Handle Client Retry and Deduplication - The Store

                      TASK
                    

Implementation

Handle client retries without duplicate execution:

Client assigns sequence number to each request
Server tracks (client_id -> latest_seq, response)
If request seq <= latest_seq, return cached response
Otherwise, process and cache new response

This makes at-least-once delivery safe for non-idempotent operations.

Sample Test Cases

First request executesTimeout: 5000ms

Input

{"src":"c0","dest":"n1","body":{"type":"init","msg_id":1,"node_id":"n1","node_ids":["n1"]}}
{"src":"c1","dest":"n1","body":{"type":"write","msg_id":2,"key":"x","value":1,"client_id":"c1","seq":1}}

Expected Output

{"src":"n1","dest":"c0","body":{"type":"init_ok","in_reply_to":1,"msg_id":0}}
{"src":"n1","dest":"c1","body":{"type":"write_ok","in_reply_to":2,"msg_id":1}}

Hints

Hint 1▾

Client assigns unique ID to each request

Hint 2▾

Server tracks latest response per client

Hint 3▾

Duplicate request returns cached response

                      OVERVIEW
                    

Theoretical Hub

Exactly-Once Semantics

Network issues cause retries. Without deduplication, a PUT might execute twice. By tracking client sessions and sequence numbers, we can detect and skip duplicates.

Session State

The deduplication table must survive leader changes. Store it in the replicated state machine. Periodically garbage collect old sessions.

Key Concepts

idempotencyclient sessionsdeduplication

main.py

python

#!/usr/bin/env python3
import sys
import json
import time
class ClientTracker:
    def __init__(self, session_timeout=300):
        self.sessions = {}  # client_id -> {seq, response, last_seen}
        self.session_timeout = session_timeout
    
    def register_request(self, client_id, seq, operation):
        # TODO: Check if duplicate
        # Return (is_dup, cached_response or None)
        pass
    
    def record_response(self, client_id, seq, response):
        # TODO: Cache response for client
        pass
    
    def gc_expired_sessions(self):
        # TODO: Remove old sessions
        pass
class DedupKVStore:
    def __init__(self, kv_store, tracker):
        self.kv = kv_store
        self.tracker = tracker
    
    def handle_request(self, client_id, seq, operation):
        # TODO: Check dedup, process or return cached
        pass