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Postgres sync engine. Sync little subsets of your Postgres data into local apps and services.
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lib/electric/postgres/lock_connection.ex
defmodule Electric.Postgres.LockConnection do
@moduledoc """
A Postgres connection that ensures an advisory lock is held for its entire duration,
useful for ensuring only a single sync service instance can be using a single
replication slot at any given time.
The connection attempts to grab the lock and waits on it until it acquires it.
When it does, it fires off an :exclusive_connection_lock_acquired message to the specified
`Electric.Connection.Manager` such that the required setup can acquired now that
the service is sure to be the only one operating on this replication stream.
"""
require Logger
@behaviour Postgrex.SimpleConnection
@type option ::
{:connection_opts, Keyword.t()}
| {:connection_manager, GenServer.server()}
| {:lock_name, String.t()}
@type options :: [option]
defmodule State do
defstruct [
:connection_manager,
:lock_acquired,
:lock_name,
:backoff
]
end
@spec start_link(options()) :: {:ok, pid()} | {:error, Postgrex.Error.t() | term()}
def start_link(opts) do
{connection_opts, init_opts} = Keyword.pop(opts, :connection_opts)
Postgrex.SimpleConnection.start_link(
__MODULE__,
init_opts,
[timeout: :infinity, auto_reconnect: false] ++
Electric.Utils.deobfuscate_password(connection_opts)
)
end
@impl true
def init(opts) do
send(self(), :acquire_lock)
metadata = [
lock_name: Keyword.fetch!(opts, :lock_name),
stack_id: Keyword.fetch!(opts, :stack_id)
]
Logger.metadata(metadata)
Electric.Telemetry.Sentry.set_tags_context(metadata)
{:ok,
%State{
connection_manager: Keyword.fetch!(opts, :connection_manager),
lock_name: Keyword.fetch!(opts, :lock_name),
lock_acquired: false,
backoff: {:backoff.init(1000, 10_000), nil}
}}
end
@impl true
def handle_info(:acquire_lock, state) do
if state.lock_acquired do
notify_lock_acquired(state)
{:noreply, state}
else
Logger.info("Acquiring lock from postgres with name #{state.lock_name}")
{:query, lock_query(state), state}
end
end
def handle_info({:timeout, tref, msg}, %{backoff: {backoff, tref}} = state) do
handle_info(msg, %{state | backoff: {backoff, nil}})
end
@impl true
def handle_result([%Postgrex.Result{columns: ["pg_advisory_lock"]}], state) do
Logger.info("Lock acquired from postgres with name #{state.lock_name}")
notify_lock_acquired(state)
{:noreply, %{state | lock_acquired: true}}
end
def handle_result(%Postgrex.Error{} = error, %State{backoff: {backoff, _}} = state) do
{time, backoff} = :backoff.fail(backoff)
tref = :erlang.start_timer(time, self(), :acquire_lock)
Logger.error(
"Failed to acquire lock #{state.lock_name} with reason #{inspect(error)} - retrying in #{inspect(time)}ms."
)
{:noreply, %{state | lock_acquired: false, backoff: {backoff, tref}}}
end
defp notify_lock_acquired(%State{connection_manager: connection_manager} = _state) do
Electric.Connection.Manager.exclusive_connection_lock_acquired(connection_manager)
end
defp lock_query(%State{lock_name: name} = _state) do
"SELECT pg_advisory_lock(hashtext('#{name}'))"
end
@impl true
def notify(_channel, _payload, _state) do
:ok
end
end