Wednesday, January 9, 2013

How to Get Your Third Party APIs to Shutup?

When programming with 3rd party libraries, sometimes we need to suppress or redirect the standard output generated by the 3rd party libraries. A very common scenario is that a third party library we use in an application generates a very verbose output which clutters up the output of our program. With most programming languages we can write a simple suppress/redirect procedure to fix this problem. Such functions are sometimes colloquially known as STFU functions. Here I'm describing a couple of STFU functions I implemented in some of my recent work.

1. AppsCake (Web interface for AppScale-Tools)
This is a Ruby based dynamic web component which uses some of the core AppScale-Tools libraries. For this project I wanted to capture the standard output of the AppScale-Tools libraries and display it on a web page. As the first step I wanted to redirect the standard output of AppScale-Tools to a separate text file. Here's what I did.
def redirect_standard_io(timestamp)
    orig_stderr = $stderr.clone
    orig_stdout = $stdout.clone
    log_path = File.join(File.expand_path(File.dirname(__FILE__)), "..", "logs")
    $stderr.reopen, "deploy-#{timestamp}.log"), "w")
    $stderr.sync = true
    $stdout.reopen, "deploy-#{timestamp}.log"), "w")
    $stdout.sync = true
    retval = yield
  rescue Exception => e
    puts "[__ERROR__] Runtime error in deployment process: #{e.message}"
    $stdout.reopen orig_stdout
    $stderr.reopen orig_stderr
    raise e
    $stdout.reopen orig_stdout
    $stderr.reopen orig_stderr
Now whenever I want to redirect the standard output and invoke the AppScale-Tools API I can do this.
redirect_standard_io(timestamp) do
   # Call AppScale-Tools API
2. Hawkeye (API fidelity test suite for AppScale)
This is a Python based framework which makes a lot of RESTful invocations using the standard Python httplib API. I wanted to trace the HTTP requests and responses that are being exchanged during the execution of the framework and log them to a separate log file. Python httplib has a verbose mode which can be enabled by passing a special flag to the HTTPConnection class and it turns out this mode logs almost all the information I need. But unfortunately it logs all this information to the standard output of the program thus messing up the output I wanted to present to users. Therefore I needed a way to redirect the standard output for all httplib API calls. Here's how that problem was solved.
http_log = open('logs/http.log', 'a')
original = sys.stdout
sys.stdout = http_log
  # Invoke httplib
  sys.stdout = original

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