Filters - Sequin

When setting up a sink, you can optionally specify one or more filters to apply to the table:

Only changes and rows that match the filters will be sent to the sink.

Operations to capture

When configuring a sink, you can specify which Postgres operations to capture:

insert
update
delete

If you backfill a table to a sink, Sequin will send read messages for each row in the table, regardless of which operations you specify.

Filter functions

Filter functions provide more advanced filtering capabilities than what’s possible with column filters alone. They allow you to:

Apply complex conditional logic to your data
Compare new and old values for changed rows
Use string pattern matching or regular expressions
Implement custom business logic for filtering

Filter functions are written in Elixir and must return a boolean value (true or false). Filter functions that return non-boolean values will fail to execute.

How filter functions work

When you create a filter function, you define an Elixir function that evaluates whether a message should be processed. For each message, your filter function receives:

def filter(action, record, changes, metadata) do
  # Your filtering logic here
  # Must return true or false
end

Parameters:

action: The operation type (e.g., “insert”, “update”, “delete”)
record: The full row/record data as a map with string keys
changes: For update operations, contains the old values that were changed
metadata: Additional information like table name, timestamp, etc.

How filters are combined

When you set up a sink with multiple types of filters, all filters are applied with a logical AND. This means the operation must match one of the selected operations (insert, update, delete) and the filter function (if specified) must return true. If any of these conditions fail, the message will not be delivered to the sink.

Example filter functions

Filter by string pattern

def filter(action, record, changes, metadata) do
  # Only process records where name starts with "A"
  String.starts_with?(record["name"], "A")
end

Filter by multiple conditions

def filter(action, record, changes, metadata) do
  # Only process VIP customers with high-value orders
  record["customer_type"] == "VIP" and record["is_deleted"] == false
end

Complex filtering with metadata

def filter(action, record, changes, metadata) do
  # Filter based on both record content and metadata
  is_high_priority = record["priority"] == "high"
  is_users_table = metadata.table_name == "users"

  # Only process high priority records from the users table
  is_high_priority and is_users_table
end

Filtering based on changes

def filter(action, record, changes, metadata) do
  # Only process updates where the status changed from "pending" to "approved"
  action == "update" and
    changes["status"] == "pending" and
    record["status"] == "approved"
end

Testing filter functions

When creating or editing a filter function, Sequin will automatically capture recent events from your database so you can test your function with real data. You’ll see a live preview showing which messages would pass or fail your filter.

Messages reference

Learn about the different fields you can use in filters.

Transforms

Learn how to transform messages before they are sent to the sink destination.

Routing

Learn how to dynamically route messages to different destinations.

​Operations to capture

​Filter functions

​How filter functions work

​How filters are combined

​Example filter functions

​Filter by string pattern

​Filter by multiple conditions

​Complex filtering with metadata

​Filtering based on changes

​Testing filter functions

​Related

Messages reference

Transforms

Routing

Operations to capture

Filter functions

How filter functions work

How filters are combined

Example filter functions

Filter by string pattern

Filter by multiple conditions

Complex filtering with metadata

Filtering based on changes

Testing filter functions

Related