/csv-blockchain-challenge

Primary LanguageTypeScript

Atix Labs technical challenge - Backend Dev

Goal

It is required to program a web service to allow users to, when invoking an API method (POST), to write on a shared log file such that each entry (line) is linked to the previous one using its hash and a proof of work.

Expected line output CSV (file on server):

prev_hash,message,nonce

where:

  • prev_hash: previous line hash (sha256) in hex format without any separators. You should use random for the first line.
  • message: message sent by the user.
  • nonce: a number that guarantees that sha256(pre_hash + message + nonce) => RegEx('^00.*'), i.e., starts with two zeroes.

Log file content example:

0000000000000000000000000000000000000000000000000000000000000000,Hola Mundo,5 0038711c83bd05b1e369e27246df4ba815a6dda04116b1b2f9a8c21ba4e1de38,Chau Mundo,71

Where effectively it holds that: sha256(pre_hash + message + nonce) => RegEx('^00.*')

For example, you can find in the second line: 0038711c83bd05b1e369e27246df4ba815a6dda04116b1b2f9a8c21ba4e1de38,Chau Mundo,71

If we want to verify the nonce is valid, we need to:

>HEX(SHA256(0038711c83bd05b1e369e27246df4ba815a6dda04116b1b2f9a8c21ba4e1de38,Chau Mundo,71))

> 00232c7d3c2283695a4029eddc1b9e8c83914515832a04f57b402fc444aa11b5

Success criteria

  • Code needs to be published on Github or Gitlab. We will be reviewing commit history.
  • It must contain a proper README file to help reviewers to understand how to launch or test the application. Also, don’t forget to keep your code clean and easy to read.
  • Users can perform requests (API calls) at the same time, the file log not necessarily must be sorted by requests income time.
  • The file:
    • The link between lines must hold for all the lines in the file, i.e hash(line(n-1)) == line(n)[0])
    • All the log lines hash must start with two zeroes.

Nice to have

  • Integration tests.
  • Proper app logs to be able to troubleshoot.
  • A single request must not block the file. All the requests must compete with each other to write down in the log file.

Context & further notes

Proof of Work: it is a cryptographic puzzle that, based on the dispersion (randomness) of hash functions, can be used to easily check a certain amount of work has been made as there is no other way to solve it besides doing bruteforce look up. It initially was created as a spam filter mechanism and nowadays is heavily used in blockchain applications.

A consequence of hash-linking entries is that if anyone wants to edit one, all the hashes after it might be recalculated, i.e, all the work already made needs to be done again therefore it’s harder (in terms of computing power) to change old entries. This is a way to encourage immutability

Installation

$ npm install

Running the app

# development
$ npm run start

# watch mode
$ npm run start:dev

# production mode
$ npm run start:prod

Writing on the LineChain

Request

POST /thing/

curl -L -X POST 'localhost:3000'  -H 'Content-Type: application/json'  --data-raw '{"message":"Hola Mundo"}'

Response

HTTP/1.1 201 Created
Date: Mon, 18 Apr 2022 15:00:20 GMT
Status: 201 Created
Connection: keep-alive
Content-Type: application/json
Content-Length: 112

{"prevHash":"0000000000000000000000000000000000000000000000000000000000000000","message":"Hola Mundo","nonce":5}

Test

# unit tests
$ npm run test

# e2e tests
$ npm run test:e2e

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