feat(grpc): site-side ICentralTransport seam + gRPC transport (T1A.3)
Introduce ICentralTransport as the site->central choke point inside SiteCommunicationActor. The seven site->central sends (notification submit/ status, audit + cached-telemetry ingest, reconcile, health, heartbeat) now delegate to an injected transport instead of owning ClusterClient.Send inline. - AkkaCentralTransport: verbatim extraction of today's ClusterClient.Send path, including the exact sender-forwarding that routes central's reply straight back to the waiting Ask. Default when no transport is injected -> behaviour unchanged, existing SiteCommunicationActorTests pass as-is. - GrpcCentralTransport + CentralChannelProvider: dial CentralControlService with sticky failover + background failback (1s-doubling-cap-60s), PSK + x-scadabridge-site via ControlPlaneCredentials, per-call deadlines mirroring today's Ask timeouts. Cross-node retry ONLY on provably-unsent connect failures; never on DeadlineExceeded. Heartbeat stays fire-and-forget. - StaticSitePskProvider: site's single own-key provider (fail-closed). - CommunicationOptions: CentralTransport flag (default Akka) + CentralGrpcEndpoints (validator: required when transport=Grpc). Host selects the impl; the ClusterClient is created only on the Akka path. Tests: actor-with-fake-transport (7 delegations + fault routing + heartbeat no-fault), AkkaCentralTransport sender-forwarding, GrpcCentralTransport over in-process TestServer (failover flip, sticky, failback, PSK+header, deadline, no-retry-on-deadline), validator. Communication.Tests 371 green, Host.Tests 391 green; the three above-seam suites pass unmodified.
This commit is contained in:
Joseph Doherty
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using Google.Protobuf.WellKnownTypes;
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using Grpc.Core;
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using Grpc.Net.Client;
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using Microsoft.Extensions.Logging;
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namespace ZB.MOM.WW.ScadaBridge.Communication.Grpc;
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/// <summary>
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/// A pair (or more) of gRPC channels to the central cluster's control-plane nodes, with the
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/// sticky-failover + background-failback policy of the design (§3.5). The site holds one channel
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/// per central endpoint, prefers the first, and stays on it until a call proves it unreachable —
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/// only then flipping to the next, and only <see cref="StatusCode.Unavailable"/> / connect
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/// failures count (a <see cref="StatusCode.DeadlineExceeded"/> never flips or retries, because the
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/// call may have run).
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/// </summary>
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/// <remarks>
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/// <para>
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/// <b>Sticky.</b> All calls go to the current channel; a healthy preferred endpoint never
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/// ping-pongs. <see cref="ReportUnavailable"/> flips to the next endpoint (round-robin) when the
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/// caller sees the current one refuse a connection.
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/// </para>
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/// <para>
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/// <b>Failback.</b> While off the preferred endpoint a background probe (a cheap <c>Heartbeat</c>
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/// ping — always answered, even by a not-yet-ready node) re-checks the preferred one. On the first
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/// success new calls return to it; in-flight calls finish where they are. The probe is
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/// event-driven: it arms on a flip and stops the moment we are back on the preferred endpoint, so
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/// a steady healthy pair spends no cycles. Its cadence backs off exponentially — 1 s, doubling,
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/// capped at 60 s — while the preferred endpoint stays down, so a genuinely dead node is not
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/// probed hard.
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/// </para>
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/// <para>
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/// <b>Auth.</b> Every channel carries the site's own preshared key and its
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/// <c>x-scadabridge-site</c> identity via <see cref="ControlPlaneCredentials"/> — the same
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/// insecure-h2c call-credentials shape the streaming client uses. The <paramref name="handlerFactory"/>
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/// seam lets a test point a channel at an in-process <c>TestServer</c>; production uses a
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/// keepalive-configured <see cref="SocketsHttpHandler"/>.
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/// </para>
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/// </remarks>
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public sealed class CentralChannelProvider : IDisposable
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{
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private static readonly TimeSpan DefaultBackoffBase = TimeSpan.FromSeconds(1);
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private static readonly TimeSpan DefaultBackoffCap = TimeSpan.FromSeconds(60);
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private static readonly TimeSpan DefaultProbeDeadline = TimeSpan.FromSeconds(5);
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private readonly IReadOnlyList<string> _endpoints;
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private readonly GrpcChannel[] _channels;
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private readonly CentralControlService.CentralControlServiceClient[] _clients;
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private readonly ILogger _logger;
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private readonly string _siteId;
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private readonly TimeSpan _backoffBase;
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private readonly TimeSpan _backoffCap;
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private readonly TimeSpan _probeDeadline;
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private readonly Timer? _failbackTimer;
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private readonly object _gate = new();
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private volatile int _current; // preferred == 0
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private int _consecutiveProbeFailures;
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private bool _disposed;
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/// <summary>Creates the provider and opens one channel per endpoint.</summary>
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/// <param name="endpoints">Central control-plane endpoints, preferred first (index 0). Must be non-empty.</param>
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/// <param name="pskProvider">Resolves this site's preshared key (site-side: a single-key provider).</param>
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/// <param name="siteId">This site's identity, sent as the <c>x-scadabridge-site</c> header.</param>
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/// <param name="options">Communication options supplying gRPC keepalive settings.</param>
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/// <param name="logger">Logger for flip/failback diagnostics.</param>
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/// <param name="handlerFactory">Test seam: per-endpoint <see cref="HttpMessageHandler"/>; null uses a production socket handler.</param>
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/// <param name="probeDeadline">Deadline for a failback probe. Null uses 5 s.</param>
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/// <param name="backoffBase">Initial failback-probe backoff. Null uses 1 s.</param>
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/// <param name="backoffCap">Maximum failback-probe backoff. Null uses 60 s.</param>
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public CentralChannelProvider(
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IReadOnlyList<string> endpoints,
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ISitePskProvider pskProvider,
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string siteId,
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CommunicationOptions options,
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ILogger logger,
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Func<string, HttpMessageHandler>? handlerFactory = null,
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TimeSpan? probeDeadline = null,
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TimeSpan? backoffBase = null,
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TimeSpan? backoffCap = null)
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{
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ArgumentNullException.ThrowIfNull(endpoints);
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ArgumentNullException.ThrowIfNull(pskProvider);
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ArgumentException.ThrowIfNullOrWhiteSpace(siteId);
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ArgumentNullException.ThrowIfNull(options);
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ArgumentNullException.ThrowIfNull(logger);
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if (endpoints.Count == 0)
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{
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throw new ArgumentException("At least one central gRPC endpoint is required.", nameof(endpoints));
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}
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_endpoints = endpoints;
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_logger = logger;
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_siteId = siteId;
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_backoffBase = backoffBase ?? DefaultBackoffBase;
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_backoffCap = backoffCap ?? DefaultBackoffCap;
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_probeDeadline = probeDeadline ?? DefaultProbeDeadline;
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_channels = new GrpcChannel[endpoints.Count];
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_clients = new CentralControlService.CentralControlServiceClient[endpoints.Count];
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for (var i = 0; i < endpoints.Count; i++)
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{
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var channelOptions = new GrpcChannelOptions
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{
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HttpHandler = handlerFactory?.Invoke(endpoints[i]) ?? new SocketsHttpHandler
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{
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KeepAlivePingDelay = options.GrpcKeepAlivePingDelay,
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KeepAlivePingTimeout = options.GrpcKeepAlivePingTimeout,
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KeepAlivePingPolicy = HttpKeepAlivePingPolicy.Always,
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EnableMultipleHttp2Connections = true,
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},
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}.WithSiteCredentials(pskProvider, siteId);
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_channels[i] = GrpcChannel.ForAddress(endpoints[i], channelOptions);
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_clients[i] = new CentralControlService.CentralControlServiceClient(_channels[i]);
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}
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// Only a multi-endpoint pair can ever fail over, so a lone endpoint needs no probe.
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if (endpoints.Count > 1)
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{
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_failbackTimer = new Timer(_ => _ = FailbackTickAsync(), null, Timeout.Infinite, Timeout.Infinite);
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}
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}
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/// <summary>The number of endpoints in the pair.</summary>
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public int EndpointCount => _endpoints.Count;
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/// <summary>The index of the endpoint calls are currently routed to (preferred == 0).</summary>
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public int CurrentIndex => _current;
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/// <summary>The endpoint address calls are currently routed to.</summary>
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public string CurrentEndpoint => _endpoints[_current];
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/// <summary>
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/// The endpoint index and client calls should use right now. Captured together so a caller can
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/// tell <see cref="ReportUnavailable"/> exactly which endpoint failed even if a concurrent flip
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/// has already moved <see cref="CurrentIndex"/>.
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/// </summary>
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/// <returns>The current endpoint index and its client.</returns>
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public (int Index, CentralControlService.CentralControlServiceClient Client) Current()
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{
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var idx = _current;
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return (idx, _clients[idx]);
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}
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/// <summary>
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/// Reports that the endpoint at <paramref name="failedIndex"/> refused a connection (an
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/// <see cref="StatusCode.Unavailable"/> / connect failure). If it is still the current endpoint
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/// and another exists, flips to the next one and — when now off the preferred endpoint — arms
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/// the failback probe. Idempotent under a concurrent flip: a stale index is ignored.
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/// </summary>
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/// <param name="failedIndex">The endpoint index the caller's failed call used.</param>
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public void ReportUnavailable(int failedIndex)
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{
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if (_endpoints.Count < 2)
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{
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return; // nothing to fail over to
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}
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lock (_gate)
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{
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if (_disposed || failedIndex != _current)
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{
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return; // a concurrent flip already moved us; do not double-flip
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}
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var next = (failedIndex + 1) % _endpoints.Count;
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_current = next;
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_logger.LogWarning(
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"Central control-plane endpoint {Failed} is unavailable; site {SiteId} failed over to {Next}.",
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_endpoints[failedIndex], _siteId, _endpoints[next]);
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if (_current != 0)
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{
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_consecutiveProbeFailures = 0;
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ArmFailback(_backoffBase);
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}
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}
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}
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private void ArmFailback(TimeSpan due)
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{
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if (_disposed)
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{
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return;
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}
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_failbackTimer?.Change(due, Timeout.InfiniteTimeSpan);
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}
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private async Task FailbackTickAsync()
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{
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int currentAtTick = _current;
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if (_disposed || currentAtTick == 0)
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{
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return; // already back on the preferred endpoint (or shutting down)
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}
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var preferred = _clients[0];
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try
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{
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await preferred.HeartbeatAsync(
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new HeartbeatDto
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{
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SiteId = _siteId,
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NodeHostname = "failback-probe",
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IsActive = false,
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Timestamp = Timestamp.FromDateTimeOffset(DateTimeOffset.UtcNow),
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},
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deadline: DateTime.UtcNow.Add(_probeDeadline)).ConfigureAwait(false);
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// The preferred endpoint answered — return new calls to it.
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lock (_gate)
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{
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if (_disposed)
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{
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return;
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}
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_current = 0;
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_consecutiveProbeFailures = 0;
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}
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_logger.LogInformation(
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"Central control-plane preferred endpoint {Preferred} is reachable again; site {SiteId} failed back.",
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_endpoints[0], _siteId);
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}
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catch (Exception ex)
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{
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lock (_gate)
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{
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if (_disposed || _current == 0)
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{
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return;
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}
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_consecutiveProbeFailures++;
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var backoff = NextBackoff(_consecutiveProbeFailures);
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_logger.LogDebug(ex,
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"Failback probe of preferred central endpoint {Preferred} failed; re-probing in {Backoff}.",
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_endpoints[0], backoff);
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ArmFailback(backoff);
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}
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}
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}
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private TimeSpan NextBackoff(int failures)
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{
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// 1 s, doubling, capped at 60 s. Guard the shift against overflow for a long outage.
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var exponent = Math.Min(failures - 1, 20);
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var scaled = _backoffBase.Ticks * (1L << exponent);
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var cap = _backoffCap.Ticks;
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return TimeSpan.FromTicks(scaled >= cap || scaled < 0 ? cap : scaled);
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}
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/// <inheritdoc />
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public void Dispose()
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{
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lock (_gate)
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{
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if (_disposed)
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{
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return;
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}
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_disposed = true;
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}
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_failbackTimer?.Dispose();
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foreach (var channel in _channels)
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{
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channel.Dispose();
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}
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}
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}
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