4 Failures In Communication

Communication fails when:

1. Packet dropped
2. Serve crashed
3. Slow server
4. Network link broken

Client sees:

• Timeout

At Least once

• client send request until receive response
• Idempotent
  • system can receive duplicates of the request, no worries

At most once

• server will only execute once, filter out duplicates
• Replies to duplicate request by result it computed the first time
• Problems
  • 1. more and more seen messages to keep track of
    • solution: Client ACKs when receives response, so server can delete keep track
  • 2. Concurrent request from same user, out of order
    • duplicate detection (like using sequence numbers) can break
  • 3. Client resends while server is still executing
    • solution: server must track “in-progress” calls to avoid running same operation twice
  • 4. Server crashes
    • solution: after restart, server must remember past requests (persistent log) to avoid re-executing duplicates

Exactly Once

• combine at least once and at most once
• We can not 100% guarantee that this process will finish.