How To Simulate Slow Connections

The Problem

With Selenium you have the luxury of cross browser testing. But what happens when you also need to test how your application behaves on a slow connection?

A Solution

With the help of a proxy server like BrowserMob Proxy we can simulate bandwidth limitations and run our tests to see if they fall down. In order to do this we'll also need to adjust our Selenium's default timeout.

A Brief Primer on BrowserMob Proxy

BrowserMob Proxy is a server which sits between your browser and the application you're testing. You connect to it by configuring a browser profile to pass through it. As listed on the BrowserMob Proxy GitHub page, here are some of the benefits it offers:

  • blacklisting and whitelisting certain URL patterns
  • simulating various bandwidth and latency
  • remapping DNS lookups
  • flushing DNS caching
  • controlling DNS and request timeouts
  • automatic BASIC authorization

NOTE: You can also capture HTTP status codes with it as well. See tip 17 for details on how to do that.

Let's dig in with an example.

An Example

First we'll need to download BrowserMob Proxy from here.

Then we can wire up our test script to pull in our requisite libraries (e.g., selenium-webdriver to drive the browser, rspec/expectations and it's matchers for our assertion, and browsermob/proxy to control the proxy server).

# filename: bandwidth_limit.rb

require 'browsermob/proxy'
require 'selenium-webdriver'
require 'rspec/expectations'
include RSpec::Matchers

Now let's configure the proxy server and bind it to a Selenium browser profile.

def configure_proxy
  server = BrowserMob::Proxy::Server.new('./browsermob-proxy-2.1.0-beta-3/bin/browsermob-proxy')
  @proxy = server.create_proxy
  profile = Selenium::WebDriver::Firefox::Profile.new
  profile.proxy = @proxy.selenium_proxy

Now we can create setup, teardown, and run methods to handle our test configuration.

def setup
  @driver = Selenium::WebDriver.for :firefox, profile: configure_proxy
  @driver.manage.timeouts.page_load = 300 # seconds
  @proxy.limit(upstream_kbps: 20, downstream_kbps: 30)

def teardown

def run

In setup we set the default timeout for Selenium to 5 minutes (e.g. 300 seconds) and specify an upload limit of 20 kilobits per second and a download limit of 30 kilobits per second (which is similar to a dial-up connection).

Now let's write our test. A simple page load and copy assertion will suffice (to make sure the page actually loaded).

run do
  @driver.get 'http://the-internet.herokuapp.com'
  expect(@driver.find_element(class: 'heading').text).to eql('Welcome to the Internet')

Expected Behavior

If you save this file and run it (e.g., ruby bandwidth_limit.rb from the command-line) here is what will happen:

  • Proxy server starts
  • Proxy server session created
  • Browser opens (connecting to the proxy server session with a large default timeout)
  • Proxy bandwidth constraints applied
  • Visit the page (which loads slowly)
  • Page eventually loads
  • Grab the text from the page heading
  • Perform an assertion against the text to make sure it loaded
  • Browser closes
  • Proxy session closes


It may not be clear in browsermob-proxy-rb what options and functionality are available since the library is light on documentation. But the code is easy to read and has good comments. It's worth looking at the client class to see what's available.

Alternatively, you can BrowserMob Proxy through it's Rest API. You can find out more about that on the BrowserMob Proxy GitHub page.

Happy Testing!

Back to the archives