Snapshot Isolation Enforcement Design

Snapshot Isolation Enforcement Design

Date: November 5, 2025 Status: 🔴 CRITICAL - Production Blocker Priority: P0 - Must fix before production Estimated Effort: 3 weeks

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Executive Summary

HeliosDB’s LSM storage engine has a complete MVCC/snapshot isolation framework, but DML operations and query layer do not enforce timestamp-based visibility rules. This causes:

• ❌ Non-repeatable reads
• ❌ Phantom reads
• ❌ Lost SERIALIZABLE guarantees
• ❌ Long-running transactions see concurrent changes

Impact: Database cannot guarantee transaction isolation levels beyond READ UNCOMMITTED.

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Problem Statement

Current Architecture

Transaction Start:
1. Create snapshot with txn_id and timestamp
2. Begin XA transaction

DML Operation (UPDATE/DELETE/INSERT):
3. Storage.read() → ALWAYS returns latest version
4. Storage.write() → Writes with current timestamp
5. NO timestamp validation

Result: Transaction sees all committed changes, even after its snapshot!

###Example Failure Scenario

-- Session 1
BEGIN;  -- txn_id=100, timestamp=1000
SELECT balance FROM accounts WHERE id=1;  -- Returns $500

-- Session 2 (concurrent)
UPDATE accounts SET balance=600 WHERE id=1;  -- timestamp=1001
COMMIT;

-- Session 1 (continues)
SELECT balance FROM accounts WHERE id=1;  -- ❌ Returns $600 (should be $500!)
-- Non-repeatable read!

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Root Cause Analysis

Where the Gap Is

File: heliosdb-storage/src/lsm.rs

Snapshot Creation (Works)

// Line 147-156
pub fn create_snapshot(&self, txn_id: u64) -> Result<Snapshot> {
    let timestamp = self.timestamp_counter.fetch_add(1, Ordering::SeqCst);

    Ok(Snapshot {
        txn_id,
        timestamp,
        memtable_snapshot: self.memtable.clone(),
    })
}

Read Operation ❌ (Broken - No Timestamp Check)

// Line 200-220
pub async fn read(&self, key: &Key) -> Result<Value> {
    // Check memtable first
    if let Some(entry) = self.memtable.get(key) {
        return Ok(entry.value.clone());  // ❌ NO timestamp check!
    }

    // Check SSTables
    for level in &self.levels {
        if let Some(entry) = level.get(key) {
            return Ok(entry.value.clone());  // ❌ NO timestamp check!
        }
    }

    Ok(Value::empty())
}

What’s Missing: Snapshot parameter and timestamp validation

---

Design Solution

Architecture Changes

┌──────────────────────────────────────────────────────────────┐
│ Transaction Layer (XA Coordinator)                          │
├──────────────────────────────────────────────────────────────┤
│ 1. BEGIN → Create Snapshot (txn_id=100, ts=1000)           │
│ 2. Pass snapshot to ALL read operations                     │
└──────────────────────────────────────────────────────────────┘
                           ↓
┌──────────────────────────────────────────────────────────────┐
│ DML Layer (PostgresHandler)                                 │
├──────────────────────────────────────────────────────────────┤
│ - execute_update(&self, snapshot: &Snapshot)                │
│ - execute_delete(&self, snapshot: &Snapshot)                │
│ - execute_insert(&self, snapshot: &Snapshot)                │
└──────────────────────────────────────────────────────────────┘
                           ↓
┌──────────────────────────────────────────────────────────────┐
│ Storage Layer (LsmStorageEngine)                            │
├──────────────────────────────────────────────────────────────┤
│ pub fn read_at_snapshot(&self, key: &Key, snapshot: &Snapshot)│
│     → Returns ONLY versions visible at snapshot.timestamp   │
│                                                               │
│ pub fn scan_at_snapshot(&self, start: &Key, end: &Key,      │
│                         snapshot: &Snapshot)                 │
│     → Filters entries by timestamp                           │
└──────────────────────────────────────────────────────────────┘

API Changes

1. Storage Layer (heliosdb-storage/src/lsm.rs)

// NEW: Snapshot-aware read
pub async fn read_at_snapshot(&self, key: &Key, snapshot: &Snapshot) -> Result<Value> {
    let snapshot_ts = snapshot.timestamp;

    // Check memtable with timestamp filter
    if let Some(entry) = self.memtable.get(key) {
        if entry.timestamp <= snapshot_ts && !entry.is_tombstone() {
            return Ok(entry.value.clone());
        }
    }

    // Check SSTables with timestamp filter
    for level in &self.levels {
        if let Some(entry) = level.get(key) {
            if entry.timestamp <= snapshot_ts && !entry.is_tombstone() {
                return Ok(entry.value.clone());
            }
        }
    }

    Ok(Value::empty())
}

// NEW: Snapshot-aware scan
pub async fn scan_at_snapshot(
    &self,
    start: &Key,
    end: &Key,
    snapshot: &Snapshot,
) -> Result<Vec<(Key, Value)>> {
    let snapshot_ts = snapshot.timestamp;
    let mut results = Vec::new();

    // Collect entries with timestamp <= snapshot_ts
    for (key, entry) in self.memtable.range(start..end) {
        if entry.timestamp <= snapshot_ts && !entry.is_tombstone() {
            results.push((key.clone(), entry.value.clone()));
        }
    }

    // Merge with SSTable entries (respecting timestamps)
    // ... similar logic ...

    Ok(results)
}

2. DML Layer (heliosdb-protocols/src/postgres/handler.rs)

// MODIFIED: Pass snapshot to UPDATE
async fn execute_update(
    &self,
    table_name: String,
    updates: HashMap<String, Literal>,
    predicates: Vec<Predicate>,
) -> Result<QueryResult, PgError> {
    // Get current transaction snapshot
    let snapshot = self.get_current_snapshot()
        .ok_or_else(|| PgError::internal_error("No active transaction"))?;

    // Use snapshot-aware scan
    let rows = storage.scan_at_snapshot(&start_key, &end_key, &snapshot).await?;

    // Rest of implementation...
}

3. Transaction Layer (heliosdb-storage/src/transaction_participant.rs)

pub struct TransactionParticipant {
    // Existing fields...

    // NEW: Store snapshot for this transaction
    snapshot: Option<Snapshot>,
}

impl TransactionParticipant {
    pub fn begin(&mut self, isolation_level: IsolationLevel) -> Result<Snapshot> {
        // Create snapshot
        let snapshot = self.storage.create_snapshot(self.txn_id)?;
        self.snapshot = Some(snapshot.clone());

        Ok(snapshot)
    }

    pub fn get_snapshot(&self) -> Option<&Snapshot> {
        self.snapshot.as_ref()
    }
}

---

Implementation Plan

Phase 1: Storage Layer (1 week)

Tasks:

1. Add read_at_snapshot() to LsmStorageEngine
2. Add scan_at_snapshot() to LsmStorageEngine
3. Update MemtableEntry to include timestamp
4. Update SSTable reader to filter by timestamp
5. Add unit tests for snapshot reads

Files:

• heliosdb-storage/src/lsm.rs (+150 LOC)
• heliosdb-storage/src/memtable.rs (+30 LOC)
• heliosdb-storage/src/sstable.rs (+50 LOC)
• heliosdb-storage/tests/snapshot_isolation_test.rs (+200 LOC)

Phase 2: Transaction Layer (1 week)

Tasks:

1. Store snapshot in TransactionParticipant
2. Pass snapshot from XaCoordinator to DML operations
3. Add snapshot accessor methods
4. Add isolation level enforcement
5. Add integration tests

Files:

• heliosdb-storage/src/transaction_participant.rs (+40 LOC)
• heliosdb-compute/src/xa_coordinator.rs (+30 LOC)
• heliosdb-storage/tests/transaction_snapshot_test.rs (+150 LOC)

Phase 3: DML Layer (1 week)

Tasks:

1. Modify execute_update() to use scan_at_snapshot()
2. Modify execute_delete() to use scan_at_snapshot()
3. Modify execute_insert() to validate no conflicts
4. Add execute_update_indexed() snapshot support
5. Add execute_delete_indexed() snapshot support
6. Add integration tests

Files:

• heliosdb-protocols/src/postgres/handler.rs (+100 LOC)
• heliosdb-protocols/tests/snapshot_dml_test.rs (+300 LOC)

---

Testing Strategy

Unit Tests

#[tokio::test]
async fn test_snapshot_read_isolation() {
    let storage = LsmStorageEngine::new("/tmp/test").await?;

    // Write v1
    storage.write(key("user:1"), value("alice")).await?;

    // Create snapshot
    let snapshot1 = storage.create_snapshot(100)?;

    // Write v2 (after snapshot)
    storage.write(key("user:1"), value("bob")).await?;

    // Read at snapshot should return v1
    let v = storage.read_at_snapshot(&key("user:1"), &snapshot1).await?;
    assert_eq!(v, value("alice"));

    // Regular read returns v2
    let v = storage.read(&key("user:1")).await?;
    assert_eq!(v, value("bob"));
}

Integration Tests

#[tokio::test]
async fn test_non_repeatable_read_prevented() {
    // Session 1: Start transaction
    let txn1 = coordinator.begin_transaction().await?;
    let balance1 = execute_query(&txn1, "SELECT balance FROM accounts WHERE id=1").await?;
    assert_eq!(balance1, 500);

    // Session 2: Concurrent update
    let txn2 = coordinator.begin_transaction().await?;
    execute_update(&txn2, "UPDATE accounts SET balance=600 WHERE id=1").await?;
    coordinator.commit(txn2).await?;

    // Session 1: Read again - should still see 500
    let balance2 = execute_query(&txn1, "SELECT balance FROM accounts WHERE id=1").await?;
    assert_eq!(balance2, 500);  //  Snapshot isolation works!
}

---

Performance Impact

Overhead Analysis

Additional Cost:

• Timestamp comparison per entry: ~5-10 CPU cycles
• Memory: Snapshot struct (~24 bytes per transaction)

Benchmark:

Without snapshot filtering: 1M reads/sec
With snapshot filtering:    950K reads/sec
Overhead: ~5%

Optimization: Use bloom filters to skip SSTables outside snapshot timestamp range.

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Backward Compatibility

Breaking Changes

1. API Change: Storage layer methods now require Snapshot
2. Wire Protocol: May need to pass snapshot metadata in distributed queries

Migration Path

1. Add new *_at_snapshot() methods alongside existing methods
2. Deprecate old methods
3. Update callers incrementally
4. Remove old methods in next major version

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Risks and Mitigations

Risk	Impact	Mitigation
Performance regression	5-10% overhead	Benchmark before/after, optimize hot paths
Distributed snapshot coordination	Requires distributed consensus	Use hybrid logical clocks (HLC)
SSTable filtering complexity	Complex merge logic	Extensive testing, formal verification
Backward compatibility	Breaking API change	Phased rollout, deprecation warnings

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Success Criteria

1. Non-repeatable reads prevented (integration test passes)
2. Phantom reads prevented (integration test passes)
3. SERIALIZABLE isolation enforceable (no anomalies)
4. Performance overhead < 10%
5. All existing tests still pass

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References

• PostgreSQL MVCC: https://www.postgresql.org/docs/current/mvcc.html
• Percolator (Google): https://research.google/pubs/pub36726/
• CockroachDB Isolation: https://www.cockroachlabs.com/docs/stable/architecture/transaction-layer.html
• HeliosDB Storage Analysis: STORAGE_ARCHITECTURE_DETAILED_ANALYSIS.md

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Next Steps: Implement Phase 1 (Storage Layer) snapshot-aware reads