Implement Chunk Leases for Primary Assignment - The Filesystem

                      TASK
                    

Implementation

A chunk lease grants one chunk server the exclusive right to define the mutation order for a chunk. This avoids per-operation consensus while maintaining consistency.

Lease lifecycle:

Grant: master assigns a 60-second lease to one chunk server (the primary)
Use: the primary defines the serial order for all mutations on that chunk
Renew: primary sends heartbeats to master; master extends the lease
Expire: if the primary fails to renew within 60s, the lease expires
Re-grant: master waits for the old lease to expire, then grants to another server

Safety guarantee: at most ONE primary exists for each chunk at any time. If the master loses contact with the primary, it simply waits for the lease to expire before granting to a new server.

Request:  {"type": "lease_grant", "msg_id": 1, "chunk_handle": "ch_001", "server": "cs1"}
Response: {"type": "lease_grant_ok", "in_reply_to": 1, "chunk_handle": "ch_001", "primary": "cs1", "expires_in_ms": 60000}

Request:  {"type": "lease_renew", "msg_id": 2, "chunk_handle": "ch_001", "server": "cs1"}
Response: {"type": "lease_renew_ok", "in_reply_to": 2, "new_expires_in_ms": 60000}

Request:  {"type": "lease_check", "msg_id": 3, "chunk_handle": "ch_001"}
Response: {"type": "lease_check_ok", "in_reply_to": 3, "primary": "cs1", "remaining_ms": 45000, "expired": false}

Sample Test Cases

Grant lease to chunk serverTimeout: 5000ms

Input

{"src":"c0","dest":"n1","body":{"type":"init","msg_id":1,"node_id":"n1","node_ids":["n1","n2","n3"]}}
{"src":"c1","dest":"n1","body":{"type":"lease_grant","msg_id":2,"chunk_handle":"ch_001","server":"n2"}}

Expected Output

{"src": "n1", "dest": "c0", "body": {"type": "init_ok", "in_reply_to": 1, "msg_id": 0}}
{"src": "n1", "dest": "c1", "body": {"type": "lease_grant_ok", "in_reply_to": 2, "chunk_handle": "ch_001", "primary": "n2", "msg_id": 1}}

Renew lease before expiryTimeout: 5000ms

Input

{"src":"c0","dest":"n1","body":{"type":"init","msg_id":1,"node_id":"n1","node_ids":["n1","n2"]}}
{"src":"c1","dest":"n1","body":{"type":"lease_grant","msg_id":2,"chunk_handle":"ch_002","server":"n2"}}
{"src":"c1","dest":"n1","body":{"type":"lease_renew","msg_id":3,"chunk_handle":"ch_002","server":"n2"}}

Expected Output

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

Hints

Hint 1▾

A lease grants one chunk server the "primary" role for 60 seconds

Hint 2▾

The primary is the only server that can define the mutation order for a chunk

Hint 3▾

The primary must renew the lease before it expires (by heartbeating the master)

Hint 4▾

If the lease expires, the master can grant it to another server — preventing split-brain

Hint 5▾

Leases avoid the cost of per-operation consensus while maintaining consistency

Resources

Leases in Distributed Systems

Classic paper on leases as a fault-tolerant mechanism for distributed caches

                      OVERVIEW
                    

Theoretical Hub

Concept overview coming soon

Key Concepts

leaseprimary electionlease renewallease expiryconsistency window

main.py

python