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:
<html>
<%= debug @post %>
<p>
<b>Title:</b>
<%=h @post.title %>
</p>
</html>
You'll see something like this:
<yaml>
--- !ruby/object:Post
attributes:
updated_at: 2008-09-05 22:55:47
body: It's a very helpful guide for debugging your Rails app.
The +to_yaml+ method converts the method to YAML format leaving it more readable, and then the +simple_format+ helper is used to render each line as in the console. This is how +debug+ method does its magic.
As a result of this, you will have something like this in your view:
<yaml>
--- !ruby/object:Post
attributes:
updated_at: 2008-09-05 22:55:47
body: It's a very helpful guide for debugging your Rails app.
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:
<html>
<%= [1, 2, 3, 4, 5].inspect %>
<p>
<b>Title:</b>
<%=h @post.title %>
</p>
</html>
Will be rendered as follows:
<pre>
[1, 2, 3, 4, 5]
Title: Rails debugging guide
</pre>
h3. 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.
Rails makes use of the +ActiveSupport::BufferedLogger+ class to write log information. You can also substitute another logger such as +Log4r+ if you wish.
When something is logged it's printed into the corresponding log if the log level of the message is equal 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
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.
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 in doing exactly that by stamping log lines with subdomains, request ids, and anything else to aid debugging such applications.
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 deeper into Rails code.
Rails has had built-in support for debugging since Rails 2.0. Inside any Rails application you can invoke the debugger by calling the +debugger+ method.
As soon as your application calls the +debugger+ 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 +(rdb:n)+. The _n_ is the thread number. The prompt will also show you the next line of code that is waiting to run.
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.
Now it's time to explore and dig into your application. A good place to start is by asking the debugger for help... so type: +help+ (You didn't see that coming, right?)
<shell>
(rdb:7) help
ruby-debug help v0.10.2
Type 'help <command-name>' for help on a specific command
The next command to learn is one of the most useful: +list+. You can abbreviate any debugging command by supplying just enough letters to distinguish them from other commands, so you can also use +l+ for the +list+ command.
This command shows you where you are in the code by printing 10 lines centered around the current line; the current line in this particular case is line 6 and is marked by +=>+.
And so on until the end of the current file. When the end of file is reached, the +list+ command will start again from the beginning of the file and continue again up to the end, treating the file as a circular buffer.
The debugger creates a context when a stopping point or an event is reached. The context has information about the suspended program which enables a debugger to inspect the frame stack, evaluate variables from the perspective of the debugged program, and contains information about 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.
<shell>
(rdb:5) where
#0 PostsController.index
at line /PathTo/project/app/controllers/posts_controller.rb:6
#1 Kernel.send
at line /PathTo/project/vendor/rails/actionpack/lib/action_controller/base.rb:1175
at line /PathTo/project/vendor/rails/actionpack/lib/action_controller/base.rb:1175
</shell>
The available variables are the same as if you were running the code line by line. After all, that's what debugging is.
Moving up and down the stack frame: You can use +up [n]+ (+u+ for abbreviated) 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.
h4. Threads
The debugger can list, stop, resume and switch between running threads by using the command +thread+ (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 plus + character and the number indicates the current thread of execution.
* +thread stop _n_+ stop thread _n_.
* +thread resume _n_+ resumes thread _n_.
* +thread switch _n_+ switches the current thread context to _n_.
This command is very helpful, among other occasions, when you are debugging concurrent threads and need to verify that there are no race conditions in your code.
h4. 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:
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).
<shell>
(rdb:11) next
Processing PostsController#index (for 127.0.0.1 at 2008-09-04 19:51:34) [GET]
TIP: You can also step into *irb* mode with the command +irb+ (of course!). This way an irb session will be started within the context you invoked it. But be warned: this is an experimental feature.
The +var+ method is the most convenient way to show variables and their values:
<shell>
var
(rdb:1) v[ar] const <object> show constants of object
(rdb:1) v[ar] g[lobal] show global variables
(rdb:1) v[ar] i[nstance] <object> show instance variables of object
(rdb:1) v[ar] l[ocal] show local variables
</shell>
This is a great way to inspect the values of the current context variables. For example:
<shell>
(rdb:9) var local
__dbg_verbose_save => false
</shell>
You can also inspect for an object method this way:
TIP: The commands +p+ (print) and +pp+ (pretty print) can be used to evaluate Ruby expressions and display the value of variables to the console.
You can use also +display+ to start watching variables. This is a good way of tracking the values of a variable while the execution goes on.
<shell>
(rdb:1) display @recent_comments
1: @recent_comments =
</shell>
The variables inside the displaying 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).
h4. Step by Step
Now you should know where you are in the running trace and be able to print the available variables. But lets continue and move on with the application execution.
You may also use +next+ which is similar to step, but function or method calls that appear within the line of code are executed without stopping. As with step, you may use plus sign to move _n_ steps.
The difference between +next+ and +step+ is that +step+ stops at the next line of code executed, doing just a single step, while +next+ moves to the next line without descending inside methods.
For example, consider this block of code with an included +debugger+ statement:
Now you can see that the +@comments+ relationship was loaded and @recent_comments defined because the line was executed.
If you want to go deeper into the stack trace you can move single +steps+, through your calling methods and into Rails code. This is one of the best ways to find bugs in your code, or perhaps in Ruby or Rails.
h4. 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 line+: set breakpoint in the _line_ in the current source file.
* +break file:line [if expression]+: set breakpoint in the _line_ number inside the _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:line.
<shell>
(rdb:5) break 10
Breakpoint 1 file /PathTo/project/vendor/rails/actionpack/lib/action_controller/filters.rb, line 10
</shell>
Use +info breakpoints _n_+ or +info break _n_+ to list breakpoints. If you supply a number, it lists that breakpoint. Otherwise it lists all breakpoints.
<shell>
(rdb:5) info breakpoints
Num Enb What
1 y at filters.rb:10
</shell>
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..
<shell>
(rdb:5) delete 1
(rdb:5) info breakpoints
No breakpoints.
</shell>
You can also enable or disable breakpoints:
* +enable breakpoints+: allow a list _breakpoints_ or all of them if no list is specified, to stop your program. This is the default state when you create a breakpoint.
* +disable breakpoints+: the _breakpoints_ will have no effect on your program.
h4. 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 is no handler for it.
To list all active catchpoints use +catch+.
h4. Resuming Execution
There are two ways to resume execution of an application that is stopped in the debugger:
* +continue+ [line-specification] (or +c+): resume program execution, at the address where your script last stopped; any breakpoints set at that address are bypassed. The optional argument line-specification allows you to specify a line number to set a one-time breakpoint which is deleted when that breakpoint is reached.
* +finish+ [frame-number] (or +fin+): 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.
h4. Editing
Two commands allow you to open code from the debugger into an editor:
* +edit [file:line]+: edit _file_ using the editor specified by the EDITOR environment variable. A specific _line_ can also be given.
* +tmate _n_+ (abbreviated +tm+): open the current file in TextMate. It uses n-th frame if _n_ is specified.
h4. Quitting
To exit the debugger, use the +quit+ command (abbreviated +q+), or its alias +exit+.
A simple quit tries to terminate all threads in effect. Therefore your server will be stopped and you will have to start it again.
The +debugger+ gem can automatically show the code you're stepping through and reload it when you change it in an editor. Here are a few of the available options:
If a Ruby object does not go out of scope, the Ruby Garbage Collector won't sweep it since it is referenced somewhere. Leaks like this can grow slowly and your application will consume more and more memory, gradually affecting the overall system performance. This tool will help you find leaks on the Ruby heap.
Make sure to run a couple hundred requests to get better data samples, then press +CTRL-C+. The server will stop and Bleak House will produce a dumpfile in +/tmp+:
<shell>
** BleakHouse: working...
** BleakHouse: complete
** Bleakhouse: run 'bleak /tmp/bleak.5979.0.dump' to analyze.
</shell>
To analyze it, just run the listed command. The top 20 leakiest lines will be listed:
If "BleakHouse":https://github.com/evan/bleak_house/ doesn't report any heap growth but you still have memory growth, you might have a broken C extension, or real leak in the interpreter. In that case, try using Valgrind to investigate further.
"Valgrind":http://valgrind.org/ is a Linux-only 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, a C extension in the interpreter calls +malloc()+ but is 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":http://blog.evanweaver.com/articles/2008/02/05/valgrind-and-ruby/ by Evan Weaver.
h3. 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:
* "Footnotes":https://github.com/josevalim/rails-footnotes: Every Rails page has footnotes that give request information and link back to your source via TextMate.
* "Query Trace":https://github.com/ntalbott/query_trace/tree/master: Adds query origin tracing to your logs.
* "Query Stats":https://github.com/dan-manges/query_stats/tree/master: A Rails plugin to track database queries.
* "Query Reviewer":http://code.google.com/p/query-reviewer/: This rails plugin not only runs "EXPLAIN" before each of your select queries in development, but provides a small DIV in the rendered output of each page with the summary of warnings for each query that it analyzed.
* "Exception Notifier":https://github.com/smartinez87/exception_notification/tree/master: Provides a mailer object and a default set of templates for sending email notifications when errors occur in a Rails application.
* "Exception Logger":https://github.com/defunkt/exception_logger/tree/master: Logs your Rails exceptions in the database and provides a funky web interface to manage them.
