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README
[rexbug logo](assets/logo_horizontal_h150px.png)
Rexbug
is a thin Elixir wrapper for :redbug
production-friendly Erlang tracing debugger.
It tries to preserve :redbug
's simple and
intuitive interface while making it more convenient to use by Elixir developers.
README
What does it do?
It's an Elixir tracing - based debugger. It allows you to connect to a live Elixir system and get information when some code inside it is executed. The "some code" can be a whole module, a specific function in the module, or some function, but only if it's called with some specific arguments. The information you can get is the function arguments, its result and the stack trace.
If you want to you can narrow the tracing down to a specific process, investigate a remote node or look at the messages sent between processes.
Rexbug is also production-system-friendly. It has sensible limits for both time and amount of trace events after which it stops tracing. This means you won't accidentally overload the system and flood your console with debug information if your trace pattern wasn't specific enough.
How does it work?
Rexbug uses unmodified :redbug
library
underneath. It translates Elixir syntax to the Erlang format expected by
:redbug
.
:redbug
in turn interacts with the
Erlang trace facility.
It will instruct the Erlang VM to generate so called
"trace messages" when certain events (such as a particular
function being called) occur.
The trace messages are either printed (i.e. human readable)
to a file or to the screen; or written to a trc file.
Using a trc file puts less stress on the system, but
there is no way to count the messages (so the msgs opt
is ignored), and the files can only be read by special tools
(such as 'bread'). Printing and trc files cannot be combined.
By default (i.e. if the :file
opt is not given), messages
are printed.
Installation
The package can be installed by adding Rexbug
to your list of dependencies
in mix.exs
:
def deps do
[{:rexbug, ">= 1.0.0"}]
end
After you've added Rexbug
to your project, there's nothing left to do - you
can start debugging it at your convenience.
Examples
Tracing a single function
The general syntax is Rexbug.start("ModuleName.function_name/_")
.
The /_
tells Rexbug we're interested in any arity of the function.
iex(3)> Rexbug.start("Map.get/_ :: return") # asking for the return value too
{105, 2}
iex(4)> Map.get(%{}, :foo)
nil
# 10:49:02 #PID<0.1057.0> IEx.Evaluator.init/4
# Map.get(%{}, :foo)
# 10:49:02 #PID<0.1057.0> IEx.Evaluator.init/4
# Map.get(%{}, :foo, nil)
# 10:49:02 #PID<0.1057.0> IEx.Evaluator.init/4
# Map.get/3 -> nil
# 10:49:02 #PID<0.1057.0> IEx.Evaluator.init/4
# Map.get/2 -> nil
redbug done, timeout - 2
Tracing a whole module
iex> Rexbug.start("Map")
{82, 41}
iex> m = Map.put(%{}, :foo, :bar) # this could have been called in any process
%{foo: :bar}
# 18:51:55 #PID<0.150.0> IEx.Evaluator.init/4
# Map.__info__(:macros)
iex> Map.get(m, :foo)
:bar
# 18:51:57 #PID<0.150.0> IEx.Evaluator.init/4
# Map.__info__(:macros)
# 18:51:57 #PID<0.150.0> IEx.Evaluator.init/4
# Map.get(%{foo: :bar}, :foo)
# 18:51:57 #PID<0.150.0> IEx.Evaluator.init/4
# Map.get(%{foo: :bar}, :foo, nil)
iex> # Rexbug tracing is going to time out now
nil
redbug done, timeout - 4
iex>
Tracing with matching function arguments
iex> Rexbug.start("Enum.member?([_, _, _], \"foo\")")
{82, 1}
iex> Enum.member?([1, 2], "foo") # first argument doesn't match
false
iex> Enum.member?([1, 2, 3], "bar") # second argument doesn't match
false
iex> Enum.member?([1, 2, 3], "foo") # will match
false
# 18:55:44 #PID<0.150.0> IEx.Evaluator.init/4
# Enum.member?([1, 2, 3], "foo")
iex> Rexbug.stop()
:stopped
redbug done, local_done - 1
iex>
Tracing messages sent and received from a process
iex> s = self()
#PID<0.193.0>
iex> proc = Process.spawn(fn ->
...> receive do
...> anything -> send(s, {:got, anything})
...> end
...> end, [])
iex> Rexbug.start([:send, :receive], procs: [proc], time: 60_000)
{1, 0}
iex> send(proc, :foo)
:foo
# 18:31:08 #PID<0.208.0> (:dead)
# <<< :foo
# 18:31:08 #PID<0.208.0> (:dead)
# #PID<0.193.0> IEx.Evaluator.init/4 <<< {:got, :foo}
iex> flush()
{:got, :foo}
:ok
redbug done, timeout - 2
Motivation
I was discussing investigating some unexpected behaviour in an Elixir project with
one of my colleagues and he rightfully suggested
using a tracing debugger to get to the bottom of it. The tool he had the most
experience with was :redbug
and it soon turned out it's possible to use from
iex
and with Elixir code, as long as you know some Erlang and are mindful
of some gotchas.
I really liked how :redbug
was designed, but wished using it with Elixir was
more streamlined...
:redbug
syntax comparison
If you want to move between :redbug
and Rexbug
or you're just curious how
they compare, here's some examples:
# tracing an Erlang module
Rexbug.start(":ets")
:redbug.start('ets')
# stopping
Rexbug.stop()
:redbug.stop()
# tracing with arguments matching (and strings)
Rexbug.start("String.starts_with?(_, \"foo\")") # you can use the ~s sigil so that you don't have to escape the quotes
:redbug.start('\'Elixir.String\':\'starts_with?\'(_, <<"foo">>)')
# selecting the actions
Rexbug.start("Map.new/_ :: return;stack")
:redbug.start('\'Elixir.Map\':new -> return;stack')
Known issues/limitations
- In the trace patterns
"Mod.fun"
implicitly translates to"Mod.fun()"
, which is equivalent to"Mod.fun/0"
. To target the function with any arity, use"Mod.fun/_"
or"Mod.fun/any"
FAQ
<!-- ### How do I use it with my mix project? -->
My app is already running and it doesn't have Rexbug in its dependencies. Can I still debug it?
Yes! You can connect to it from a node that has Rexbug in its path and work from there.
The app:
nietaki@shiny:~$ iex --sname production --cookie monster
Erlang/OTP 20 [erts-9.0] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:10] [hipe] [kernel-poll:false]
Interactive Elixir (1.4.5) - press Ctrl+C to exit (type h() ENTER for help)
iex(production@shiny)1> Rexbug.help() # the node doesn't know about Rexbug
** (UndefinedFunctionError) function Rexbug.help/0 is undefined (module Rexbug is not available)
Rexbug.help()
iex(production@shiny)2> Stream.interval(1000) |> Enum.each(&Integer.mod(&1, 3))
Your local shell:
nietaki@shiny:~$ iex --sname investigator --cookie monster -pa ~/repos/rexbug/_build/dev/lib/rexbug/ebin/ -pa ~/repos/rexbug/_build/dev/lib/redbug/ebin/
Erlang/OTP 20 [erts-9.0] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:10] [hipe] [kernel-poll:false]
Interactive Elixir (1.4.5) - press Ctrl+C to exit (type h() ENTER for help)
iex(investigator@shiny)1> opts = [target: :production@shiny, msgs: 4]
[target: :production@shiny, msgs: 4]
iex(investigator@shiny)2> Rexbug.start("Integer.mod/2", opts)
{63, 1}
% 23:53:44 <9548.89.0>({'Elixir.IEx.Evaluator',init,4})
% 'Elixir.Integer':mod(46, 3)
% 23:53:45 <9548.89.0>({'Elixir.IEx.Evaluator',init,4})
% 'Elixir.Integer':mod(47, 3)
% 23:53:46 <9548.89.0>({'Elixir.IEx.Evaluator',init,4})
% 'Elixir.Integer':mod(48, 3)
% 23:53:47 <9548.89.0>({'Elixir.IEx.Evaluator',init,4})
% 'Elixir.Integer':mod(49, 3)
redbug done, msg_count - 4
iex(investigator@shiny)3>
Instead of pointing to the Rexbug
and :redbug
beam files you can just clone
this repo and run iex -S mix
in the root directory:
nietaki@shiny:rexbug (master=)$ iex --sname investigator --cookie monster -S mix
Erlang/OTP 19 [erts-8.3] [source] [64-bit] [smp:8:8] [async-threads:10] [hipe] [kernel-poll:false]
Interactive Elixir (1.4.4) - press Ctrl+C to exit (type h() ENTER for help)
iex(investigator@shiny)1> opts = [target: :production@shiny, msgs: 4]
[target: :production@shiny, msgs: 4]
iex(investigator@shiny)2> Rexbug.start("Integer.mod/2", opts)
(...)
How does Rexbug compare with other Elixir debuggers?
Good question! There are other projects that give you similar capabilities, like dbg by @fishcakez or exrun, both of which look great and are definitely more battle-tested than Rexbug.
I'll try to add a brief and unbiased (as much as I can) comparison after I've spent some time playing with them so I can do make sure I know what I'm talking about.
Why "translate" the syntax instead of forking :redbug
and caling its internals directly?
There's a number of reasons:
- The performance overhead should be irrelevant. You pay the small additional cost
once every time you run
Rexbug.start/2
and it should be negligible compared to whatever system you're debugging. - Since
:redbug
is included as-is you can still use it directly and benefit from any new features it might get. Also if your team is split between people more comfortable in Erlang and Elixir, everyone can use what they prefer. - "time to market" - doing this was the simplest way I could think of to get to a relatively polished library.
- This approach didn't seem to limit the possible featureset. All the
:redbug
features can still be provided.
In general there weren't enough reasons to do it the other way. I don't rule out the possibility of a future rewrite, which wouldn't be too drastic anyways.