Client Session Management

Module: crates/kimberlite-vsr/src/client_sessions.rs TLA+ Spec: specs/tla/ClientSessions.tla Kani Proofs: crates/kimberlite-vsr/src/kani_proofs.rs (Proofs #26-29) VOPR Scenarios: 3 scenarios (ClientSessionCrash, ClientSessionViewChangeLockout, ClientSessionEviction)

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Overview

Kimberlite’s client session management fixes two critical bugs found in the VRR (Viewstamped Replication Revisited) paper. These bugs cause request collisions and client lockouts in production systems.

VRR Bug #1: Successive Client Crashes

Problem: When a client crashes and restarts, it resets its request number to 0. If the server still has the old session cached, it returns the reply for request #0 from the previous client incarnation, not the current one.

Example:

1. Client connects, sends request #0 → gets reply "A"
2. Client crashes, cache entry remains
3. Client reconnects, sends request #0 → gets reply "A" (WRONG!)
   Should execute new request, not return cached "A"

Impact: Clients get wrong results from previous session, causing data corruption.

Fix: Explicit session registration. Each client must register before sending requests. Registration creates a new session with a fresh request number space.

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VRR Bug #2: Uncommitted Request Table Updates

Problem: The VRR paper updates the client table when a request is prepared (not yet committed). During a view change, the new leader doesn’t have uncommitted prepares, so it rejects requests from clients whose table was updated but not committed. Client is locked out.

Example:

1. Primary prepares client request #5 → updates client table to request_number = 5
2. View change occurs before commit
3. New primary doesn't have request #5
4. Client retries request #5 → rejected (not > 5 in new primary's table)
   Client cannot make progress!

Impact: Clients become permanently locked out after view changes, requiring manual intervention.

Fix: Separate committed vs uncommitted tracking. Only update the committed table after a request is actually committed. Track uncommitted requests separately and discard them on view change.

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Solution Architecture

Separate Tracking

pub struct ClientSessions {
    /// Committed sessions with cached replies (survives view changes)
    committed: HashMap<ClientId, CommittedSession>,

    /// Uncommitted sessions (discarded on view changes)
    uncommitted: HashMap<ClientId, UncommittedSession>,

    /// Priority queue for deterministic eviction (oldest first)
    eviction_queue: BinaryHeap<Reverse<SessionEviction>>,

    /// Configuration (max_sessions limit)
    config: ClientSessionsConfig,
}

Key Operations

1. Session Registration (Bug #1 Fix)

let client_id = sessions.register_client();
// Each registration creates a new session ID
// Even if request numbers overlap, different IDs prevent collisions

2. Uncommitted Tracking (Bug #2 Fix - Part 1)

// During prepare phase: record as uncommitted
sessions.record_uncommitted(client_id, request_number, preparing_op)?;

3. Commit (Bug #2 Fix - Part 2)

// After consensus: move from uncommitted to committed
sessions.commit_request(
    client_id,
    request_number,
    committed_op,
    reply_op,
    cached_effects,
    commit_timestamp,
)?;

4. View Change (Bug #2 Fix - Part 3)

// New leader discards uncommitted sessions
sessions.discard_uncommitted();
// Committed sessions preserved, uncommitted gone

5. Deterministic Eviction

// When max_sessions exceeded, evict oldest by commit_timestamp
// All replicas evict same session (deterministic)
evict_oldest();

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Implementation Details

CommittedSession

pub struct CommittedSession {
    /// Highest committed request number for this client
    pub request_number: u64,

    /// Op number where committed
    pub committed_op: OpNumber,

    /// Op number to return as reply
    pub reply_op: OpNumber,

    /// Cached effects (for idempotent retry)
    pub cached_effects: Vec<Effect>,

    /// Timestamp when committed (for deterministic eviction)
    pub commit_timestamp: Timestamp,
}

UncommittedSession

pub struct UncommittedSession {
    /// Request number being prepared (not yet committed)
    pub request_number: u64,

    /// Op number where preparing
    pub preparing_op: OpNumber,
}

Configuration

pub struct ClientSessionsConfig {
    /// Maximum concurrent sessions (default: 100,000)
    pub max_sessions: usize,
}

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Formal Verification

TLA+ Specification (specs/tla/ClientSessions.tla)

Properties Verified:

1. NoRequestCollision: Client crash with request number reset doesn’t return wrong cached replies
2. NoClientLockout: View change doesn’t prevent valid client requests from being processed
3. DeterministicEviction: All replicas evict same sessions (by commit_timestamp)
4. RequestNumberMonotonic: Within a client session, request numbers only increase
5. CommittedSessionsSurviveViewChange: View changes preserve committed sessions
6. NoDuplicateCommits: A client cannot commit the same request number twice

Model checked: TLC verifies all invariants hold.

Kani Proofs (4 proofs)

1. Proof 26: No request collision after client crash

   • Property: Separate committed/uncommitted prevents wrong cached replies
   • Verified: Client crash with reset returns correct (not cached) reply

2. Proof 27: Committed and uncommitted sessions are independent

   • Property: Uncommitted sessions don’t interfere with committed lookups
   • Verified: Duplicate detection only checks committed sessions

3. Proof 28: View change transfers only committed sessions

   • Property: Uncommitted sessions discarded, committed preserved
   • Verified: discard_uncommitted() clears only uncommitted

4. Proof 29: Session eviction is deterministic

   • Property: Eviction by commit_timestamp produces same result across replicas
   • Verified: Oldest session (by timestamp) always evicted first

Production Assertions (6 assertions)

All use assert!() (not debug_assert!()) for production enforcement:

1. Committed slot monotonicity (record_uncommitted:323)

   • request_number > committed.request_number
   • Prevents VRR Bug #1 (request collisions)

2. No duplicate commits (commit_request:381)

   • existing.request_number != request_number
   • A client cannot commit same request number twice

3. Session capacity enforcement (commit_request:403)

   • committed.len() <= max_sessions + 1
   • Prevents unbounded memory growth

4. Eviction verification (commit_request:414)

   • committed.len() <= max_sessions after eviction
   • Ensures eviction actually worked

5. Backups clear uncommitted (discard_uncommitted:444)

   • uncommitted.is_empty() after discard
   • Prevents VRR Bug #2 (client lockout)

6. Eviction determinism (evict_oldest:462)

   • committed.len() == count_before - 1
   • Exactly one session evicted (deterministic)

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VOPR Testing (3 scenarios)

1. ClientSessionCrash (VRR Bug #1)

Test: Successive client crashes with request number reset Verify: No wrong cached replies returned Config: 15s runtime, 40K events, 10% gray failures (simulates crashes)

2. ClientSessionViewChangeLockout (VRR Bug #2)

Test: Uncommitted sessions during view change Verify: Clients not locked out after view change Config: 20s runtime, 35K events, 15% drop probability (triggers view changes)

3. ClientSessionEviction

Test: Deterministic eviction under load (100K sessions) Verify: All replicas evict same sessions (oldest by commit_timestamp) Config: 15s runtime, 100K events, 3 tenants for session pressure

All scenarios pass: 1M iterations per scenario, 0 violations

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Performance Characteristics

• Memory per session: ~120 bytes (committed) or ~40 bytes (uncommitted)
• Registration overhead: O(1) - increment counter
• Duplicate check: O(1) - HashMap lookup
• Record uncommitted: O(1) - HashMap insert
• Commit: O(log N) - priority queue insert for eviction
• Eviction: O(log N) - priority queue pop
• View change: O(U) - clear U uncommitted sessions

Typical overhead: <1% for 100K concurrent sessions

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Integration with VSR

Primary Replica

// On client request
if let Some(cached) = sessions.check_duplicate(client_id, request_number) {
    // Return cached reply (idempotent retry)
    return Ok(cached.cached_effects.clone());
}

// On prepare
sessions.record_uncommitted(client_id, request_number, preparing_op)?;

// On commit
let timestamp = clock.realtime_synchronized().unwrap();
sessions.commit_request(
    client_id,
    request_number,
    committed_op,
    reply_op,
    effects,
    timestamp,
)?;

Backup Replica

// On view change: new leader discards uncommitted
sessions.discard_uncommitted();

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Debugging Guide

Common Issues

Issue: Client gets wrong cached reply Diagnosis: Check if client properly registered session Fix: Ensure register_client() called before sending requests

Issue: Client locked out after view change Diagnosis: Uncommitted session not discarded Fix: Verify discard_uncommitted() called during view change

Issue: Memory growth from sessions Diagnosis: Eviction not working or max_sessions too high Fix: Check eviction_queue, verify commit_timestamp ordering

Assertions That Catch Bugs

Assertion	What It Catches	VRR Bug
request_number > committed.request_number	Request number collision	Bug #1
existing.request_number != request_number	Duplicate commit	Bug #1
uncommitted.is_empty() after view change	Uncommitted not discarded	Bug #2
committed.len() <= max_sessions	Eviction failure	Eviction
committed.len() == count_before - 1	Non-deterministic eviction	Eviction

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References

Academic Papers

• Liskov, B., & Cowling, J. (2012). “Viewstamped Replication Revisited” (VRR paper with bugs)

Industry Implementations

• TigerBeetle: src/vsr/client_sessions.zig (inspiration for fixes)
• FoundationDB: Client request deduplication (similar approach)

Internal Documentation

• docs/concepts/compliance.md - Idempotency for HIPAA/GDPR
• docs/traceability_matrix.md - TLA+ → Rust → VOPR traceability

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Future Work

• Cross-cluster session transfer (for disaster recovery)
• Session persistence (survive replica crashes, not just view changes)
• Adaptive max_sessions (scale based on load)
• Session TTL (auto-expire inactive sessions)

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Implementation Status: Complete (Phase 1.2 - v0.3.0) Verification: 4 Kani proofs, 3 VOPR scenarios, 1 TLA+ spec, 6 production assertions VRR Bugs Fixed: Bug #1 (request collisions), Bug #2 (client lockout)