Module ranked_register 

Ranked register recipe for Paxos-style ballot fencing

Ranked Register for FoundationDB

A shared memory abstraction that encapsulates Paxos ballots, based on Chockler & Malkhi’s “Active Disk Paxos with infinitely many processes” (PODC 2002). A ranked register is a mutable register with conflict detection via ranks, supporting unbounded processes with finite storage.

Operations

Operation	Who	Effect
read(rank)	Leader	Updates max_read_rank (installs fence), returns current value
write(rank, value)	Leader	Commits only if rank is high enough
value()	Followers	Plain read, no fence installed

Composing with Leader Election

The ranked register is designed to work with the leader election recipe. The leader election’s ballot serves as the rank for register operations, providing automatic fencing against stale leaders.

use foundationdb::recipes::leader_election::LeaderElection;
use foundationdb::recipes::ranked_register::{RankedRegister, Rank};
use foundationdb::tuple::Subspace;

let election = LeaderElection::new(Subspace::all().subspace(&"my-election"));
let register = RankedRegister::new(Subspace::all().subspace(&"my-state"));

// In the leader's main loop:
// db.run(|txn, _| async move {
//     let result = election.run_election_cycle(&txn, process_id, priority, now).await?;
//     match result {
//         ElectionResult::Leader(state) => {
//             let rank = Rank::from(state.ballot);
//             // Read current state (installs fence at this ballot)
//             let current = register.read(&txn, rank).await?;
//             // Mutate and write back
//             register.write(&txn, rank, b"new_value").await?;
//         }
//         ElectionResult::Follower(_) => {
//             // Safe read — doesn't interfere with leader's writes
//             let current = register.value(&txn).await?;
//         }
//     }
//     Ok(())
// }).await?;

Why This Works

• Leader election’s ballot is monotonically increasing
• A deposed leader has a lower ballot than the new leader
• read(rank) installs a fence at the ballot value
• Any write with a lower rank is automatically rejected
• value() is safe for followers — it never installs a fence

Structs

Rank

A rank value for ordering register operations

RankedRegister

A ranked register backed by FoundationDB

ReadResult

Result of a ranked read operation

RegisterState

Internal state of the ranked register as stored in FoundationDB

Enums

RankedRegisterError

Ranked register specific errors

WriteResult

Result of a ranked write operation

Type Aliases

Result

Result type for ranked register operations