Where Does Rack Log To

Where does RACK log to?

It depends. Many developers define their app log file to app/servername.log or just to the current path where the Rack app is loaded.

Yes you can change it's path.

Usually you get a config.ru file with something like:

    log = File.new("sinatra.log", "a+")
    $stdout.reopen(log)
    $stderr.reopen(log)

    # optionally to sync logs while the server is running
    $stderr.sync = true
    $stdout.sync = true

and/or

    configure do
      LOGGER = Logger.new("sinatra.log")
      enable :logging, :dump_errors
      set :raise_errors, true
    end

in this case the log file is located under appdir/sinatra.log. But remember this code can be anywhere in your Rack app, so please seek for "log" in your application directory.

    $ cd projectname
    $ grep -ri 'log' *

have fun and post here your config.ru and/or the mainprojectfile.rb.

How do I get SInatra and Rack to output to my Logger instance?

I'm not using WEBrick but Thin (set :server, 'thin') Anyway according to Sinatra configuration I redirect rack.errors to my $logger:

class IOToLog < IO
  def initialize(logger)
    @logger = logger
  end
  def write(string)
    @logger.debug {"SINATRA #{string.strip}"}
  end
end
IOTOLOG = IOToLog.new($logger)

before {
  env["rack.errors"] =  IOTOLOG
}

and the output is:

2017-06-21 15:44:46.166 DEBUG SINATRA 127.0.0.1 - - [21/Jun/2017:15:44:46 +0200] "GET /wd/hub/status HTTP/1.1" 200 83 0.0004
2017-06-21 15:44:51.167 DEBUG SINATRA 127.0.0.1 - - [21/Jun/2017:15:44:51 +0200] "GET /wd/hub/status HTTP/1.1" 200 83 0.0004

BTW I've tried env["rack.logger"] = $logger but with no luck.

Use Rack::CommonLogger in Sinatra

Rack::CommonLogger won't provide a logger to your main app, it will just logs the request like Apache would do.

Check the code by yourself: https://github.com/rack/rack/blob/master/lib/rack/common_logger.rb

All Rack apps have the call method that get's invoked with the HTTP Request env, if you check the call method of this middleware this is what happens:

def call(env)
  began_at = Time.now
  status, header, body = @app.call(env)
  header = Utils::HeaderHash.new(header)
  log(env, status, header, began_at)
  [status, header, body]
end

The @app in this case is the main app, the middleware is just registering the time the request began at, then it class your middleware getting the [status, header, body] triple, and then invoke a private log method with those parameters, returning the same triple that your app returned in the first place.

The logger method goes like:

def log(env, status, header, began_at)
  now = Time.now
  length = extract_content_length(header)

  logger = @logger || env['rack.errors']
  logger.write FORMAT % [
    env['HTTP_X_FORWARDED_FOR'] || env["REMOTE_ADDR"] || "-",
    env["REMOTE_USER"] || "-",
    now.strftime("%d/%b/%Y %H:%M:%S"),
    env["REQUEST_METHOD"],
    env["PATH_INFO"],
    env["QUERY_STRING"].empty? ? "" : "?"+env["QUERY_STRING"],
    env["HTTP_VERSION"],
    status.to_s[0..3],
    length,
    now - began_at ]
end

As you can tell, the log method just grabs some info from the request env, and logs in on a logger that is specified on the constructor call, if there is no logger instance then it goes to the rack.errors logger (it seems there is one by default)

The way to use it (in your config.ru):

logger = Logger.new('log/app.log')

use Rack::CommonLogger, logger
run YourApp

If you want to have a common logger in all your app, you could create a simple logger middleware:

class MyLoggerMiddleware

  def initialize(app, logger)
    @app, @logger = app, logger
  end

  def call(env)
    env['mylogger'] = @logger
    @app.call(env)
  end

end

To use it, on your config.ru:

logger = Logger.new('log/app.log')
use Rack::CommonLogger, logger
use MyLoggerMiddleware, logger
run MyApp

Hope this helps.

How to filter sensitive information when logging with Sinatra and Rack Logger

You can try to intercept the call to write and filter out sensitive messages like so :

logger = Logger.new("my_common.log")
logger.instance_eval do
  def write(msg)
    self.send(:<<, msg) if !msg.match /SUPER SENSITIVE INFO HERE/
  end
end

then, configure Rack::CommonLogger to use this instance of the logger:

config.middleware.use Rack::CommonLogger, logger

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