Catch Exceptions Application Wide - ruby-on-rails

Question

Is there a way of overriding default Exception handling (default rescue), so that I can write custom logic there, for example sending stacktrace to our Slack channel.
Basically I want to make something what NewRelic does with their gem: https://github.com/newrelic/rpm but more local version of it.
Thanks

Answer 1

The simplest thing you could do is instrument all error raising using a TracePoint:
trace = TracePoint.new(:raise) do |tp|
myLogger.log [tp.lineno, tp.event, tp.raised_exception]
end
trace.enable
Using traces used to cause significant code slowdown. I think the new API in ruby 2.0 reduced the slowdown notably. According to my naive benchmark:
count = 10_000_000
t = Time.now
count.times do |i|
begin
0 / 0
rescue
end
end
puts "trial 1: #{Time.now - t}"
t = Time.now
count.times do |i|
"hi"
begin
0 / 0
rescue
end
end
puts "trial 2: #{Time.now - t}"
trace = TracePoint.new(:raise) do |tp|
"hi"
end
trace.enable
t = Time.now
count.times do |i|
begin
0 / 0
rescue
end
end
puts "trial 3: #{Time.now - t}"
#=>trial 1: 10.110471094
#=>trial 2: 9.971755759
#=>trial 3: 11.608365399
Tracepoint only adds 1 second (or 10%) slowdown in 10,000,000 executions of raise. That being said, TracePoint still isn't considered a "production-worthy technique" because it does add overhead and can be hard to predict (eg there are a lot of obscure exceptions in ruby).
If you're wondering how new relic manages to instrument code without overhead:
...
class_eval <<-EOC
def #{with_method_name}(*args, &block)
perform_action_with_newrelic_trace(#{argument_list.join(',')}) do
#{without_method_name}(*args, &block)
end
end
EOC
...
It uses several hundred lines of meta-programming to capture specific methods, deconstruct them, and redefine them with instrumentation inside. This technique requires a lot of code and (I would assume) extra memory and time at start-up, but has the advantage of no additional overhead once the methods are assigned.

Answer 2

Try something like this in application_controller.rb
rescue_from Exception, with: :rescue500 if Rails.env.production?
rescue_from ActiveRecord::RecordNotFound, with: :record_not_found unless Rails.env.development?
def record_not_found
render json: {code: 404, errors: ["Record not found"]}, status: 404
end
def rescue404
render json: {code: 404, errors: ["No round matches"]}, status: 404
end
def rescue500(ex)
Rails.logger.error("\n\nEXCEPTION: #{ex.inspect}\n")
Rails.logger.info(ex)
render json: {code: 500, errors: [ex.message] }, status: 500
end