# WalEx
Postgres [Change Data Capture
(CDC)](https://en.wikipedia.org/wiki/Change_data_capture) 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?](https://bbhoss.io/posts/announcing-cainophile/#why-capture-changes)
## Credit
This library steals liberally from
[realtime](https://github.com/supabase/realtime) from Supabase, which in turn
draws heavily on [cainophile](https://github.com/cainophile/cainophile).
## Installation
If [available in Hex](https://hex.pm/docs/publish), the package can be installed
by adding `walex` to your list of dependencies in `mix.exs`:
```elixir
def deps do
[
{:walex, "~> 1.0.1"}
]
end
```
## PostgreSQL Configuration
### Logical Replication
WalEx only supports PostgreSQL. To get started, you first need to configure
PostgreSQL for [logical replication](https://www.crunchydata.com/blog/data-to-go-postgres-logical-replication):
```sql
ALTER SYSTEM SET wal_level = 'logical';
```
Docker Compose:
```bash
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](https://www.postgresql.org/docs/current/sql-createpublication.html)
for the tables you want to receive changes for:
All tables:
```sql
CREATE PUBLICATION events FOR ALL TABLES;
```
Or specific tables:
```sql
CREATE PUBLICATION events FOR TABLE user_account, todo;
```
### Replica Identity
WalEx supports all of the settings for [REPLICA
IDENTITY](https://www.postgresql.org/docs/current/sql-altertable.html#SQL-CREATETABLE-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:
```sql
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 Parameter
Group with the following settings:
```text
rds.replication = 1
max_replication_slots = 5
max_slot_wal_keep_size = 2048
```
## Usage
Config:
```elixir
# config.exs
config :walex, WalEx,
hostname: "localhost",
username: "postgres",
password: "postgres",
port: "5432",
database: "postgres",
publication: "events",
subscriptions: [:user_account, :todo],
modules: ExampleApp.UserAcountEvent, ExampleApp.TodoEvent]
```
Supervisor:
```elixir
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:
```elixir
defmodule ExampleApp.UserAccountEvent do
import WalEx.{Event, TransactionFilter}
@behaviour WalEx.Event
def process(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** _behaviour_ returns an `Event` Struct with changes provided by the
[map_diff](https://github.com/Qqwy/elixir-map_diff) library (UPDATE example
where _name_ field was changed):
```elixir
%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](https://github.com/elixir-lang/ex_doc)
and published on [HexDocs](https://hexdocs.pm). Once published, the docs can
be found at [https://hexdocs.pm/walex](https://hexdocs.pm/walex).