feat(browser): stream bounded live session updates (#26)

src/FinalFactory.Rendezvous.Server/Browser/SessionBrowserService.cs

@@ -12,6 +12,8 @@ internal sealed record BrowserServiceResult<T>(RendezvousErrorCode Error, T? Val
 internal sealed class SessionBrowserService(
     IEphemeralRendezvousStore store,
     SessionBrowserCursorCodec cursors,
+    SessionStreamCursorCodec streamCursors,
+    SessionChangeJournal changes,
     IWallClock clock)
 {
     public BrowserServiceResult<BrowseSessionsResponse> Browse(
@@ -45,9 +47,25 @@ internal sealed class SessionBrowserService(
             request.PageSize + 1,
             after,
             request.ExcludeFull);
-        StoreResult<IReadOnlyList<StoredListing>> found = store.BrowseVisibleListings(
-            query,
-            cancellationToken);
+        StoreResult<IReadOnlyList<StoredListing>> found = default!;
+        long streamRevision = 0;
+        bool stableSnapshot = false;
+        for (int attempt = 0; attempt < 3; attempt++)
+        {
+            long before = changes.CurrentRevision;
+            found = store.BrowseVisibleListings(query, cancellationToken);
+            long afterRevision = changes.CurrentRevision;
+            if (before == afterRevision)
+            {
+                streamRevision = afterRevision;
+                stableSnapshot = true;
+                break;
+            }
+        }
+        if (!stableSnapshot)
+        {
+            return new(RendezvousErrorCode.ServiceUnavailable);
+        }
         if (!found.Succeeded || found.Value is null)
         {
             return new(found.Code == StoreResultCode.ServiceUnavailable
@@ -65,7 +83,12 @@ internal sealed class SessionBrowserService(
             string? nextCursor = hasMore
                 ? cursors.Encode(query, items[^1].ListingId, clock.UtcNow)
                 : null;
-            BrowseSessionsResponse response = new() { Items = items, NextCursor = nextCursor };
+            BrowseSessionsResponse response = new()
+            {
+                Items = items,
+                NextCursor = nextCursor,
+                StreamCursor = streamCursors.Encode(query, streamRevision, clock.UtcNow),
+            };
             if (JsonSerializer.SerializeToUtf8Bytes(response, ContractJson.Options).Length
                 <= ContractLimits.BrowserResponseMaxBytes)
             {
@@ -76,7 +99,10 @@ internal sealed class SessionBrowserService(
             hasMore = true;
         }
 
-        return new(RendezvousErrorCode.None, new BrowseSessionsResponse());
+        return new(RendezvousErrorCode.None, new BrowseSessionsResponse
+        {
+            StreamCursor = streamCursors.Encode(query, streamRevision, clock.UtcNow),
+        });
     }
 
     public BrowserServiceResult<GetSessionResponse> Get(

src/FinalFactory.Rendezvous.Server/Browser/SessionChangeJournal.cs

@@ -0,0 +1,276 @@
+using FinalFactory.Rendezvous.Contracts;
+using FinalFactory.Rendezvous.Server.State;
+
+namespace FinalFactory.Rendezvous.Server.Browser;
+
+internal sealed record SessionChangeJournalOptions
+{
+    public int ReplayCapacity { get; init; } = 4096;
+    public int MaximumSubscribers { get; init; } = 256;
+    public int MaximumSubscribersPerTenant { get; init; } = 64;
+    public int MaximumBatchSize { get; init; } = 128;
+    public TimeSpan CoalesceInterval { get; init; } = TimeSpan.FromMilliseconds(50);
+    public TimeSpan KeepaliveInterval { get; init; } = TimeSpan.FromSeconds(15);
+    public TimeSpan MaximumConnectionDuration { get; init; } = TimeSpan.FromMinutes(5);
+
+    public void Validate()
+    {
+        if (ReplayCapacity is < 64 or > 65_536
+            || MaximumSubscribers is < 1 or > 4096
+            || MaximumSubscribersPerTenant < 1
+            || MaximumSubscribersPerTenant > MaximumSubscribers
+            || MaximumBatchSize is < 1 or > 1024
+            || CoalesceInterval < TimeSpan.Zero
+            || CoalesceInterval > TimeSpan.FromSeconds(1)
+            || KeepaliveInterval < TimeSpan.FromSeconds(1)
+            || KeepaliveInterval > TimeSpan.FromMinutes(1)
+            || MaximumConnectionDuration < KeepaliveInterval
+            || MaximumConnectionDuration > TimeSpan.FromMinutes(30))
+        {
+            throw new ArgumentOutOfRangeException(nameof(SessionChangeJournalOptions));
+        }
+    }
+}
+
+internal sealed record SessionChange(
+    long Revision,
+    SessionListingProjection? Before,
+    SessionListingProjection? After)
+{
+    public SessionListingId ListingId => (After ?? Before)!.Listing.ListingId;
+}
+
+internal sealed record SessionChangeBatch(
+    long CurrentRevision,
+    bool RequiresReset,
+    IReadOnlyList<SessionChange> Changes);
+
+internal sealed class SessionListingProjection
+{
+    private SessionListingProjection(SessionListing listing, bool visible)
+    {
+        Listing = listing;
+        Visible = visible;
+    }
+
+    public SessionListing Listing { get; }
+    public bool Visible { get; }
+
+    public static SessionListingProjection From(StoredListing stored) => new(
+        new SessionListing
+        {
+            ListingId = stored.Definition.ListingId,
+            GameId = stored.Definition.Scope.GameId,
+            EnvironmentId = stored.Definition.Scope.EnvironmentId,
+            RegionId = stored.Definition.RegionId,
+            ProtocolVersion = stored.Definition.ProtocolVersion,
+            BuildVersion = stored.Definition.BuildVersion,
+            DisplayName = stored.Definition.DisplayName,
+            Visibility = stored.Definition.Visibility,
+            PublisherTrustMode = stored.Definition.TrustMode,
+            Capacity = new SessionCapacity
+            {
+                CurrentPlayers = stored.Definition.CurrentPlayers,
+                MaximumPlayers = stored.Definition.MaximumPlayers,
+            },
+            Metadata = new Dictionary<string, string>(stored.Definition.Metadata, StringComparer.Ordinal),
+            DedicatedFallback = StoredListing.CopyEndpoint(stored.Definition.DedicatedFallback),
+        },
+        stored.HasFreshPresence && stored.Definition.Visibility == ListingVisibility.Public);
+
+    public bool Matches(VisibleListingQuery query) => Visible
+        && Listing.GameId == query.Scope.GameId
+        && Listing.EnvironmentId == query.Scope.EnvironmentId
+        && Listing.ProtocolVersion == query.ProtocolVersion
+        && (!query.RegionId.HasValue || Listing.RegionId == query.RegionId.Value)
+        && (!query.ExcludeFull
+            || Listing.Capacity.CurrentPlayers < Listing.Capacity.MaximumPlayers);
+
+    public static bool Equivalent(SessionListingProjection? left, SessionListingProjection? right)
+    {
+        if (ReferenceEquals(left, right))
+        {
+            return true;
+        }
+        if (left is null || right is null || left.Visible != right.Visible)
+        {
+            return false;
+        }
+
+        SessionListing a = left.Listing;
+        SessionListing b = right.Listing;
+        return a.ListingId == b.ListingId
+            && a.GameId == b.GameId
+            && a.EnvironmentId == b.EnvironmentId
+            && a.RegionId == b.RegionId
+            && a.ProtocolVersion == b.ProtocolVersion
+            && string.Equals(a.BuildVersion, b.BuildVersion, StringComparison.Ordinal)
+            && string.Equals(a.DisplayName, b.DisplayName, StringComparison.Ordinal)
+            && a.Visibility == b.Visibility
+            && a.PublisherTrustMode == b.PublisherTrustMode
+            && a.Capacity.CurrentPlayers == b.Capacity.CurrentPlayers
+            && a.Capacity.MaximumPlayers == b.Capacity.MaximumPlayers
+            && a.Metadata.Count == b.Metadata.Count
+            && a.Metadata.All(item => b.Metadata.TryGetValue(item.Key, out string? value)
+                && string.Equals(item.Value, value, StringComparison.Ordinal))
+            && EndpointEquals(a.DedicatedFallback, b.DedicatedFallback);
+    }
+
+    private static bool EndpointEquals(NetworkEndpoint? left, NetworkEndpoint? right) =>
+        left is null && right is null
+        || left is not null && right is not null
+        && left.AddressFamily == right.AddressFamily
+        && string.Equals(left.Address, right.Address, StringComparison.Ordinal)
+        && left.Port == right.Port;
+}
+
+internal sealed class SessionChangeJournal
+{
+    private readonly object _gate = new();
+    private readonly SessionChangeJournalOptions _options;
+    private readonly Queue<SessionChange> _changes = [];
+    private TaskCompletionSource<long> _changed = NewSignal();
+    private long _revision;
+    private int _subscribers;
+    private readonly Dictionary<TenantScope, int> _subscribersByTenant = [];
+
+    public SessionChangeJournal(SessionChangeJournalOptions options)
+    {
+        ArgumentNullException.ThrowIfNull(options);
+        options.Validate();
+        _options = options;
+    }
+
+    public SessionChangeJournalOptions Options => _options;
+
+    public long CurrentRevision
+    {
+        get
+        {
+            lock (_gate)
+            {
+                return _revision;
+            }
+        }
+    }
+
+    public void Publish(StoredListing? before, StoredListing? after)
+    {
+        SessionListingProjection? previous = before is null ? null : SessionListingProjection.From(before);
+        SessionListingProjection? current = after is null ? null : SessionListingProjection.From(after);
+        if (SessionListingProjection.Equivalent(previous, current)
+            || previous is { Visible: false } && current is null
+            || previous is null && current is { Visible: false })
+        {
+            return;
+        }
+
+        TaskCompletionSource<long> signal;
+        long revision;
+        lock (_gate)
+        {
+            revision = ++_revision;
+            _changes.Enqueue(new SessionChange(revision, previous, current));
+            while (_changes.Count > _options.ReplayCapacity)
+            {
+                _changes.Dequeue();
+            }
+            signal = _changed;
+            _changed = NewSignal();
+        }
+        signal.TrySetResult(revision);
+    }
+
+    public SessionChangeBatch ReadAfter(long revision)
+    {
+        lock (_gate)
+        {
+            long oldest = _changes.TryPeek(out SessionChange? first)
+                ? first.Revision
+                : _revision + 1;
+            if (revision < oldest - 1 || revision > _revision)
+            {
+                return new(_revision, true, []);
+            }
+
+            SessionChange[] changes = _changes
+                .Where(change => change.Revision > revision)
+                .Take(_options.MaximumBatchSize)
+                .ToArray();
+            return new(_revision, false, changes);
+        }
+    }
+
+    public async Task<bool> WaitForChangeAsync(
+        long revision,
+        TimeSpan timeout,
+        CancellationToken cancellationToken)
+    {
+        Task<long> signal;
+        lock (_gate)
+        {
+            if (_revision > revision)
+            {
+                return true;
+            }
+            signal = _changed.Task;
+        }
+
+        try
+        {
+            await signal.WaitAsync(timeout, cancellationToken).ConfigureAwait(false);
+            return true;
+        }
+        catch (TimeoutException)
+        {
+            return false;
+        }
+    }
+
+    public bool TrySubscribe(TenantScope scope, out IDisposable? lease)
+    {
+        lock (_gate)
+        {
+            if (_subscribers >= _options.MaximumSubscribers
+                || _subscribersByTenant.GetValueOrDefault(scope)
+                    >= _options.MaximumSubscribersPerTenant)
+            {
+                lease = null;
+                return false;
+            }
+            _subscribers++;
+            _subscribersByTenant[scope] = _subscribersByTenant.GetValueOrDefault(scope) + 1;
+            lease = new Subscription(this, scope);
+            return true;
+        }
+    }
+
+    private void Release(TenantScope scope)
+    {
+        lock (_gate)
+        {
+            _subscribers--;
+            int remaining = _subscribersByTenant[scope] - 1;
+            if (remaining == 0)
+            {
+                _subscribersByTenant.Remove(scope);
+            }
+            else
+            {
+                _subscribersByTenant[scope] = remaining;
+            }
+        }
+    }
+
+    private static TaskCompletionSource<long> NewSignal() => new(
+        TaskCreationOptions.RunContinuationsAsynchronously);
+
+    private sealed class Subscription(
+        SessionChangeJournal owner,
+        TenantScope scope) : IDisposable
+    {
+        private SessionChangeJournal? _owner = owner;
+
+        public void Dispose() => Interlocked.Exchange(ref _owner, null)?.Release(scope);
+    }
+}

src/FinalFactory.Rendezvous.Server/Browser/SessionStreamCursorCodec.cs

@@ -0,0 +1,108 @@
+using System.Security.Cryptography;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using FinalFactory.Rendezvous.Contracts;
+using FinalFactory.Rendezvous.Server.State;
+
+namespace FinalFactory.Rendezvous.Server.Browser;
+
+internal sealed class SessionStreamCursorCodec : IDisposable
+{
+    private const string Prefix = "rvs1";
+    private readonly EphemeralCursorProtector _protector = new();
+
+    public string Encode(VisibleListingQuery query, long revision, DateTimeOffset now)
+    {
+        ArgumentOutOfRangeException.ThrowIfNegative(revision);
+        SessionStreamCursorPayload payload = new()
+        {
+            GameId = query.Scope.GameId.Value,
+            EnvironmentId = query.Scope.EnvironmentId.Value,
+            ProtocolVersion = query.ProtocolVersion,
+            RegionId = query.RegionId?.Value,
+            ExcludeFull = query.ExcludeFull,
+            Revision = revision,
+            ExpiresAtUnixSeconds = now.AddMinutes(10).ToUnixTimeSeconds(),
+        };
+        byte[] encoded = JsonSerializer.SerializeToUtf8Bytes(payload, ContractJson.Options);
+        try
+        {
+            return _protector.Protect(Prefix, encoded);
+        }
+        finally
+        {
+            CryptographicOperations.ZeroMemory(encoded);
+        }
+    }
+
+    public bool TryDecode(
+        string? cursor,
+        VisibleListingQuery query,
+        DateTimeOffset now,
+        out long revision)
+    {
+        revision = 0;
+        if (cursor is null || !_protector.TryUnprotect(Prefix, cursor, out byte[] encodedPayload))
+        {
+            return false;
+        }
+
+        SessionStreamCursorPayload? payload;
+        try
+        {
+            payload = JsonSerializer.Deserialize<SessionStreamCursorPayload>(
+                encodedPayload,
+                ContractJson.Options);
+        }
+        catch (JsonException)
+        {
+            payload = null;
+        }
+        finally
+        {
+            CryptographicOperations.ZeroMemory(encodedPayload);
+        }
+
+        if (payload is null
+            || payload.Revision < 0
+            || payload.ExpiresAtUnixSeconds <= now.ToUnixTimeSeconds()
+            || !string.Equals(payload.GameId, query.Scope.GameId.Value, StringComparison.Ordinal)
+            || !string.Equals(payload.EnvironmentId, query.Scope.EnvironmentId.Value, StringComparison.Ordinal)
+            || payload.ProtocolVersion != query.ProtocolVersion
+            || !string.Equals(payload.RegionId, query.RegionId?.Value, StringComparison.Ordinal)
+            || payload.ExcludeFull != query.ExcludeFull)
+        {
+            return false;
+        }
+
+        revision = payload.Revision;
+        return true;
+    }
+
+    public void Dispose() => _protector.Dispose();
+
+    public override string ToString() => "[SessionStreamCursorCodec: key and cursors redacted]";
+}
+
+internal sealed class SessionStreamCursorPayload
+{
+    [JsonRequired]
+    public string GameId { get; set; } = string.Empty;
+
+    [JsonRequired]
+    public string EnvironmentId { get; set; } = string.Empty;
+
+    [JsonRequired]
+    public uint ProtocolVersion { get; set; }
+
+    public string? RegionId { get; set; }
+
+    [JsonRequired]
+    public bool ExcludeFull { get; set; }
+
+    [JsonRequired]
+    public long Revision { get; set; }
+
+    [JsonRequired]
+    public long ExpiresAtUnixSeconds { get; set; }
+}

src/FinalFactory.Rendezvous.Server/Browser/SessionStreamService.cs

@@ -0,0 +1,172 @@
+using FinalFactory.Rendezvous.Contracts;
+using FinalFactory.Rendezvous.Server.State;
+
+namespace FinalFactory.Rendezvous.Server.Browser;
+
+internal sealed record SessionStreamReadResult(
+    bool RequiresReset,
+    IReadOnlyList<SessionStreamEvent> Events);
+
+internal sealed class SessionStreamSubscription : IDisposable
+{
+    private IDisposable? _lease;
+
+    public SessionStreamSubscription(
+        VisibleListingQuery query,
+        long revision,
+        bool requiresReset,
+        IDisposable lease)
+    {
+        Query = query;
+        Revision = revision;
+        RequiresReset = requiresReset;
+        _lease = lease;
+    }
+
+    public VisibleListingQuery Query { get; }
+    public long Revision { get; set; }
+    public bool RequiresReset { get; set; }
+
+    public void Dispose() => Interlocked.Exchange(ref _lease, null)?.Dispose();
+}
+
+internal sealed class SessionStreamService(
+    SessionChangeJournal changes,
+    SessionStreamCursorCodec cursors,
+    IWallClock clock)
+{
+    public BrowserServiceResult<SessionStreamSubscription> Subscribe(
+        BrowseSessionsRequest request,
+        string? cursor)
+    {
+        ArgumentNullException.ThrowIfNull(request);
+        RendezvousErrorCode validation = Validate(request);
+        if (validation != RendezvousErrorCode.None)
+        {
+            return new(validation);
+        }
+        VisibleListingQuery query = new(
+            new TenantScope(request.GameId, request.EnvironmentId),
+            request.ProtocolVersion,
+            request.RegionId,
+            ContractLimits.BrowserPageMaxItems,
+            ExcludeFull: request.ExcludeFull);
+        if (!changes.TrySubscribe(query.Scope, out IDisposable? lease) || lease is null)
+        {
+            return new(RendezvousErrorCode.CapacityExceeded);
+        }
+        bool validCursor = cursors.TryDecode(cursor, query, clock.UtcNow, out long revision);
+        if (!validCursor)
+        {
+            revision = changes.CurrentRevision;
+        }
+        return new(RendezvousErrorCode.None, new SessionStreamSubscription(
+            query,
+            revision,
+            requiresReset: !validCursor,
+            lease));
+    }
+
+    public SessionStreamReadResult Read(SessionStreamSubscription subscription)
+    {
+        ArgumentNullException.ThrowIfNull(subscription);
+        if (subscription.RequiresReset)
+        {
+            subscription.RequiresReset = false;
+            return new(true, []);
+        }
+
+        SessionChangeBatch batch = changes.ReadAfter(subscription.Revision);
+        if (batch.RequiresReset)
+        {
+            subscription.Revision = batch.CurrentRevision;
+            return new(true, []);
+        }
+        if (batch.Changes.Count == 0)
+        {
+            return new(false, []);
+        }
+
+        Dictionary<SessionListingId, PendingDelta> coalesced = [];
+        foreach (SessionChange change in batch.Changes)
+        {
+            bool beforeMatches = change.Before?.Matches(subscription.Query) == true;
+            bool afterMatches = change.After?.Matches(subscription.Query) == true;
+            if (!beforeMatches && !afterMatches)
+            {
+                continue;
+            }
+            coalesced[change.ListingId] = afterMatches
+                ? new(change.Revision, SessionStreamEventKind.SessionUpsert, change.After!.Listing)
+                : new(change.Revision, SessionStreamEventKind.SessionRemove, null);
+        }
+
+        subscription.Revision = batch.Changes[^1].Revision;
+        SessionStreamEvent[] events = coalesced
+            .OrderBy(static item => item.Value.Revision)
+            .Select(item => ToEvent(item.Key, item.Value, subscription.Query))
+            .ToArray();
+        return new(false, events);
+    }
+
+    public async Task<bool> WaitForChangeAsync(
+        SessionStreamSubscription subscription,
+        CancellationToken cancellationToken)
+    {
+        bool changed = await changes.WaitForChangeAsync(
+            subscription.Revision,
+            changes.Options.KeepaliveInterval,
+            cancellationToken).ConfigureAwait(false);
+        if (changed && changes.Options.CoalesceInterval > TimeSpan.Zero)
+        {
+            await Task.Delay(changes.Options.CoalesceInterval, cancellationToken)
+                .ConfigureAwait(false);
+        }
+        return changed;
+    }
+
+    public SessionStreamEvent ResetEvent(SessionStreamSubscription subscription) => new()
+    {
+        Kind = SessionStreamEventKind.Reset,
+        Cursor = cursors.Encode(subscription.Query, subscription.Revision, clock.UtcNow),
+    };
+
+    public SessionStreamEvent KeepaliveEvent(SessionStreamSubscription subscription) => new()
+    {
+        Kind = SessionStreamEventKind.Keepalive,
+        Cursor = cursors.Encode(subscription.Query, subscription.Revision, clock.UtcNow),
+    };
+
+    public TimeSpan MaximumConnectionDuration => changes.Options.MaximumConnectionDuration;
+
+    private SessionStreamEvent ToEvent(
+        SessionListingId listingId,
+        PendingDelta delta,
+        VisibleListingQuery query) => new()
+        {
+            Kind = delta.Kind,
+            Cursor = cursors.Encode(query, delta.Revision, clock.UtcNow),
+            Session = delta.Session,
+            ListingId = delta.Kind == SessionStreamEventKind.SessionRemove ? listingId : null,
+        };
+
+    private static RendezvousErrorCode Validate(BrowseSessionsRequest request)
+    {
+        RendezvousErrorCode version = ContractValidation.ValidateContractVersion(request.ContractVersion);
+        if (version != RendezvousErrorCode.None)
+        {
+            return version;
+        }
+        return string.IsNullOrEmpty(request.GameId.Value)
+            || string.IsNullOrEmpty(request.EnvironmentId.Value)
+            || request.ProtocolVersion == 0
+            || request.RegionId.HasValue && string.IsNullOrEmpty(request.RegionId.Value.Value)
+            ? RendezvousErrorCode.InvalidRequest
+            : RendezvousErrorCode.None;
+    }
+
+    private sealed record PendingDelta(
+        long Revision,
+        SessionStreamEventKind Kind,
+        SessionListing? Session);
+}

src/FinalFactory.Rendezvous.Server/FinalFactory.Rendezvous.Server.csproj

@@ -47,6 +47,9 @@
     <Compile Include="Browser/EphemeralCursorProtector.cs" />
     <Compile Include="Browser/SessionBrowserCursorCodec.cs" />
     <Compile Include="Browser/SessionBrowserService.cs" />
+    <Compile Include="Browser/SessionChangeJournal.cs" />
+    <Compile Include="Browser/SessionStreamCursorCodec.cs" />
+    <Compile Include="Browser/SessionStreamService.cs" />
     <Compile Include="ConnectionOutcomes/ConnectionOutcomeService.cs" />
     <Compile Include="Deployment/DeploymentOptions.cs" />
     <Compile Include="Deployment/GracefulDrainService.cs" />

src/FinalFactory.Rendezvous.Server/Http/ContractEndpoints.cs

@@ -1,4 +1,5 @@
 using System.Net;
+using System.Text.Json;
 using FinalFactory.Rendezvous.Contracts;
 using FinalFactory.Rendezvous.Server.Abuse;
 using FinalFactory.Rendezvous.Server.Browser;
@@ -69,6 +70,12 @@ internal static class ContractEndpoints
             .Produces<ApiError>(StatusCodes.Status429TooManyRequests)
             .Produces<ApiError>(StatusCodes.Status503ServiceUnavailable)
             .WithName("BrowseSessions");
+        sessions.MapGet("/stream", StreamSessions)
+            .Produces<SessionStreamEvent>(StatusCodes.Status200OK, contentType: "text/event-stream")
+            .Produces<ApiError>(StatusCodes.Status400BadRequest)
+            .Produces<ApiError>(StatusCodes.Status429TooManyRequests)
+            .Produces<ApiError>(StatusCodes.Status503ServiceUnavailable)
+            .WithName("StreamSessions");
         sessions.MapGet("/{listingId}", GetSession)
             .Produces<GetSessionResponse>()
             .Produces<ApiError>(StatusCodes.Status400BadRequest)
@@ -349,6 +356,123 @@ internal static class ContractEndpoints
         }
     }
 
+    private static async Task<IResult> StreamSessions(
+        [FromQuery] int contractVersion,
+        [FromQuery] string gameId,
+        [FromQuery] string environmentId,
+        [FromQuery] uint protocolVersion,
+        [FromQuery] string? regionId,
+        [FromQuery] bool? excludeFull,
+        [FromQuery] string? streamCursor,
+        [FromHeader(Name = "Last-Event-ID")] string? lastEventId,
+        [FromServices] SessionStreamService streams,
+        [FromServices] AbuseProtectionService abuseProtection,
+        HttpContext httpContext,
+        CancellationToken cancellationToken)
+    {
+        if (!GameId.TryParse(gameId, out GameId parsedGameId)
+            || !EnvironmentId.TryParse(environmentId, out EnvironmentId parsedEnvironmentId)
+            || regionId is not null && !RegionId.TryParse(regionId, out _))
+        {
+            return Error(RendezvousErrorCode.InvalidRequest);
+        }
+        if (!TryAcquireIdentity(
+                abuseProtection,
+                httpContext,
+                "StreamSessions",
+                Tenant(parsedGameId, parsedEnvironmentId),
+                null,
+                null,
+                out AbuseProtectionService.AbuseLease? abuseLease))
+        {
+            return RateLimited(httpContext);
+        }
+
+        using (abuseLease)
+        {
+            BrowserServiceResult<SessionStreamSubscription> subscribed = streams.Subscribe(new()
+            {
+                ContractVersion = contractVersion,
+                GameId = parsedGameId,
+                EnvironmentId = parsedEnvironmentId,
+                ProtocolVersion = protocolVersion,
+                RegionId = regionId is null ? null : new RegionId(regionId),
+                ExcludeFull = excludeFull ?? false,
+            }, string.IsNullOrEmpty(lastEventId) ? streamCursor : lastEventId);
+            if (!subscribed.Succeeded || subscribed.Value is null)
+            {
+                return Error(subscribed.Error);
+            }
+
+            using SessionStreamSubscription subscription = subscribed.Value;
+            using CancellationTokenSource duration = CancellationTokenSource.CreateLinkedTokenSource(
+                cancellationToken);
+            duration.CancelAfter(streams.MaximumConnectionDuration);
+            HttpResponse response = httpContext.Response;
+            response.StatusCode = StatusCodes.Status200OK;
+            response.ContentType = "text/event-stream";
+            response.Headers.CacheControl = "no-cache, no-store";
+            response.Headers["X-Accel-Buffering"] = "no";
+            await response.StartAsync(duration.Token).ConfigureAwait(false);
+            try
+            {
+                while (!duration.IsCancellationRequested)
+                {
+                    SessionStreamReadResult read = streams.Read(subscription);
+                    if (read.RequiresReset)
+                    {
+                        await WriteSseAsync(response, streams.ResetEvent(subscription), duration.Token)
+                            .ConfigureAwait(false);
+                        await response.Body.FlushAsync(duration.Token).ConfigureAwait(false);
+                        break;
+                    }
+                    if (read.Events.Count > 0)
+                    {
+                        foreach (SessionStreamEvent item in read.Events)
+                        {
+                            await WriteSseAsync(response, item, duration.Token).ConfigureAwait(false);
+                        }
+                        await response.Body.FlushAsync(duration.Token).ConfigureAwait(false);
+                        continue;
+                    }
+
+                    bool changed = await streams.WaitForChangeAsync(subscription, duration.Token)
+                        .ConfigureAwait(false);
+                    if (!changed)
+                    {
+                        await WriteSseAsync(
+                            response,
+                            streams.KeepaliveEvent(subscription),
+                            duration.Token).ConfigureAwait(false);
+                        await response.Body.FlushAsync(duration.Token).ConfigureAwait(false);
+                    }
+                }
+            }
+            catch (OperationCanceledException) when (duration.IsCancellationRequested)
+            {
+            }
+            return Results.Empty;
+        }
+    }
+
+    private static async Task WriteSseAsync(
+        HttpResponse response,
+        SessionStreamEvent item,
+        CancellationToken cancellationToken)
+    {
+        string eventName = item.Kind switch
+        {
+            SessionStreamEventKind.SessionUpsert => "session_upsert",
+            SessionStreamEventKind.SessionRemove => "session_remove",
+            SessionStreamEventKind.Reset => "reset",
+            _ => "keepalive",
+        };
+        string data = JsonSerializer.Serialize(item, ContractJson.Options);
+        await response.WriteAsync(
+            $"id: {item.Cursor}\nevent: {eventName}\ndata: {data}\n\n",
+            cancellationToken).ConfigureAwait(false);
+    }
+
     private static IResult GetSession(
         SessionListingId listingId,
         [FromQuery] int contractVersion,

src/FinalFactory.Rendezvous.Server/Program.cs

@@ -255,6 +255,7 @@ builder.Services.Configure<HostOptions>(options =>
     options.ShutdownTimeout = TimeSpan.FromSeconds(deploymentOptions.DrainDeadlineSeconds + 10));
 
 SystemRendezvousClock rendezvousClock = new();
+SessionChangeJournal sessionChanges = new(new SessionChangeJournalOptions());
 EphemeralStoreOptions stateOptions = new()
 {
     GracefulDrainLifetime = TimeSpan.FromSeconds(deploymentOptions.DrainDeadlineSeconds),
@@ -262,8 +263,10 @@ EphemeralStoreOptions stateOptions = new()
 InMemoryEphemeralRendezvousStore stateStore = new(
     stateOptions,
     rendezvousClock,
-    rendezvousClock);
+    rendezvousClock,
+    sessionChanges);
 builder.Services.AddSingleton(stateStore);
+builder.Services.AddSingleton(sessionChanges);
 builder.Services.AddSingleton<IEphemeralRendezvousStore>(stateStore);
 builder.Services.AddSingleton<IWallClock>(rendezvousClock);
 builder.Services.AddSingleton<IMonotonicClock>(rendezvousClock);
@@ -297,7 +300,9 @@ else
     builder.Services.AddSingleton(SessionLeaseTiming.From(stateOptions));
     builder.Services.AddSingleton<SessionLeaseService>();
     builder.Services.AddSingleton<SessionBrowserCursorCodec>();
+    builder.Services.AddSingleton<SessionStreamCursorCodec>();
     builder.Services.AddSingleton<SessionBrowserService>();
+    builder.Services.AddSingleton<SessionStreamService>();
     builder.Services.AddSingleton<JoinAttemptCursorCodec>();
     builder.Services.AddSingleton<JoinAttemptService>();
     builder.Services.AddSingleton<ConnectionOutcomeMetrics>();

src/FinalFactory.Rendezvous.Server/Sessions/SessionLeaseService.cs

@@ -240,7 +240,15 @@ internal sealed class SessionLeaseService(
         }
 
         StoredListing ownedListing = listing!;
-        PublisherAuthorizationResult authorized = AuthorizeExisting(principal, ownedListing, request.Metadata);
+        PublisherAuthorizationResult authorized = authorization.Authorize(
+            principal,
+            ownedListing.Definition.Scope.GameId,
+            ownedListing.Definition.Scope.EnvironmentId,
+            request.RegionId ?? ownedListing.Definition.RegionId,
+            request.ProtocolVersion ?? ownedListing.Definition.ProtocolVersion,
+            request.Visibility ?? ownedListing.Definition.Visibility,
+            request.Metadata,
+            clock.UtcNow);
         if (!authorized.IsAllowed || authorized.Context is null)
         {
             return new(MapAuthorization(authorized.Error));
@@ -261,7 +269,10 @@ internal sealed class SessionLeaseService(
             request.Capacity.CurrentPlayers,
             request.Capacity.MaximumPlayers,
             request.Metadata,
-            request.DedicatedFallback), cancellationToken);
+            request.DedicatedFallback,
+            request.RegionId,
+            request.ProtocolVersion,
+            request.Visibility), cancellationToken);
         return updated.Succeeded
             ? new(RendezvousErrorCode.None, true)
             : new(updated.Code.ToContractError());
@@ -391,6 +402,9 @@ internal sealed class SessionLeaseService(
             || !ContractValidation.IsDisplayNameValid(request.DisplayName)
             || !ContractValidation.IsCapacityValid(request.Capacity)
             || !ContractValidation.IsMetadataValid(request.Metadata)
+            || request.RegionId.HasValue && string.IsNullOrEmpty(request.RegionId.Value.Value)
+            || request.ProtocolVersion.HasValue && request.ProtocolVersion.Value == 0
+            || request.Visibility.HasValue && !Enum.IsDefined(request.Visibility.Value)
             || request.DedicatedFallback is not null
                 && !ContractValidation.IsNetworkEndpointValid(request.DedicatedFallback)
             ? RendezvousErrorCode.InvalidRequest

src/FinalFactory.Rendezvous.Server/State/EphemeralStateContracts.cs

@@ -228,7 +228,10 @@ internal sealed record UpdateListingCommand(
     int CurrentPlayers,
     int MaximumPlayers,
     IReadOnlyDictionary<string, string> Metadata,
-    NetworkEndpoint? DedicatedFallback);
+    NetworkEndpoint? DedicatedFallback,
+    RegionId? RegionId = null,
+    uint? ProtocolVersion = null,
+    ListingVisibility? Visibility = null);
 
 internal sealed record DeleteListingCommand(
     SessionListingId ListingId,

src/FinalFactory.Rendezvous.Server/State/InMemoryEphemeralRendezvousStore.cs

@@ -1,4 +1,5 @@
 using FinalFactory.Rendezvous.Contracts;
+using FinalFactory.Rendezvous.Server.Browser;
 
 namespace FinalFactory.Rendezvous.Server.State;
 
@@ -8,6 +9,7 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
     private readonly object _gate = new();
     private readonly EphemeralStoreOptions _options;
     private readonly IMonotonicClock _monotonicClock;
+    private readonly SessionChangeJournal? _sessionChanges;
     private readonly DateTimeOffset _wallOrigin;
     private readonly TimeSpan _monotonicOrigin;
     private readonly Dictionary<SessionListingId, ListingEntry> _listings = [];
@@ -45,7 +47,8 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
     public InMemoryEphemeralRendezvousStore(
         EphemeralStoreOptions options,
         IWallClock wallClock,
-        IMonotonicClock monotonicClock)
+        IMonotonicClock monotonicClock,
+        SessionChangeJournal? sessionChanges = null)
     {
         ArgumentNullException.ThrowIfNull(options);
         ArgumentNullException.ThrowIfNull(wallClock);
@@ -53,6 +56,7 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
         options.Validate();
         _options = options;
         _monotonicClock = monotonicClock;
+        _sessionChanges = sessionChanges;
         _wallOrigin = wallClock.UtcNow;
         _monotonicOrigin = monotonicClock.Elapsed;
         InstanceId = Guid.NewGuid();
@@ -225,7 +229,9 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
             now + _options.IdempotencyLifetime);
         _idempotency.Add(idempotencyKey, idempotency);
         EnqueueDeadline(_idempotencyExpiries, idempotencyKey, idempotency.Deadline);
-        return new(StoreResultCode.Success, Snapshot(entry));
+        StoredListing created = Snapshot(entry);
+        _sessionChanges?.Publish(null, created);
+        return new(StoreResultCode.Success, created);
     }, cancellationToken);
 
     public StoreResult<StoredListing> RenewLease(
@@ -277,6 +283,9 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
             || command.MaximumPlayers is <= 0 or > ContractLimits.SessionCapacityMaxPlayers
             || command.CurrentPlayers < 0
             || command.CurrentPlayers > command.MaximumPlayers
+            || command.RegionId.HasValue && string.IsNullOrEmpty(command.RegionId.Value.Value)
+            || command.ProtocolVersion.HasValue && command.ProtocolVersion.Value == 0
+            || command.Visibility.HasValue && !Enum.IsDefined(command.Visibility.Value)
             || !ContractValidation.IsMetadataValid(command.Metadata)
             || command.DedicatedFallback is not null
                 && !ContractValidation.IsNetworkEndpointValid(command.DedicatedFallback))
@@ -302,8 +311,12 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
             return new(StoreResultCode.NotFound);
         }
 
+        StoredListing before = Snapshot(entry);
         entry.Definition = StoredListing.Freeze(entry.Definition with
         {
+            RegionId = command.RegionId ?? entry.Definition.RegionId,
+            ProtocolVersion = command.ProtocolVersion ?? entry.Definition.ProtocolVersion,
+            Visibility = command.Visibility ?? entry.Definition.Visibility,
             BuildVersion = command.BuildVersion,
             DisplayName = command.DisplayName,
             CurrentPlayers = command.CurrentPlayers,
@@ -312,7 +325,9 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
             DedicatedFallback = command.DedicatedFallback,
         });
         entry.Version++;
-        return new(StoreResultCode.Success, Snapshot(entry));
+        StoredListing after = Snapshot(entry);
+        _sessionChanges?.Publish(before, after);
+        return new(StoreResultCode.Success, after);
     }, cancellationToken);
 
     public StoreResult<bool> DeleteListing(
@@ -382,6 +397,7 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
             return new(StoreResultCode.CapacityExceeded);
         }
 
+        StoredListing before = Snapshot(entry);
         bool isNewPresence = !_presence.ContainsKey(command.Handle);
         PresenceEntry presence = new(
             command.PublicEndpoint,
@@ -396,7 +412,9 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
                 command.Handle,
                 presence.Deadline);
         }
-        return new(StoreResultCode.Success, Snapshot(entry));
+        StoredListing after = Snapshot(entry);
+        _sessionChanges?.Publish(before, after);
+        return new(StoreResultCode.Success, after);
     }, cancellationToken, eagerCleanup: false);
 
     public StoreResult<IReadOnlyList<StoredListing>> BrowseVisibleListings(
@@ -996,6 +1014,7 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
 
     private void ClearActiveState()
     {
+        StoredListing[] removedListings = _listings.Values.Select(Snapshot).ToArray();
         _listings.Clear();
         _listingCountsByOwner.Clear();
         _listingExpiries.Clear();
@@ -1017,6 +1036,10 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
         _idempotencyExpiries.Clear();
         _replay.Clear();
         _replayExpiries.Clear();
+        foreach (StoredListing listing in removedListings)
+        {
+            _sessionChanges?.Publish(listing, null);
+        }
     }
 
     private void RemoveListing(SessionListingId listingId)
@@ -1026,6 +1049,7 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
             return;
         }
 
+        StoredListing removed = Snapshot(listing);
         _leases.Remove(listing.Definition.LeaseId);
         DecrementCount(_listingCountsByOwner, listing.Definition.OwnerSubject);
         _presenceHandles.Remove(listing.Definition.HostPresenceHandle);
@@ -1045,6 +1069,8 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
                 RemoveOutcome(attemptId);
             }
         }
+
+        _sessionChanges?.Publish(removed, null);
     }
 
     private void RemoveAttempt(JoinAttemptId attemptId)
@@ -1186,6 +1212,12 @@ internal sealed class InMemoryEphemeralRendezvousStore : IEphemeralRendezvousSto
             }
             else if (_presence.Remove(candidate.Key))
             {
+                if (_presenceHandles.TryGetValue(candidate.Key, out SessionListingId listingId)
+                    && _listings.TryGetValue(listingId, out ListingEntry? listing))
+                {
+                    StoredListing after = Snapshot(listing);
+                    _sessionChanges?.Publish(after with { HasFreshPresence = true }, after);
+                }
                 removed++;
             }
         }