About

The Trailblazer project started off with a single gem called trailblazer. Currently, the framework consists of around 40 gems. The main gem trailblazer is an umbrella gem with the sole reason to pull all default dependencies for you. Feel free to pick and choose what you need for your applications.

Overview

The following list of gems is an overview of the most important core components in Trailblazer.

Along with the core gems goes a rich list of eco-system gems.

2.1 Migration

Call API

• Call API
• Operation.call

In versions before 2.1, the automatic merging of the params part and the additional options was confusing many new users and an unnecessary step.

# old style
result = Memo::Create.( params, "current_user" => current_user )

The first argument (params) was merged into the second argument using the key “params”. You now pass one hash to call and hence do the merging yourself.

# new style
result = Memo::Create.( params: params, current_user: current_user )

The new call API is much more consistent and takes away another thing we kept explaining to new users - an indicator for a flawed API.

require "trailblazer/deprecation/call"

• Call API
• Step Signature

In case your steps expose a signature as follows, you’re safe.

class Memo::Create < Trailblazer::Operation
  step :create_model

  def create_model(options, params:, **)
    # ..
  end
end

By convention, we renamed option to ctx, but it is completely up to you to adopt this.

Nevertheless, the “old style” signatures won’t work anymore.

class Memo::Create < Trailblazer::Operation
  def create_model(options)
    # ..
  end

  # or

  def create_model(model:, **)
    # ..
  end
end

Neither a single options nor keyword-arguments-only are gonna fly as the new step signature is more restrictive and always requires you to maintain the ctx (or options, if you will) as the first positional argument, then keyword arguments.

Double splat operator ** at the end will be required to in order to discard unused kw args.

• Call API
• pass/fail

Steps declared as success or failure are now renamed to pass and fail respectively.

class Memo::Create < Trailblazer::Operation
  success :model
  failure :handle_some_error
end

Change it as follows.

class Memo::Create < Trailblazer::Operation
  pass :model
  fail :handle_some_error
end

If you are using Rubocop it will probably start complaining about unreachable code because it just so happens that fail is also a Ruby Kernel’s method. One solution to this could be to add a custom rule to .rubocop.yml like this:

Lint/UnreachableCode:
  Exclude:
    - '*/**/concepts/**/operation/**/*'
Style/SignalException:
  Exclude:
    - '*/**/concepts/**/operation/**/*'

There is also a trick that will allow you to do this rename before actually migrating to 2.1. You can put this in an initializer:

Trailblazer::Operation::Pipetree::DSL.send(:alias_method, :pass, :success)
Trailblazer::Operation::Pipetree::DSL.send(:alias_method, :fail, :failure)

This way you could introduce this change before actual migration to 2.1. Just don’t forget to remove it after updating gems to 2.1.

• Call API
• pass_fast/fail_fail

Now every step that may end up in pass_fast or fail_fast signals need an extra option that indicates fast track usage. Change this operation:

class Memo::Create < Trailblazer::Operation
  step :create

  def create(ctx, **)
    Railway.pass_fast! if ctx[:model].save
  end
end

Use the :fast_track option to let Trailblazer know about the potential new signal being emitted.

class Memo::Create < Trailblazer::Operation
  step :create, fast_track: true # notice the extra option :fast_track

  def create(ctx, **)
    Railway.pass_fast! if ctx[:model].save
  end
end

Context

The keys for ctx used to be mixed up, some where "longer.strings", some were :symbols. The new context implementation Context::IndifferentAccess now allows to use both.

ctx["model"]
ctx[:model]  # => same!

This also works for namespaced keys, which you still might find helpful.

ctx["contract.default"]
ctx[:"contract.default"]  # => same!

The default implementation of the context object can be set by overriding Context.implementation. For example, if you want the old behavior back.

class Trailblazer::Context
  def self.implementation
    Context # old behavior.
  end
end

Note that the override might be deprecated in favor of a dependency injection.

Nested

The Nested macro allows to, well, nest activities or operations, providing a neat way to encapsulate and reuse logic.

• Nested
• Deprecation

In 2.1, the [Subprocess macro] is the standard way for nesting. The Nested macro should only be used when you use the dynamic version where the nested operation is computed at runtime using :builder.

An exception will warn you about the inappropriate usage.

[Trailblazer] Using the `Nested()` macro with operations and activities is deprecated. Replace `Nested(Create)` with `Subprocess(Create)`.

• Nested
• input and output

Both the :input and :output options that used to go with Nested(Present, :input: ...) are now a generic option in Trailblazer. Move them behind the macro parenthesis.

# 2.0
Nested(Present, input: :my_input, output: :my_output)

# 2.1
Nested(Present), input: :my_input, output: :my_output

An exception will stop compilation if you fail to obey.

ArgumentError: unknown keyword: input

Behold of another tiny API change! The :output filter signature has changed for a better.

In Trailblazer 2.0, the following signature was allowed.

# 2.0
Nested(Present,
  output: ->(ctx, mutable_data:, **) {
    {model: mutable_data[:article]} # you used the {mutable_data} keyword argument.
  }
)

Mind the arguments being passed to :output. The first positional is the original outer context, the context from the nested operation comes as a mutable_data: keyword argument.

In 2.1, those two have swapped.

# 2.1
Nested(Present),
  output: ->(inner_ctx, article:, **) {
    # {inner_ctx} is {mutable_data}
    {model: article} # you used the {mutable_data} keyword argument.
  }

The inner context from the nested operation comes first, as a positional argument. Note how you can conveniently use keyword arguments to access variables from this inner ctx (e.g. article:). Keep in mind, the naming is completely up to you. We use inner_ctx/ctx and original_ctx in our code.

If you also need the original context in the :output filter, use the :output_with_outer_ctx option.

# 2.1
Nested(Present),
  output_with_outer_ctx: true
  output: ->(inner_ctx, original_ctx, article:, **) {
    {
      model:  article,
      errors: original_ctx[:errors].merge(inner_ctx[:errors]) # you can access the outer ctx!
    }
  }

The original, outer ctx comes in as a second positional argument.

Check the Variable Mapping docs for a complete explanation.

• Nested
• Fast Track

Don’t forget to declare fast track usage if you expect it from within nested operation, like this:

Nested(Present), input: :my_input, output: :my_output, fast_track: true

Another difference is that in 2.0, when you were using pass_fast in nested operations, it would stop only the nested operation from executing. After this the outer one would continue executing.

Now returning pass_fast in nested operation will stop both, inner and outer operation with success as a final result. If you rely on old behaviour you can still have it with proper signals mapping:

Nested(Present), input: :my_input,
                 output: :my_output,
                 fast_track: true,
                 Output(:pass_fast) => Track(:success), # pass_fast now mapped to `just` a success
                 Output(:fail_fast) => End(:failure)

Macro API

Macros are functions that add a task along with specific options to the activity. In TRB 2.0, those (historically camel-cased) functions returned an array with two elements.

module MyMacro
  def self.NormalizeParams(name: :myparams, merge_hash: {})
    task = ->((ctx, flow_options), _) do
      ctx[name] = ctx[:params].merge(merge_hash)

      return Trailblazer::Activity::Right, [ctx, flow_options]
    end

    [task, name: name] # old API
  end
end

In 2.1, a hash is returned. Note that :name is :id now.

module MyMacro
  def self.NormalizeParams(name: :myparams, merge_hash: {})
    task = ->((ctx, flow_options), _) do
      ctx[name] = ctx[:params].merge(merge_hash)

      return Trailblazer::Activity::Right, [ctx, flow_options]
    end

    # new API
    {
      task: task,
      id:   name
    }
  end
end

This allows for a much richer macro experience where you might add additional steps via a macro, use DSL options such as :before and :after and add taskWrap extensions. [macro API]

A common error if you don’t return a hash from your macro is a call on Array.

NoMethodError:
   undefined method `call' for #<Array:0x000055663116be98>
 # gems/trailblazer-context-0.4.0/lib/trailblazer/option.rb:9:in `call!'

• Macro API
• Signature and Return

In Trailblazer 2.0, macros were automatically using the task interface. Mind the arguments and the return value.

task = ->(ctx, current_user:, model:, **) {
  # do something
  # return true/false
}

In Trailblazer 2.1, macros use the circuit interface per default. If you want the simpler task interface, you need to wrap your task using Binary().

def MyMacro()
  task = ->(ctx, current_user:, model:, **) {
    # do something
    # return true/false
  }

  {
    task: Trailblazer::Activity::TaskBuilder.Binary(task), # wrap the callable.
    id: "my_macro"
  }

A common error if you don’t wrap your macro is an ArgumentError.

ArgumentError:
       missing keywords: current_user, model

• Macro API
• Step id

In case you used the same macro twice in one operation, like this for example:

class Create < Trailblazer::Operation
  step Contract::Persist(method: :sync)
  step Contract::Persist(method: :save!)
end

You will have to specify id explictly for one of them:

class Create < Trailblazer::Operation
  step Contract::Persist(method: :sync)
  step Contract::Persist(method: :save!), id: "some_custom_unique_id"
end

Contract DSL

It was possible to define contracts on the operation level using a DSL.

class Create < Trailblazer::Operation
  contract do
    property :id
  end

  step Contract::Build()
  step Contract::Validate()
end

Since the usability doesn’t outweigh the complexity needed to implement such DSL, we decided to remove that functionality for now.

Instead, use an explicit inline class and the :constant option.

class Create < Trailblazer::Operation
  class Form < Reform::Form
    property :id
  end

  step Contract::Build(constant: Form)
  step Contract::Validate()
end

Trailblazer loader

Usage of a trailblazer-loader is now discouraged as it’s slower than the ones provided by Rails and it’s error prone.

In short, we decided to adopt the Rails naming scheme and change operation names from User::Create to User::Operation::Create, so the file name and class path are in sync.

Read the details here.

We highly recommend changing this with your upgrade as it highly improves the dev experience.

Developer

Trailblazer provides a rich set of developer tools to ease debugging and make modelling processes a pleasant experience.

The developer gem contains tools to help you visualize and debug Trailblazer code. Its development features such as tracing or exception handling are advanced tools and will become your best friends very quickly.

Constant

We advise you to alias the Developer constant to something handy, such as ::Dev in your project. For instance, in Rails you’d have the following line in a config/initializers/trailblazer.rb initializer.

# config/initializers/trailblazer.rb
require "trailblazer/developer"
Dev = Trailblazer::Developer

Wtf?

Use wtf? if you want to

• Debug an exception happening somewhere deep in your code.
• Find out which step changed the track to failure.
• Focus on specific step(s) to find out what ctx mutations are happening inside them.

This will run your activity with tracing enabled, and in case of an exception, print the trace path to the closest point where the exception was thrown.

Your activity will be called using the TaskWrap invoker, a possible exception will be caught and the closest task found. It then prints the “stack trace”, however, on a much higher level based on tasks, not methods.

• Wtf?
• focus_on

You can focus on specific steps and variables to find out what ctx comes in and what goes out. The focus_on option allows us to capture any key(s) from ctx and print any mutations happening within given steps.

You need to pass one or more step names (either default name or given by explicit :id) to capture the mutations.

class Memo::Create < Trailblazer::Activity::Path
  step :validate
  step :create, id: :create_memo
  # ...
end

Dev.wtf?(Memo::Create, [ctx, { focus_on: { steps: [:validate, :create_memo] } }])

By default, focusing will capture whole ctx for given steps. But you can also filter on it by passing one or more keys using variables.

Dev.wtf?(Memo::Create, [ctx, { focus_on: { variables: [:params], steps: :validate } }])

• Wtf?
• default_inspector

Internally, when focus_on option is passed, we call Object#inspect method to collect before-after data from the ctx for given steps. This data is then used while rendering the trace. In case you want to customize the data collection behaviour with your own logic, you can pass the default_inspector.

Dev.wtf?(
  Memo::Create,
  [
    { params: { text: 'Hydrate!', value: nil } },
    {
      focus_on: { steps: :validate, variables: :params },
      default_inspector: ->(value){ value.nil? ? 'UNKNOWN' : value.inspect }
    }
  ]
)

Inspection runs deeply when ctx contains nested structures like hashes or arrays. It’s concept is very simple and handled in Dev::Trace::Inspector module.

• Wtf?
• color_map

The color_map option allows you to customize default coloring scheme being used to render passed or failed steps.

Render

It often is helpful to visualize an activity. One way is the render helper.

puts Trailblazer::Developer.render(Memo::Create)

The rendered output prints each task, its outputs and where they’re connected to.

Render Linear

Client

The Developer::Client module provides functions to work with activities and workflows created in the PRO editor.

• Client
• Import

Notes

• IDs are extracted from the label of the element. They get chomped to remove a potential newline at the end and reduce confusion.
• It is currently not possible to assign semantics to arrows via the label. In the meantime, use the :query option. # TODO

Context

Context aka ctx (or plain old options) is a core argument-specific data structure for Trailblazer. It provides a generic, ordered read/write interface that collects mutable runtime-computed data while providing access to any compile-time information. It is extracted into its own gem and can also be used independently.

ctx

ctx can be initialized when an operation is invoked at the run time or by defining dependencies at the compile time. Inside the operation, it gets passed down to every step with it’s argument position depending on step’s interface. It will contain whatever the most recently executed step has changed and hopefully contains what you’re expecting.

ctx’s purpose is to hold the state of your activity which can also be passed down to other nested activities using Subprocess. You can filter what such activities can or can not “see” using variable mapping. After operation’s execution using public call, the Result object wraps the context for convenient access.

In order to provide the generic interface, scoping and debugging capabilities, the “Hash” argument you provide to an operation is initialized as an instance of Trailblazer::Context::Container to build up the final ctx. This allows us to support more features on top of it like indifferent access, aliasing etc

• ctx
• Indifferent Access

ctx mimics as “Hash” and also allows you to use Strings or Symbols interchangeably as keys; similar to the params hash in Rails.

result = Memo::Create.(params: { text: "Enjoy an IPA" })

result[:params]     # => { text: "Enjoy an IPA" }
result['params']    # => { text: "Enjoy an IPA" }

All keys are stored as Symbols by default in order to allow them to be accessible as keyword arguments. Note that ctx doesn’t provide interchangeable keys for any nested hashes because of the performance reasons.

• ctx
• Aliases

Most commonly found keys in ctx are 'contract.default', 'contract.default.params', 'policy.default' etc. It sometimes becomes cumbersome to access them from ctx as they can’t be defined as keyword arguments in steps.

To overcome this, it is possible to define a shorter versions of context keys using aliases. By providing aliases mapping in flow_options[:context_options], context will maintain any mutations being made on the origianl keys with the aliases and vice versa.

options = { params: { text: "Enjoy an IPA" } }
flow_options = {
  context_options: {
    aliases: { 'contract.default': :contract, 'policy.default': :policy },
    container_class: Trailblazer::Context::Container::WithAliases,
  }
}

# Sorry, this feature is only reliable in Ruby > 2.7
if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new("3.0.0")
  result = AliasesExample::Memo::Create.(options, flow_options)
else # Ruby 2.6 etc
  result = AliasesExample::Memo::Create.call_with_flow_options(options, flow_options)
end

result['contract.default']  # => Memo::Contract::Create
result[:contract]           # => Memo::Contract::Create

flow_options are passed to the nested operations via Subprocess and all given aliases will also be applied in them by default.

class Memo::Create < Trailblazer::Operation
  # ...

  pass :sync

  def sync(ctx, contract:, **)
    # ctx['contract.default'] == ctx[:contract]
    contract.sync
  end
end

Option

Trailblazer::Option is one of the core concept behind operation’s step API, reform’s populator API etc. It makes us possible to accept any kind of callable objects at compile time and execute them at runtime.

class Song::Create < Trailblazer::Operation
  step Authorize				# Module callable
  step :model					# Method callable
  step ->(ctx, model:, **) { puts model }	# Proc callable
end

It is also a replacement over declarative-option and has been extracted out from trailblazer-context by identifying common callable patterns.

Callables

Trailblazer::Option() accepts Symbol, lambda and any other type of callable as an argument. It will be wrapped accordingly to make an executable, so you can evaluate it at runtime.

Passing a Symbol will be treated as a method that’s called on the given exec_context.

option = Trailblazer::Option(:object_id)
option.(exec_context: Object.new) # => 1234567

Same with the objects responding to .call or #call method.

class CallMe
  def self.call(message:, **options)
    message
  end
end

option = Trailblazer::Option(CallMe)
option.(keyword_arguments: { message: "hello!" }, exec_context: nil) # => "hello!"

And of course, passing lambdas. They get executed within given exec_context when set.

option = Trailblazer::Option(-> { object_id })
option.(exec_context: Object.new) # => 1234567