Ruby worker for Cadence

A pure Ruby library for defining and running Cadence workflows and activities.

To find more about Cadence please visit https://cadenceworkflow.io/.

Getting Started

NOTE: Make sure you have both Cadence and TChannel Proxy up and running. Head over to this section for installation instructions.

Clone this repository:

> git clone git@github.com:coinbase/cadence-ruby.git

Include this gem to your Gemfile:

gem 'cadence-ruby', path: 'path/to/a/cloned/cadence-ruby/'

Define an activity:

class HelloActivity < Cadence::Activity
  def execute(name)
    puts "Hello #{name}!"

    return
  end
end

Define a workflow:

require 'path/to/hello_activity'

class HelloWorldWorkflow < Cadence::Workflow
  def execute
    HelloActivity.execute!('World')

    return
  end
end

Configure your Cadence connection:

Cadence.configure do |config|
  config.host = 'localhost'
  config.port = 6666 # this should point to the tchannel proxy
  config.domain = 'ruby-samples'
  config.task_list = 'hello-world'
end

Register domain with the Cadence service:

Cadence.register_domain('ruby-samples', 'A safe space for playing with Cadence Ruby')

Configure and start your worker process:

require 'cadence/worker'

worker = Cadence::Worker.new
worker.register_workflow(HelloWorldWorkflow)
worker.register_activity(HelloActivity)
worker.start

And finally start your workflow:

require 'path/to/hello_world_workflow'

Cadence.start_workflow(HelloWorldWorkflow)

Congratulation you've just created and executed a distributed workflow!

To view more details about your execution, point your browser to http://localhost:8088/domain/ruby-samples/workflows?range=last-3-hours&status=CLOSED.

There are plenty of runnable examples demonstrating various features of this library available, make sure to check them out.

Installing dependencies

In order to run your Ruby workers you need to have the Cadence service and the TChannel Proxy running. Below are the instructions on setting these up:

Cadence

Cadence service handles all the persistence, fault tolerance and coordination of your workflows and activities. To set it up locally, download and boot the Docker Compose file from the official repo:

> curl -O https://raw.githubusercontent.com/uber/cadence/master/docker/docker-compose.yml

> docker-compose up

TChannel Proxy

Right now the Cadence service only communicates with the workers using Thrift over TChannel. Unfortunately there isn't a working TChannel protocol implementation for Ruby, so in order to connect to the Cadence service a simple proxy was created. You can run it using:

> cd proxy

> bin/proxy

The code and detailed instructions can be found here.

Workflows

A workflow is defined using pure Ruby code, however it should contain only a high-level deterministic outline of the steps (their composition) that need to be executed to complete a workflow. The actual work should be defined in your activities.

NOTE: Keep in mind that your workflow code can get run multiple times (replayed) during the same execution, which is why it must NOT contain any non-deterministic code (network requests, DB queries, etc) as it can break your workflows.

Here's an example workflow:

class RenewSubscriptionWorkflow < Cadence::Workflow
  def execute(user_id)
    subscription = FetchUserSubscriptionActivity.execute!(user_id)
    subscription ||= CreateUserSubscriptionActivity.execute!(user_id)

    return if subscription[:active]

    ChargeCreditCardActivity.execute!(subscription[:price], subscription[:card_token])

    RenewedSubscriptionActivity.execute!(subscription[:id])
    SendSubscriptionRenewalEmailActivity.execute!(user_id, subscription[:id])
  rescue CreditCardNotChargedError => e
    CancelSubscriptionActivity.execute!(subscription[:id])
    SendSubscriptionCancellationEmailActivity.execute!(user_id, subscription[:id])
  end
end

In this simple workflow we are checking if a user has an active subscription and then attempt to charge their credit card to renew an expired subscription, notifying the user of the outcome. All the work is encapsulated in activities, while the workflow itself is responsible for calling the activities in the right order, passing values between them and handling failures.

There is a couple of ways to execute an activity from your workflow:

# Calls the activity by its class and blocks the execution until activity is
# finished. The return value of your activity will get assigned to the result
result = MyActivity.execute!(arg1, arg2)

# Here's a non-blocking version of the execute, returning back the future that
# will get fulfilled when activity completes. This approach allows modelling
# asynchronous workflows with activities executed in parallel
future = MyActivity.execute(arg1, arg2)
result = future.get

# Full versions of the calls from above, but has more flexibility (shown below)
result = workflow.execute_activity!(MyActivity, arg1, arg2)
future = workflow.execute_activity(MyActivity, arg1, arg2)

# In case your workflow code does not have access to activity classes (separate
# process, activities implemented in a different language, etc), you can
# simply reference them by their names
workflow.execute_activity('MyActivity', arg1, arg2, options: { domain: 'my-domain', task_list: 'my-task-list' })

Besides calling activities workflows can:

• Use timers
• Receive signals
• Execute other (child) workflows
• Respond to queries [not yet implemented]

Activities

An activity is a basic unit of work that performs the desired action (potentially causing side-effects). It can return a result or raise an error. It is defined like so:

class CloseUserAccountActivity < Cadence::Activity
  class UserNotFound < Cadence::ActivityException; end

  def execute(user_id)
    user = User.find_by(id: user_id)

    raise UserNotFound, 'User with specified ID does not exist' unless user

    user.close_account
    user.save

    AccountClosureEmail.deliver(user)

    return
  end
end

It is important to make your activities idempotent, because they can get retried by Cadence (in case a timeout is reached or your activity has thrown an error). You normally want to avoid generating additional side effects during subsequent activity execution.

To achieve this there are two methods (returning a UUID token) available from your activity class:

• activity.run_idem — unique within for the current workflow execution (scoped to run_id)
• activity.workflow_idem — unique across all execution of the workflow (scoped to workflow_id)

Both tokens will remain the same across multiple retry attempts of the activity.

Asynchronous completion

When dealing with asynchronous business logic in your activities, you might need to wait for an external event to complete your activity (e.g. a callback or a webhook). This can be achieved by manually completing your activity using a provided async_token from activity's context:

class AsyncActivity < Cadence::Activity
  def execute(user_id)
    user = User.find_by(id: user_id)

    # Pass the async_token to complete your activity later
    ExternalSystem.verify_user(user, activity.async_token)

    activity.async # prevents activity from completing immediately
  end
end

Later when a confirmation is received you'll need to complete your activity manually using the token provided:

Cadence.complete_activity(async_token, result)

Similarly you can fail the activity by calling:

Cadence.fail_activity(async_token, MyError.new('Something went wrong'))

This doesn't change the behaviour from the workflow's perspective — as any other activity the result will be returned or an error raised.

NOTE: Make sure to configure your timeouts accordingly and not to set heartbeat timeout (off by default) since you won't be able to emit heartbeats and your async activities will keep timing out.

Similar behaviour can also be achieved in other ways (one which might be more preferable in your specific use-case), e.g.:

• by polling for a result within your activity (long-running activities with heartbeat)
• using retry policy to keep retrying activity until a result is available
• completing your activity after the initial call is made, but then waiting on a completion signal from your workflow

Worker

Worker is a process that communicates with the Cadence server and manages Workflow and Activity execution. To start a worker:

require 'cadence/worker'

worker = Cadence::Worker.new
worker.register_workflow(HelloWorldWorkflow)
worker.register_activity(SomeActivity)
worker.register_activity(SomeOtherActivity)
worker.start

A call to worker.start will take over the current process and will keep it unning until a TERM or INT signal is received. By only registering a subset of your workflows/activities with a given worker you can split processing across as many workers as you need.

Starting a workflow

All communication is handled via Cadence service, so in order to start a workflow you need to send a message to Cadence:

Cadence.start_workflow(HelloWorldWorkflow)

Optionally you can pass input and other options to the workflow:

Cadence.start_workflow(RenewSubscriptionWorkflow, user_id, options: { workflow_id: user_id })

Passing in a workflow_id allows you to prevent concurrent execution of a workflow — a subsequent call with the same workflow_id will always get rejected while it is still running, raising CadenceThrift::WorkflowExecutionAlreadyStartedError. You can adjust the behaviour for finished workflows by supplying the workflow_id_reuse_policy: argument with one of these options:

• :allow_failed will allow re-running workflows that have failed (terminated, cancelled, timed out or failed)
• :allow will allow re-running any finished workflows both failed and completed
• :reject will reject any subsequent attempt to run a workflow

Execution Options

There are lots of ways in which you can configure your Workflows and Activities. The common ones (domain, task_list, timeouts and retry policy) can be defined in one of these places (in the order of precedence):

1. Inline when starting or registering a workflow/activity (use options: argument)
2. In your workflow/activity class definitions by calling a class method (e.g. domain 'my-domain')
3. Globally, when configuring your Cadence library via Cadence.configure

Breaking Changes

Since the workflow execution has to be deterministic, breaking changes can not be simply added and deployed — this will undermine the consistency of running workflows and might lead to unexpected behaviour. However, breaking changes are often needed and these include:

• Adding new activities, timers, child workflows, etc.
• Remove existing activities, timers, child workflows, etc.
• Rearranging existing activities, timers, child workflows, etc.
• Adding/removing signal handlers

In order to add a breaking change you can use workflow.has_release?(release_name) method in your workflows, which is guaranteed to return a consistent result whether or not it was called prior to shipping the new release. It is also consistent for all the subsequent calls with the same release_name — all of them will return the original result. Consider the following example:

class MyWorkflow < Cadence::Workflow
  def execute
    ActivityOld1.execute!

    workflow.sleep(10)

    ActivityOld2.execute!

    return
  end
end

which got updated to:

class MyWorkflow < Cadence::Workflow
  def execute
    Activity1.execute!

    if workflow.has_release?(:fix_1)
      ActivityNew1.execute!
    end

    workflow.sleep(10)

    if workflow.has_release?(:fix_1)
      ActivityNew2.execute!
    else
      ActivityOld.execute!
    end

    if workflow.has_release?(:fix_2)
      ActivityNew3.execute!
    end

    return
  end
end

If the release got deployed while the original workflow was waiting on a timer, ActivityNew1 and ActivityNew2 won't get executed, because they are part of the same change (same release_name), however ActivityNew3 will get executed, since the release wasn't yet checked at the time. And for every new execution of the workflow — all new activities will get executed, while ActivityOld will not.

Later on you can clean it up and drop all the checks if you don't have any older workflows running or expect them to ever be executed (e.g. reset).

NOTE: Releases with different names do not depend on each other in any way.

Testing

It is crucial to properly test your workflows and activities before running them in production. The provided testing framework is still limited in functionality, but will allow you to test basic use-cases.

The testing framework is not required automatically when you require cadence-ruby, so you have to do this yourself (it is strongly recommended to only include this in your test environment, spec_helper.rb or similar):

require 'cadence/testing'

This will allow you to execute workflows locally by running HelloWorldWorkflow.execute_locally. Any arguments provided will forwarded to your #execute method.

In case of a higher level end-to-end integration specs, where you need to execute a Cadence workflow as part of your code, you can enable local testing:

Cadence::Testing.local!

This will treat every Cadence.start_workflow call as local and perform your workflows inline. It also works with a block, restoring the original mode back after the execution:

Cadence::Testing.local! do
  Cadence.start_workflow(HelloWorldWorkflow)
end

Make sure to check out example integration specs for more details.

TODO

There's plenty of work to be done, but most importanly we need:

• Write specs for everything
• Implement support for missing features

LICENSE

Copyright 2020 Coinbase, Inc.

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.