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# EctoJob
A transactional job queue built with Ecto, PostgreSQL and GenStage
## Goals
- Transactional job processing
- Retries
- Scheduled jobs
- Multiple queues
- Low latency concurrent processing
- Avoid frequent database polling
- Library of functions, not a full OTP application
## Getting Started
Add `:ecto_job` to your `dependencies`
```elixir
{:ecto_job, ~> "0.1"}
```
## Installation
Add a migration to install the notification function and create the a job queue table:
```
mix ecto.gen.migration create_job_queue
```
```elixir
defmodule MyApp.Repo.Migrations.CreateJobQueue do
use Ecto.Migration
def up do
EctoJob.Migrations.Install.up()
EctoJob.Migrations.CreateJobTable.up("jobs")
end
def down do
EctoJob.Migrations.CreateJobTable.down("jobs")
EctoJob.Migrations.Install.down()
end
end
```
Add a module for the queue, mix in `EctoJob.JobQueue`.
This will declare an `Ecto.Schema` to use with the table created in the migration, and a `start_link` function allowing the worker supervision tree to be started conveniently.
```elixir
defmodule MyApp.JobQueue do
use EctoJob.JobQueue, table_name: "jobs"
end
```
Add `perform/2` function to the job queue module, this is where jobs from the queue will be dispatched.
```elixir
defmodule MyApp.JobQueue do
use EctoJob.JobQueue, table_name: "jobs"
def perform(multi = %Ecto.Multi{}, job = %{}) do
... job logic here ...
end
end
```
Add your new `JobQueue` module as a supervisor to the application supervision tree to run the worker supervisor:
```elixir
def start(_type, _args) do
import Supervisor.Spec
children = [
supervisor(MyApp.Repo, []),
supervisor(MyApp.JobQueue, [[repo: MyApp.Repo, max_demand: 100]])
]
opts = [strategy: :one_for_one, name: MyApp.Supervisor]
Supervisor.start_link(children, opts)
end
```
If you want to run the workers on a separate node to the enqueuers, just leave your `JobQueue` module out of the supervision tree.
## Usage
### Enqueueing jobs
Jobs are Ecto schemas, with each queue backed by a different table.
A job can be inserted into the Repo directly by constructing a job with the `new/2` function:
```elixir
%{"type" => "SendEmail", "address" => "joe@gmail.com", "body" => "Welcome!"}
|> MyApp.JobQueue.new()
|> MyApp.Repo.insert()
```
The primary benefit of `EctoJob` is the ability to enqueue and process jobs transactionally.
To achieve this, a job can be added to an `Ecto.Multi`, along with other application updates, using the `enqueue/3` function:
```elixir
Ecto.Multi.new()
|> Ecto.Multi.insert(:add_user, User.insert_changeset(%{name: "Joe", email: "joe@gmail.com"}))
|> MyApp.JobQueue.enqueue(:email_job, %{"type" => "SendEmail", "address" => "joe@gmail.com", "body" => "Welcome!"})
|> MyApp.Repo.transaction()
```
### Handling Jobs
All jobs sent to a queue are eventually dispatched to the `perform/2` function defined in the queue module.
The first argument supplied is an `Ecto.Multi` which has been initialized with a `delete` operation, marking the job as complete.
The `Ecto.Multi` struct must be passed to the `Ecto.Repo.transaction` function to complete the job, along with any other application updates.
```elixir
defmodule MyApp.JobQueue do
use EctoJob.JobQueue, table_name: "jobs"
def perform(multi = %Ecto.Multi{}, job = %{"type" => "SendEmail", "recipient" => recipient, "body" => body}) do
multi
|> Ecto.Multi.run(:send, fn _ -> EmailService.send(recipient, body))
|> Ecto.Multi.insert(:stats, %EmailSendStats{recipient: recipient})
|> MyApp.Repo.transaction()
end
end
```
When a queue handles multiple job types, it is useful to pattern match on the job and delegate to separate modules:
```elixir
defmodule MyApp.JobQueue do
use EctoJob.JobQueue, table_name: "jobs"
def perform(multi = %Ecto.Multi{}, job = %{"type" => "SendEmail"), do: MyApp.SendEmail.perform(multi, job)
def perform(multi = %Ecto.Multi{}, job = %{"type" => "CustomerReport"), do: MyApp.CustomerReport.perform(multi, job)
def perform(multi = %Ecto.Multi{}, job = %{"type" => "SyncWithCRM"), do: MyApp.CRMSync.perform(multi, job)
...
end
```
## How it works
Each job queue is represented as a PostgreSQL table and Ecto schema.
Jobs are added to the queue by inserting into the table, using `Ecto.Repo.transaction` to transactionally enqueue jobs with other application updates.
A `GenStage` producer responds to demand for jobs by efficiently pulling jobs from the queue in batches.
When there is insufficient jobs in the queue, the demand for jobs is buffered.
As jobs are inserted into the queue, `pg_notify` notifies the producer that new work is available,
allowing the producer to dispatch jobs immediately if there is pending demand.
A `GenStage` `ConsumerSupervisor` subscribes to the producer, and spawns a new `Task` for each job.
The callback for each job receives an `Ecto.Multi` structure, pre-populated with a `delete`
command to remove the job from the queue.
Application code then add additional commands to the `Ecto.Multi` and submit it to the
`Repo` with a call to `transaction`, ensuring that application updates are performed atomically with the job removal.
Scheduled jobs and Failed jobs are reactivated by polling the database once per minute.
## Job Lifecycle
Jobs scheduled to run at a future time start in the "SCHEDULED" state.
Scheduled jobs transition to "AVAILABLE" after the scheduled time has passed.
Jobs that are intended to run immediately start in an "AVAILABLE" state.
The producer will update a batch of jobs setting the state to "RESERVED", with an expiry of 5 minutes.
Once a consumer is given a job, it increments the attempt counter and updates the state to "IN_PROGRESS", with an expiry of 5 minutes.
If the job is being retried, the expiry will be 5 minutes * the attempt counter.
If successful, the consumer will delete the job from the queue.
If unsuccessful, the job remains in the "IN_PROGRESS" state until it expires.
Jobs in the "RESERVED" or "IN_PROGRESS" state past the expiry time will be returned to the "AVAILABLE" state.
Expired jobs in the "IN_PROGRESS" state with attempts >= MAX_ATTEMPTS move to a "FAILED" state.
Failed jobs are kept in the database so that application developers can handle the failure.
