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-developer 0.1.0

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?

The iconic wtf? method is one way to invoke an operation with tracing turned on. During the execution, invoked steps are traced and printed in the console as a tree-view, helping you to understand the code path the operation took. It is one of Trailblazer’s most-loved features and was introduced in Trailblazer 2.1.

When working with a Trailblazer::Operation simply invoke its #wtf? method.

result = Memo::Operation::Create.wtf?(params: {title: "Remember me..."})

On the console, you can see the printed trace in all its beauty.

This method is especially helpful 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.

• Wtf?
• Exception

In case of an exception somewhere in the operation or its nested activities, wtf? will print the trace path to the closest point where the exception was thrown.

The original exception will be visible on the console, too.

• Wtf?
• Activity

The #wtf? class method is only available for Trailblazer::Operation subclasses. You will get an exception if you try to use it with Trailblazer::Activity::Railway and other activities.

NoMethodError: undefined method `wtf?' for Memo::Operation::Create:Class

However, tracing can be used with low-level Activity classes, too.

module Memo::Operation
  class Create < Trailblazer::Activity::Railway # Note that this is not an {Operation}!
    step :extract_markdown
    step :model
    # ...
  end
end

You have to use the clumsier Developer.wtf? method for tracing.

signal, (ctx, _) = Trailblazer::Developer.wtf?(
  Memo::Operation::Create, [{params: {title: "Remember me.."}}, {}]
)

As always with Activity subclasses, it operates with the circuit interface.

• 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

Graph

To find out the structure and composition details about any activity, use the Introspect API.

• Graph
• Find

You may use Graph#find with a block for a more complex search.

node = graph.find { |node| node.task.class == Trailblazer::Activity::TaskBuilder }

Alternatively, using the Graph#find method with an ID provided, you can retrieve a particular node in the activity’s circuit.

Consider the following activity.

class Memo::Update < Trailblazer::Activity::Railway
  step :find_model
  step :validate, Output(:failure) => End(:validation_error)
  step :save
  fail :log_error
end

You can introspect a certain element by using find(id).

graph = Trailblazer::Developer::Introspect.Graph(Memo::Update)

node = graph.find(:validate)

Note that all queries go via a Graph instance.

• Graph
• Node API

The returned node instance exposes several inspect helpers.

The ID of the element is retrieved by using #id.

# puts node.id.inspect #=> :validate

To see what actual task sits behind this circuit’s node, use #task. This is always the low-level task that responds to a circuit interface. It might be a wrapper around your actual logic, provided by the DSL.

# puts node.task       #=> #Trailblazer::Activity::TaskBuilder::Task user_proc=validate>

Outgoing connections from a task can be queried using #outgoings. This returns an array of all outgoing arrows.

# left, right = node.outgoings # returns array

• Graph
• Outgoing

An Outgoing provides several interesting fields.

You can retrieve the connected element by using #task. It returns the actual low-level task.

# puts left.task #=> #Trailblazer::Activity::End semantic=:validation_error>

The #output method returns the Output instance that connects the task to the next element.

# puts left.output.signal   #=> Trailblazer::Activity::Left
# puts left.output.semantic #=> :failure

The Output#signal field returns the signal object the task returns to trigger that connection. To see the semantic of this output, use Output#semantic.

• Graph
• Outputs

The Node#outputs method returns an array of Output objects defining each output of outgoing connections of a specific node.

outputs = node.outputs
left = outputs[0] #=> output object