MP 8: Versioned State Server

Due Date: Completed and turned in via git before April 4, 2022 at 11:59pm
Points: MP 8 is worth 50 points
Semester-Long Details: Programming Environment and MP Policy


In lecture, you learned how cloud-based applications will use data stores that are not just in their local memory. In this MP, you will write two solutions to the same problem:

  • One solution you will store your data locally in memory (ex: any Python data structure you want),
  • The other solution will store your data remotely in a MongoDB data store.

You will utilize a Docker container to launch a MongoDB instance and connect to it without installing it locally. Finally, you will run this on your CS 240 VM.

Initial Files

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

git fetch release
git merge release/mp8 -m "Merging initial files"

MP Overview Session

An MP Overview was held by Eunice on Monday, March 28 at 11:00am.

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="2021-11-05" to the server via a PUT /date request with the content 2021-11-05.
  • Since this is the first time the key date has been added, it is version 1.
  • You now add the key/value date="2021-11-12" to the server via another PUT /date request with the content 2021-11-12.
  • 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": "2021-11-12", "version": 2 }.
  • You request “Version 1” of the content via a GET /date/1 request. The data returned is: { "value": "2021-11-05", 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 You need to populate it to create a server that supports the following four API(s):

1. 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").

2. 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": "2021-11-12", "version": 2 }

3. 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.

4. 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.


The MP is divided into two parts. In part 1, you’ll implement a versioned state store by using in-memory variables. In part 2, you’ll use a mongodb data store to store your key and data.

Part 1: Implementing a Versioned State Store

In this part, you’ll complete the in local-store/ to implement a versioned state server.

Running Your Server

As usual, launch the flask app in your local-store directory using the standard command:

python -m flask run

Testing Your Code

You can debug your code by running your server and using curl commands in the terminal:

  • PUT: curl -X PUT localhost:5000/<Key Name> -d '<Key Value>' -H "Content-Type: text/plain"
  • GET: curl localhost:5000/<Key Name>
  • GET (with version): curl localhost:5000/<Key Name>/<Version>
  • DELETE: curl -X DELETE localhost:5000/<Key Name>

Alternatively, you can also use applications like Postman for sending HTTP requests.

You can run the test suite we provide by running python -m pytest

Part 2: Using MongoDB NoSQL Data Store

In this part, you’ll complete the in mongodb-nosql/ to implement a versioned state server using MongoDB as your data store.

To transition your code to a mongodb data store, you will want to install the mongodb Python library using either:

  • python -m pip install pymongo or pip install pymongo

Additionally, you will want to run a mongodb server to connect to. The quickest way to get one running is to use docker:

  • docker run --rm -it -p 27017:27017 mongo

The documentation for mongodb can be found here:

  • Every database is made up of “collections”. Similarly, you will need to “create” a collection. Note, just by using a collection will create it!
  • Your database MUST be named mp8-state-server. This is important to allow us to verify the contents of the database. (The already sets this value in db = mongo["mp8-state-server"])
  • You can use just one collection for the entire MP, or one collection per key, and both designs are great.

Testing Your Code

The tests for this part can be run using curl in the same way as the tests for the previous part. However, you must now:

  • Ensure the mongodb server is running as a Docker container,
  • Launch your app in your mongodb-nosql directory.
  • Run your curl tests.

You can run the test suite we provide by running python -m pytest


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 -u
git commit -m "MP submission"
git push

You can verify your code was successfully submitted by viewing your git repo on your github account: You should also be able to see if your code passed our test cases from there.