Commanded Ecto projections

Read model projections for Commanded CQRS/ES applications using Ecto for persistence.

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Changelog

MIT License

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Overview

• Getting started
  • Schema Prefix
• Usage
  • Supervision
  • error/3 callback
  • after_update/3 callback
  • Rebuilding a projection
• Contributing
• Need help?

Getting started

You should already have Ecto installed and configured before proceeding. Please follow the Ecto Getting Started guide to get going first.

1. Add commanded_ecto_projections to your list of dependencies in mix.exs:

   def deps do
     [
       {:commanded_ecto_projections, "~> 0.7"},
     ]
   end

2. Configure commanded_ecto_projections with the Ecto repo used by your application:

   config :commanded_ecto_projections,
     repo: MyApp.Projections.Repo

   Or alternatively in case of umbrella application define it later per projection:

   defmodule MyApp.ExampleProjector do
     use Commanded.Projections.Ecto,
       name: "example_projection",
       repo: MyApp.Projections.Repo
   
     ...
   end

3. Generate an Ecto migration in your app:

   $ mix ecto.gen.migration create_projection_versions

4. Modify the generated migration, in priv/repo/migrations, to create the projection_versions table:

   defmodule CreateProjectionVersions do
     use Ecto.Migration
   
     def change do
       create table(:projection_versions, primary_key: false) do
         add :projection_name, :text, primary_key: true
         add :last_seen_event_number, :bigint
   
         timestamps()
       end
     end
   end

5. Run the Ecto migration:

   $ mix ecto.migrate

Schema Prefix

When using a prefix for your schemas you might also want to change the prefix for the ProjectionVersion schema. There are two options to do this:

1. provide a global prefix via the config

config :commanded_ecto_projections,
  schema_prefix: "example_schema_prefix"

2. provide the prefix on a projection by projection basis

defmodule MyApp.ExampleProjector do
  use Commanded.Projections.Ecto,
    name: "example_projection",
    schema_prefix: "example_schema_prefix"
end

Usage

Use Ecto schemas to define your read model:

defmodule ExampleProjection do
  use Ecto.Schema

  schema "example_projections" do
    field :name, :string
  end
end

For each read model you will need to define a module that uses the Commanded.Projections.Ecto macro and configures the domain events to be projected. The :name option passed to the use invocation specifies the name of the subscription to be used. It can be any string that is unique among subscriptions.

The project/3 macro expects the domain event, metadata and function that takes and returns an Ecto.Multi data structure for grouping multiple Repo operations. These will all be executed within a single transaction. You can use Ecto.Multi to insert, update, and delete data.

You can also use project/2 if you do not need to use the event metadata.

defmodule MyApp.ExampleProjector do
  use Commanded.Projections.Ecto, name: "example_projection"

  project %AnEvent{name: name}, _metadata, fn multi ->
    Ecto.Multi.insert(multi, :example_projection, %ExampleProjection{name: name})
  end

  project %AnotherEvent{name: name}, fn multi ->
    Ecto.Multi.insert(multi, :example_projection, %ExampleProjection{name: name})
  end
end

If you want to skip a projection event, you can return the multi transaction without further modifying it:

project %ItemUpdated{uuid: uuid} = event, _metadata, fn multi ->
  case Repo.get(ItemProjection, uuid) do
    nil -> multi
    item -> Ecto.Multi.update(multi, :item, update_changeset(event, item))
  end
end

Supervision

Your projector module must be included in your application supervision tree:

defmodule MyApp.Projections.Supervisor do
  use Supervisor

  def start_link do
    Supervisor.start_link(__MODULE__, nil)
  end

  def init(_) do
    children = [
      # projections
      worker(MyApp.ExampleProjector, [], id: :example_projector),
    ]

    supervise(children, strategy: :one_for_one)
  end
end

error/3 callback

The Commanded.Projections.Ecto macro defines a Commanded event handler which means you can take advantage of the error/3 callback function to handle any errors returned from a project function. The function is passed the error returned by the event handler (e.g. {:error, error}), the event causing the error, and a context map containing state passed between retries. Use the context map to track any transient state you need to access between retried failures, such as the number of failed attempts.

You can return one of the following responses depending upon the error severity:

• {:retry, context} - retry the failed event, provide a context map containing any state passed to subsequent failures. This could be used to count the number of failures, stopping after too many.

• {:retry, delay, context} - retry the failed event, after sleeping for the requested delay (in milliseconds). Context is a map as described in {:retry, context} above.

• :skip - skip the failed event by acknowledging receipt.

• {:stop, reason} - stop the projector with the given reason.

Error handling example

Here's an example projector module where an error tagged tuple is explicitly returned from a project function, but you can also handle exceptions caused by faulty Ecto.Multi database operations in a similar manner since the errors are caught and returned as tagged tuples (e.g. {:error, %Ecto.ConstraintError{}}).

defmodule MyApp.ExampleProjector do
  use Commanded.Projections.Ecto, name: "MyApp.ExampleProjector"

  require Logger

  alias Commanded.Event.FailureContext

  project %AnEvent{}, fn _multi ->
    {:error, :failed}
  end

  def error({:error, :failed} = error, %AnEvent{}, %FailureContext{}) do
    :skip
  end

  def error({:error, %Ecto.ConstraintError{} = error}, _event, _failure_context) do
    Logger.error(fn -> "Failed due to constraint error: " <> inspect(error) end)
    :skip
  end

  def error({:error, _error} = error, _event, _failure_context) do
    :skip
  end
end

after_update/3 callback

You can define an after_update/3 function in a projector to be called after each projected event. It receives the event, its associated metadata, and all changes from Ecto.Multi executed in the database transaction.

defmodule MyApp.ExampleProjector do
  use Commanded.Projections.Ecto, name: "MyApp.ExampleProjector"

  project %AnEvent{name: name}, fn multi ->
    Ecto.Multi.insert(multi, :example_projection, %ExampleProjection{name: name})
  end

  def after_update(event, metadata, changes) do
    # ... use event, metadata, or `Ecto.Multi` changes
    :ok
  end
end

You could use this function to notify subscribers that the read model has been updated (e.g. pub/sub to Phoenix channels).

Rebuilding a projection

The projection_versions table is used to ensure that events are only projected once.

To rebuild a projection you will need to:

1. Delete the row containing the last seen event for the projection name:

   delete from projection_versions
   where projection_name = 'example_projection';

2. Truncate the tables that are being populated by the projection, and restart their identity:

   truncate table
     example_projections,
     other_projections
   restart identity;

You will also need to reset the event store subscription for the commanded event handler. This is specific to whichever event store you are using.

Contributing

Pull requests to contribute new or improved features, and extend documentation are most welcome. Please follow the existing coding conventions.

You should include unit tests to cover any changes. Run mix test to execute the test suite:

mix deps.get
MIX_ENV=test mix do ecto.create, ecto.migrate
mix test

Contributors

• Andrey Akulov
• Ben Smith
• Florian Ebeling
• Sascha Wolf
• Tobiasz Małecki

Need help?

Please open an issue if you encounter a problem, or need assistance. You can also seek help in the Gitter chat room for Commanded.

For commercial support, and consultancy, please contact Ben Smith.