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Backend Thinking

Backend Role

Transform business requirements to action, which usually involves:

• Service:
  • ZaloPay use microservices architecture, mostly written using Go and Java
• API:
  • HTTP (Client-Server) and GRPC (Server-Server)
• Database/Cache/Storage/Message Broker
  • MySQL/Redis/S3/Kafka
  • CRUD
• Docs
  • Mostly design notes and diagrams which show how to implement business requirements

After successfully do all of that, next step is:

• Testing
  • Unit tests, Integration tests
• Observation
  • Log
  • Metrics
  • Tracing

In ZaloPay, each team has its own responsibilites/domains, aka many different services.

Ideally each team can choose custom backend techstack if they want, but mostly boils down to Java or Go. Some teams use Python for scripting, data processing, ...

Example: UM (Team User Management Core) has 10+ Java services and 30+ Go services.

The question is for each new business requirements, what should we do:

• Create new services with new APIs?
• Add new APIs to existing services?
• Update existing APIs?
• Change configs?
• Don't do anything?

Example: Business requirements says: Must match/compare user EKYC data with Bank data (name, dob, id, ...). TODO

TODO: How to split services?

Technical side

How do services communicate with each other?

API

First is through API, this is the direct way, you send a request then you wait for response.

HTTP: GET/POST/...

Example: TODO use curl

GRPC: use proto file as constract.

Example: TODO: show sample proto file

There are no hard rules on how to design APIs, only some best practices, like REST API, ...

Correct answer will always be: "It depends". Depends on:

• Your audience (android, ios, web client or another internal service)
• Your purpose (allow to do what?)
• Your current techstack (technology limitation?)
• Your team (bias, prefer, ...?)
• ...

Why do we use HTTP for Client-Server and GRPC for Server-Server?

• HTTP for Client-Server is pretty standard. Easy for client to debug, ...
• Before ZaloPay switch to GRPC for Server-Server, we use HTTP. The reason for switch is mainly performance.

References

• Best practices for REST API design
• ZaloPay API
• stripe API
• moov API

Message Broker

Second way is by Message Broker, the most well known is Kafka.

Main idea is decoupling.

Imaging service A need to call services B, C, D, E after doing some action, but B just died. We must handle B errors gracefully if B is not that important (not affect main flow of A). Imaging not only B, but multi B, like B1, B2, B3, ... Bn, this is so depressed to continue.

Message Broker is a way to detach B from A.

Dumb exaplain be like: each time A do something, A produces message to Message Broker, than A forgets about it. Then all B1, B2 can consume A's message if they want and do something with it, A does not know and does not need to know about it.

References

• Using Apache Kafka to process 1 trillion inter-service messages

My own experiences:

• Whatever you design, you stick with it consistently. Don't use different name for same object/value in your APIs.
• Don't trust client blindly, everything can be fake, everything must be validated. We can not know the request is actually from our client or some hacker computer. (Actually we can but this is out of scope, and require lots of advance work)
• Don't delete/rename/change old fields because you want and you can, please think it through before do it. Because back compability is very hard, old apps should continue to function if user don't upgrade. Even if we rollout new version, it takes time.

Pro tip: Use proto to define models (if you can) to take advantage of detecting breaking changes.

Coding principle

Known concept

TODO:

Challenge

TODO: Scale problem

Damage control

TODO: Take care incident

Bonus

TODO

Draft

single point of failure ownership, debugging