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Cainophile

Cainophile is a library to assist you in building Change data capture (CDC) systems in Elixir. With Cainophile, you can quickly and easily stream every change made to your PostgreSQL database, with no plugins, Java, or Zookeeper required. You can read more in the announcement.

Installation

The package can be installed by adding cainophile to your list of dependencies in mix.exs:

def deps do
  [
    {:cainophile, "~> 0.1.0"}
  ]
end

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

PostgreSQL Configuration

Currently, Cainophile only supports PostgreSQL, but other database support may be added later. To get started, you first need to configure PostgreSQL for logical replication:

ALTER SYSTEM SET wal_level = 'logical';

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:

CREATE PUBLICATION example_publication FOR ALL TABLES;

Replica Identity

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

Usage

The library is built to be added to your Application's Supervisor tree with a registered name, or simply started and linked to your own GenServer worker that will be responsible for consuming the changes.

Supervisor Usage:

defmodule ExampleApp.Application do
  use Application

  def start(_type, _args) do
    # List all child processes to be supervised
    children = [
      {
        Cainophile.Adapters.Postgres,
        register: Cainophile.ExamplePublisher, # name this process will be registered globally as, for usage with Cainophile.Adapters.Postgres.subscribe/2
        epgsql: %{ # All epgsql options are supported here
          host: 'localhost',
          username: "username",
          database: "yourdb",
          password: "yourpassword"
        },
        slot: "example", # :temporary is also supported if you don't want Postgres keeping track of what you've acknowledged
        wal_position: {"0", "0"}, # You can provide a different WAL position if desired, or default to allowing Postgres to send you what it thinks you need
        publications: ["example_publication"]
      }
    ]

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

Other usage:

Cainophile.Adapters.Postgres.start_link(
  register: Cainophile.ExamplePublisher, # name this process will be registered globally as, for usage with Cainophile.Adapters.Postgres.subscribe/2
  epgsql: %{ # All epgsql options are supported here
    host: 'localhost',
    username: "username",
    database: "yourdb",
    password: "yourpassword"
  },
  slot: "example", # :temporary is also supported if you don't want Postgres keeping track of what you've acknowledged
  wal_position: {"0", "0"}, # You can leave this 
  publications: ["example_publication"]
)

Then, you can subscribe to changes with Cainophile.Adapters.Postgres.subscribe/2:

Cainophile.Adapters.Postgres.subscribe(Cainophile.ExamplePublisher, self())

This will asyncronously deliver changes as messages to your process. See Cainophile.Changes for what they'll look like.

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