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Hamms is designed to elicit failures in your HTTP Client. Connection failures, malformed response data, slow servers, fat headers, and more!


You can either install hamms via pip:

pip install hamms

Or clone this project:

git clone


  1. Start hamms by running it from the command line:

     python -m hamms

    Or use the HammsServer class to start and stop the server on command.

    from hamms import HammsServer
    class MyTest(object):
        def setUp(self):
            self.hs = HammsServer()
        def tearDown(self):
  2. Make requests and test your client. See the reference below for a list of supported failure modes.

By default, Hamms uses ports 5500-5600. You can customize the port range by passing the beginning_port parameter to HammsServer.start().


Connection level errors

Connect to the ports listed below to enact the various failure modes.

  • 5500 - Nothing is listening on the port. Note, your machine will likely send back a TCP reset (closing the connection) immediately.

    To simulate a connection failure that just hangs forever (a connection timeout), connect to a bad host on a real server, for example, or use a port in the 10.* range, for example

  • 5501 - The port accepts traffic but never sends back data

  • 5502 - The port sends back an empty string immediately upon connection

  • 5503 - The port sends back an empty string after the client sends data

  • 5504 - The port sends back a malformed response ("foo bar") immediately upon connection

  • 5505 - The port sends back a malformed response ("foo bar") after the client sends data

  • 5506 - The client accepts the request, and sends back one byte every 5 seconds

  • 5507 - The client accepts the request, and sends back one byte every 30 seconds

  • 5508 - Send a request to localhost:5508?sleep=<float> to sleep for float number of seconds. If no value is provided, sleep for 5 seconds.

  • 5509 - Send a request to localhost:5509?status=<int> to return a response with HTTP status code status. If no value is provided, return status code 200.

  • 5510 - The server will send a response with a Content-Length: 3 header, however the response is actually 1 MB in size. This can break clients that reuse a socket.

  • 5511 - Send a request to localhost:5511?size=<int> to return a Cookie header that is n bytes long. By default, return a 63KB header. 1KB larger will break many popular clients (curl, requests, for example)

  • 5512 - Use this port to test retry logic in your client - to ensure that it retries on failure.

    The server maintains a counter for incoming requests. Each time a new request is made, a 500 error is served and the counter is decremented. When the counter reaches zero, a 200 response is served. This server accepts two query arguments:

    • key - Specify a key to create a new counter. Continue making requests with key=<key> to decrement that particular counter. If no key is provided, 'default' is used.
    • tries - Specify the number of tries before success, as an integer. If no number is provided, you will get a success on the 3rd try.

    The server will let you know the key and how many tries are remaining until you get a successful response. Example error response:

    Content-Length: 116
    Content-Type: application/json
    Date: Wed, 19 Nov 2014 00:59:19 GMT
    Server: TwistedWeb/14.0.2
        "error": "The server had an error. Try again 1 more time",
        "key": "foobar",
        "success": false,
        "tries_remaining": 1

    Example usage:

    r = requests.get('http://localhost:5512?key=special-key')
    assert_equal(r.status_code, 500)
    r = requests.get('http://localhost:5512?key=special-key')
    assert_equal(r.status_code, 500)
    # Third time is the charm
    r = requests.get('http://localhost:5512?key=special-key')
    assert_equal(r.status_code, 200)
    # Set tries=1 to serve a 200 right away.
    r = requests.get('http://localhost:5512?key=my-key&tries=1')
    assert_equal(r.status_code, 200)

    You can see the status of all available counters by making a GET request to http://localhost:5512/counters, or reset a counter by making a POST request to http://localhost:5512/counters with the key you want to reset.

  • 5513 - Send a request to localhost:5513?failrate=<float>. The server will drop requests with a frequency of failrate.

  • 5514 - The server will try as hard as it can to return a content type that is not parseable by the Accept header provided by the request. Specify a Accept: application/json header in your request and the server will return data with the text/morse content type. The server will try these content-types in turn:

  • text/morse

  • application/json

  • text/html

  • text/csv

If your Accept header indicates it can accept all of these content-types, the server will return text/morse.

  • 5515 - The server will return a response with a content-type that matches the request, but it will be incomplete. The server will advertise an incorrect, too long Content-Length, and the response body will not be complete. The practical effect is that the server will hang halfway through the response download. The server can return partial responses with the following content-types:

  • application/json

  • text/html

  • text/plain

  • text/xml

If your server indicates an Accept header value of */*, or the server cannot find a matching content-type, the server will returnn an incomplete json response.

  • 5516 - Same semantics as port 5515, but the server will close the connection partway through, instead of hanging indefinitely.

Not implemented yet

  • The server sends back a response without a content-type
  • The server sends back a response with the wrong content-type
  • The server randomly drops bytes from a valid response.
  • Sending back byte data
  • Handshake timeout
  • Invalid certificate
  • TLS v1.0 and higher only
  • TLS v1.2 and higher only
  • Server closes connection


Donations free up time to make improvements to the library, and respond to bug reports. You can send donations via Paypal's "Send Money" feature to Donations are not tax deductible in the USA.