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Django Scaler

Degrade gracefully by automatically replacing heavy pages with static pages while a server is taking strain.

Contents

Installation

  1. Install or add django-scaler to your Python path.
  2. Add scaler to your INSTALLED_APPS setting.
  3. Add scaler.middleware.ScalerMiddleware to the top of your MIDDLEWARE_CLASSES setting.
  4. Add (r'^scaler/', include('scaler.urls')) to urlpatterns.

Overview

Servers may at times get overloaded due to a variety of reasons. When that happens you don't want expensive requests to bring down your entire site. The site must redirect expensive requests to a "server busy" page while the server is under load, and then automatically remove the redirects once the load has dropped enough.

django-scaler addresses this situation in two ways. Firstly, it knows which requests to redirect by itself. Secondly, it can be instructed to redirect the N most expensive requests. It stores response time data in in-memory caches enabling it to make these decisions.

Usage

Pasted from test_settings.py:

DJANGO_SCALER = {
    'server_busy_url_name': 'server-busy',

    # How many response times to consider for an URL. A small value means slow
    # response times are quickly acted upon, but it may be overly aggressive.
    # A large value means an URL must be slow for a number of requests before
    # it is acted upon. The default is 100.
    'trend_size': 10,

    # How much slower than average the trend must be before redirection kicks
    # in. The default is 4.0.
    'slow_threshold': 2.0,

    # How many seconds to keep redirecting an URL before serving normally. The
    # default is 60.
    'redirect_for': 10,

    # A function that returns how many of the slowest URLs must be redirected.
    # Depending on the site, data and load on the server this may be a large
    # number. This allows external processes to instruct the middleware to
    # redirect. The default is 0.
    'redirect_n_slowest_function': lambda: 0,

    # A function that returns what percentage of the slowest URLs must be
    # redirected. Depending on the site, data and load on the server this may
    # approach 100. The default is 0.
    'redirect_percentage_slowest_function': lambda: 0,

    # A function that returns a list of regexes. URLs matching the regexes are
    # redirected. Each regex is a simple string. Do not prefix with r''. The
    # default is an empty list.
    'redirect_regexes_function': lambda: [],
}