Wallet

Implementation Notes

• a REST api is provided with minimal endpoints (echo)
• I made use of a sql database (postgres) to achieve consistency
• consistency is achieved by optimistic concurrency and unique keys (via sql database)
• idempotency is achieved by "fingerprint" value (api returns http conflict 409 in case of retry)
• implemented a cli style app (cobra)
• unit tests are written for service package
• integration tests are written for api and db packages

How to Run

• You need go:1.17, docker and docker-compose
• First run the database server $ docker-compose up -d
• Create the database and tables $ go run . db --initdb
• Run unit tests $ go test ./... -v
• Run integration tests $ go test ./... -v -tags integration
• Run the api $ go run . api
• Repository can also be opened and run on GitPod (free, but login required).

Enpoints

Create Wallet

• POST /wallets
• externalid value should be unique, enables idempotency
• labels is a string dictionary to attach metadata
• response json includes id (int)
• request:

{
    "externalId" : "userid-123",
    "labels" : {
        "somekey" : "somevalue"
    }
}

Get Wallet

• GET /wallets/:id
• response:

{
    "id": 9180,
    "labels" : {
        "somekey" : "somevalue"
    },
    "externalId" : "userid-123",
    "created" : "2021-09-07T01:42:00"
}

Add Transaction

• POST /wallets/:id/transactions
• amount may be negative or positive (should be <= -1.0 or >=1.0)
• fingerprint should be unique, enables idempotency
• labels is a string dictionary to attach metadata
• request1:

{
    "amount" : 10.0,             
    "fingerprint" : "TX123A001",
    "labels" : { 
        "couponId" : "10004871",
        "reason": "prize"
    },
    "description" : "won ticket #10004870"
}

• request2:

{
    "amount" : -5.0,             
    "fingerprint" : "job-0018254",
    "labels" : { 
        "paypal_referenceid" : "PP00CX098", 
        "reason": "withdraw",
        "withdraw_target" : "paypal",
        "withdraw_jobid" : "job-0018254"
    },
    "description" : "won ticket #10004870"
}

Get Latest Transaction

• GET /wallets/:id/transactions/latest
• returs latest transaction of the requested wallet
• refno: a transaction sequence number per wallet, strengthens consistency
• response:

{
    "id" : "{uuid}",
    "refno" : 2,
    "amount" : 10.0,             
    "fingerprint" : "TX123A001",
    "labels" : {                 
        "couponId" : "10004871", 
    },
    "description" : "won ticket #10004870",
    "created" : "{timestamp}",
    "oldbalance" : 25.0,
    "newbalance" : 35.0
}

Get Balance

• GET /wallets/:id/balance
• returs current balance of the requested wallet
• only return a float value; not a json object

35.0

Other Notes

Run tests

$ go test ./...  -v
$ go test ./...  -v -tags integration
# if integration tests run; unit tests will not run (using build tags)
# Integration tests use same database with api : walletdb.
# You can use different database for integration tests
$ DB_DATABASE="postgresql://postgres:1234@localhost/testdb" go test ./... -v -tags integration

Create and Drop database

go run . db --initdb
go run . db --dropdb

Environment Variables

$ DB_DATABASE="postgresql://postgres:1234@localhost/testdb"
$ LOGLEVEL=info

Refactor TODO

• In api integrationt tests, only happy path is implemented. Implement the rest.
• Use pgx connection pooling in repository
• Run validations also at wallet service (validations only run at api handlers at the moment)

Missing & Possible Features

• Void a recent transaction
• Get wallet by external id
• Get transaction by fingerprint
• Search wallets by label
• Search transactions by label
• List transactions by wallet
• Publish external events (WalletCreated, TransactionCreated, BalanceChanged, TransactionVoid)
• Technical:
  • enable api authentication
  • enable OpenAPI/Swagger definitions and discovery
  • enable healtcheck and metrics endpoints (may include custom metrics)
  • implement tracing for critical endpoints

Design Question: Withdraw Money to Paypal Account

I designed and sketched a timeline flow. You can find at places below:

• Design Url: https://excalidraw.com/#json=4622116301307904,MA0O2YePaZ-fPR8wce1xzw
• Also the file named "withdraw-money-design.svg" in the repository: