MP 6: Versioned State Server

Due Date: Completed and turned in via git before November 12, 2020 at 8:00am
Points: MP 6 is worth 25 points


Maintaining the state of a system is an important aspect of many web services. For this MP, you will build a simple version-based state server using HTTP actions.

Initial Files

In your CS 240 directory, merge the initial starting files with the following commands:

git fetch release
git merge release/mp6 -m "Merging initial files"

Machine Problem


Your web server must store key-value pairs of data and maintain a version number for each key-value. An example of a possible use-case for this MP is as follows:

  • You add the key/value pair date="2020-11-05" (the day the MP is released) to the server via a PUT /date request with the content 2020-11-05.
  • Since this is the first time the key date has been added, it is version 1.
  • You now add the key/value date="2020-11-12" (the day the MP is due) to the server via another PUT /date request with the content 2020-11-12.
  • Since this is the second time the key date has been added, it is version 2.
  • You request the value contained in date via a GET /date request. The data returned is a JSON with the latest version, which is: { value: "2020-11-12", version: 2 }.
  • You request “Version 1” of the content via a GET /date/1 request. The data returned is: { value: "2020-11-05", version: 1 }.
  • You request “Version 3” of the content via a GET /date/3 request. Since there is no version 3, the server returns an HTTP/404 Not Found response.
  • You delete the date key, and all versions of it, from the server via a DELETE /date request. (Future versions of date must begin back at 1, there should be no history of date ever existing after being deleted.)


In this MP, a nearly blank Python file is provided for Use your skills from MP4, MP5, or provided MP5 courses-microservice as a starting point if you’re uncertain about creating a flask server from an empty file.

PUT /<key> – Adds a versioned object

  • Request: PUT /<key>
  • Action: Adds the content of the request as a versioned value for the key <key>. You should assume the content is a UTF-8 string. The first version of the data is version 1, second version is 2, and so-on.
  • Returns: HTTP/200 if successful.
  • Notes:
    • In flask, you can get the contents of the request as a UTF-8 string with the following code: value ="utf-8").

GET /<key> – Retrieves the latest version of a key

  • Request: GET /<key>
  • Action: Returns a JSON containing the latest value stored for the key as value (string) and the version number as version (number).
  • Returns: HTTP/200 if successful, or HTTP/404 if the key was not found.
  • Example JSON: { value: "2020-11-12", version: 2 }

GET /<key>/<version> – Retrieves a specific version of a key

  • Request: GET /<key>/<version>
  • Action: Same as GET /<key>, except requesting a specific version instead of the latest.
  • Returns: HTTP/200 if successful, or HTTP/404 if the key or version was not found.

DELETE /<key> – Completely deletes a key

  • Request: DELETE /<key>
  • Action: Deletes a key and all versions of the key. No record of the key ever existing should remain. Future versions of the key begin again at 1.
  • Returns: HTTP/200 if successful.

Running Your Server

As usual, launch the flask app using the standard command:

python -m flask run

We have provided a test script,, which you can run while the server is running:


If all tests pass successfully, you will see no output. If any assertion errors occur, you can inspect the test case to find out what was tested.


When you have completed your program, double-check that your server runs as expected. When you are ready, submit the code via the following git commands:

git add -A
git commit -m "MP submission"
git push origin master

You can verify your code was successfully submitted by viewing your git repo via the web interface here: