Simple and reliable 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 via the Event DSL. For example:
You can learn more about CDC and what you can do with it here: Why capture changes?
This library borrows liberally from realtime from Supabase, which in turn draws heavily on cainophile.
If available in Hex, the package can be installed
by adding walex
to your list of dependencies in mix.exs
:
def deps do
[
{:walex, "~> 4.1.0"}
]
end
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" ]
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;
Or just specific tables:
CREATE PUBLICATION events FOR TABLE user, todo;
Filter based on row conditions (Postgres v15+ only):
CREATE PUBLICATION user_event FOR TABLE user WHERE (active IS TRUE);
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 REPLICA IDENTITY FULL;
ALTER TABLE todo REPLICA IDENTITY FULL;
Also, be mindful of replication gotchas.
Amazon (AWS) RDS Postgres allows you to configure logical replication.
When creating a new Postgres database on RDS, you'll need to set a Parameter Group with the following settings:
rds.logical_replication = 1
max_replication_slots = 5
max_slot_wal_keep_size = 2048
# config.exs
config :my_app, WalEx,
hostname: "localhost",
username: "postgres",
password: "postgres",
port: "5432",
database: "postgres",
publication: "events",
subscriptions: ["user", "todo"],
# WalEx assumes your module names match this pattern: MyApp.Events.User, MyApp.Events.ToDo, etc
# but you can also specify custom modules like so:
# modules: [MyApp.CustomModule, MyApp.OtherCustomModule],
name: MyApp
It is also possible to just define the URL configuration for the database
# config.exs
config :my_app, WalEx,
url: "postgres://username:password@hostname:port/database"
publication: "events",
subscriptions: ["user", "todo"],
name: MyApp
You can also dynamically update the config at runtime:
WalEx.Configs.add_config(MyApp, :subscriptions, ["new_subscriptions_1", "new_subscriptions_2"])
WalEx.Configs.remove_config(MyApp, :subscriptions, "subscriptions")
WalEx.Configs.replace_config(MyApp, :password, "new_password")
defmodule MyApp.Application do
use Application
def start(_type, _args) do
children = [
{WalEx.Supervisor, Application.get_env(:my_app, WalEx)}
]
opts = [strategy: :one_for_one, name: MyApp.Supervisor]
Supervisor.start_link(children, opts)
end
end
Returned change data is a List of %Event{} structs with changes. UPDATE event example where name field was changed):
[
%Walex.Event{
name: :user,
type: :update,
source: %WalEx.Event.Source{
name: "WalEx",
version: "3.8.0",
db: "todos",
schema: "public",
table: "user",
columns: %{
id: "integer",
name: "varchar",
created_at: "timestamptz"
}
},
new_record: %{
id: 1234,
name: "Chase Pursley",
created_at: #DateTime<2023-08-18 14:09:05.988369-04:00 -04 Etc/UTC-4>
},
# we don't show old_record for update to reduce payload size
# however, you can see any old values that changed under "changes"
old_record: nil,
changes: %{
name: %{
new_value: "Chase Pursley",
old_value: "Chase"
}
},
timestamp: ~U[2023-12-18 15:50:08.329504Z]
}
]
If your app is named MyApp and you have a subscription called :user (which represents a database table), WalEx assumes you have a module called MyApp.Events.User
that uses WalEx Event. But you can also define any custom module, just be sure to add it to the modules config.
Note that the result of events
is a list. This is because WalEx returns a List of transactions for a particular table when there's a change event. Often times this will just contain one result, but it could be many (for example, if you use database triggers to update a column after an insert).
defmodule MyApp.Events.User do
use WalEx.Event, name: MyApp
# any subscribed event
on_event(:all, fn events ->
IO.inspect(events: events)
end)
# any user event
on_event(:user, fn users ->
IO.inspect(on_event: users)
# do something with users data
end)
# any user insert event
on_insert(:user, fn users ->
IO.inspect(on_insert: users)
end)
on_update(:user, fn users ->
IO.inspect(on_update: users)
end)
on_delete(:user, fn users ->
IO.inspect(on_delete: users)
end)
A common scenario is where you want to "unsubscribe" from specific records (for example, temporarily for a migration or data fix). One way to accomplish this is to have a column with a value like event_subscribe: false
. Then you can ignore specific events by specifying their key and value to unwatched_records.
Another scenario is you might not care when just certain fields change. For example, maybe a database trigger sets updated_at after a record is updated. Or a count changes, or several do that you don't need to react to. In this case, you can ignore the event change by adding them to unwatched_fields.
Additional filter helpers available in the WalEx.TransactionFilter module.
defmodule MyApp.Events.User do
use WalEx.Event, name: MyApp
@filters %{
unwatched_records: %{event_subscribe: false},
unwatched_fields: ~w(event_id updated_at todos_count)a
}
on_insert(:user, @filters, fn users ->
IO.inspect(on_insert: users)
# resulting users data is filtered
end)
end
You can also provide a list of functions (as atoms) to be applied to each Event (after optional filters are applied). Each function is run as an async Task on each event. The functions must be defined in the current module and take a single event argument. Use with caution!
defmodule MyApp.Events.User do
use WalEx.Event, name: MyApp
@filters %{unwatched_records: %{event_subscribe: false}}
@functions ~w(send_welcome_email add_to_crm clear_cache)a
on_insert(:user, @filters, @functions, fn users ->
IO.inspect(on_insert: users)
# resulting users data is first filtered then functions are applied
end)
def send_welcome_email(user) do
# logic for sending welcome email to new user
end
def add_to_crm(user) do
# logic for adding user to crm system
end
def clear_cache(user) do
# logic for clearing user cache
end
end
By default WalEx will create a temporary replication slot in Postgres.
This means that if the connection between WalEx and Postgres gets interrupted (crash / disconnection / etc.), the replication slot will get dropped by Postgres. This makes using WalEx safer as there is not risk of filling up the disk of the Postgres writer instance in case of downtime.
The downside being that this event-loss is more than likely. If this is a no-go, WalEx also supports durable replication.
# config.exs
config :my_app, WalEx,
# ...
durable_slot: true,
slot_name: my_app_replication_slot
Only a single process can be connected to a durable slot at once,
in case the slot is already used WalEx.Supervisor
will fail to start with a RuntimeError
.
Be warned that there are many additional potential gotchas (a detailed guide is planned).
WalEx receives events from Postgres in WalEx.Replication.Server
and then cast
those to WalEx.Replication.Publisher
.
It's then WalEx.Replication.Publisher
that is responsible to join these events together and process them.
In the event where you'd expect Postgres to overwhelm WalEx and potentially cause OOMs, WalEx provides a config option that should help you implement back-pressure.
As it's a quite advanced use, with many strong requirements, it's recommended instead to increase the amount of RAM of your instance.
Never the less, if it's not an option or would like to control the consumption rate of events, WalEx provide the following configuration option:
config :my_app, WalEx,
# ...
message_middleware: fn message, app_name -> ... end
message_middleware
allows you to define the way WalEx.Replication.Server
and WalEx.Replication.Publisher
communicate.
If for instance you'd like to store these events to disk before processing them you would need to:
message_middleware
callback. It should serialize messages and store them to diskWalEx.Replication.Publisher.process_message_async(message, app_name)
WalEx.Replication.Publisher.process_message_sync(message, app_name)
Any back-pressure implementation needs to guarantee:
You'll need a local Postgres instance running
MIX_ENV=test mix walex.setup
MIX_ENV=test mix test