local cache is running already. However,
you don't need to define any additional cache module for the primary
storage, instead, the adapter initializes it automatically (it adds the
primary storage as part of the supervision tree) based on the given
options within the `primary_storage_adapter:` argument.
## Usage
When used, the Cache expects the `:otp_app` and `:adapter` as options.
The `:otp_app` should point to an OTP application that has the cache
configuration. For example:
defmodule MyApp.PartitionedCache do
use Nebulex.Cache,
otp_app: :my_app,
adapter: Nebulex.Adapters.Partitioned
end
Optionally, you can configure the desired primary storage adapter with the
option `:primary_storage_adapter`; defaults to `Nebulex.Adapters.Local`.
defmodule MyApp.PartitionedCache do
use Nebulex.Cache,
otp_app: :my_app,
adapter: Nebulex.Adapters.Partitioned,
primary_storage_adapter: Nebulex.Adapters.Local
end
Also, you can provide a custom keyslot function:
defmodule MyApp.PartitionedCache do
use Nebulex.Cache,
otp_app: :my_app,
adapter: Nebulex.Adapters.Partitioned,
primary_storage_adapter: Nebulex.Adapters.Local
@behaviour Nebulex.Adapter.Keyslot
@impl true
def hash_slot(key, range) do
key
|> :erlang.phash2()
|> :jchash.compute(range)
end
end
Where the configuration for the cache must be in your application environment,
usually defined in your `config/config.exs`:
config :my_app, MyApp.PartitionedCache,
keyslot: MyApp.PartitionedCache,
primary: [
gc_interval: 3_600_000,
backend: :shards
]
If your application was generated with a supervisor (by passing `--sup`
to `mix new`) you will have a `lib/my_app/application.ex` file containing
the application start callback that defines and starts your supervisor.
You just need to edit the `start/2` function to start the cache as a
supervisor on your application's supervisor:
def start(_type, _args) do
children = [
{MyApp.PartitionedCache, []},
...
]
See `Nebulex.Cache` for more information.
## Options
This adapter supports the following options and all of them can be given via
the cache configuration:
* `:primary` - The options that will be passed to the adapter associated
with the local primary storage. These options will depend on the local
adapter to use.
* `:keyslot` - Defines the module implementing `Nebulex.Adapter.Keyslot`
behaviour.
* `:task_supervisor_opts` - Start-time options passed to
`Task.Supervisor.start_link/1` when the adapter is initialized.
* `:join_timeout` - Interval time in milliseconds for joining the
running partitioned cache to the cluster. This is to ensure it is
always joined. Defaults to `:timer.seconds(180)`.
## Shared options
Almost all of the cache functions outlined in `Nebulex.Cache` module
accept the following options:
* `:timeout` - The time-out value in milliseconds for the command that
will be executed. If the timeout is exceeded, then the current process
will exit. For executing a command on remote nodes, this adapter uses
`Task.await/2` internally for receiving the result, so this option tells
how much time the adapter should wait for it. If the timeout is exceeded,
the task is shut down but the current process doesn't exit, only the
result associated with that task is skipped in the reduce phase.
## Telemetry events
This adapter emits all recommended Telemetry events, and documented
in `Nebulex.Cache` module (see **"Adapter-specific events"** section).
Since the partitioned adapter depends on the configured primary storage
adapter (local cache adapter), this one may also emit Telemetry events.
Therefore, there will be events emitted by the partitioned adapter as well
as the primary storage adapter. For example, for the cache defined before
`MyApp.PartitionedCache`, these would be the emitted events:
* `[:my_app, :partitioned_cache, :command, :start]`
* `[:my_app, :partitioned_cache, :primary, :command, :start]`
* `[:my_app, :partitioned_cache, :command, :stop]`
* `[:my_app, :partitioned_cache, :primary, :command, :stop]`
* `[:my_app, :partitioned_cache, :command, :exception]`
* `[:my_app, :partitioned_cache, :primary, :command, :exception]`
As you may notice, the telemetry prefix by default for the partitioned cache
is `[:my_app, :partitioned_cache]`, and the prefix for its primary storage
`[:my_app, :partitioned_cache, :primary]`.
See also the [Telemetry guide](http://hexdocs.pm/nebulex/telemetry.html)
for more information and examples.
## Adapter-specific telemetry events
This adapter exposes following Telemetry events:
* `telemetry_prefix ++ [:bootstrap, :started]` - Dispatched by the adapter
when the bootstrap process is started.
* Measurements: `%{system_time: non_neg_integer}`
* Metadata:
```
%{
adapter_meta: %{optional(atom) => term},
cluster_nodes: [node]
}
```
* `telemetry_prefix ++ [:bootstrap, :stopped]` - Dispatched by the adapter
when the bootstrap process is stopped.
* Measurements: `%{system_time: non_neg_integer}`
* Metadata:
```
%{
adapter_meta: %{optional(atom) => term},
cluster_nodes: [node],
reason: term
}
```
* `telemetry_prefix ++ [:bootstrap, :exit]` - Dispatched by the adapter
when the bootstrap has received an exit signal.
* Measurements: `%{system_time: non_neg_integer}`
* Metadata:
```
%{
adapter_meta: %{optional(atom) => term},
cluster_nodes: [node],
reason: term
}
```
* `telemetry_prefix ++ [:bootstrap, :joined]` - Dispatched by the adapter
when the bootstrap has joined the cache to the cluster.
* Measurements: `%{system_time: non_neg_integer}`
* Metadata:
```
%{
adapter_meta: %{optional(atom) => term},
cluster_nodes: [node]
}
```
## Stats
This adapter depends on the primary storage adapter for the stats support.
Therefore, it is important to ensure the underlying primary storage adapter
does support stats, otherwise, you may get unexpected errors.
## Extended API
This adapter provides some additional convenience functions to the
`Nebulex.Cache` API.
Retrieving the primary storage or local cache module:
MyCache.__primary__()
Retrieving the cluster nodes associated with the given cache `name`:
MyCache.nodes()
Get a cluster node based on the given `key`:
MyCache.get_node("mykey")
Joining the cache to the cluster:
MyCache.join_cluster()
Leaving the cluster (removes the cache from the cluster):
MyCache.leave_cluster()
## Caveats of partitioned adapter
For `c:Nebulex.Cache.get_and_update/3` and `c:Nebulex.Cache.update/4`,
they both have a parameter that is the anonymous function, and it is compiled
into the module where it is created, which means it necessarily doesn't exists
on remote nodes. To ensure they work as expected, you must provide functions
from modules existing in all nodes of the group.
"""
# Provide Cache Implementation
@behaviour Nebulex.Adapter
@behaviour Nebulex.Adapter.Entry
@behaviour Nebulex.Adapter.Queryable
@behaviour Nebulex.Adapter.Stats
# Inherit default transaction implementation
use Nebulex.Adapter.Transaction
# Inherit default persistence implementation
use Nebulex.Adapter.Persistence
# Inherit default keyslot implementation
use Nebulex.Adapter.Keyslot
import Nebulex.Adapter
import Nebulex.Helpers
alias Nebulex.Cache.Cluster
alias Nebulex.RPC
## Nebulex.Adapter
@impl true
defmacro __before_compile__(env) do
otp_app = Module.get_attribute(env.module, :otp_app)
opts = Module.get_attribute(env.module, :opts)
primary = Keyword.get(opts, :primary_storage_adapter, Nebulex.Adapters.Local)
quote do
defmodule Primary do
@moduledoc """
This is the cache for the primary storage.
"""
use Nebulex.Cache,
otp_app: unquote(otp_app),
adapter: unquote(primary)
end
@doc """
A convenience function for getting the primary storage cache.
"""
def __primary__, do: Primary
@doc """
A convenience function for getting the cluster nodes.
"""
def nodes do
Cluster.get_nodes(get_dynamic_cache())
end
@doc """
A convenience function to get the node of the given `key`.
"""
def get_node(key) do
with_meta(get_dynamic_cache(), fn _adapter, %{name: name, keyslot: keyslot} ->
Cluster.get_node(name, key, keyslot)
end)
end
@doc """
A convenience function for joining the cache to the cluster.
"""
def join_cluster do
Cluster.join(get_dynamic_cache())
end
@doc """
A convenience function for removing the cache from the cluster.
"""
def leave_cluster do
Cluster.leave(get_dynamic_cache())
end
end
end
@impl true
def init(opts) do
# Required options
telemetry_prefix = Keyword.fetch!(opts, :telemetry_prefix)
telemetry = Keyword.fetch!(opts, :telemetry)
cache = Keyword.fetch!(opts, :cache)
name = opts[:name] || cache
# Maybe use stats
stats = get_boolean_option(opts, :stats)
# Primary cache options
primary_opts =
Keyword.merge(
[telemetry_prefix: telemetry_prefix ++ [:primary], telemetry: telemetry, stats: stats],
Keyword.get(opts, :primary, [])
)
# Maybe put a name to primary storage
primary_opts =
if opts[:name],
do: [name: normalize_module_name([name, Primary])] ++ primary_opts,
else: primary_opts
# Keyslot module for selecting nodes
keyslot =
opts
|> get_option(:keyslot, "an atom", &is_atom/1, __MODULE__)
|> assert_behaviour(Nebulex.Adapter.Keyslot, "keyslot")
# Maybe task supervisor for distributed tasks
{task_sup_name, children} = task_sup_child_spec(name, opts)
# Prepare metadata
adapter_meta = %{
telemetry_prefix: telemetry_prefix,
telemetry: telemetry,
name: name,
primary_name: primary_opts[:name],
task_sup: task_sup_name,
keyslot: keyslot,
stats: stats
}
# Prepare child_spec
child_spec =
Nebulex.Adapters.Supervisor.child_spec(
name: normalize_module_name([name, Supervisor]),
strategy: :rest_for_one,
children: [
{cache.__primary__, primary_opts},
{__MODULE__.Bootstrap, {Map.put(adapter_meta, :cache, cache), opts}}
| children
]
)
{:ok, child_spec, adapter_meta}
end
if Code.ensure_loaded?(:erpc) do
defp task_sup_child_spec(_name, _opts) do
{nil, []}
end
else
defp task_sup_child_spec(name, opts) do
# task supervisor to execute parallel and/or remote commands
task_sup_name = normalize_module_name([name, TaskSupervisor])
task_sup_opts = Keyword.get(opts, :task_supervisor_opts, [])
children = [
{Task.Supervisor, [name: task_sup_name] ++ task_sup_opts}
]
{task_sup_name, children}
end
end
## Nebulex.Adapter.Entry
@impl true
defspan get(adapter_meta, key, opts) do
call(adapter_meta, key, :get, [key, opts], opts)
end
@impl true
defspan get_all(adapter_meta, keys, opts) do
map_reduce(
keys,
adapter_meta,
:get_all,
[opts],
Keyword.get(opts, :timeout),
{
%{},
fn
{:ok, res}, _, acc when is_map(res) ->
Map.merge(acc, res)
_, _, acc ->
acc
end
}
)
end
@impl true
defspan put(adapter_meta, key, value, _ttl, on_write, opts) do
case on_write do
:put ->
:ok = call(adapter_meta, key, :put, [key, value, opts], opts)
true
:put_new ->
call(adapter_meta, key, :put_new, [key, value, opts], opts)
:replace ->
call(adapter_meta, key, :replace, [key, value, opts], opts)
end
end
@impl true
defspan put_all(adapter_meta, entries, _ttl, on_write, opts) do
case on_write do
:put ->
do_put_all(:put_all, adapter_meta, entries, opts)
:put_new ->
do_put_all(:put_new_all, adapter_meta, entries, opts)
end
end
def do_put_all(action, adapter_meta, entries, opts) do
reducer = {
{true, []},
fn
{:ok, :ok}, {_, {_, _, [_, _, [kv, _]]}}, {bool, acc} ->
{bool, Enum.reduce(kv, acc, &[elem(&1, 0) | &2])}
{:ok, true}, {_, {_, _, [_, _, [kv, _]]}}, {bool, acc} ->
{bool, Enum.reduce(kv, acc, &[elem(&1, 0) | &2])}
{:ok, false}, _, {_, acc} ->
{false, acc}
{:error, _}, _, {_, acc} ->
{false, acc}
end
}
entries
|> map_reduce(
adapter_meta,
action,
[opts],
Keyword.get(opts, :timeout),
reducer
)
|> case do
{true, _} ->
true
{false, keys} ->
:ok = Enum.each(keys, &delete(adapter_meta, &1, []))
action == :put_all
end
end
@impl true
defspan delete(adapter_meta, key, opts) do
call(adapter_meta, key, :delete, [key, opts], opts)
end
@impl true
defspan take(adapter_meta, key, opts) do
call(adapter_meta, key, :take, [key, opts], opts)
end
@impl true
defspan has_key?(adapter_meta, key) do
call(adapter_meta, key, :has_key?, [key])
end
@impl true
defspan update_counter(adapter_meta, key, amount, _ttl, _default, opts) do
call(adapter_meta, key, :incr, [key, amount, opts], opts)
end
@impl true
defspan ttl(adapter_meta, key) do
call(adapter_meta, key, :ttl, [key])
end
@impl true
defspan expire(adapter_meta, key, ttl) do
call(adapter_meta, key, :expire, [key, ttl])
end
@impl true
defspan touch(adapter_meta, key) do
call(adapter_meta, key, :touch, [key])
end
## Nebulex.Adapter.Queryable
@impl true
defspan execute(adapter_meta, operation, query, opts) do
reducer =
case operation do
:all -> &List.flatten/1
_ -> &Enum.sum/1
end
adapter_meta.task_sup
|> RPC.multi_call(
Cluster.get_nodes(adapter_meta.name),
__MODULE__,
:with_dynamic_cache,
[adapter_meta, operation, [query, opts]],
opts
)
|> handle_rpc_multi_call(operation, reducer)
end
@impl true
defspan stream(adapter_meta, query, opts) do
Stream.resource(
fn ->
Cluster.get_nodes(adapter_meta.name)
end,
fn
[] ->
{:halt, []}
[node | nodes] ->
elements =
rpc_call(
adapter_meta.task_sup,
node,
__MODULE__,
:eval_stream,
[adapter_meta, query, opts],
opts
)
{elements, nodes}
end,
& &1
)
end
## Nebulex.Adapter.Persistence
@impl true
defspan dump(adapter_meta, path, opts) do
super(adapter_meta, path, opts)
end
@impl true
defspan load(adapter_meta, path, opts) do
super(adapter_meta, path, opts)
end
## Nebulex.Adapter.Transaction
@impl true
defspan transaction(adapter_meta, opts, fun) do
super(adapter_meta, Keyword.put(opts, :nodes, Cluster.get_nodes(adapter_meta.name)), fun)
end
@impl true
defspan in_transaction?(adapter_meta) do
super(adapter_meta)
end
## Nebulex.Adapter.Stats
@impl true
defspan stats(adapter_meta) do
with_dynamic_cache(adapter_meta, :stats, [])
end
## Helpers
@doc """
Helper function to use dynamic cache for internal primary cache storage
when needed.
"""
def with_dynamic_cache(%{cache: cache, primary_name: nil}, action, args) do
apply(cache.__primary__, action, args)
end
def with_dynamic_cache(%{cache: cache, primary_name: primary_name}, action, args) do
cache.__primary__.with_dynamic_cache(primary_name, fn ->
apply(cache.__primary__, action, args)
end)
end
@doc """
Helper to perform `stream/3` locally.
"""
def eval_stream(meta, query, opts) do
meta
|> with_dynamic_cache(:stream, [query, opts])
|> Enum.to_list()
end
## Private Functions
defp get_node(%{name: name, keyslot: keyslot}, key) do
Cluster.get_node(name, key, keyslot)
end
defp call(adapter_meta, key, action, args, opts \\ []) do
adapter_meta
|> get_node(key)
|> rpc_call(adapter_meta, action, args, opts)
end
defp rpc_call(node, %{task_sup: task_sup} = meta, fun, args, opts) do
rpc_call(task_sup, node, __MODULE__, :with_dynamic_cache, [meta, fun, args], opts)
end
if Code.ensure_loaded?(:erpc) do
defp rpc_call(supervisor, node, mod, fun, args, opts) do
RPC.call(supervisor, node, mod, fun, args, opts[:timeout] || 5000)
end
else
defp rpc_call(supervisor, node, mod, fun, args, opts) do
case RPC.call(supervisor, node, mod, fun, args, opts[:timeout] || 5000) do
{:badrpc, remote_ex} ->
raise remote_ex
response ->
response
end
end
end
defp group_keys_by_node(enum, adapter_meta) do
Enum.reduce(enum, %{}, fn
{key, _} = entry, acc ->
node = get_node(adapter_meta, key)
Map.put(acc, node, [entry | Map.get(acc, node, [])])
key, acc ->
node = get_node(adapter_meta, key)
Map.put(acc, node, [key | Map.get(acc, node, [])])
end)
end
defp map_reduce(
enum,
%{task_sup: task_sup} = meta,
action,
args,
timeout,
reducer
) do
groups =
enum
|> group_keys_by_node(meta)
|> Enum.map(fn {node, group} ->
{node, {__MODULE__, :with_dynamic_cache, [meta, action, [group | args]]}}
end)
RPC.multi_call(task_sup, groups, timeout: timeout, reducer: reducer)
end
defp handle_rpc_multi_call({res, []}, _action, fun) do
fun.(res)
end
defp handle_rpc_multi_call({responses, errors}, action, _) do
raise Nebulex.RPCMultiCallError, action: action, responses: responses, errors: errors
end
end
defmodule Nebulex.Adapters.Partitioned.Bootstrap do
@moduledoc false
use GenServer
import Nebulex.Helpers
alias Nebulex.Cache.Cluster
alias Nebulex.Telemetry
# Default join timeout
@join_timeout :timer.seconds(180)
# State
defstruct [:adapter_meta, :join_timeout]
## API
@doc false
def start_link({%{name: name}, _} = state) do
GenServer.start_link(
__MODULE__,
state,
name: normalize_module_name([name, Bootstrap])
)
end
## GenServer Callbacks
@impl true
def init({adapter_meta, opts}) do
# Trap exit signals to run cleanup job
_ = Process.flag(:trap_exit, true)
# Bootstrap started
:ok = dispatch_telemetry_event(:started, adapter_meta)
# Ensure joining the cluster when the cache supervision tree is started
:ok = Cluster.join(adapter_meta.name)
# Bootstrap joined the cache to the cluster
:ok = dispatch_telemetry_event(:joined, adapter_meta)
# Build initial state
state = build_state(adapter_meta, opts)
# Start bootstrap process
{:ok, state, state.join_timeout}
end
@impl true
def handle_info(message, state)
def handle_info(:timeout, %__MODULE__{adapter_meta: adapter_meta} = state) do
# Ensure it is always joined to the cluster
:ok = Cluster.join(adapter_meta.name)
# Bootstrap joined the cache to the cluster
:ok = dispatch_telemetry_event(:joined, adapter_meta)
{:noreply, state, state.join_timeout}
end
def handle_info({:EXIT, _from, reason}, %__MODULE__{adapter_meta: adapter_meta} = state) do
# Bootstrap received exit signal
:ok = dispatch_telemetry_event(:exit, adapter_meta, %{reason: reason})
{:stop, reason, state}
end
@impl true
def terminate(reason, %__MODULE__{adapter_meta: adapter_meta}) do
# Ensure leaving the cluster when the cache stops
:ok = Cluster.leave(adapter_meta.name)
# Bootstrap stopped or terminated
:ok = dispatch_telemetry_event(:stopped, adapter_meta, %{reason: reason})
end
## Private Functions
defp build_state(adapter_meta, opts) do
# Join timeout to ensure it is always joined to the cluster
join_timeout =
get_option(
opts,
:join_timeout,
"an integer > 0",
&(is_integer(&1) and &1 > 0),
@join_timeout
)
%__MODULE__{adapter_meta: adapter_meta, join_timeout: join_timeout}
end
defp dispatch_telemetry_event(event, adapter_meta, meta \\ %{}) do
Telemetry.execute(
adapter_meta.telemetry_prefix ++ [:bootstrap, event],
%{system_time: System.system_time()},
Map.merge(meta, %{
adapter_meta: adapter_meta,
cluster_nodes: Cluster.get_nodes(adapter_meta.name)
})
)
end
end
