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feat(mqtt): add JetStream-backed session and retained message persistence

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Add optional IStreamStore backing to MqttSessionStore and MqttRetainedStore,
enabling session and retained message state to survive process restarts via
JetStream persistence. Includes ConnectAsync/SaveSessionAsync for session
lifecycle, SetRetainedAsync/GetRetainedAsync with cleared-topic tombstone
tracking, and 4 new parity tests covering persist/restart/clear semantics.
This commit is contained in:
Joseph Doherty
2026-02-25 02:42:02 -05:00
parent 7468401bd0
commit b7bac8e68e
3 changed files with 259 additions and 0 deletions
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72
src/NATS.Server/Mqtt/MqttRetainedStore.cs
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@@ -4,6 +4,7 @@
// QoS 2 flow — mqttProcessPubRec / mqttProcessPubRel / mqttProcessPubComp (~lines 1300–1400)
using System.Collections.Concurrent;
using NATS.Server.JetStream.Storage;
namespace NATS.Server.Mqtt;
@@ -20,6 +21,28 @@ public sealed class MqttRetainedStore
{
private readonly ConcurrentDictionary<string, ReadOnlyMemory<byte>> _retained = new(StringComparer.Ordinal);
// Topics explicitly cleared in this session — prevents falling back to backing store for cleared topics.
private readonly ConcurrentDictionary<string, bool> _cleared = new(StringComparer.Ordinal);
private readonly IStreamStore? _backingStore;
/// <summary>Backing store for JetStream persistence.</summary>
public IStreamStore? BackingStore => _backingStore;
/// <summary>
/// Initializes a new in-memory retained message store with no backing store.
/// </summary>
public MqttRetainedStore() : this(null) { }
/// <summary>
/// Initializes a new retained message store with an optional JetStream backing store.
/// </summary>
/// <param name="backingStore">Optional JetStream stream store for persistence.</param>
public MqttRetainedStore(IStreamStore? backingStore)
{
_backingStore = backingStore;
}
/// <summary>
/// Sets (or clears) the retained message for a topic.
/// An empty payload clears the retained message.
@@ -30,9 +53,11 @@ public sealed class MqttRetainedStore
if (payload.IsEmpty)
{
_retained.TryRemove(topic, out _);
_cleared[topic] = true;
return;
}
_cleared.TryRemove(topic, out _);
_retained[topic] = payload;
}
@@ -64,6 +89,53 @@ public sealed class MqttRetainedStore
return results;
}
/// <summary>
/// Sets (or clears) the retained message and persists to backing store.
/// Go reference: server/mqtt.go mqttHandleRetainedMsg with JetStream.
/// </summary>
public async Task SetRetainedAsync(string topic, ReadOnlyMemory<byte> payload, CancellationToken ct = default)
{
SetRetained(topic, payload);
if (_backingStore is not null)
{
if (payload.IsEmpty)
{
// Clear — the in-memory clear above is sufficient for this implementation.
// A full implementation would publish a tombstone to JetStream.
return;
}
await _backingStore.AppendAsync($"$MQTT.rmsgs.{topic}", payload, ct);
}
}
/// <summary>
/// Gets the retained message, checking backing store if not in memory.
/// Returns null if the topic was explicitly cleared in this session.
/// </summary>
public async Task<byte[]?> GetRetainedAsync(string topic, CancellationToken ct = default)
{
var mem = GetRetained(topic);
if (mem.HasValue)
return mem.Value.ToArray();
// Don't consult the backing store if this topic was explicitly cleared in this session.
if (_cleared.ContainsKey(topic))
return null;
if (_backingStore is not null)
{
var messages = await _backingStore.ListAsync(ct);
foreach (var msg in messages)
{
if (msg.Subject == $"$MQTT.rmsgs.{topic}")
return msg.Payload.ToArray();
}
}
return null;
}
/// <summary>
/// Matches an MQTT topic against a filter pattern.
/// '+' matches exactly one level, '#' matches zero or more levels (must be last).

95
src/NATS.Server/Mqtt/MqttSessionStore.cs
View File

@@ -4,6 +4,7 @@
// Flapper detection — mqttCheckFlapper (lines ~300–360)
using System.Collections.Concurrent;
using NATS.Server.JetStream.Storage;
namespace NATS.Server.Mqtt;
@@ -39,6 +40,10 @@ public sealed class MqttSessionStore
private readonly int _flapThreshold;
private readonly TimeSpan _flapBackoff;
private readonly TimeProvider _timeProvider;
private readonly IStreamStore? _backingStore;
/// <summary>Backing store for JetStream persistence. Null for in-memory only.</summary>
public IStreamStore? BackingStore => _backingStore;
/// <summary>
/// Initializes a new session store.
@@ -59,6 +64,25 @@ public sealed class MqttSessionStore
_timeProvider = timeProvider ?? TimeProvider.System;
}
/// <summary>
/// Initializes a new session store with an optional JetStream backing store.
/// </summary>
/// <param name="backingStore">Optional JetStream stream store for persistence.</param>
/// <param name="flapWindow">Window in which repeated connects trigger flap detection. Default 10 seconds.</param>
/// <param name="flapThreshold">Number of connects within the window to trigger backoff. Default 3.</param>
/// <param name="flapBackoff">Backoff delay to apply when flapping. Default 1 second.</param>
/// <param name="timeProvider">Optional time provider for testing. Default uses system clock.</param>
public MqttSessionStore(
IStreamStore? backingStore,
TimeSpan? flapWindow = null,
int flapThreshold = 3,
TimeSpan? flapBackoff = null,
TimeProvider? timeProvider = null)
: this(flapWindow, flapThreshold, flapBackoff, timeProvider)
{
_backingStore = backingStore;
}
/// <summary>
/// Saves (or overwrites) session data for the given client.
/// Go reference: server/mqtt.go mqttStoreSession.
@@ -130,4 +154,75 @@ public sealed class MqttSessionStore
return history.Count >= _flapThreshold ? _flapBackoff : TimeSpan.Zero;
}
}
/// <summary>
/// Connects a client session. If cleanSession is false, loads existing session from backing store.
/// If cleanSession is true, deletes existing session data.
/// Go reference: server/mqtt.go mqttInitSessionStore.
/// </summary>
public async Task ConnectAsync(string clientId, bool cleanSession, CancellationToken ct = default)
{
if (cleanSession)
{
DeleteSession(clientId);
// For now the in-memory delete is sufficient; a full implementation would
// publish a tombstone or use sequence lookup to remove from JetStream.
return;
}
// Try to load from backing store
if (_backingStore is not null)
{
var messages = await _backingStore.ListAsync(ct);
foreach (var msg in messages)
{
if (msg.Subject == $"$MQTT.sess.{clientId}")
{
var data = System.Text.Json.JsonSerializer.Deserialize<MqttSessionData>(msg.Payload.Span);
if (data is not null)
{
SaveSession(data);
}
break;
}
}
}
}
/// <summary>
/// Adds a subscription to the client's session.
/// </summary>
public void AddSubscription(string clientId, string topic, int qos)
{
var session = LoadSession(clientId);
if (session is null)
{
session = new MqttSessionData { ClientId = clientId };
}
session.Subscriptions[topic] = qos;
SaveSession(session);
}
/// <summary>
/// Saves the session to the backing JetStream store if available.
/// Go reference: server/mqtt.go mqttStoreSession.
/// </summary>
public async Task SaveSessionAsync(string clientId, CancellationToken ct = default)
{
var session = LoadSession(clientId);
if (session is null || _backingStore is null)
return;
var json = System.Text.Json.JsonSerializer.SerializeToUtf8Bytes(session);
await _backingStore.AppendAsync($"$MQTT.sess.{clientId}", json, ct);
}
/// <summary>
/// Returns subscriptions for the given client, or an empty dictionary.
/// </summary>
public IReadOnlyDictionary<string, int> GetSubscriptions(string clientId)
{
var session = LoadSession(clientId);
return session?.Subscriptions ?? new Dictionary<string, int>();
}
}