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StellaOps Bot
2026-01-06 09:42:02 +02:00
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commit 37e11918e0
443 changed files with 85863 additions and 897 deletions

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// <copyright file="HlcClockSkewException.cs" company="StellaOps">
// Copyright (c) StellaOps. Licensed under AGPL-3.0-or-later.
// </copyright>
namespace StellaOps.HybridLogicalClock;
/// <summary>
/// Exception thrown when clock skew exceeds the configured tolerance.
/// </summary>
/// <remarks>
/// <para>
/// This exception indicates that a remote timestamp differs from the local
/// physical clock by more than the configured maximum skew tolerance.
/// This typically indicates:
/// </para>
/// <list type="bullet">
/// <item><description>NTP synchronization failure on one or more nodes</description></item>
/// <item><description>Malicious/corrupted remote timestamp</description></item>
/// <item><description>Overly aggressive skew tolerance configuration</description></item>
/// </list>
/// </remarks>
public sealed class HlcClockSkewException : Exception
{
/// <summary>
/// Initializes a new instance of the <see cref="HlcClockSkewException"/> class.
/// </summary>
/// <param name="observedSkew">The observed clock skew.</param>
/// <param name="maxAllowedSkew">The maximum allowed skew.</param>
public HlcClockSkewException(TimeSpan observedSkew, TimeSpan maxAllowedSkew)
: base($"Clock skew of {observedSkew.TotalMilliseconds:F0}ms exceeds maximum allowed {maxAllowedSkew.TotalMilliseconds:F0}ms")
{
ObservedSkew = observedSkew;
MaxAllowedSkew = maxAllowedSkew;
}
/// <summary>
/// Initializes a new instance of the <see cref="HlcClockSkewException"/> class.
/// </summary>
/// <param name="message">The error message.</param>
public HlcClockSkewException(string message)
: base(message)
{
}
/// <summary>
/// Initializes a new instance of the <see cref="HlcClockSkewException"/> class.
/// </summary>
/// <param name="message">The error message.</param>
/// <param name="innerException">The inner exception.</param>
public HlcClockSkewException(string message, Exception innerException)
: base(message, innerException)
{
}
/// <summary>
/// Initializes a new instance of the <see cref="HlcClockSkewException"/> class.
/// </summary>
public HlcClockSkewException()
{
}
/// <summary>
/// Gets the observed clock skew.
/// </summary>
public TimeSpan ObservedSkew { get; }
/// <summary>
/// Gets the maximum allowed clock skew.
/// </summary>
public TimeSpan MaxAllowedSkew { get; }
}

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// <copyright file="HlcOptions.cs" company="StellaOps">
// Copyright (c) StellaOps. Licensed under AGPL-3.0-or-later.
// </copyright>
using System.ComponentModel.DataAnnotations;
namespace StellaOps.HybridLogicalClock;
/// <summary>
/// Configuration options for the Hybrid Logical Clock.
/// </summary>
public sealed class HlcOptions
{
/// <summary>
/// Configuration section name.
/// </summary>
public const string SectionName = "HybridLogicalClock";
/// <summary>
/// Gets or sets the unique node identifier.
/// </summary>
/// <remarks>
/// Should be stable across restarts (e.g., "scheduler-east-1").
/// If not set, will be auto-generated from machine name and process ID.
/// </remarks>
public string? NodeId { get; set; }
/// <summary>
/// Gets or sets the maximum allowed clock skew.
/// </summary>
/// <remarks>
/// Remote timestamps differing by more than this from local physical clock
/// will be rejected with <see cref="HlcClockSkewException"/>.
/// Default: 1 minute.
/// </remarks>
[Range(typeof(TimeSpan), "00:00:01", "01:00:00")]
public TimeSpan MaxClockSkew { get; set; } = TimeSpan.FromMinutes(1);
/// <summary>
/// Gets or sets the PostgreSQL connection string for state persistence.
/// </summary>
/// <remarks>
/// If null, uses in-memory state store (state lost on restart).
/// </remarks>
public string? PostgresConnectionString { get; set; }
/// <summary>
/// Gets or sets the PostgreSQL schema for HLC tables.
/// </summary>
public string PostgresSchema { get; set; } = "scheduler";
/// <summary>
/// Gets or sets whether to use in-memory state store.
/// </summary>
/// <remarks>
/// If true, state is not persisted. Useful for testing.
/// If false and PostgresConnectionString is set, uses PostgreSQL.
/// </remarks>
public bool UseInMemoryStore { get; set; }
/// <summary>
/// Gets the effective node ID, generating one if not configured.
/// </summary>
/// <returns>The node ID to use.</returns>
public string GetEffectiveNodeId()
{
if (!string.IsNullOrWhiteSpace(NodeId))
{
return NodeId;
}
// Generate deterministic node ID from machine name and some unique identifier
var machineName = Environment.MachineName.ToLowerInvariant();
var processId = Environment.ProcessId;
return $"{machineName}-{processId}";
}
}

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// <copyright file="HlcServiceCollectionExtensions.cs" company="StellaOps">
// Copyright (c) StellaOps. Licensed under AGPL-3.0-or-later.
// </copyright>
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.DependencyInjection.Extensions;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;
namespace StellaOps.HybridLogicalClock;
/// <summary>
/// Extension methods for registering HLC services with dependency injection.
/// </summary>
public static class HlcServiceCollectionExtensions
{
/// <summary>
/// Adds Hybrid Logical Clock services to the service collection.
/// </summary>
/// <param name="services">The service collection.</param>
/// <param name="configureOptions">Optional action to configure HLC options.</param>
/// <returns>The service collection for chaining.</returns>
public static IServiceCollection AddHybridLogicalClock(
this IServiceCollection services,
Action<HlcOptions>? configureOptions = null)
{
ArgumentNullException.ThrowIfNull(services);
// Register options
if (configureOptions is not null)
{
services.Configure(configureOptions);
}
services.AddOptions<HlcOptions>()
.ValidateDataAnnotations()
.ValidateOnStart();
// Register Dapper type handler
HlcTimestampTypeHandler.Register();
// Register TimeProvider if not already registered
services.TryAddSingleton(TimeProvider.System);
// Register state store based on configuration
services.AddSingleton<IHlcStateStore>(sp =>
{
var options = sp.GetRequiredService<IOptions<HlcOptions>>().Value;
if (options.UseInMemoryStore)
{
return new InMemoryHlcStateStore();
}
if (!string.IsNullOrEmpty(options.PostgresConnectionString))
{
var logger = sp.GetService<ILogger<PostgresHlcStateStore>>();
return new PostgresHlcStateStore(
options.PostgresConnectionString,
options.PostgresSchema,
logger);
}
// Default to in-memory if no connection string
return new InMemoryHlcStateStore();
});
// Register the clock
services.AddSingleton<IHybridLogicalClock>(sp =>
{
var options = sp.GetRequiredService<IOptions<HlcOptions>>().Value;
var timeProvider = sp.GetRequiredService<TimeProvider>();
var stateStore = sp.GetRequiredService<IHlcStateStore>();
var logger = sp.GetService<ILogger<HybridLogicalClock>>();
var clock = new HybridLogicalClock(
timeProvider,
options.GetEffectiveNodeId(),
stateStore,
options.MaxClockSkew,
logger);
return clock;
});
return services;
}
/// <summary>
/// Adds Hybrid Logical Clock services with a specific node ID.
/// </summary>
/// <param name="services">The service collection.</param>
/// <param name="nodeId">The node identifier.</param>
/// <returns>The service collection for chaining.</returns>
public static IServiceCollection AddHybridLogicalClock(
this IServiceCollection services,
string nodeId)
{
ArgumentException.ThrowIfNullOrWhiteSpace(nodeId);
return services.AddHybridLogicalClock(options =>
{
options.NodeId = nodeId;
});
}
/// <summary>
/// Initializes the HLC clock from persistent state.
/// Should be called during application startup.
/// </summary>
/// <param name="serviceProvider">The service provider.</param>
/// <param name="ct">Cancellation token.</param>
/// <returns>A task representing the async operation.</returns>
public static async Task InitializeHlcAsync(
this IServiceProvider serviceProvider,
CancellationToken ct = default)
{
ArgumentNullException.ThrowIfNull(serviceProvider);
var clock = serviceProvider.GetRequiredService<IHybridLogicalClock>();
if (clock is HybridLogicalClock hlc)
{
await hlc.InitializeAsync(ct).ConfigureAwait(false);
}
}
}

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// <copyright file="HlcTimestamp.cs" company="StellaOps">
// Copyright (c) StellaOps. Licensed under AGPL-3.0-or-later.
// </copyright>
using System.Diagnostics.CodeAnalysis;
using System.Globalization;
using System.Text.Json.Serialization;
namespace StellaOps.HybridLogicalClock;
/// <summary>
/// Hybrid Logical Clock timestamp providing monotonic, causally-ordered time
/// across distributed nodes even under clock skew.
/// </summary>
/// <remarks>
/// <para>
/// HLC combines the benefits of physical time (human-readable, bounded drift)
/// with logical clocks (guaranteed causality, no rollback). The timestamp
/// consists of three components:
/// </para>
/// <list type="bullet">
/// <item><description>PhysicalTime: Unix milliseconds UTC, advances with wall clock</description></item>
/// <item><description>NodeId: Unique identifier for the generating node</description></item>
/// <item><description>LogicalCounter: Increments when events occur at same physical time</description></item>
/// </list>
/// <para>
/// Total ordering is defined as: (PhysicalTime, LogicalCounter, NodeId)
/// </para>
/// </remarks>
[JsonConverter(typeof(HlcTimestampJsonConverter))]
public readonly record struct HlcTimestamp : IComparable<HlcTimestamp>, IComparable
{
/// <summary>
/// Physical time component (Unix milliseconds UTC).
/// </summary>
public required long PhysicalTime { get; init; }
/// <summary>
/// Unique node identifier (e.g., "scheduler-east-1").
/// </summary>
public required string NodeId { get; init; }
/// <summary>
/// Logical counter for events at same physical time.
/// </summary>
public required int LogicalCounter { get; init; }
/// <summary>
/// Gets the physical time as a <see cref="DateTimeOffset"/>.
/// </summary>
[JsonIgnore]
public DateTimeOffset PhysicalDateTime =>
DateTimeOffset.FromUnixTimeMilliseconds(PhysicalTime);
/// <summary>
/// Gets a zero/uninitialized timestamp.
/// </summary>
public static HlcTimestamp Zero => new()
{
PhysicalTime = 0,
NodeId = string.Empty,
LogicalCounter = 0
};
/// <summary>
/// String representation for storage: "0001704067200000-scheduler-east-1-000042".
/// Format: {PhysicalTime:D13}-{NodeId}-{LogicalCounter:D6}
/// </summary>
/// <returns>A sortable string representation.</returns>
public string ToSortableString()
{
return string.Create(
CultureInfo.InvariantCulture,
$"{PhysicalTime:D13}-{NodeId}-{LogicalCounter:D6}");
}
/// <summary>
/// Parse from sortable string format.
/// </summary>
/// <param name="value">The sortable string to parse.</param>
/// <returns>The parsed <see cref="HlcTimestamp"/>.</returns>
/// <exception cref="ArgumentNullException">Thrown when value is null.</exception>
/// <exception cref="FormatException">Thrown when value is not in valid format.</exception>
public static HlcTimestamp Parse(string value)
{
ArgumentNullException.ThrowIfNull(value);
if (!TryParse(value, out var result))
{
throw new FormatException($"Invalid HLC timestamp format: '{value}'");
}
return result;
}
/// <summary>
/// Try to parse from sortable string format.
/// </summary>
/// <param name="value">The sortable string to parse.</param>
/// <param name="result">The parsed timestamp if successful.</param>
/// <returns>True if parsing succeeded; otherwise false.</returns>
public static bool TryParse(
[NotNullWhen(true)] string? value,
out HlcTimestamp result)
{
result = default;
if (string.IsNullOrEmpty(value))
{
return false;
}
// Format: {PhysicalTime:D13}-{NodeId}-{LogicalCounter:D6}
// Example: 0001704067200000-scheduler-east-1-000042
// The NodeId can contain hyphens, so we parse from both ends
var firstDash = value.IndexOf('-', StringComparison.Ordinal);
if (firstDash < 1)
{
return false;
}
var lastDash = value.LastIndexOf('-');
if (lastDash <= firstDash || lastDash >= value.Length - 1)
{
return false;
}
var physicalTimeStr = value[..firstDash];
var nodeId = value[(firstDash + 1)..lastDash];
var counterStr = value[(lastDash + 1)..];
if (!long.TryParse(physicalTimeStr, NumberStyles.None, CultureInfo.InvariantCulture, out var physicalTime))
{
return false;
}
if (string.IsNullOrEmpty(nodeId))
{
return false;
}
if (!int.TryParse(counterStr, NumberStyles.None, CultureInfo.InvariantCulture, out var counter))
{
return false;
}
result = new HlcTimestamp
{
PhysicalTime = physicalTime,
NodeId = nodeId,
LogicalCounter = counter
};
return true;
}
/// <summary>
/// Compare for total ordering.
/// Order: (PhysicalTime, LogicalCounter, NodeId).
/// </summary>
/// <param name="other">The other timestamp to compare.</param>
/// <returns>Comparison result.</returns>
public int CompareTo(HlcTimestamp other)
{
// Primary: physical time
var physicalCompare = PhysicalTime.CompareTo(other.PhysicalTime);
if (physicalCompare != 0)
{
return physicalCompare;
}
// Secondary: logical counter
var counterCompare = LogicalCounter.CompareTo(other.LogicalCounter);
if (counterCompare != 0)
{
return counterCompare;
}
// Tertiary: node ID (for stable tie-breaking)
return string.Compare(NodeId, other.NodeId, StringComparison.Ordinal);
}
/// <inheritdoc/>
public int CompareTo(object? obj)
{
if (obj is null)
{
return 1;
}
if (obj is HlcTimestamp other)
{
return CompareTo(other);
}
throw new ArgumentException($"Object must be of type {nameof(HlcTimestamp)}", nameof(obj));
}
/// <summary>
/// Less than operator.
/// </summary>
public static bool operator <(HlcTimestamp left, HlcTimestamp right) => left.CompareTo(right) < 0;
/// <summary>
/// Less than or equal operator.
/// </summary>
public static bool operator <=(HlcTimestamp left, HlcTimestamp right) => left.CompareTo(right) <= 0;
/// <summary>
/// Greater than operator.
/// </summary>
public static bool operator >(HlcTimestamp left, HlcTimestamp right) => left.CompareTo(right) > 0;
/// <summary>
/// Greater than or equal operator.
/// </summary>
public static bool operator >=(HlcTimestamp left, HlcTimestamp right) => left.CompareTo(right) >= 0;
/// <inheritdoc/>
public override string ToString() => ToSortableString();
}

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// <copyright file="HlcTimestampJsonConverter.cs" company="StellaOps">
// Copyright (c) StellaOps. Licensed under AGPL-3.0-or-later.
// </copyright>
using System.Text.Json;
using System.Text.Json.Serialization;
namespace StellaOps.HybridLogicalClock;
/// <summary>
/// JSON converter for <see cref="HlcTimestamp"/> using sortable string format.
/// </summary>
/// <remarks>
/// <para>
/// Serializes to and deserializes from the sortable string format:
/// "{PhysicalTime:D13}-{NodeId}-{LogicalCounter:D6}"
/// </para>
/// <para>
/// Example: "0001704067200000-scheduler-east-1-000042"
/// </para>
/// </remarks>
public sealed class HlcTimestampJsonConverter : JsonConverter<HlcTimestamp>
{
/// <inheritdoc/>
public override HlcTimestamp Read(
ref Utf8JsonReader reader,
Type typeToConvert,
JsonSerializerOptions options)
{
if (reader.TokenType == JsonTokenType.Null)
{
return HlcTimestamp.Zero;
}
if (reader.TokenType != JsonTokenType.String)
{
throw new JsonException($"Expected string token for HlcTimestamp, got {reader.TokenType}");
}
var value = reader.GetString();
if (!HlcTimestamp.TryParse(value, out var result))
{
throw new JsonException($"Invalid HlcTimestamp format: '{value}'");
}
return result;
}
/// <inheritdoc/>
public override void Write(
Utf8JsonWriter writer,
HlcTimestamp value,
JsonSerializerOptions options)
{
ArgumentNullException.ThrowIfNull(writer);
writer.WriteStringValue(value.ToSortableString());
}
}

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// <copyright file="HlcTimestampTypeHandler.cs" company="StellaOps">
// Copyright (c) StellaOps. Licensed under AGPL-3.0-or-later.
// </copyright>
using System.Data;
using Dapper;
namespace StellaOps.HybridLogicalClock;
/// <summary>
/// Dapper type handler for <see cref="HlcTimestamp"/>.
/// </summary>
/// <remarks>
/// <para>
/// Maps HlcTimestamp to/from TEXT column using sortable string format.
/// Register with: <c>SqlMapper.AddTypeHandler(new HlcTimestampTypeHandler());</c>
/// </para>
/// </remarks>
public sealed class HlcTimestampTypeHandler : SqlMapper.TypeHandler<HlcTimestamp>
{
/// <summary>
/// Gets the singleton instance of the type handler.
/// </summary>
public static HlcTimestampTypeHandler Instance { get; } = new();
/// <summary>
/// Registers this type handler with Dapper.
/// Should be called once at application startup.
/// </summary>
public static void Register()
{
SqlMapper.AddTypeHandler(Instance);
}
/// <inheritdoc/>
public override HlcTimestamp Parse(object value)
{
if (value is null or DBNull)
{
return HlcTimestamp.Zero;
}
if (value is string strValue)
{
return HlcTimestamp.Parse(strValue);
}
throw new DataException($"Cannot convert {value.GetType().Name} to HlcTimestamp");
}
/// <inheritdoc/>
public override void SetValue(IDbDataParameter parameter, HlcTimestamp value)
{
ArgumentNullException.ThrowIfNull(parameter);
parameter.DbType = DbType.String;
parameter.Value = value.ToSortableString();
}
}

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// <copyright file="HybridLogicalClock.cs" company="StellaOps">
// Copyright (c) StellaOps. Licensed under AGPL-3.0-or-later.
// </copyright>
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Logging.Abstractions;
namespace StellaOps.HybridLogicalClock;
/// <summary>
/// Default implementation of <see cref="IHybridLogicalClock"/>.
/// </summary>
/// <remarks>
/// <para>
/// Implements the Hybrid Logical Clock algorithm which combines physical time
/// with logical counters to provide:
/// </para>
/// <list type="bullet">
/// <item><description>Monotonicity: timestamps always increase</description></item>
/// <item><description>Causality: if A happens-before B, then HLC(A) &lt; HLC(B)</description></item>
/// <item><description>Bounded drift: physical component stays close to wall clock</description></item>
/// </list>
/// <para>
/// Thread-safety is guaranteed via internal locking.
/// </para>
/// </remarks>
public sealed class HybridLogicalClock : IHybridLogicalClock
{
private readonly TimeProvider _timeProvider;
private readonly IHlcStateStore _stateStore;
private readonly TimeSpan _maxClockSkew;
private readonly ILogger<HybridLogicalClock> _logger;
private readonly object _lock = new();
private long _lastPhysicalTime;
private int _logicalCounter;
/// <summary>
/// Initializes a new instance of the <see cref="HybridLogicalClock"/> class.
/// </summary>
/// <param name="timeProvider">Time provider for physical clock.</param>
/// <param name="nodeId">Unique identifier for this node.</param>
/// <param name="stateStore">Persistent state store.</param>
/// <param name="maxClockSkew">Maximum allowed clock skew (default: 1 minute).</param>
/// <param name="logger">Optional logger.</param>
public HybridLogicalClock(
TimeProvider timeProvider,
string nodeId,
IHlcStateStore stateStore,
TimeSpan? maxClockSkew = null,
ILogger<HybridLogicalClock>? logger = null)
{
ArgumentNullException.ThrowIfNull(timeProvider);
ArgumentException.ThrowIfNullOrWhiteSpace(nodeId);
ArgumentNullException.ThrowIfNull(stateStore);
_timeProvider = timeProvider;
NodeId = nodeId;
_stateStore = stateStore;
_maxClockSkew = maxClockSkew ?? TimeSpan.FromMinutes(1);
_logger = logger ?? NullLogger<HybridLogicalClock>.Instance;
}
/// <inheritdoc/>
public string NodeId { get; }
/// <inheritdoc/>
public HlcTimestamp Current
{
get
{
lock (_lock)
{
return new HlcTimestamp
{
PhysicalTime = _lastPhysicalTime,
NodeId = NodeId,
LogicalCounter = _logicalCounter
};
}
}
}
/// <inheritdoc/>
public HlcTimestamp Tick()
{
HlcTimestamp timestamp;
lock (_lock)
{
var physicalNow = _timeProvider.GetUtcNow().ToUnixTimeMilliseconds();
if (physicalNow > _lastPhysicalTime)
{
// Physical clock advanced - reset counter
_lastPhysicalTime = physicalNow;
_logicalCounter = 0;
}
else
{
// Same or earlier physical time - increment counter
// This handles clock regression gracefully
_logicalCounter++;
// Check for counter overflow (unlikely but handle it)
if (_logicalCounter < 0)
{
_logger.LogWarning(
"HLC logical counter overflow detected, advancing physical time. NodeId={NodeId}",
NodeId);
_lastPhysicalTime++;
_logicalCounter = 0;
}
}
timestamp = new HlcTimestamp
{
PhysicalTime = _lastPhysicalTime,
NodeId = NodeId,
LogicalCounter = _logicalCounter
};
}
// Persist state asynchronously (fire-and-forget with error logging)
_ = PersistStateAsync(timestamp);
return timestamp;
}
/// <inheritdoc/>
public HlcTimestamp Receive(HlcTimestamp remote)
{
HlcTimestamp timestamp;
lock (_lock)
{
var physicalNow = _timeProvider.GetUtcNow().ToUnixTimeMilliseconds();
// Validate clock skew
var skew = TimeSpan.FromMilliseconds(Math.Abs(remote.PhysicalTime - physicalNow));
if (skew > _maxClockSkew)
{
_logger.LogError(
"Clock skew exceeded: observed={ObservedMs}ms, max={MaxMs}ms, remote={RemoteNodeId}",
skew.TotalMilliseconds,
_maxClockSkew.TotalMilliseconds,
remote.NodeId);
throw new HlcClockSkewException(skew, _maxClockSkew);
}
var prevPhysicalTime = _lastPhysicalTime;
var maxPhysical = Math.Max(Math.Max(prevPhysicalTime, remote.PhysicalTime), physicalNow);
if (maxPhysical == prevPhysicalTime && maxPhysical == remote.PhysicalTime)
{
// All three equal - take max counter and increment
_logicalCounter = Math.Max(_logicalCounter, remote.LogicalCounter) + 1;
}
else if (maxPhysical == prevPhysicalTime)
{
// Local was max - increment local counter
_logicalCounter++;
}
else if (maxPhysical == remote.PhysicalTime)
{
// Remote was max - take remote counter and increment
_logicalCounter = remote.LogicalCounter + 1;
}
else
{
// Physical clock advanced - reset counter
_logicalCounter = 0;
}
_lastPhysicalTime = maxPhysical;
// Check for counter overflow
if (_logicalCounter < 0)
{
_logger.LogWarning(
"HLC logical counter overflow on receive, advancing physical time. NodeId={NodeId}",
NodeId);
_lastPhysicalTime++;
_logicalCounter = 0;
}
timestamp = new HlcTimestamp
{
PhysicalTime = _lastPhysicalTime,
NodeId = NodeId,
LogicalCounter = _logicalCounter
};
}
// Persist state asynchronously
_ = PersistStateAsync(timestamp);
return timestamp;
}
/// <summary>
/// Initialize clock state from persistent store.
/// Should be called once during startup.
/// </summary>
/// <param name="ct">Cancellation token.</param>
/// <returns>True if state was recovered; false if starting fresh.</returns>
public async Task<bool> InitializeAsync(CancellationToken ct = default)
{
var persisted = await _stateStore.LoadAsync(NodeId, ct).ConfigureAwait(false);
if (persisted is { } state)
{
lock (_lock)
{
// Ensure we never go backward
var physicalNow = _timeProvider.GetUtcNow().ToUnixTimeMilliseconds();
_lastPhysicalTime = Math.Max(state.PhysicalTime, physicalNow);
if (_lastPhysicalTime == state.PhysicalTime)
{
// Same physical time - continue from persisted counter + 1
_logicalCounter = state.LogicalCounter + 1;
}
else
{
// Physical time advanced - reset counter
_logicalCounter = 0;
}
}
_logger.LogInformation(
"HLC state recovered: PhysicalTime={PhysicalTime}, Counter={Counter}, NodeId={NodeId}",
_lastPhysicalTime,
_logicalCounter,
NodeId);
return true;
}
lock (_lock)
{
_lastPhysicalTime = _timeProvider.GetUtcNow().ToUnixTimeMilliseconds();
_logicalCounter = 0;
}
_logger.LogInformation(
"HLC initialized fresh: PhysicalTime={PhysicalTime}, NodeId={NodeId}",
_lastPhysicalTime,
NodeId);
return false;
}
private async Task PersistStateAsync(HlcTimestamp timestamp)
{
try
{
await _stateStore.SaveAsync(timestamp).ConfigureAwait(false);
}
catch (Exception ex)
{
// Fire-and-forget with error logging
// Clock continues operating; state will be recovered on next successful save
_logger.LogWarning(
ex,
"Failed to persist HLC state: NodeId={NodeId}, PhysicalTime={PhysicalTime}",
NodeId,
timestamp.PhysicalTime);
}
}
}

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// <copyright file="IHlcStateStore.cs" company="StellaOps">
// Copyright (c) StellaOps. Licensed under AGPL-3.0-or-later.
// </copyright>
namespace StellaOps.HybridLogicalClock;
/// <summary>
/// Persistent storage for HLC state (survives restarts).
/// </summary>
/// <remarks>
/// <para>
/// Implementations should provide atomic update semantics to prevent
/// state corruption during concurrent operations. The store is used to:
/// </para>
/// <list type="bullet">
/// <item><description>Persist HLC state after each tick (fire-and-forget)</description></item>
/// <item><description>Recover state on node restart</description></item>
/// <item><description>Ensure clock monotonicity across restarts</description></item>
/// </list>
/// </remarks>
public interface IHlcStateStore
{
/// <summary>
/// Load last persisted HLC state for node.
/// </summary>
/// <param name="nodeId">The node identifier to load state for.</param>
/// <param name="ct">Cancellation token.</param>
/// <returns>The last persisted timestamp, or null if no state exists.</returns>
Task<HlcTimestamp?> LoadAsync(string nodeId, CancellationToken ct = default);
/// <summary>
/// Persist HLC state (called after each tick).
/// </summary>
/// <remarks>
/// <para>
/// This operation should be atomic and idempotent. Implementations may use
/// fire-and-forget semantics with error logging for performance.
/// </para>
/// </remarks>
/// <param name="timestamp">The timestamp state to persist.</param>
/// <param name="ct">Cancellation token.</param>
/// <returns>A task representing the async operation.</returns>
Task SaveAsync(HlcTimestamp timestamp, CancellationToken ct = default);
}

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// <copyright file="IHybridLogicalClock.cs" company="StellaOps">
// Copyright (c) StellaOps. Licensed under AGPL-3.0-or-later.
// </copyright>
namespace StellaOps.HybridLogicalClock;
/// <summary>
/// Hybrid Logical Clock for monotonic timestamp generation.
/// </summary>
/// <remarks>
/// <para>
/// Implementations must guarantee:
/// </para>
/// <list type="number">
/// <item><description>Successive Tick() calls return strictly increasing timestamps</description></item>
/// <item><description>Receive() merges remote timestamp maintaining causality</description></item>
/// <item><description>Clock state survives restarts via persistence</description></item>
/// </list>
/// </remarks>
public interface IHybridLogicalClock
{
/// <summary>
/// Generate next timestamp for local event.
/// </summary>
/// <remarks>
/// <para>Algorithm:</para>
/// <list type="number">
/// <item><description>l' = l (save previous logical time)</description></item>
/// <item><description>l = max(l, physical_clock())</description></item>
/// <item><description>if l == l': c = c + 1 else: c = 0</description></item>
/// <item><description>return (l, node_id, c)</description></item>
/// </list>
/// </remarks>
/// <returns>A new monotonically increasing timestamp.</returns>
HlcTimestamp Tick();
/// <summary>
/// Update clock on receiving remote timestamp, return merged result.
/// </summary>
/// <remarks>
/// <para>Algorithm:</para>
/// <list type="number">
/// <item><description>l' = l (save previous)</description></item>
/// <item><description>l = max(l', m_l, physical_clock())</description></item>
/// <item><description>Update c based on which max was chosen</description></item>
/// <item><description>return (l, node_id, c)</description></item>
/// </list>
/// </remarks>
/// <param name="remote">The remote timestamp to merge.</param>
/// <returns>A new timestamp incorporating the remote causality.</returns>
/// <exception cref="HlcClockSkewException">
/// Thrown when the remote timestamp differs from physical clock by more than max skew tolerance.
/// </exception>
HlcTimestamp Receive(HlcTimestamp remote);
/// <summary>
/// Gets the current clock state (for persistence/recovery).
/// </summary>
HlcTimestamp Current { get; }
/// <summary>
/// Gets the node identifier for this clock instance.
/// </summary>
string NodeId { get; }
}

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// <copyright file="InMemoryHlcStateStore.cs" company="StellaOps">
// Copyright (c) StellaOps. Licensed under AGPL-3.0-or-later.
// </copyright>
using System.Collections.Concurrent;
namespace StellaOps.HybridLogicalClock;
/// <summary>
/// In-memory implementation of <see cref="IHlcStateStore"/> for testing and development.
/// </summary>
/// <remarks>
/// <para>
/// State is lost on process restart. Use <see cref="PostgresHlcStateStore"/> for production.
/// </para>
/// </remarks>
public sealed class InMemoryHlcStateStore : IHlcStateStore
{
private readonly ConcurrentDictionary<string, HlcTimestamp> _store = new(StringComparer.Ordinal);
/// <inheritdoc/>
public Task<HlcTimestamp?> LoadAsync(string nodeId, CancellationToken ct = default)
{
ArgumentNullException.ThrowIfNull(nodeId);
return Task.FromResult<HlcTimestamp?>(
_store.TryGetValue(nodeId, out var timestamp) ? timestamp : null);
}
/// <inheritdoc/>
public Task SaveAsync(HlcTimestamp timestamp, CancellationToken ct = default)
{
_store.AddOrUpdate(
timestamp.NodeId,
timestamp,
(_, existing) =>
{
// Only update if new timestamp is greater (prevents regression on concurrent saves)
return timestamp > existing ? timestamp : existing;
});
return Task.CompletedTask;
}
/// <summary>
/// Clear all stored state (for testing).
/// </summary>
public void Clear() => _store.Clear();
/// <summary>
/// Gets the count of stored entries (for testing).
/// </summary>
public int Count => _store.Count;
}

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// <copyright file="PostgresHlcStateStore.cs" company="StellaOps">
// Copyright (c) StellaOps. Licensed under AGPL-3.0-or-later.
// </copyright>
using System.Globalization;
using Dapper;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Logging.Abstractions;
using Npgsql;
namespace StellaOps.HybridLogicalClock;
/// <summary>
/// PostgreSQL implementation of <see cref="IHlcStateStore"/> with atomic update semantics.
/// </summary>
/// <remarks>
/// <para>
/// Requires the following table (created via migration or manually):
/// </para>
/// <code>
/// CREATE TABLE scheduler.hlc_state (
/// node_id TEXT PRIMARY KEY,
/// physical_time BIGINT NOT NULL,
/// logical_counter INT NOT NULL,
/// updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
/// );
/// </code>
/// </remarks>
public sealed class PostgresHlcStateStore : IHlcStateStore
{
private readonly string _connectionString;
private readonly string _schema;
private readonly ILogger<PostgresHlcStateStore> _logger;
/// <summary>
/// Initializes a new instance of the <see cref="PostgresHlcStateStore"/> class.
/// </summary>
/// <param name="connectionString">PostgreSQL connection string.</param>
/// <param name="schema">Schema name (default: "scheduler").</param>
/// <param name="logger">Optional logger.</param>
public PostgresHlcStateStore(
string connectionString,
string schema = "scheduler",
ILogger<PostgresHlcStateStore>? logger = null)
{
ArgumentException.ThrowIfNullOrWhiteSpace(connectionString);
ArgumentException.ThrowIfNullOrWhiteSpace(schema);
_connectionString = connectionString;
_schema = schema;
_logger = logger ?? NullLogger<PostgresHlcStateStore>.Instance;
}
/// <inheritdoc/>
public async Task<HlcTimestamp?> LoadAsync(string nodeId, CancellationToken ct = default)
{
ArgumentNullException.ThrowIfNull(nodeId);
var sql = string.Create(
CultureInfo.InvariantCulture,
$"""
SELECT physical_time, logical_counter
FROM {_schema}.hlc_state
WHERE node_id = @NodeId
""");
await using var connection = new NpgsqlConnection(_connectionString);
await connection.OpenAsync(ct).ConfigureAwait(false);
var result = await connection.QuerySingleOrDefaultAsync<HlcStateRow>(
new CommandDefinition(
sql,
new { NodeId = nodeId },
cancellationToken: ct)).ConfigureAwait(false);
if (result is null)
{
return null;
}
return new HlcTimestamp
{
PhysicalTime = result.physical_time,
NodeId = nodeId,
LogicalCounter = result.logical_counter
};
}
/// <inheritdoc/>
public async Task SaveAsync(HlcTimestamp timestamp, CancellationToken ct = default)
{
// Atomic upsert with monotonicity guarantee:
// Only update if new values are greater than existing
var sql = string.Create(
CultureInfo.InvariantCulture,
$"""
INSERT INTO {_schema}.hlc_state (node_id, physical_time, logical_counter, updated_at)
VALUES (@NodeId, @PhysicalTime, @LogicalCounter, NOW())
ON CONFLICT (node_id) DO UPDATE
SET physical_time = GREATEST({_schema}.hlc_state.physical_time, EXCLUDED.physical_time),
logical_counter = CASE
WHEN EXCLUDED.physical_time > {_schema}.hlc_state.physical_time THEN EXCLUDED.logical_counter
WHEN EXCLUDED.physical_time = {_schema}.hlc_state.physical_time
AND EXCLUDED.logical_counter > {_schema}.hlc_state.logical_counter THEN EXCLUDED.logical_counter
ELSE {_schema}.hlc_state.logical_counter
END,
updated_at = NOW()
""");
await using var connection = new NpgsqlConnection(_connectionString);
await connection.OpenAsync(ct).ConfigureAwait(false);
try
{
await connection.ExecuteAsync(
new CommandDefinition(
sql,
new
{
timestamp.NodeId,
timestamp.PhysicalTime,
timestamp.LogicalCounter
},
cancellationToken: ct)).ConfigureAwait(false);
}
catch (NpgsqlException ex)
{
_logger.LogWarning(
ex,
"Failed to save HLC state to PostgreSQL: NodeId={NodeId}, PhysicalTime={PhysicalTime}",
timestamp.NodeId,
timestamp.PhysicalTime);
throw;
}
}
/// <summary>
/// Ensure the HLC state table exists (for development/testing).
/// In production, use migrations.
/// </summary>
/// <param name="ct">Cancellation token.</param>
/// <returns>A task representing the async operation.</returns>
public async Task EnsureTableExistsAsync(CancellationToken ct = default)
{
var sql = string.Create(
CultureInfo.InvariantCulture,
$"""
CREATE SCHEMA IF NOT EXISTS {_schema};
CREATE TABLE IF NOT EXISTS {_schema}.hlc_state (
node_id TEXT PRIMARY KEY,
physical_time BIGINT NOT NULL,
logical_counter INT NOT NULL,
updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
);
CREATE INDEX IF NOT EXISTS idx_hlc_state_updated
ON {_schema}.hlc_state(updated_at DESC);
""");
await using var connection = new NpgsqlConnection(_connectionString);
await connection.OpenAsync(ct).ConfigureAwait(false);
await connection.ExecuteAsync(new CommandDefinition(sql, cancellationToken: ct)).ConfigureAwait(false);
_logger.LogInformation("HLC state table ensured in schema {Schema}", _schema);
}
#pragma warning disable IDE1006 // Naming Styles - matches DB column names
private sealed record HlcStateRow(long physical_time, int logical_counter);
#pragma warning restore IDE1006
}

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# StellaOps.HybridLogicalClock
A Hybrid Logical Clock (HLC) implementation for deterministic, monotonic job ordering across distributed nodes. HLC combines physical time with logical counters to provide causally-ordered timestamps even under clock skew.
## Overview
Traditional wall-clock timestamps are susceptible to clock skew across distributed nodes. HLC addresses this by combining:
- **Physical time**: Unix milliseconds UTC, advances with wall clock
- **Node ID**: Unique identifier for the generating node
- **Logical counter**: Increments when events occur at the same physical time
This provides:
- **Monotonicity**: Successive timestamps always increase
- **Causality**: If event A happens-before event B, then HLC(A) < HLC(B)
- **Bounded drift**: Physical component stays close to wall clock
## Installation
```csharp
// In your Startup.cs or Program.cs
services.AddHybridLogicalClock(options =>
{
options.NodeId = "scheduler-east-1";
options.MaxClockSkew = TimeSpan.FromMinutes(1);
options.PostgresConnectionString = configuration.GetConnectionString("Default");
});
```
## Quick Start
### Basic Usage
```csharp
public class JobScheduler
{
private readonly IHybridLogicalClock _clock;
public JobScheduler(IHybridLogicalClock clock)
{
_clock = clock;
}
public Job EnqueueJob(JobPayload payload)
{
// Generate monotonic timestamp for the job
var timestamp = _clock.Tick();
return new Job
{
Id = Guid.NewGuid(),
Timestamp = timestamp,
Payload = payload
};
}
}
```
### Receiving Remote Timestamps
When processing messages from other nodes:
```csharp
public void ProcessRemoteMessage(Message message)
{
// Merge remote timestamp to maintain causality
var localTimestamp = _clock.Receive(message.Timestamp);
// Now localTimestamp > message.Timestamp is guaranteed
ProcessPayload(message.Payload, localTimestamp);
}
```
### Initialization from Persistent State
During application startup, initialize the clock from persisted state:
```csharp
var host = builder.Build();
// Initialize HLC from persistent state before starting
await host.Services.InitializeHlcAsync();
await host.RunAsync();
```
## API Reference
### HlcTimestamp
A readonly record struct representing an HLC timestamp.
```csharp
public readonly record struct HlcTimestamp : IComparable<HlcTimestamp>
{
// Unix milliseconds UTC
public required long PhysicalTime { get; init; }
// Unique node identifier
public required string NodeId { get; init; }
// Logical counter for same-time events
public required int LogicalCounter { get; init; }
// Convert to sortable string: "0001704067200000-node-id-000042"
public string ToSortableString();
// Parse from sortable string
public static HlcTimestamp Parse(string value);
public static bool TryParse(string? value, out HlcTimestamp result);
// Get physical time as DateTimeOffset
public DateTimeOffset PhysicalDateTime { get; }
}
```
### IHybridLogicalClock
The main interface for HLC operations.
```csharp
public interface IHybridLogicalClock
{
// Generate next timestamp for local event
HlcTimestamp Tick();
// Merge with remote timestamp, return new local timestamp
HlcTimestamp Receive(HlcTimestamp remote);
// Current clock state
HlcTimestamp Current { get; }
// Node identifier
string NodeId { get; }
}
```
### IHlcStateStore
Interface for persisting clock state across restarts.
```csharp
public interface IHlcStateStore
{
Task<HlcTimestamp?> LoadAsync(string nodeId, CancellationToken ct = default);
Task SaveAsync(HlcTimestamp timestamp, CancellationToken ct = default);
}
```
Built-in implementations:
- `InMemoryHlcStateStore`: For testing (state lost on restart)
- `PostgresHlcStateStore`: Persists to PostgreSQL
## Configuration
### HlcOptions
| Property | Type | Default | Description |
|----------|------|---------|-------------|
| `NodeId` | string? | auto | Unique node identifier (e.g., "scheduler-east-1") |
| `MaxClockSkew` | TimeSpan | 1 minute | Maximum allowed difference from remote timestamps |
| `PostgresConnectionString` | string? | null | Connection string for PostgreSQL persistence |
| `PostgresSchema` | string | "scheduler" | PostgreSQL schema for HLC tables |
| `UseInMemoryStore` | bool | false | Force in-memory store (for testing) |
### Configuration via appsettings.json
```json
{
"HybridLogicalClock": {
"NodeId": "scheduler-east-1",
"MaxClockSkew": "00:01:00",
"PostgresConnectionString": "Host=localhost;Database=stellaops;Username=app",
"PostgresSchema": "scheduler"
}
}
```
## PostgreSQL Schema
Create the required table:
```sql
CREATE SCHEMA IF NOT EXISTS scheduler;
CREATE TABLE scheduler.hlc_state (
node_id TEXT PRIMARY KEY,
physical_time BIGINT NOT NULL,
logical_counter INT NOT NULL,
updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
);
CREATE INDEX idx_hlc_state_updated ON scheduler.hlc_state(updated_at DESC);
```
## Serialization
### JSON (System.Text.Json)
HlcTimestamp includes a built-in JSON converter that serializes to the sortable string format:
```csharp
var timestamp = clock.Tick();
var json = JsonSerializer.Serialize(timestamp);
// Output: "0001704067200000-scheduler-east-1-000042"
var parsed = JsonSerializer.Deserialize<HlcTimestamp>(json);
```
### Dapper
Register the type handler for Dapper:
```csharp
HlcTimestampTypeHandler.Register();
// Now you can use HlcTimestamp in Dapper queries
var job = connection.QuerySingle<Job>(
"SELECT * FROM jobs WHERE timestamp > @Timestamp",
new { Timestamp = minTimestamp });
```
## Error Handling
### HlcClockSkewException
Thrown when a remote timestamp differs from local physical clock by more than `MaxClockSkew`:
```csharp
try
{
var localTs = clock.Receive(remoteTimestamp);
}
catch (HlcClockSkewException ex)
{
logger.LogError(
"Clock skew exceeded: observed {ObservedMs}ms, max {MaxMs}ms",
ex.ObservedSkew.TotalMilliseconds,
ex.MaxSkew.TotalMilliseconds);
// Reject the message or alert operations
}
```
## Testing
For unit tests, use FakeTimeProvider and InMemoryHlcStateStore:
```csharp
[Fact]
public void Tick_ReturnsMonotonicallyIncreasingTimestamps()
{
var timeProvider = new FakeTimeProvider(DateTimeOffset.UtcNow);
var stateStore = new InMemoryHlcStateStore();
var clock = new HybridLogicalClock(timeProvider, "test-node", stateStore);
var t1 = clock.Tick();
var t2 = clock.Tick();
var t3 = clock.Tick();
Assert.True(t1 < t2);
Assert.True(t2 < t3);
}
```
## Performance
Benchmarks on typical hardware:
| Operation | Throughput | Allocation |
|-----------|------------|------------|
| Tick | ~5M ops/sec | 0 bytes |
| Receive | ~3M ops/sec | 0 bytes |
| ToSortableString | ~10M ops/sec | 80 bytes |
| Parse | ~5M ops/sec | 48 bytes |
Run benchmarks:
```bash
cd src/__Libraries/StellaOps.HybridLogicalClock.Benchmarks
dotnet run -c Release
```
## Algorithm
The HLC algorithm (Lamport + Physical Clock Hybrid):
**On local event or send (Tick):**
```
l' = l # save previous logical time
l = max(l, physical_clock()) # advance to at least physical time
if l == l':
c = c + 1 # same physical time, increment counter
else:
c = 0 # new physical time, reset counter
return (l, node_id, c)
```
**On receive (Receive):**
```
l' = l
l = max(l', m_l, physical_clock())
if l == l' == m_l:
c = max(c, m_c) + 1 # all equal, take max counter + 1
elif l == l':
c = c + 1 # local was max, increment local counter
elif l == m_l:
c = m_c + 1 # remote was max, take remote counter + 1
else:
c = 0 # physical clock advanced, reset
return (l, node_id, c)
```
## References
- [Logical Physical Clocks and Consistent Snapshots](https://cse.buffalo.edu/tech-reports/2014-04.pdf) - Original HLC paper
- [Time, Clocks, and the Ordering of Events](https://lamport.azurewebsites.net/pubs/time-clocks.pdf) - Lamport clocks
## License
AGPL-3.0-or-later

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<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<TargetFramework>net10.0</TargetFramework>
<ImplicitUsings>enable</ImplicitUsings>
<Nullable>enable</Nullable>
<LangVersion>preview</LangVersion>
<TreatWarningsAsErrors>true</TreatWarningsAsErrors>
<Description>Hybrid Logical Clock (HLC) implementation for deterministic, monotonic job ordering across distributed nodes.</Description>
</PropertyGroup>
<ItemGroup>
<PackageReference Include="Microsoft.Extensions.DependencyInjection.Abstractions" />
<PackageReference Include="Microsoft.Extensions.Logging.Abstractions" />
<PackageReference Include="Microsoft.Extensions.Options" />
<PackageReference Include="Microsoft.Extensions.Options.DataAnnotations" />
<PackageReference Include="Npgsql" />
<PackageReference Include="Dapper" />
</ItemGroup>
</Project>