Implement the following plan:

MultiPaxos Replicated KV Store Emulation

Context

The project needs a new emulation that replicates the existing kv_store using MultiPaxos consensus. This demonstrates how a distributed consensus protocol can be used to replicate state across multiple nodes while preserving the same client-facing semantics (put fails if key exists, get returns value or error, delete fails if key missing). The client is given a Subject pointing to the leader directly (no leader discovery).

Architecture

Three actor types + client:

1. Acceptor - Paxos voting participant (prepare/promise, accept/accepted)
2. Leader (proposer + learner + state machine) - Receives client requests, assigns log slots, runs Paxos Phase 2, applies committed commands to local KV store, replies to clients, broadcasts commits to replicas
3. Replica (passive learner) - Receives committed commands from leader, applies to local KV copy
4. Client - Same as kv_store/client.gleam but 5 requests, targeting leader

Message Flow

Client --[ClientPut/Get/Delete]--> Leader
  Leader assigns slot N
  Leader --[Accept(slot N, ballot, cmd)]--> all Acceptors (synchronous calls)
  Leader waits for quorum of Accepted responses
  Leader applies command to local KV store
  Leader --[reply]--> Client
  Leader --[Commit(slot N, cmd)]--> all Replicas

Key Design Decisions

• Phase 1 once at startup: Leader runs prepare/promise during init. With stable leader assumption, succeeds immediately.
• Synchronous Phase 2: Leader calls acceptors sequentially with actor.call during request handling. Simple, correct, educational.
• GETs go through the log: For linearizability, reads are also assigned slots and go through consensus.
• Replicas are passive: They receive committed commands for demonstration of replication.

File Structure

multipaxos_kv/
  gleam.toml
  src/
    multipaxos_kv.gleam              -- main entry point
    multipaxos_kv/
      types.gleam                    -- shared types (Command, Ballot, OpResult)
      acceptor.gleam                 -- Paxos acceptor actor
      leader.gleam                   -- proposer + learner + KV state machine + client API
      replica.gleam                  -- passive learner
      client.gleam                   -- 5 random requests to leader
  test/
    multipaxos_kv_test.gleam         -- basic test

Implementation Steps

1. Create project

cd /Users/apanda/code/agent-spec-code
gleam new --skip-git --skip-github multipaxos_kv
cd multipaxos_kv
gleam add gleam_otp gleam_erlang argv

2. types.gleam - Shared types

• Ballot(round: Int, leader_id: Int) with comparison function
• Command = Put(key, value) | Get(key) | Delete(key) | Noop
• OpResult = OpOk | OpError(reason: String)

3. acceptor.gleam - Paxos acceptor

• State: id, promised_ballot: Option(Ballot), accepted: Dict(Int, #(Ballot, Command)), self
• Messages: Prepare(ballot, reply_with), Accept(slot, ballot, command, reply_with)
• Responses: Promise(accepted_entries) | PrepareRejected(highest), Accepted(slot) | AcceptRejected(slot, highest)
• Logic: standard Paxos prepare/accept with ballot comparison

4. leader.gleam - Core logic (most complex file)

• State: id, ballot, acceptors, replicas, quorum_size, next_slot, log: Dict(Int, Command), store: Dict(String, String), last_applied, self
• Client messages: ClientPut(key, value, reply_with), ClientGet(key, reply_with), ClientDelete(key, reply_with) - same signatures as kv_store/server
• Init: Run Phase 1 (prepare) to all acceptors, collect promises
• Request handling: Assign slot, run Phase 2 synchronously (call each acceptor), on quorum: commit to log, apply to store, reply to client, broadcast to replicas
• Public API: put(leader, key, value), get(leader, key), delete(leader, key) matching kv_store/server signatures
• KV semantics: Identical to kv_store/server.gleam (put rejects existing keys, delete rejects missing keys)

5. replica.gleam - Passive learner

• State: id, log: Dict(Int, Command), store: Dict(String, String), last_applied, self
• Message: Commit(slot: Int, command: Command)
• Applies committed commands in log order to local store, prints state changes

6. client.gleam - Adapted from kv_store

• Same structure as kv_store/client.gleam
• 5 requests instead of 10
• Targets Subject(leader.Message) instead of Subject(server.Message)
• Calls leader.put, leader.get (and optionally leader.delete)
• Same random key pool ["a","b","c","d","e"], same random values

7. multipaxos_kv.gleam - Main

• Start 3 acceptors, 2 replicas, 1 leader, N clients (default 3)
• Parse num_clients from CLI args
• Send Start to all clients, sleep 3s

Reference Files

• /Users/apanda/code/agent-spec-code/kv_store/src/kv_store/server.gleam - KV semantics to replicate
• /Users/apanda/code/agent-spec-code/kv_store/src/kv_store/client.gleam - Client pattern to adapt
• /Users/apanda/code/agent-spec-code/kv_store/src/kv_store.gleam - Main wiring pattern
• /Users/apanda/code/agent-spec-code/.claude/skills/gleam-otp.md - Actor patterns

Verification

1. gleam build - should compile without errors
2. gleam run - should show:
3. Acceptors starting
4. Replicas starting
5. Leader starting with Phase 1 completion
6. Clients issuing 5 requests each with Paxos consensus trace
7. Replicas applying committed commands
8. gleam format - ensure code is formatted

If you need specific details from before exiting plan mode (like exact code snippets, error messages, or content you generated), read the full transcript at: /Users/apanda/.claude/projects/-Users-apanda-code-agent-spec-code/7f5e4a48-7a64-493f-ad71-afbc3a310b33.jsonl

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Did you remember to add a commit for all this?

Yes please, split plan.md out to a different commit