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Marc Schier f358078b1c [Client] Central channel manager: ref-counted shared channels, coalesced reconnect, IRetryBudget (#3288) (#3852)
# Description

Introduces a central `IClientChannelManager` that owns client-side
transport channels with reference counting, sharing, coalesced
reconnect, and asynchronous participant notification. Sessions,
discovery clients, and registration clients targeting the same
`ConfiguredEndpoint` (with the same reverse-connect identity) now share
a single underlying `ITransportChannel`; reconnect is transparent to
callers; and the previous `AttachChannel`/`DetachChannel` +
`SessionReconnectHandler` patterns are obsoleted.

## What lands

### Core types (`Stack/Opc.Ua.Core/Stack/Client/`)

- `ManagedChannelKey`, `ChannelState`, `ChannelStateChange`,
`ParticipantReconnectResult`
- `IReconnectParticipant`, `IManagedTransportChannel`,
`IClientChannelManager`
- `IChannelReconnectPolicy` + `ExponentialBackoffChannelReconnectPolicy`
(default mirrors historical `SessionReconnectHandler`: 500 ms → 30 s
exponential)
- `IRetryBudget` + `RetryBudget` — shared retry deadline so the
two-level retry no longer compounds multiplicatively
- `ClientChannelManager` — single non-partial sealed class with
SOLID-extracted internal types under
`Stack/Opc.Ua.Core/Stack/Client/Channels/Internal/`: `ChannelEntry`
(behind `IChannelEntryHost`), `ManagedTransportChannelLease`,
`ClientChannel`, `ClientChannelManagerMetrics` (IMeter),
`ClientChannelManagerCertRotation` (auto `ReconnectAllAsync` on
`CertificateManager` rotation), `ClientChannelManagerDiagnostics`
(ActivitySource + EventSource). Refcount, coalesced reconnect, and
transparent faulted-entry recovery via `SwapFaultedEntryAsync` (swaps a
lease's underlying entry on subsequent `ReconnectAsync` when the prior
cycle Faulted/Closed, preserves participant, back-off via policy
`GetDelay(SwapCount)`)

### Three-state lifecycle gate

`Disconnected` → `TransportConnecting` / `TransportReconnecting` →
`TransportConnectedSessionReactivating` → `Ready` (or `Faulted` /
`Closed`). Only `Ready` releases the service-call gate; the manager
bypasses the gate internally for participant `ActivateSession` traffic
during reactivation via an `AsyncLocal` scope.

### Session integration (`Libraries/Opc.Ua.Client/Session/`)

- New `Session.CreateAsync(IClientChannelManager, …)` overload
- `Session` implements `IReconnectParticipant`; reactivation handler
distinguishes `Reactivated` / `RequiresSessionRecreate` /
`TransientFailure` / `FatalForParticipant` / `FatalForChannel`
- `Session.ReconnectAsync(ct)` automatically delegates to the channel
manager when wired
- `Session.RecreateInPlaceAsync` swaps managed-channel leases for
failover-to-different-endpoint via a new atomic
`IClientChannelManager.GetAsync(endpoint, participantFactory, …)`
overload; same-key recreates delegate to manager reconnect;
explicit-channel/connection callers retain legacy behavior
- KA failure routes through
`IClientChannelManager.ReconnectAsync(channel)`

### ManagedSession integration

- `WithChannelManager(...)` builder + ctor param; DI-resolved
automatically
- Two-level retry: channel-mgr handles transparent reconnect; outer
`ConnectionStateMachine` + `IReconnectPolicy` only triggers on terminal
channel `Faulted`. Outer-state churn suppressed during channel-mgr
cycles via `_channelReconnectInProgress` flag wired through
`IManagedTransportChannel.StateChanged`
- New `ManagedSession.ChannelStateChanged` event surfaces transparent
reconnects
- `ConnectionStateChangedEventArgs.UnderlyingChannelState` populated
when outer reconnect was triggered by a channel fault
- Shared `IRetryBudget` propagated so the layers enforce a single
max-total-time

### Discovery / Registration / GDS

Channel-manager-aware `CreateAsync` overloads on `DiscoveryClient`,
`RegistrationClient`, `ServerPushConfigurationClient`,
`LocalDiscoveryServerClient`, `GlobalDiscoveryServerClient`. Each
registers a minimal `IReconnectParticipant`; co-located sessions and
discovery probes targeting the same endpoint share a channel.

### HTTPS transport (`Stack/Opc.Ua.Bindings.Https/`)

- New `IOpcUaHttpClientFactory` abstraction
- HTTPS / OPC-HTTPS channels migrated to `IHttpClientFactory` +
`Microsoft.Extensions.Http.Resilience` (10.6.0; AOT-compatible on net10
via source generators; legacy direct-HTTP path retained for older TFMs /
no-DI consumers)

### DI

`AddOpcUa().AddClient(...)` registers `IClientChannelManager` and the
named HTTPS `HttpClient` + standard resilience handler automatically.
`ManagedSessionBuilder` resolves the manager from DI when present.

### Obsoletions (functional, `[Obsolete]` with guidance)

- `IClientBase.AttachChannel` / `DetachChannel`
- `SessionReconnectHandler`
- `SessionExtensions.ReconnectAsync(connection, ct)` and `(channel, ct)`
legacy overloads

### Tests

- Core: `ClientChannelManagerManagedTests`,
`ClientChannelManagerCertRotationTests`, `RetryBudgetTests`,
`OpcUaHttpClientFactoryTests` — **56 / 56 pass**
- Client: `ManagedSession*Tests` updated; legacy
`SessionReconnectHandlerTests` + `SessionExtensionsTests` preserved with
`#pragma warning disable CS0618` — **129 / 129 pass**
- Sessions integration: `ChannelManagerSharing` / `TransparentReconnect`
/ `SessionLifecycle` / `CertRotation` — **7 pass, 1 skipped**
(`ChannelManagerExhaustionEscalatesAndRecoversWhenServerReturns`
un-`[Ignore]`d and implemented in this PR via `SwapFaultedEntryAsync`;
remaining skip is end-to-end `RequiresSessionRecreate` plumbing —
carry-forward)
- Stress (consolidated `Tests/Opc.Ua.Stress.Tests/Subscriptions/` +
`Channels/`): chaos / contract / soak / lifecycle / leak-accuracy / gap
/ participant-result-aggregation tests merged from the
previously-separate `Opc.Ua.Channels.Stress.Tests` project; nightly run
via `.github/workflows/stress-test.yml`

Builds clean (0 warnings / 0 errors) across all six TFMs: `net472`,
`net48`, `netstandard2.1`, `net8.0`, `net9.0`, `net10.0`.

### Docs

- `Docs/Sessions.md` new § 4 documents the channel manager, three-state
gating, two-level retry with shared `IRetryBudget`, HTTPS resilience
layering
- `Docs/MigrationGuide.md` migration recipes for the four obsoleted API
groups
- `Docs/DependencyInjection.md` channel-manager and HTTPS factory
registration

## Carry-forward (separate PRs)

- End-to-end `ParticipantReconnectResult.RequiresSessionRecreate` →
`Session.RecreateAsync` lifecycle wiring (currently incomplete — flagged
in the 1 remaining skipped integration test)

## Related Issues

- Fixes #3288
- Builds on #3282 (PR that introduced the initial
`ITransportChannelManager` skeleton)

## Checklist

- [x] I have signed the
[CLA](https://opcfoundation.org/license/cla/ContributorLicenseAgreementv1.0.pdf)
and read the
[CONTRIBUTING](https://github.com/OPCFoundation/UA-.NETStandard/blob/master/CONTRIBUTING.md)
doc.
- [x] I have added tests that prove my fix is effective or that my
feature works and increased code coverage.
- [x] I have added all necessary documentation.
- [x] I have verified that my changes do not introduce (new) build or
analyzer warnings.
- [ ] I ran **all** tests locally using the **UA.slnx** solution against
at least .net **framework** and .net **10**, and all passed.
*(Channel-manager-specific tests + ManagedSession/client tests verified
across all 6 TFMs; full UA.slnx run is pending CI.)*
- [ ] I fixed **all** failing and flaky tests in the CI pipelines and
**all** CodeQL warnings.
- [ ] I have addressed **all** PR feedback received.
2026-06-13 16:34:48 +02:00

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