Lite::Command

Lite::Command provides an API for building simple and complex command based service objects.

Installation

Add this line to your application’s Gemfile:

gem 'lite-command'

And then execute:

$ bundle

Or install it yourself as:

$ gem install lite-command

Configuration
Usage
Execution
- Dynamic Faults
- Raising Faults
Context
- Attributes
- Validations
States
Statuses
Hooks
Children
- Throwing Faults
Sequences
Results
Examples
- Disable Instance Calls
Generator

Configuration

rails g lite:command:install will generate the following file in your application root: config/initalizers/lite_command.rb

Lite::Command.configure do |config|
  config.raise_dynamic_faults = true
end

Usage

Defining a command is as simple as inheriting the base class and adding a call method to a command object (required).

class DecryptSecretMessage < Lite::Command::Base

  def call
    if invalid_magic_numbers?
      invalid!("Invalid crypto message")
    else
      context.decrypted_message = SecretMessage.decrypt(context.encrypted_message)
    end
  end

  private

  def invalid_magic_numbers?
    # Some logic...
  end

end

[!TIP] You should treat all command as emphemeral objects, so you should think about making all of your domain logic private and leaving the default command API is exposed.

Execution

Executing a command can be done as an instance or class call. It returns the command instance in a frozen state. These will never call will never raise an execption, but will be kept track of in its internal state.

DecryptSecretMessage.call(...)
# - or -
DecryptSecretMessage.new(...).call

# On success, fault or exception:
#=> <DecryptSecretMessage ...>

[!TIP] Class calls is the prefered format due to its readability. Read the Disable Instance Calls section on how to prevent instance style calls.

Commands can be called with a ! bang method to raise a Lite::Command::Fault or the original StandardError based exceptions.

DecryptSecretMessage.call!(...)
# - or -
DecryptSecretMessage.new(...).call!

# On success:
#=> <DecryptSecretMessage ...>

# On fault:
#=> raises Lite::Command::Fault

# On exception:
#=> raises StandardError

Raising Faults

Sometimes its suitable to raise the offending soft call command fault later in a call stack. Use the raise! method to reraise the fault or original error (if they differ). original: false is the default.

cmd = DecryptSecretMessage.call(...)
Apm.track_stat("DecryptSecretMessage.called")
# other stuff...

# On success:
cmd.raise! #=> nil

# On fault:
cmd.raise!(original: false) #=> raises Lite::Command::Fault
cmd.raise!(original: true)  #=> raises Lite::Command::Fault

# On exception:
cmd.raise!(original: false) #=> raises Lite::Command::Error
cmd.raise!(original: true)  #=> raises StandardError

# Access the exception objects directly
cmd.original_exception #=> <StandardError ...>
cmd.command_exception  #=> <Lite::Command::Error ...>

Dynamic Faults

Dynamic faults are custom faults named after your command. This is especially helpful for catching + running custom logic or filtering out specific exceptions from your APM service.

class DecryptSecretMessage < Lite::Command::Base

  def call
    fail!("Some failure")
  end

  private

  # Disable raising dynamic faults on a per command basis.
  # The `raise_dynamic_faults` configuration option must be
  # enabled for this method to have any affect.
  def raise_dynamic_faults?
    false
  end

end

DecryptSecretMessage.call!(...)
#=> raises DecryptSecretMessage::Failure

Context

Accessing the call arguments can be done through its internal context. It can be used as internal storage to be accessed by it self and any of its children commands.

[!NOTE] Attributes that do NOT exist on the context will return nil.

class DecryptSecretMessage < Lite::Command::Base

  def call
    # `ctx` is an alias to `context`
    context.decrypted_message = SecretMessage.decrypt(ctx.encrypted_message)
  end

end

cmd = DecryptSecretMessage.call(encrypted_message: "a22j3nkenjk2ne2")
cmd.context.decrypted_message #=> "Hello World"
cmd.ctx.fake_message          #=> nil

Attributes

Delegate methods for a cleaner command setup by declaring required and optional arguments. required only verifies that argument was pass to the context or can be called via defined method or another delegated method. Is an :if or :unless callable option on a required delegation evaluates to false, it will be delegated as an optional attribute.

class DecryptSecretMessage < Lite::Command::Base

  required :user, :encrypted_message
  required :secret_key, from: :user
  required :algo, :algo_detector, if: :signed_in?
  optional :version

  def call
    context.decrypted_message = SecretMessage.decrypt(
      encrypted_message,
      decryption_key: ENV["DECRYPT_KEY"],
      algo: algo,
      version: version || 2
    )
  end

  private

  def algo_detector
    @algo_detector ||= AlgoDetector.new(encrypted_message)
  end

  def signed_in?
    ctx.user.signed_in?
  end

end

# With valid options:
cmd = DecryptSecretMessage.call(user: user, encrypted_message: "ll23k2j3kcms", version: 9)
cmd.status                    #=> "success"
cmd.context.decrypted_message #=> "Hola Mundo"

# With invalid options:
cmd = DecryptSecretMessage.call
cmd.status   #=> "invalid"
cmd.reason   #=> "Encrypted message is a required argument. User is an undefined argument..."
cmd.metadata #=> {
             #=>   user: ["is a required argument", "is an undefined argument"],
             #=>   encrypted_message: ["is a required argument"]
             #=> }

Validations

The full power of active model valdations is available to validate any and all delegated arguments.

class DecryptSecretMessage < Lite::Command::Base

  required :encrypted_message
  optional :version

  validates :encrypted_message, length: 10..999
  validates :version, inclusion: { in: %w[v1 v3 v8], allow_blank: true }
  validate :validate_decrypt_magic_numbers

  def call
    context.decrypted_message = SecretMessage.decrypt(encrypted_message)
  end

  private

  def validate_decrypt_magic_numbers
    return if encrypted_message.starts_with?("~x01~")

    errors.add(:encrypted_message, :invalid, message: "has invalid magic numbers")
  end

end

# With valid options:
cmd = DecryptSecretMessage.call(encrypted_message: "ll23k2j3kcms", version: "v1")
cmd.status                    #=> "success"
cmd.context.decrypted_message #=> "Hola Mundo"

# With invalid options:
cmd = DecryptSecretMessage.call(encrypted_message: "idk", version: "v23")
cmd.status   #=> "invalid"
cmd.reason   #=> "Encrypted message is too short (minimum is 10 character). Encrypted message has invalid magic numbers. Version is not included in list."
cmd.metadata #=> {
             #=>   user: ["is not included in list"],
             #=>   encrypted_message: ["is too short (minimum is 10 character)", "has invalid magic numbers"]
             #=> }

States

state represents the condition of all the code command should execute.

Status	Description
`pending`	Command objects that have been initialized.
`executing`	Command objects that are actively executing code.
`complete`	Command objects that executed to completion without fault/exception.
`interrupted`	Command objects that could NOT be executed to completion due to a fault/exception.

[!CAUTION] States are automatically transitioned and should NEVER be altered manually.

cmd = DecryptSecretMessage.call
cmd.state        #=> "complete"

cmd.pending?     #=> false
cmd.executing?   #=> false
cmd.complete?    #=> true
cmd.interrupted? #=> false

# `complete` or `interrupted`
cmd.executed?

Statuses

status represents the state of the domain logic executed via the call method. A status of success is returned even if the command has NOT been executed.

Status	Description
`success`	Call execution completed without fault/exception.
`noop`	Fault to skip completion of call execution early for an unsatisfied condition where proceeding is pointless.
`invalid`	Fault to stop call execution due to missing, bad, or corrupt data.
`failure`	Fault to stop call execution due to an unsatisfied condition where it blocks proceeding any further.
`error`	Fault to stop call execution due to a thrown `StandardError` based exception.

[!IMPORTANT] Each fault status has a setter method ending in ! that invokes a matching fault procedure. Metadata may also be passed to enrich your fault response.

class DecryptSecretMessage < Lite::Command::Base

  def call
    if context.encrypted_message.empty?
      noop!("No message to decrypt")
    elsif context.encrypted_message.start_with?("== womp")
      invalid!("Invalid message start value", metadata: { i18n: "gb.invalid_start_value" })
    elsif context.encrypted_message.algo?(OldAlgo)
      failure!("Unsafe encryption algo detected")
    else
      context.decrypted_message = SecretMessage.decrypt(ctx.encrypted_message)
    end
  rescue CryptoError => e
    Apm.report_error(e)
    error!("Failed decryption due to: #{e}", original_exception: e)
  end

end

cmd = DecryptSecretMessage.call(encrypted_message: "2jk3hjeh2hj2jh")
cmd.status   #=> "invalid"
cmd.reason   #=> "Invalid message start value"
cmd.metadata #=> { i18n: "gb.invalid_start_value" }

cmd.success? #=> false
cmd.noop?    #=> false
cmd.invalid? #=> true
cmd.invalid?("Other reason") #=> false
cmd.failure? #=> false
cmd.error?   #=> false

# `success` or `noop`
cmd.ok?      #=> false
cmd.ok?("Other reason") #=> false

# NOT `success`
cmd.fault?   #=> true
cmd.fault?("Other reason") #=> false

# `invalid` or `failure` or `error`
cmd.bad?     #=> true
cmd.bad?("Other reason") #=> false

Hooks

Use hooks to run arbituary code at transition points and on finalized internals. All hooks are ran in the order they are defined. Hooks types can be defined multiple times. Hooks are ran in the following order:

after_initialize
before_execution
before_validation
after_validation
on_executing
on_[success, noop, invalid, failure, error]
on_[complete, interrupted]
after_execution

Lifecycle Hooks

Define before and after callbacks to call around execution.

class DecryptSecretMessage < Lite::Command::Base

  after_initialize  :some_method
  before_validation :some_method
  after_validation  :some_method
  before_execution  :some_method
  after_execution   :some_method

  def call
    # ...
  end

end

Status Hooks

Define one or more callbacks that are called after execution for specific statuses.

class DecryptSecretMessage < Lite::Command::Base

  on_success :some_method
  on_noop    :some_method
  on_invalid :some_method
  on_failure :some_method
  on_error   :some_method

  def call
    # ...
  end

end

State Hooks

Define one or more callbacks that are called during transitions between states.

class DecryptSecretMessage < Lite::Command::Base

  on_pending     :some_method
  on_executing   :some_method
  on_complete    :some_method
  on_interrupted :some_method

  def call
    # ...
  end

end

Children

When building complex commands, its best that you pass the parents context to the child command (unless neccessary) so that it gains automated indexing and the parents cmd_id.

class DecryptSecretMessage < Lite::Command::Base

  def call
    context.merge!(decryption_key: ENV["DECRYPT_KEY"])
    ValidateSecretMessage.call(context)
  end

end

Throwing Faults

Throwing faults allows you to bubble up child faults up to the parent. Use it to create branches within your logic and create clean tracing of your command results. You can use throw! as a catch-all or any of the bang status method failure!. Any reason and metadata will be bubbled up from the original fault.

class DecryptSecretMessage < Lite::Command::Base

  def call
    context.merge!(decryption_key: ENV["DECRYPT_KEY"])
    cmd = ValidateSecretMessage.call(context)

    if cmd.invalid?("Invalid magic numbers")
      failure!(cmd) # Manually throw a specific fault
    elsif command.fault?
      throw!(cmd) # Automatically throws a matching fault
    else
      context.decrypted_message = SecretMessage.decrypt(ctx.encrypted_message)
    end
  end

end

Sequences

A sequence is a command that calls commands in a linear fashion. This is useful for composing multiple steps into one call.

[!NOTE] Sequences only stop processing on invalid, failure, and error faults. This is due to the the idea the noop performs no work, so its no different than just passing the context forward. To change this behavior, just override the ok? method with you logic, eg: just success

[!WARNING] Do NOT define a call method in this class. The sequence logic is automatically defined by the sequence class.

class ProcessCheckout < Lite::Command::Sequence

  required :user

  step FinalizeInvoice
  step ChargeCard, if: :card_available?
  step SendConfirmationEmail, SendConfirmationText
  step NotifyWarehouse, unless: proc { ctx.invoice.fullfilled_by_amazon? }

  # Do NOT define a call method.

  private

  def card_available?
    user.has_card?
  end

end

seq = ProcessCheckout.call(...)
# <ProcessCheckout ...>

Results

During any point in the lifecyle of a command, to_hash can be called to dump out the current values. The index value is auto-incremented and the cmd_id is static when its passed to child commands. This helps with debugging and logging.

command = DecryptSecretMessage.call(...)
command.to_hash #=> {
                #=>   index: 1,
                #=>   cmd_id: "018c2b95-b764-7615-a924-cc5b910ed1e5",
                #=>   command: "FailureCommand",
                #=>   outcome: "failure",
                #=>   state: "interrupted",
                #=>   status: "failure",
                #=>   reason: "Command stopped due to some failure",
                #=>   metadata: {
                #=>     errors: { name: ["is too short"] },
                #=>     i18n_key: "command.failure"
                #=>   },
                #=>   caused_by: 3,
                #=>   caused_exception: "[ChildCommand::Failure] something is wrong from within",
                #=>   thrown_by: 2,
                #=>   thrown_exception: "[FailureCommand::Failure] something is wrong from within",
                #=>   runtime: 0.0123
                #=> }

Examples

Disable Instance Calls

class DecryptSecretMessage < Lite::Command::Base

  private_class_method :new

  def call
    # ...
  end

end

DecryptSecretMessage.new(...).call
#=> raise NoMethodError

Generator

rails g command NAME will generate the following file:

app/commands/[NAME]_command.rb

If a ApplicationCommand file in the app/commands directory is available, the generator will create file that inherit from ApplicationCommand if not it will fallback to Lite::Command::Base.

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/[USERNAME]/lite-command. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.

License

The gem is available as open source under the terms of the MIT License.

Code of Conduct

Everyone interacting in the Lite::Command project’s codebases, issue trackers, chat rooms and mailing lists is expected to follow the code of conduct.