walex

Postgres change events (CDC) in Elixir

Programming language: Elixir

License: MIT License

Tags: ORM And Datamapping    

README

WalEx

Postgres Change Data Capture (CDC) in Elixir.

WalEx allows you to listen to change events on your Postgres tables then perform callback-like actions with the data. For example:

• Stream database changes to an external data processing service
• Send a user a welcome email after they create a new account
• Augment an existing Postgres-backed application with business logic
• Send events to third party services (analytics, CRM, Zapier/n8n)
• Update index / invalidate cache whenever a record is changed

You can learn more about CDC and what you can do with it here: Why capture changes?

Credit

This library steals liberally from realtime from Supabase, which in turn draws heavily on cainophile.

Installation

If available in Hex, the package can be installed by adding walex to your list of dependencies in mix.exs:

def deps do
  [
    {:walex, "~> 0.7.1"}
  ]
end

PostgreSQL Configuration

Logical Replication

WalEx only supports PostgreSQL. To get started, you first need to configure PostgreSQL for logical replication:

ALTER SYSTEM SET wal_level = 'logical';

Docker Compose:

command: [ "postgres", "-c", "wal_level=logical" ]

Publication

When you change the wal_level variable, you'll need to restart your PostgreSQL server. Once you've restarted, go ahead and create a publication for the tables you want to receive changes for:

All tables:

CREATE PUBLICATION events FOR ALL TABLES;

Specific tables:

CREATE PUBLICATION events FOR TABLE user_account, todo;

Replica Identity

WalEx supports all of the settings for REPLICA IDENTITY. Use FULL if you can use it, as it will make tracking differences easier as the old data will be sent alongside the new data. You'll need to set this for each table.

Specific tables:

ALTER TABLE user_account REPLICA IDENTITY FULL;
ALTER TABLE todo REPLICA IDENTITY FULL;

AWS RDS

Amazon (AWS) RDS Postgres allows you to configure logical replication.

• https://debezium.io/documentation/reference/1.4/connectors/postgresql.html#setting-up-postgresql
• https://dev.to/vumdao/how-to-change-rds-postgresql-configurations-2kmk

When creating a new Postgres database on RDS, you'll need to set a Paramater Group with the following settings:

rds.replication = 1
max_replication_slots = 5
max_slot_wal_keep_size = 2048

Usage

Config:

# config.exs

config :walex,
  db_host: "localhost",
  db_user: "postgres",
  db_name: "postgres",
  db_password: "postgres",
  db_port: "5432",
  db_ssl: true,
  db_ip_version: "ipv6",
  # :temporary atom is also supported if you don't want Postgres keeping track of what you've acknowledged
  slot_name:  "example",
  max_replication_lag_in_mb: 0,
  publications: ["events"],
  # specify the changes you want to subscribe to. If you exclude, it will subscribe to all change events
  subscriptions: [:user_account, :todo],
  # include your modules for processing events (supervised)
  modules: [ExampleApp.UserAcountEvent, ExampleApp.TodoEvent]

Supervisor:

defmodule ExampleApp.Application do
  use Application

  def start(_type, _args) do
    # List all child processes to be supervised
    children = [
      WalEx.Supervisor
    ]

    opts = [strategy: :one_for_one, name: ExampleApp.Supervisor]
    Supervisor.start_link(children, opts)
  end
end

Example Module:

defmodule ExampleApp.UserAccountEvent do
  import WalEx.{Event, TransactionFilter}

  def process_event(txn) do
    cond do
      insert_event?(:user_account, txn) ->
        {:ok, user_account} = event(:user_account, txn)
        IO.inspect(user_account_insert_event: user_account)
        # do something with user_account data

      update_event?(:user_account, txn) ->
        {:ok, user_account} = event(:user_account, txn)
        IO.inspect(user_account_update_event: user_account)

      # you can also specify the relation
      delete_event?("public.user_account", txn) ->
        {:ok, user_account} = event(:user_account, txn)
        IO.inspect(user_account_delete_event: user_account)

      true ->
        nil
    end
  end
end

Additional filter helpers available in the [WalEx.TransactionFilter](lib/walex/transaction_filter.ex) module.

The process_event returns an Event Struct with changes provided by the map_diff library (UPDATE example where name field was changed):

%Event{
  type: :update,
   # the new record
  record: %{
    id: 1234,
    name: "Chase",
    ...
  },
  # changes provided by the map_diff library,
  changes: %{
    name: %{
      added: "Chase Pursley",
      changed: :primitive_change,
      removed: "Chase"
    }
  },
  commit_timestamp: ~U[2021-12-06 14:32:49Z]
}

Documentation can be generated with ExDoc and published on HexDocs. Once published, the docs can be found at https://hexdocs.pm/walex.