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

Table of Contents

• Setup
• Execution
  • Dynamic Faults
• Context
  • Attributes
• States
• Statuses
• Callbacks
  • State Hooks
  • Execution Hooks
  • Status Hooks
• Children
  • Throwing Faults
• Sequences
• Results
• Examples
  • Disable Instance Calls
  • ActiveModel Validations
• Generator

Setup

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

class CalculatePower < Lite::Command::Base

  def call
    if all_even_numbers?
      context.result = ctx.a ** ctx.b
    else
      invalid!("All values must be even")
    end
  end

  private

  def all_even_numbers?
    # Some logic...
  end

end

Tip

You should make all of your domain logic private so that only the 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.

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

# On success, fault and exception:
#=> <CalculatePower ...>

Tip

Class calls is the prefered format due to its readability.

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

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

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

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

# On exception:
#=> raises StandardError

Dynamic Faults

You can enable dynamic faults named after your command. This is especially helpful for catching + running custom logic or filtering out specific errors from you APM service.

class CalculatePower < Lite::Command::Base

  def call
    fail!("Some failure")
  end

  private

  def raise_dynamic_faults?
    true
  end

end

CalculatePower.call!(...)
#=> raises CalculatePower::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 CalculatePower < Lite::Command::Base

  def call
    # `ctx` is an alias to `context`
    context.result = ctx.a ** ctx.b
  end

end

cmd = CalculatePower.call(a: 2, b: 3)
cmd.context.result #=> 8
cmd.ctx.fake       #=> nil

Attributes

Delegate methods for a cleaner command setup, type checking and argument requirements. Setup a contract by using the attribute method which automatically delegates to context.

Options	Values	Default	Description
from	Symbol, String	:context	The object containing the attribute.
types	Symbol, String, Array, Proc		The allowed class types of the attribute value.
required	Symbol, String, Boolean, Proc	false	The attribute must be passed to the context or delegatable (no matter the value).
filled	Symbol, String, Boolean, Proc, Hash	false	The attribute value must be not be nil. Prevent empty values using { empty: false }

Note

If optioned with some similar to filled: true, types: [String, NilClass] then NilClass for the types option will be removed automatically.

class CalculatePower < Lite::Command::Base

  attribute :remote_storage, required: true, filled: true, types: RemoteStorage

  attribute :a, :b
  attribute :c, :d, from: :remote_storage, types: [Integer, Float]
  attribute :x, :y, from: :local_storage, filled: { empty: false }, if: :signed_in?

  def call
    context.result =
      (a.to_i ** b.to_i) +
      (c.to_i + d.to_i) -
      (x.to_i + y.to_i)
  end

  private

  def local_storage
    @local_storage ||= LocalStorage.new(x: 1, y: 1, z: 99)
  end

  def signed_in?
    ctx.user.signed_in?
  end

end

# With valid options:
rs  = RemoteStorage.new(c: 2, d: 2, j: 99)
cmd = CalculatePower.call(a: 2, b: 2, remote_storage: rs)
cmd.status         #=> "success"
cmd.context.result #=> 6

# With invalid options:
cmd = CalculatePower.call
cmd.status   #=> "invalid"
cmd.reason   #=> "Invalid context attributes"
cmd.metadata #=> {
             #=>   context: ["a is required", "remote_storage must be filled"],
             #=>   remote_storage: ["d type invalid"]
             #=>   local_storage: ["is not defined or an attribute"]
             #=> }

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 = CalculatePower.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 CalculatePower < Lite::Command::Base

  def call
    if ctx.a.nil? || ctx.b.nil?
      invalid!("An a and b parameter must be passed")
    elsif ctx.a < 1 || ctx.b < 1
      failure!("Parameters must be >= 1")
    elsif ctx.a == 1 || ctx.b == 1
      noop!(
        "Anything to the power of 1 is 1",
        { i18n: "some.key" }
      )
    else
      ctx.result = ctx.a ** ctx.b
    end
  rescue DivisionError => e
    error!("Cathcing it myself")
  end

end

cmd = CalculatePower.call(a: 1, b: 3)
cmd.status   #=> "noop"
cmd.reason   #=> "Anything to the power of 1 is 1"
cmd.metadata #=> { i18n: "some.key" }

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

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

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

Callbacks

Use callbacks to run arbituary code at transition points and on finalized internals. The following is an example of the hooks called for a failed command with a successful child command.

-> 1. FooCommand.on_pending
-> 2. FooCommand.on_before_execution
-> 3. FooCommand.on_executing
---> 3a. BarCommand.on_pending
---> 3b. BarCommand.on_before_execution
---> 3c. BarCommand.on_executing
---> 3d. BarCommand.on_after_execution
---> 3e. BarCommand.on_success
---> 3f. BarCommand.on_complete
-> 4. FooCommand.on_after_execution
-> 5. FooCommand.on_failure
-> 6. FooCommand.on_interrupted

Status Hooks

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

class CalculatePower < Lite::Command::Base

  def call
    # ...
  end

  private

  def on_pending
    # eg: Append additional contextual data
  end

  def on_executing
    # eg: Insert inspection debugger
  end

  def on_complete
    # eg: Log message for posterity
  end

  def on_interrupted
    # eg: Report to APM with tags and metadata
  end

end

Execution Hooks

Define before and after callbacks to call around execution.

class CalculatePower < Lite::Command::Base

  def call
    # ...
  end

  private

  def on_before_execution
    # eg: Append additional contextual data
  end

  def on_after_execution
    # eg: Store results to database
  end

end

Status Hooks

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

class CalculatePower < Lite::Command::Base

  def call
    # ...
  end

  private

  def on_success
    # eg: Increment KPI counter
  end

  def on_noop(fault)
    # eg: Log message for posterity
  end

  def on_invalid(fault)
    # eg: Send metadata errors to frontend
  end

  def on_failure(fault)
    # eg: Rollback record changes
  end

  def on_error(fault_or_exception)
    # eg: Report to APM with tags and metadata
  end

end

Note

The on_success callback does NOT take any arguments.

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 CalculatePower < Lite::Command::Base

  def call
    context.merge!(some_other: "required value")
    CalculateSqrt.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 CalculatePower < Lite::Command::Base

  def call
    command = CalculateSqrt.call(context.merge!(some_other: "required value"))

    if command.noop?("Sqrt of 1 is 1")
      # Manually throw a specific fault
      invalid!(command)
    elsif command.fault?
      # Automatically throws a matching fault
      throw!(command)
    else
      # Success, do nothing
    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

  attribute :user, required: true, filled: true

  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 = CalculatePower.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 failure",
                #=>   metadata: {
                #=>     errors: { name: ["is too short"] },
                #=>     i18n_key: "command.failure"
                #=>   },
                #=>   caused_by: 1,
                #=>   thrown_by: 1,
                #=>   runtime: 0.0123
                #=> }

Examples

Disable Instance Calls

class CalculatePower < Lite::Command::Base

  private_class_method :new

  def call
    # ...
  end

end

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

ActiveModel Validations

class CalculatePower < Lite::Command::Base
  include ActiveModel::Validations

  validates :a, :b, presence: true

  def call
    # ...
  end

  def read_attribute_for_validation(key)
    context.public_send(key)
  end

  private

  def on_before_execution
    return if valid?

    invalid!(
      errors.full_messages.to_sentence,
      errors.to_hash
    )
  end

end

CalculatePower.call!

# With `validate!`
#=> raise ActiveRecord::RecordInvalid

# With `valid?`
#=> raise Lite::Command::Invalid

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.