Lock

Problem:

Implement a lock service

1. Understand the requirements and establish the design scope

- Support 2 operations: Acquire a lock and Release a lock
- Should work in distributed environment
- Performant, high availibility, consistency
- A lock automatically releases when the expiration has reached

2. High level design

- Client --> Lock Service

- Acquire(key): acquires a lock with the given key 
- Release(key): releases a lock held on the given key

- Usecases:
+ client1 acquires a lock on "ABC" key -> client1 holds a lock on "ABC" key
+ client2 acquires a lock on "ABC" key -> fails dues to client1 is holding a lock on "ABC" key
+ client3 acquires a lock on "CDF" key -> lock acquired on "CDF" key
+ client1 finishes the work and release the lock on "ABC" key
+ client2 acquires a lock on "ABC key -> client2 holds a lock on "ABC" key

3. Deep dive

- What if client1 crashes while holding a lock on "ABC" key -> ?
- Should we queue clients who are trying to acquire a lock on a specified key as another alternative -> ?
- If the Lock Service crashes all lock information currently stores in the memory will be lost -> Persist to disk?

4. Wrap up

5. Test

-- server
docker run --name redis-instance -p 6379:6379 -d redis:7.0.8
-- cli
docker run -it --rm redis redis-cli -h redis-instance