sentinel-coordination 0.1.3

Lightweight DB-backed coordination primitive with leases and fencing tokens

Sentinel

A lightweight coordination primitive for backend systems. Useful for coordinating workers, preventing duplicate jobs, and ensuring safe execution with fencing tokens.

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Installation

pip install sentinel-coordination

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Features

• DB-backed leases
• Fencing tokens (prevents stale worker writes)
• Simple API (acquire, heartbeat, release, lease)

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Example

import sentinel

with sentinel.lease(conn, "order_123", ttl_ms=5000, get_conn=get_conn):
    process_order()

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API

acquire

result = sentinel.acquire(conn, key, ttl_ms=5000)

Returns:

• acquired (bool)
• owner_id (str)
• expires_at (timestamp)
• fencing_token (int)

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heartbeat

result = sentinel.heartbeat(conn, key, owner_id, ttl_ms=5000)

Returns:

• extended (bool)

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release

result = sentinel.release(conn, key, owner_id)

Returns:

• released (bool)

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lease (recommended)

with sentinel.lease(conn, key, ttl_ms=5000, get_conn=get_conn):
    do_work()

Handles:

• acquire
• automatic heartbeat
• release on exit

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Fencing Tokens (Important)

Each acquire returns a monotonically increasing fencing token.

To use fencing tokens safely:

1. Store fencing_token in your application data
2. Include a condition in all writes

Example:

UPDATE orders
SET status = 'paid', fencing_token = %s
WHERE id = %s AND fencing_token <= %s;

This ensures:

• stale workers cannot overwrite newer ones
• race conditions are safe

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Status

⚠️ Beta (v0.1.0)

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Limitations

• No retries yet
• No async support
• TTL tuning required

Long-running / stuck jobs

The lease() helper maintains ownership via continuous heartbeats.

If a job:

• hangs
• enters an infinite loop
• or gets stuck without crashing

it will continue renewing the lease indefinitely.

This prevents other workers from taking over.

Recommendation:

• Set appropriate timeouts at the application level
• Avoid unbounded blocking operations
• Consider external watchdogs for critical workflows

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Philosophy

Sentinel does not eliminate race conditions. It makes them safe.