Fencing Tokens

Monotonic counters that protect against stale lock holders corrupting data.

Technical Deep Dive

For the complete fence token specification including format contracts, overflow handling, and cross-backend consistency requirements, see specs/interface.md § Fence Token Types.

What are Fencing Tokens?

Every successful lock acquisition returns a fence token—a strictly increasing number that identifies the lock generation:

const result = await backend.acquire({ key: "document:123", ttlMs: 30000 });

if (result.ok) {
  const { lockId, fence } = result;
  console.log(fence); // "000000000000001"
}

Fence tokens are 15-digit zero-padded strings that compare lexicographically (ADR-004):

const first = "000000000000001";
const second = "000000000000002";

console.log(second > first); // true (string comparison works!)
console.log(first === second); // false

Key properties:

• Monotonic: Each acquisition increments the counter (fence2 > fence1)
• Persistent: Counters survive backend restarts
• Per-key: Each lock key has its own independent counter
• Lexicographic: Direct string comparison (>, <, ===) reflects chronological order
• No helpers needed: Fixed 15-digit format enables direct comparison without utility functions

The Stale Lock Problem

Distributed locks can expire while work is still in progress. Without fencing tokens, a stale lock holder can corrupt data:

// ❌ Without fencing: stale writes can corrupt data
await lock(
  async () => {
    const data = await fetchData(); // Process 1 acquires lock
    await slowNetworkCall(); // Takes 40s (lock TTL was 30s!)
    await writeData(data); // Process 1 lost lock; Process 2 already wrote!
  },
  { key: "document:123", ttlMs: 30000 },
);

Timeline without fencing:

1. 0s: Process 1 acquires lock, starts work
2. 30s: Lock expires (TTL reached)
3. 31s: Process 2 acquires lock, completes quickly
4. 40s: Process 1 (stale) overwrites Process 2's correct data

Timeline with fencing:

1. 0s: Process 1 acquires lock with fence: "001"
2. 30s: Lock expires
3. 31s: Process 2 acquires lock with fence: "002"
4. 40s: Process 1 attempts write with fence: "001" → rejected (stale)

Fencing tokens let your backend reject stale operations, even if the lock holder doesn't realize it lost the lock.

Using Fencing Tokens

Basic Pattern: Check-and-Write

Store the latest fence token with your data. Reject writes from older fences:

const result = await backend.acquire({ key: "document:123", ttlMs: 30000 });

if (result.ok) {
  try {
    const { fence } = result;

    // Fetch current document
    const doc = await getDocument("123");

    // Check if our fence is newer (we haven't lost the lock)
    if (doc.fence && fence <= doc.fence) {
      throw new Error(`Stale lock: our fence ${fence} <= current ${doc.fence}`);
    }

    // Safe: we hold the newest lock
    await updateDocument("123", {
      data: processData(),
      fence, // Store fence with the data
    });
  } finally {
    await backend.release({ lockId: result.lockId });
  }
}

Database-Enforced Fencing

For Firestore, use transactions to atomically verify the fence:

const result = await backend.acquire({ key: "document:123", ttlMs: 30000 });

if (result.ok) {
  const { fence } = result;

  await db.runTransaction(async (trx) => {
    const docRef = db.collection("documents").doc("123");
    const doc = await trx.get(docRef);
    const currentFence = doc.data()?.fence || "000000000000000";

    // Reject stale writes atomically
    if (fence <= currentFence) {
      throw new Error("Stale fence token");
    }

    // Safe: atomic check-and-write
    await trx.update(docRef, {
      data: newData,
      fence,
      updatedAt: Date.now(),
    });
  });
}

External API Protection

Pass fence tokens to external services that support conditional writes:

const result = await backend.acquire({ key: "job:456", ttlMs: 60000 });

if (result.ok) {
  const { fence } = result;

  // External API with conditional update
  await apiClient.updateResource({
    resourceId: "456",
    data: newData,
    ifMatch: fence, // Only update if server fence matches
  });
}

TypeScript Guarantees

Both Redis and Firestore backends provide fencing tokens at compile time—no runtime assertions needed:

import { createRedisBackend } from "syncguard/redis";

const backend = createRedisBackend(redis);
const result = await backend.acquire({ key: "resource:123", ttlMs: 30000 });

if (result.ok) {
  // TypeScript knows fence exists!
  const fence = result.fence; // Type: string (no undefined)

  // Direct comparison works
  if (fence > lastKnownFence) {
    await updateWithFence(data, fence);
  }
}

No boilerplate required:

• ✅ result.fence is required (TypeScript enforces this)
• ❌ No need for hasFence() or optional chaining (result.fence?)
• ❌ No runtime assertions or type guards needed

Generic Code with Unknown Backends

If you're writing generic functions that accept any backend type, use the hasFence() helper:

import { hasFence } from "syncguard";
import type { LockBackend, BackendCapabilities } from "syncguard";

function processWithAnyBackend<C extends BackendCapabilities>(
  backend: LockBackend<C>,
  result: AcquireResult<C>,
) {
  if (hasFence(result)) {
    // Type guard for generic contexts
    console.log("Fence token:", result.fence);
  }
}

Most application code uses typed backends directly and doesn't need hasFence().

Common Patterns

Monotonic Version Counter

Use fences as version numbers for optimistic concurrency:

const result = await backend.acquire({ key: "counter", ttlMs: 30000 });

if (result.ok) {
  const { fence } = result;

  // Fetch current value
  const current = await redis.get("counter:value");
  const currentFence = await redis.get("counter:fence");

  // Increment only if we hold the newest lock
  if (!currentFence || fence > currentFence) {
    await redis.set("counter:value", parseInt(current || "0") + 1);
    await redis.set("counter:fence", fence);
  }
}

Heartbeat with Fence Validation

For long-running tasks, validate fence tokens before each operation:

const result = await backend.acquire({ key: "batch-job", ttlMs: 60000 });

if (!result.ok) throw new Error("Failed to acquire lock");

let currentFence = result.fence;

// Extend every 30s
const heartbeat = setInterval(async () => {
  const extended = await backend.extend({
    lockId: result.lockId,
    ttlMs: 60000,
  });

  if (!extended.ok) {
    clearInterval(heartbeat);
    throw new Error("Lost lock ownership");
  }

  // Note: extend doesn't return a new fence (same lock instance)
}, 30000);

try {
  for (const batch of largeBatches) {
    // Check if our fence is still valid before each batch
    const info = await backend.lookup({ key: "batch-job" });
    if (!info || info.fence !== currentFence) {
      throw new Error("Lock fence changed (lost ownership)");
    }

    await processBatch(batch, currentFence);
  }
} finally {
  clearInterval(heartbeat);
  await backend.release({ lockId: result.lockId });
}

Comparing Fences from Different Sources

Fences are lexicographically ordered strings—direct comparison works:

const fenceA = "000000000000100";
const fenceB = "000000000000200";

// String comparison reflects chronological order
console.log(fenceB > fenceA); // true
console.log(fenceA === fenceB); // false

// Sort fences chronologically
const fences = ["000000000000003", "000000000000001", "000000000000002"];
const sorted = fences.sort(); // ["001", "002", "003"]

Why Zero-Padded Strings? (ADR-004)

15-digit zero-padding ensures lexicographic order matches numeric order. Without padding, "2" > "10" (lexicographic), but we need "000000000000002" < "000000000000010" (correct). The fixed-width format eliminates the need for comparison helper functions—direct string comparison just works.

When to Use Fencing

✅ Use Fencing When

• Stale writes risk data corruption: Payment processing, inventory updates
• External systems need protection: APIs that support conditional writes
• Lock TTL might expire mid-work: Long-running batch jobs
• Multiple writers compete: Document editing, configuration updates

❌ Skip Fencing When

• Single writer per key: Only one process ever writes to the resource
• Short critical sections: Lock expires well after work completes (no stale risk)
• Idempotent operations: Writing the same data twice is safe
• Eventually consistent data: Last-write-wins semantics are acceptable

Example: Fencing not needed

// Idempotent: writing status="completed" multiple times is safe
await lock(
  async () => {
    await updateJobStatus(jobId, "completed");
  },
  { key: `job:${jobId}` },
);

Example: Fencing required

// Not idempotent: balance changes must be fenced
const result = await backend.acquire({ key: `account:${id}`, ttlMs: 30000 });

if (result.ok) {
  const { fence } = result;
  const account = await getAccount(id);

  // Reject if our fence is stale
  if (fence <= account.fence) {
    throw new Error("Stale lock");
  }

  await updateAccount(id, {
    balance: account.balance - amount,
    fence,
  });
}

Fence Token Format

SyncGuard uses 15-digit zero-padded decimal strings across all backends:

"000000000000001"; // First lock
"000000000000002"; // Second lock
"000000000009999"; // 9,999th lock

Why 15 digits?

• Guarantees full safety within Lua's 53-bit precision limit (2^53-1 ≈ 9.007e15)
• Eliminates ALL risk of floating-point rounding errors in Redis
• Consistent format across Redis and Firestore
• Lexicographic comparison works correctly
• JSON-safe (no precision loss from BigInt serialization)

Practical limits:

• Overflow protection: FENCE_THRESHOLDS.MAX = "900000000000000" (9×10¹⁴ operations per key)
• Warning threshold: FENCE_THRESHOLDS.WARN = "090000000000000" (9×10¹³ operations per key)
• At 1 million locks/day, takes ~2,466 years to reach the overflow limit
• Backends automatically enforce these limits (ADR-004)

Counter Lifecycle

Fence counters are persistent and survive backend restarts. They accumulate over the lifetime of the backend instance. For most applications (bounded key spaces), memory impact is negligible (~50-100 bytes per unique key).

Overflow Protection

SyncGuard enforces strict overflow limits to prevent fence counter wraparound:

Automatic Protection (ADR-004):

• Hard limit: Operations fail with LockError("Internal") when fence exceeds FENCE_THRESHOLDS.MAX
• Early warning: Logs warnings when fence exceeds FENCE_THRESHOLDS.WARN (10% before limit)
• Cross-backend consistency: Both Redis and Firestore enforce identical limits using canonical constants from common/constants.ts

When overflow occurs:

try {
  const result = await backend.acquire({ key, ttlMs: 30000 });
} catch (error) {
  if (error instanceof LockError && error.code === "Internal") {
    // Fence counter has reached operational limit (FENCE_THRESHOLDS.MAX)
    // Action: Rotate to a new key namespace or contact operations
  }
}

Operational Response:

1. Monitor warnings: Watch for log messages about approaching limits
2. Key rotation: If a key approaches the limit, migrate to a new key namespace
3. Production planning: At 1M locks/day, you have ~2,466 years before overflow

Overflow Recovery

If overflow occurs, you must rotate to a new key namespace. Resetting fence counters breaks monotonicity guarantees and can cause data corruption. Never reset production fence counters.

Troubleshooting

Q: How do I reset a fence counter?

You can't reset counters safely in production—monotonicity is the security guarantee. If you need to reset for testing:

• Redis: DEL syncguard:fence:resource:123
• Firestore: Delete the document in the fence_counters collection

Never reset production counters. Use key rotation instead.

Q: What happens if two backends use the same key?

Each backend maintains independent counters. Don't mix backends for the same key—fence tokens won't be comparable.

Q: Do I need to clean up old fences?

No. Counters are per-key and accumulate slowly. Deleting fence counters breaks monotonicity and can cause data corruption.

Q: What if I see overflow warnings in production?

Warnings appear when fence counters exceed FENCE_THRESHOLDS.WARN (9×10¹³). This is extremely rare—it means the key has been locked ~90 trillion times. Contact operations to plan key namespace rotation before reaching the hard limit at FENCE_THRESHOLDS.MAX (9×10¹⁴).