rails--rails/guides/source/debugging_rails_applications.md

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Debugging Rails Applications

This guide introduces techniques for debugging Ruby on Rails applications.

After reading this guide, you will know:

• The purpose of debugging.
• How to track down problems and issues in your application that your tests aren't identifying.
• The different ways of debugging.
• How to analyze the stack trace.

---

View Helpers for Debugging

One common task is to inspect the contents of a variable. Rails provides three different ways to do this:

• debug
• to_yaml
• inspect

debug

The debug helper will return a <pre> tag that renders the object using the YAML format. This will generate human-readable data from any object. For example, if you have this code in a view:

<%= debug @article %>
<p>
  <b>Title:</b>
  <%= @article.title %>
</p>

You'll see something like this:

--- !ruby/object Article
attributes:
  updated_at: 2008-09-05 22:55:47
  body: It's a very helpful guide for debugging your Rails app.
  title: Rails debugging guide
  published: t
  id: "1"
  created_at: 2008-09-05 22:55:47
attributes_cache: {}


Title: Rails debugging guide

to_yaml

Alternatively, calling to_yaml on any object converts it to YAML. You can pass this converted object into the simple_format helper method to format the output. This is how debug does its magic.

<%= simple_format @article.to_yaml %>
<p>
  <b>Title:</b>
  <%= @article.title %>
</p>

The above code will render something like this:

--- !ruby/object Article
attributes:
updated_at: 2008-09-05 22:55:47
body: It's a very helpful guide for debugging your Rails app.
title: Rails debugging guide
published: t
id: "1"
created_at: 2008-09-05 22:55:47
attributes_cache: {}

Title: Rails debugging guide

inspect

Another useful method for displaying object values is inspect, especially when working with arrays or hashes. This will print the object value as a string. For example:

<%= [1, 2, 3, 4, 5].inspect %>
<p>
  <b>Title:</b>
  <%= @article.title %>
</p>

Will render:

[1, 2, 3, 4, 5]

Title: Rails debugging guide

The Logger

It can also be useful to save information to log files at runtime. Rails maintains a separate log file for each runtime environment.

What is the Logger?

Rails makes use of the ActiveSupport::Logger class to write log information. Other loggers, such as Log4r, may also be substituted.

You can specify an alternative logger in config/application.rb or any other environment file, for example:

config.logger = Logger.new(STDOUT)
config.logger = Log4r::Logger.new("Application Log")

Or in the Initializer section, add any of the following

Rails.logger = Logger.new(STDOUT)
Rails.logger = Log4r::Logger.new("Application Log")

TIP: By default, each log is created under Rails.root/log/ and the log file is named after the environment in which the application is running.

Log Levels

When something is logged, it's printed into the corresponding log if the log level of the message is equal to or higher than the configured log level. If you want to know the current log level, you can call the Rails.logger.level method.

The available log levels are: :debug, :info, :warn, :error, :fatal, and :unknown, corresponding to the log level numbers from 0 up to 5, respectively. To change the default log level, use

config.log_level = :warn # In any environment initializer, or
Rails.logger.level = 0 # at any time

This is useful when you want to log under development or staging without flooding your production log with unnecessary information.

TIP: The default Rails log level is debug in all environments.

Sending Messages

To write in the current log use the logger.(debug|info|warn|error|fatal|unknown) method from within a controller, model, or mailer:

logger.debug "Person attributes hash: #{@person.attributes.inspect}"
logger.info "Processing the request..."
logger.fatal "Terminating application, raised unrecoverable error!!!"

Here's an example of a method instrumented with extra logging:

class ArticlesController < ApplicationController
  # ...

  def create
    @article = Article.new(article_params)
    logger.debug "New article: #{@article.attributes.inspect}"
    logger.debug "Article should be valid: #{@article.valid?}"

    if @article.save
      logger.debug "The article was saved and now the user is going to be redirected..."
      redirect_to @article, notice: 'Article was successfully created.'
    else
      render :new, status: :unprocessable_entity
    end
  end

  # ...

  private
    def article_params
      params.require(:article).permit(:title, :body, :published)
    end
end

Here's an example of the log generated when this controller action is executed:

Started POST "/articles" for 127.0.0.1 at 2018-10-18 20:09:23 -0400
Processing by ArticlesController#create as HTML
  Parameters: {"utf8"=>"✓", "authenticity_token"=>"XLveDrKzF1SwaiNRPTaMtkrsTzedtebPPkmxEFIU0ordLjICSnXsSNfrdMa4ccyBjuGwnnEiQhEoMN6H1Gtz3A==", "article"=>{"title"=>"Debugging Rails", "body"=>"I'm learning how to print in logs.", "published"=>"0"}, "commit"=>"Create Article"}
New article: {"id"=>nil, "title"=>"Debugging Rails", "body"=>"I'm learning how to print in logs.", "published"=>false, "created_at"=>nil, "updated_at"=>nil}
Article should be valid: true
   (0.0ms)  begin transaction
  ↳ app/controllers/articles_controller.rb:31
  Article Create (0.5ms)  INSERT INTO "articles" ("title", "body", "published", "created_at", "updated_at") VALUES (?, ?, ?, ?, ?)  [["title", "Debugging Rails"], ["body", "I'm learning how to print in logs."], ["published", 0], ["created_at", "2018-10-19 00:09:23.216549"], ["updated_at", "2018-10-19 00:09:23.216549"]]
  ↳ app/controllers/articles_controller.rb:31
   (2.3ms)  commit transaction
  ↳ app/controllers/articles_controller.rb:31
The article was saved and now the user is going to be redirected...
Redirected to http://localhost:3000/articles/1
Completed 302 Found in 4ms (ActiveRecord: 0.8ms)

Adding extra logging like this makes it easy to search for unexpected or unusual behavior in your logs. If you add extra logging, be sure to make sensible use of log levels to avoid filling your production logs with useless trivia.

Verbose Query Logs

When looking at database query output in logs, it may not be immediately clear why multiple database queries are triggered when a single method is called:

irb(main):001:0> Article.pamplemousse
  Article Load (0.4ms)  SELECT "articles".* FROM "articles"
  Comment Load (0.2ms)  SELECT "comments".* FROM "comments" WHERE "comments"."article_id" = ?  [["article_id", 1]]
  Comment Load (0.1ms)  SELECT "comments".* FROM "comments" WHERE "comments"."article_id" = ?  [["article_id", 2]]
  Comment Load (0.1ms)  SELECT "comments".* FROM "comments" WHERE "comments"."article_id" = ?  [["article_id", 3]]
=> #<Comment id: 2, author: "1", body: "Well, actually...", article_id: 1, created_at: "2018-10-19 00:56:10", updated_at: "2018-10-19 00:56:10">

After running ActiveRecord::Base.verbose_query_logs = true in the bin/rails console session to enable verbose query logs and running the method again, it becomes obvious what single line of code is generating all these discrete database calls:

irb(main):003:0> Article.pamplemousse
  Article Load (0.2ms)  SELECT "articles".* FROM "articles"
  ↳ app/models/article.rb:5
  Comment Load (0.1ms)  SELECT "comments".* FROM "comments" WHERE "comments"."article_id" = ?  [["article_id", 1]]
  ↳ app/models/article.rb:6
  Comment Load (0.1ms)  SELECT "comments".* FROM "comments" WHERE "comments"."article_id" = ?  [["article_id", 2]]
  ↳ app/models/article.rb:6
  Comment Load (0.1ms)  SELECT "comments".* FROM "comments" WHERE "comments"."article_id" = ?  [["article_id", 3]]
  ↳ app/models/article.rb:6
=> #<Comment id: 2, author: "1", body: "Well, actually...", article_id: 1, created_at: "2018-10-19 00:56:10", updated_at: "2018-10-19 00:56:10">

Below each database statement you can see arrows pointing to the specific source filename (and line number) of the method that resulted in a database call. This can help you identify and address performance problems caused by N+1 queries: single database queries that generates multiple additional queries.

Verbose query logs are enabled by default in the development environment logs after Rails 5.2.

WARNING: We recommend against using this setting in production environments. It relies on Ruby's Kernel#caller method which tends to allocate a lot of memory in order to generate stacktraces of method calls.

Tagged Logging

When running multi-user, multi-account applications, it's often useful to be able to filter the logs using some custom rules. TaggedLogging in Active Support helps you do exactly that by stamping log lines with subdomains, request ids, and anything else to aid debugging such applications.

logger = ActiveSupport::TaggedLogging.new(Logger.new(STDOUT))
logger.tagged("BCX") { logger.info "Stuff" }                            # Logs "[BCX] Stuff"
logger.tagged("BCX", "Jason") { logger.info "Stuff" }                   # Logs "[BCX] [Jason] Stuff"
logger.tagged("BCX") { logger.tagged("Jason") { logger.info "Stuff" } } # Logs "[BCX] [Jason] Stuff"

Impact of Logs on Performance

Logging will always have a small impact on the performance of your Rails app, particularly when logging to disk. Additionally, there are a few subtleties:

Using the :debug level will have a greater performance penalty than :fatal, as a far greater number of strings are being evaluated and written to the log output (e.g. disk).

Another potential pitfall is too many calls to Logger in your code:

logger.debug "Person attributes hash: #{@person.attributes.inspect}"

In the above example, there will be a performance impact even if the allowed output level doesn't include debug. The reason is that Ruby has to evaluate these strings, which includes instantiating the somewhat heavy String object and interpolating the variables.

Therefore, it's recommended to pass blocks to the logger methods, as these are only evaluated if the output level is the same as — or included in — the allowed level (i.e. lazy loading). The same code rewritten would be:

logger.debug {"Person attributes hash: #{@person.attributes.inspect}"}

The contents of the block, and therefore the string interpolation, are only evaluated if debug is enabled. This performance savings are only really noticeable with large amounts of logging, but it's a good practice to employ.

INFO: This section was written by Jon Cairns at a StackOverflow answer and it is licensed under cc by-sa 4.0.

Debugging with the byebug gem

When your code is behaving in unexpected ways, you can try printing to logs or the console to diagnose the problem. Unfortunately, there are times when this sort of error tracking is not effective in finding the root cause of a problem. When you actually need to journey into your running source code, the debugger is your best companion.

The debugger can also help you if you want to learn about the Rails source code but don't know where to start. Just debug any request to your application and use this guide to learn how to move from the code you have written into the underlying Rails code.

Setup

You can use the byebug gem to set breakpoints and step through live code in Rails. To install it, just run:

$ gem install byebug

Inside any Rails application you can then invoke the debugger by calling the byebug method.

Here's an example:

class PeopleController < ApplicationController
  def new
    byebug
    @person = Person.new
  end
end

The Shell

As soon as your application calls the byebug method, the debugger will be started in a debugger shell inside the terminal window where you launched your application server, and you will be placed at the debugger's prompt (byebug). Before the prompt, the code around the line that is about to be run will be displayed and the current line will be marked by '=>', like this:

[1, 10] in /PathTo/project/app/controllers/articles_controller.rb
    3:
    4:   # GET /articles
    5:   # GET /articles.json
    6:   def index
    7:     byebug
=>  8:     @articles = Article.find_recent
    9:
   10:     respond_to do |format|
   11:       format.html # index.html.erb
   12:       format.json { render json: @articles }

(byebug)

If you got there by a browser request, the browser tab containing the request will be hung until the debugger has finished and the trace has finished processing the entire request.

For example:

=> Booting Puma
=> Rails 6.0.0 application starting in development
=> Run `bin/rails server --help` for more startup options
Puma starting in single mode...
* Version 3.12.1 (ruby 2.5.7-p206), codename: Llamas in Pajamas
* Min threads: 5, max threads: 5
* Environment: development
* Listening on tcp://localhost:3000
Use Ctrl-C to stop
Started GET "/" for 127.0.0.1 at 2014-04-11 13:11:48 +0200
  ActiveRecord::SchemaMigration Load (0.2ms)  SELECT "schema_migrations".* FROM "schema_migrations"
Processing by ArticlesController#index as HTML

[3, 12] in /PathTo/project/app/controllers/articles_controller.rb
    3:
    4:   # GET /articles
    5:   # GET /articles.json
    6:   def index
    7:     byebug
=>  8:     @articles = Article.find_recent
    9:
   10:     respond_to do |format|
   11:       format.html # index.html.erb
   12:       format.json { render json: @articles }
(byebug)

Now it's time to explore your application. A good place to start is by asking the debugger for help. Type: help

(byebug) help

  break      -- Sets breakpoints in the source code
  catch      -- Handles exception catchpoints
  condition  -- Sets conditions on breakpoints
  continue   -- Runs until program ends, hits a breakpoint or reaches a line
  debug      -- Spawns a subdebugger
  delete     -- Deletes breakpoints
  disable    -- Disables breakpoints or displays
  display    -- Evaluates expressions every time the debugger stops
  down       -- Moves to a lower frame in the stack trace
  edit       -- Edits source files
  enable     -- Enables breakpoints or displays
  finish     -- Runs the program until frame returns
  frame      -- Moves to a frame in the call stack
  help       -- Helps you using byebug
  history    -- Shows byebug's history of commands
  info       -- Shows several information about the program being debugged
  interrupt  -- Interrupts the program
  irb        -- Starts an IRB session
  kill       -- Sends a signal to the current process
  list       -- Lists lines of source code
  method     -- Shows methods of an object, class or module
  next       -- Runs one or more lines of code
  pry        -- Starts a Pry session
  quit       -- Exits byebug
  restart    -- Restarts the debugged program
  save       -- Saves current byebug session to a file
  set        -- Modifies byebug settings
  show       -- Shows byebug settings
  source     -- Restores a previously saved byebug session
  step       -- Steps into blocks or methods one or more times
  thread     -- Commands to manipulate threads
  tracevar   -- Enables tracing of a global variable
  undisplay  -- Stops displaying all or some expressions when program stops
  untracevar -- Stops tracing a global variable
  up         -- Moves to a higher frame in the stack trace
  var        -- Shows variables and its values
  where      -- Displays the backtrace

(byebug)

To see the previous ten lines you should type list- (or l-).

(byebug) l-

[1, 10] in /PathTo/project/app/controllers/articles_controller.rb
   1  class ArticlesController < ApplicationController
   2    before_action :set_article, only: [:show, :edit, :update, :destroy]
   3
   4    # GET /articles
   5    # GET /articles.json
   6    def index
   7      byebug
   8      @articles = Article.find_recent
   9
   10     respond_to do |format|

This way you can move inside the file and see the code above the line where you added the byebug call. Finally, to see where you are in the code again you can type list=

(byebug) list=

[3, 12] in /PathTo/project/app/controllers/articles_controller.rb
    3:
    4:   # GET /articles
    5:   # GET /articles.json
    6:   def index
    7:     byebug
=>  8:     @articles = Article.find_recent
    9:
   10:     respond_to do |format|
   11:       format.html # index.html.erb
   12:       format.json { render json: @articles }
(byebug)

The Context

When you start debugging your application, you will be placed in different contexts as you go through the different parts of the stack.

The debugger creates a context when a stopping point or an event is reached. The context has information about the suspended program which enables the debugger to inspect the frame stack, evaluate variables from the perspective of the debugged program, and know the place where the debugged program is stopped.

At any time you can call the backtrace command (or its alias where) to print the backtrace of the application. This can be very helpful to know how you got where you are. If you ever wondered about how you got somewhere in your code, then backtrace will supply the answer.

(byebug) where
--> #0  ArticlesController.index
      at /PathToProject/app/controllers/articles_controller.rb:8
    #1  ActionController::BasicImplicitRender.send_action(method#String, *args#Array)
      at /PathToGems/actionpack-5.1.0/lib/action_controller/metal/basic_implicit_render.rb:4
    #2  AbstractController::Base.process_action(action#NilClass, *args#Array)
      at /PathToGems/actionpack-5.1.0/lib/abstract_controller/base.rb:181
    #3  ActionController::Rendering.process_action(action, *args)
      at /PathToGems/actionpack-5.1.0/lib/action_controller/metal/rendering.rb:30
...

The current frame is marked with -->. You can move anywhere you want in this trace (thus changing the context) by using the frame n command, where n is the specified frame number. If you do that, byebug will display your new context.

(byebug) frame 2

[176, 185] in /PathToGems/actionpack-5.1.0/lib/abstract_controller/base.rb
   176:       # is the intended way to override action dispatching.
   177:       #
   178:       # Notice that the first argument is the method to be dispatched
   179:       # which is *not* necessarily the same as the action name.
   180:       def process_action(method_name, *args)
=> 181:         send_action(method_name, *args)
   182:       end
   183:
   184:       # Actually call the method associated with the action. Override
   185:       # this method if you wish to change how action methods are called,
(byebug)

The available variables are the same as if you were running the code line by line. After all, that's what debugging is.

You can also use up [n] and down [n] commands in order to change the context n frames up or down the stack respectively. n defaults to one. Up in this case is towards higher-numbered stack frames, and down is towards lower-numbered stack frames.

Threads

The debugger can list, stop, resume, and switch between running threads by using the thread command (or the abbreviated th). This command has a handful of options:

• thread: shows the current thread.
• thread list: is used to list all threads and their statuses. The current thread is marked with a plus (+) sign.
• thread stop n: stops thread n.
• thread resume n: resumes thread n.
• thread switch n: switches the current thread context to n.

This command is very helpful when you are debugging concurrent threads and need to verify that there are no race conditions in your code.

Inspecting Variables

Any expression can be evaluated in the current context. To evaluate an expression, just type it!

This example shows how you can print the instance variables defined within the current context:

[3, 12] in /PathTo/project/app/controllers/articles_controller.rb
    3:
    4:   # GET /articles
    5:   # GET /articles.json
    6:   def index
    7:     byebug
=>  8:     @articles = Article.find_recent
    9:
   10:     respond_to do |format|
   11:       format.html # index.html.erb
   12:       format.json { render json: @articles }

(byebug) instance_variables
[:@_action_has_layout, :@_routes, :@_request, :@_response, :@_lookup_context,
 :@_action_name, :@_response_body, :@marked_for_same_origin_verification,
 :@_config]

As you may have figured out, all of the variables that you can access from a controller are displayed. This list is dynamically updated as you execute code. For example, run the next line using next (you'll learn more about this command later in this guide).

(byebug) next

[5, 14] in /PathTo/project/app/controllers/articles_controller.rb
   5     # GET /articles.json
   6     def index
   7       byebug
   8       @articles = Article.find_recent
   9
=> 10      respond_to do |format|
   11        format.html # index.html.erb
   12        format.json { render json: @articles }
   13      end
   14    end
   15
(byebug)

And then ask again for the instance_variables:

(byebug) instance_variables
[:@_action_has_layout, :@_routes, :@_request, :@_response, :@_lookup_context,
 :@_action_name, :@_response_body, :@marked_for_same_origin_verification,
 :@_config, :@articles]

Now @articles is included in the instance variables, because the line defining it was executed.

TIP: You can also step into irb mode with the command irb (of course!). This will start an irb session within the context you invoked it.

The var method is the most convenient way to show variables and their values. Let's have byebug help us with it.

(byebug) help var

  [v]ar <subcommand>

  Shows variables and its values


  var all      -- Shows local, global and instance variables of self.
  var args     -- Information about arguments of the current scope
  var const    -- Shows constants of an object.
  var global   -- Shows global variables.
  var instance -- Shows instance variables of self or a specific object.
  var local    -- Shows local variables in current scope.

This is a great way to inspect the values of the current context variables. For example, to check that we have no local variables currently defined:

(byebug) var local
(byebug)

You can also inspect for an object method this way:

(byebug) var instance Article.new
@_start_transaction_state = nil
@aggregation_cache = {}
@association_cache = {}
@attributes = #<ActiveRecord::AttributeSet:0x007fd0682a9b18 @attributes={"id"=>#<ActiveRecord::Attribute::FromDatabase:0x007fd0682a9a00 @name="id", @value_be...
@destroyed = false
@destroyed_by_association = nil
@marked_for_destruction = false
@new_record = true
@readonly = false
@transaction_state = nil

You can also use display to start watching variables. This is a good way of tracking the values of a variable while the execution goes on.

(byebug) display @articles
1: @articles = nil

The variables inside the displayed list will be printed with their values after you move in the stack. To stop displaying a variable use undisplay n where n is the variable number (1 in the last example).

Step by Step

Now you should know where you are in the running trace and be able to print the available variables. But let's continue and move on with the application execution.

Use step (abbreviated s) to continue running your program until the next logical stopping point and return control to the debugger. next is similar to step, but while step stops at the next line of code executed, doing just a single step, next moves to the next line without descending inside methods.

For example, consider the following situation:

Started GET "/" for 127.0.0.1 at 2014-04-11 13:39:23 +0200
Processing by ArticlesController#index as HTML

[1, 6] in /PathToProject/app/models/article.rb
   1: class Article < ApplicationRecord
   2:   def self.find_recent(limit = 10)
   3:     byebug
=> 4:     where('created_at > ?', 1.week.ago).limit(limit)
   5:   end
   6: end

(byebug)

If we use next, we won't go deep inside method calls. Instead, byebug will go to the next line within the same context. In this case, it is the last line of the current method, so byebug will return to the next line of the caller method.

(byebug) next
[4, 13] in /PathToProject/app/controllers/articles_controller.rb
    4:   # GET /articles
    5:   # GET /articles.json
    6:   def index
    7:     @articles = Article.find_recent
    8:
=>  9:     respond_to do |format|
   10:       format.html # index.html.erb
   11:       format.json { render json: @articles }
   12:     end
   13:   end

(byebug)

If we use step in the same situation, byebug will literally go to the next Ruby instruction to be executed -- in this case, Active Support's week method.

(byebug) step

[49, 58] in /PathToGems/activesupport-5.1.0/lib/active_support/core_ext/numeric/time.rb
   49:
   50:   # Returns a Duration instance matching the number of weeks provided.
   51:   #
   52:   #   2.weeks # => 2 weeks
   53:   def weeks
=> 54:     ActiveSupport::Duration.weeks(self)
   55:   end
   56:   alias :week :weeks
   57:
   58:   # Returns a Duration instance matching the number of fortnights provided.
(byebug)

This is one of the best ways to find bugs in your code.

TIP: You can also use step n or next n to move forward n steps at once.

Breakpoints

A breakpoint makes your application stop whenever a certain point in the program is reached. The debugger shell is invoked in that line.

You can add breakpoints dynamically with the command break (or just b). There are 3 possible ways of adding breakpoints manually:

• break n: set breakpoint in line number n in the current source file.
• break file:n [if expression]: set breakpoint in line number n inside file named file. If an expression is given it must evaluated to true to fire up the debugger.
• break class(.|\#)method [if expression]: set breakpoint in method (. and # for class and instance method respectively) defined in class. The expression works the same way as with file:n.

For example, in the previous situation

[4, 13] in /PathToProject/app/controllers/articles_controller.rb
    4:   # GET /articles
    5:   # GET /articles.json
    6:   def index
    7:     @articles = Article.find_recent
    8:
=>  9:     respond_to do |format|
   10:       format.html # index.html.erb
   11:       format.json { render json: @articles }
   12:     end
   13:   end

(byebug) break 11
Successfully created breakpoint with id 1

Use info breakpoints to list breakpoints. If you supply a number, it lists that breakpoint. Otherwise it lists all breakpoints.

(byebug) info breakpoints
Num Enb What
1   y   at /PathToProject/app/controllers/articles_controller.rb:11

To delete breakpoints: use the command delete n to remove the breakpoint number n. If no number is specified, it deletes all breakpoints that are currently active.

(byebug) delete 1
(byebug) info breakpoints
No breakpoints.

You can also enable or disable breakpoints:

• enable breakpoints [n [m [...]]]: allows a specific breakpoint list or all breakpoints to stop your program. This is the default state when you create a breakpoint.
• disable breakpoints [n [m [...]]]: make certain (or all) breakpoints have no effect on your program.

Catching Exceptions

The command catch exception-name (or just cat exception-name) can be used to intercept an exception of type exception-name when there would otherwise be no handler for it.

To list all active catchpoints use catch.

Resuming Execution

There are two ways to resume execution of an application that is stopped in the debugger:

• continue [n]: resumes program execution at the address where your script last stopped; any breakpoints set at that address are bypassed. The optional argument n allows you to specify a line number to set a one-time breakpoint which is deleted when that breakpoint is reached.
• finish [n]: execute until the selected stack frame returns. If no frame number is given, the application will run until the currently selected frame returns. The currently selected frame starts out the most-recent frame or 0 if no frame positioning (e.g. up, down, or frame) has been performed. If a frame number is given it will run until the specified frame returns.

Editing

Two commands allow you to open code from the debugger into an editor:

• edit [file:n]: edit file named file using the editor specified by the EDITOR environment variable. A specific line n can also be given.

Quitting

To exit the debugger, use the quit command (abbreviated to q). Or, type q! to bypass the Really quit? (y/n) prompt and exit unconditionally.

A simple quit tries to terminate all threads in effect. Therefore your server will be stopped and you will have to start it again.

Settings

byebug has a few available options to tweak its behavior:

(byebug) help set

  set <setting> <value>

  Modifies byebug settings

  Boolean values take "on", "off", "true", "false", "1" or "0". If you
  don't specify a value, the boolean setting will be enabled. Conversely,
  you can use "set no<setting>" to disable them.

  You can see these environment settings with the "show" command.

  List of supported settings:

  autosave       -- Automatically save command history record on exit
  autolist       -- Invoke list command on every stop
  width          -- Number of characters per line in byebug's output
  autoirb        -- Invoke IRB on every stop
  basename       -- <file>:<line> information after every stop uses short paths
  linetrace      -- Enable line execution tracing
  autopry        -- Invoke Pry on every stop
  stack_on_error -- Display stack trace when `eval` raises an exception
  fullpath       -- Display full file names in backtraces
  histfile       -- File where cmd history is saved to. Default: ./.byebug_history
  listsize       -- Set number of source lines to list by default
  post_mortem    -- Enable/disable post-mortem mode
  callstyle      -- Set how you want method call parameters to be displayed
  histsize       -- Maximum number of commands that can be stored in byebug history
  savefile       -- File where settings are saved to. Default: ~/.byebug_save

TIP: You can save these settings in an .byebugrc file in your home directory. The debugger reads these global settings when it starts. For example:

set callstyle short
set listsize 25

Debugging with the web-console gem

Web Console is a bit like byebug, but it runs in the browser. In any page you are developing, you can request a console in the context of a view or a controller. The console would be rendered next to your HTML content.

Console

Inside any controller action or view, you can invoke the console by calling the console method.

For example, in a controller:

class PostsController < ApplicationController
  def new
    console
    @post = Post.new
  end
end

Or in a view:

<% console %>

<h2>New Post</h2>

This will render a console inside your view. You don't need to care about the location of the console call; it won't be rendered on the spot of its invocation but next to your HTML content.

The console executes pure Ruby code: You can define and instantiate custom classes, create new models, and inspect variables.

NOTE: Only one console can be rendered per request. Otherwise web-console will raise an error on the second console invocation.

Inspecting Variables

You can invoke instance_variables to list all the instance variables available in your context. If you want to list all the local variables, you can do that with local_variables.

Settings

• config.web_console.allowed_ips: Authorized list of IPv4 or IPv6 addresses and networks (defaults: 127.0.0.1/8, ::1).
• config.web_console.whiny_requests: Log a message when a console rendering is prevented (defaults: true).

Since web-console evaluates plain Ruby code remotely on the server, don't try to use it in production.

Debugging Memory Leaks

A Ruby application (on Rails or not), can leak memory — either in the Ruby code or at the C code level.

In this section, you will learn how to find and fix such leaks by using tools such as Valgrind.

Valgrind

Valgrind is an application for detecting C-based memory leaks and race conditions.

There are Valgrind tools that can automatically detect many memory management and threading bugs, and profile your programs in detail. For example, if a C extension in the interpreter calls malloc() but doesn't properly call free(), this memory won't be available until the app terminates.

For further information on how to install Valgrind and use with Ruby, refer to Valgrind and Ruby by Evan Weaver.

Find a Memory Leak

There is an excellent article about detecting and fixing memory leaks at Derailed, which you can read here.

Plugins for Debugging

There are some Rails plugins to help you to find errors and debug your application. Here is a list of useful plugins for debugging:

• Query Trace Adds query origin tracing to your logs.
• Exception Notifier Provides a mailer object and a default set of templates for sending email notifications when errors occur in a Rails application.
• Better Errors Replaces the standard Rails error page with a new one containing more contextual information, like source code and variable inspection.
• RailsPanel Chrome extension for Rails development that will end your tailing of development.log. Have all information about your Rails app requests in the browser — in the Developer Tools panel. Provides insight to db/rendering/total times, parameter list, rendered views and more.
• Pry An IRB alternative and runtime developer console.

References

• byebug Homepage
• web-console Homepage