LeaderElection.Postgres 2.0.1

LeaderElection.Postgres

A PostgreSQL-backed implementation of the distributed leader election pattern for .NET. This implementation uses extremely fast, native PostgreSQL "Advisory Locks" (pg_try_advisory_lock) to ensure safe, atomic leadership acquisition tied directly to the lifetime of the database connection.

Features

• Async/Await Support: Full async/await pattern support for better performance and scalability
• Comprehensive Error Handling: Robust error handling with retry logic and exponential backoff
• Structured Logging: Integration with Microsoft.Extensions.Logging for observability
• Configuration Options: Flexible configuration through options pattern
• Event-Driven: Leadership change and error events for reactive programming
• Graceful Shutdown: Proper cleanup and resource disposal
• Session-Level Locks: Fully avoids row-level contention and deadlocks by locking in shared memory

Quick Start

1. Install the Package

dotnet add package LeaderElection
dotnet add package LeaderElection.Postgres

2. Configure Services

using LeaderElection.Postgres;
using Npgsql;

var builder = WebApplication.CreateBuilder(args);

// Register a NpgsqlDataSource (from Npgsql.DependencyInjection) specifically for
// Leader Election use...
const string postgresLeaderElectionDataSource = "PostgresLeaderElectionDataSource";
builder.Services.AddNpgsqlDataSource(
    // Use short CommandTimeout to avoid long waits if the database becomes unresponsive.
    "Host=localhost;Database=mydb;Username=myuser;Password=mypass;Timeout=5;CommandTimeout=3;",
    serviceKey: postgresLeaderElectionDataSource,
);

builder.Services.AddPostgresLeaderElection(builder =>
    builder
        .WithRegisteredDataSource(postgresLeaderElectionDataSource)
        .WithSettings(options =>
        {
            options.LockId = 1337; // A unique 64-bit integer representing this specific lock

            // Use a short RenewInterval to quickly detect and recover from failed leaders.
            // Note that the actual detection time will be at least the sum of the CommandTimeout
            // and RenewInterval, so keep CommandTimeout low as well.
            // Alternatively, you can specify a KeepAlive interval in the connection string
            // (e.g., "KeepAlive=5") to have the Npgsql driver detect a broken connection.
            options.RenewInterval = TimeSpan.FromSeconds(5); // aggressive renew
        })
);

3. Use in Your Service

public class Worker : BackgroundService
{
    private readonly ILeaderElection _leaderElection;
    private readonly ILogger<Worker> _logger;

    public Worker(ILeaderElection leaderElection, ILogger<Worker> logger)
    {
        _leaderElection = leaderElection;
        _logger = logger;
    }

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        // Start the background election loop
        await _leaderElection.StartAsync(stoppingToken);

        while (!stoppingToken.IsCancellationRequested)
        {
            // Only runs if this instance currently holds the Postgres advisory lock
            await _leaderElection.RunTaskIfLeaderAsync(async ct =>
            {
                _logger.LogInformation("Instance is leader. Doing work...");
                await Task.Delay(1000, ct);
            }, stoppingToken);

            await Task.Delay(5000, stoppingToken);
        }
    }
}

Configuration (PostgresSettings)

PostgreSQL Specifics

Instead of executing slow UPDATE queries on rows with expiration timestamps, this package executes SELECT pg_try_advisory_lock(@LockId); under the hood.

This requests a session-level lock from PostgreSQL itself. If the application crashes, the host node dies, or the network connection drops, PostgreSQL instantly cleans up the connection and drops the lock. This allows other backup nodes to acquire leadership immediately without having to wait for traditional polling or lease-expiration timers.

Additionally, to prevent silent network partitions from locking up the system, the leader election class periodically executes a lightweight SELECT 1; to verify the physical connection is still healthy and the lock holds true.

PostgreSQL Requirements

• The database is required to implement a Single-Primary topology since this Leader Election algorithm relies on advisory locks, which are not replicated in multi-host configurations.

• In multi-host configurations (e.g., host=host1,host2,etc), an explicit TargetSessionAttributes=read-write (or primary) is required in order to ensure all leader-election candidates are connected to the same instance. An error will be thrown if the connection string contains multiple hosts without the required TargetSessionAttributes.

• Multiplexing is not supported. An error will be thrown if the connection string contains Multiplexing=true.

• The PostgreSQL user must have permissions to execute pg_try_advisory_lock and pg_advisory_unlock functions, which are typically granted by default to all users.

Contributing

1. Fork the repository
2. Create a feature branch
3. Make your changes
4. Add tests
5. Submit a pull request

License

MIT License

Copyright (c) 2025 Greg James

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Icon

"Business icon pack leader icon" by mr icons is licensed under CC BY 4.0