Ports the sister project's live-proven fix (ScadaBridge cf3bd52f). OtOpcUa ran
the identical configuration it indicts: keep-oldest with down-if-alone = on.
Akka.NET 1.5.62's KeepOldest.OldestDecision only lets the down-if-alone branch
rescue a side holding >= 2 members. A two-node cluster losing a peer is always a
1-vs-1 split, so the branch never fires, the lone survivor falls through to
DownReachable and downs ITSELF, and run-coordinated-shutdown-when-down then
terminates it. down-if-alone is a 3+-node feature and does not do what its name
suggests for a pair: a crash of the oldest node is a total outage, which is the
exact failure the redundancy pair exists to absorb.
Cluster:SplitBrainResolverStrategy now selects the provider, defaulting to
auto-down: Akka's AutoDowning with auto-down-unreachable-after = 15s, so the
leader among the reachable members downs the unreachable peer and a crash of
either node fails over in place. keep-oldest remains available for deployments
that would rather take an outage than ever run dual-active during a real
partition. An unrecognised value fails the host at startup rather than falling
through to the fatal default.
The tests assert the EFFECTIVE configuration — they start a real host through
WithOtOpcUaClusterBootstrap and read akka.cluster.downing-provider-class back off
the running ActorSystem. This is not incidental. The first draft inlined the
three calls the bootstrap makes instead of calling it, which pinned the test's
own wiring: sabotaging production's HOCON precedence left all 36 green. The
prior file had the same shape at a smaller scale, asserting only that
BuildClusterOptions returned a KeepOldestOption — true, and true of a
configuration that cannot fail over. Positive control: making BuildDowningHocon
emit nothing for auto-down turns exactly the two effective-config guards red.
Measured while verifying rather than assumed: HoconAddMode.Append also wins here,
purely because it is added last, so the mode name is not the guarantee. The
comment now says so instead of asserting a precedence rule that does not hold.
Not yet live-drilled on OtOpcUa. The docker-dev rig is a single six-node mesh
where the 1-vs-1 pathology cannot occur; the kill-the-oldest drill belongs with
the per-cluster mesh work that makes every mesh exactly two nodes (design doc
6.2 / Phase 0a). Recorded as an outstanding gate in docs/Redundancy.md.
Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW
Akka.Cluster.Hosting's WithClustering enables an SBR downing provider by default
(applies SplitBrainResolverOption.Default when ClusterOptions.SplitBrainResolver is
null), which reads the pre-existing akka.conf keep-oldest block. So the cluster was
NOT running NoDowning before Critical 1 and hard-crash failover already worked — the
typed KeepOldestOption is reinforcing/explicit-in-code, not the sole activator.
Corrects the inaccurate 'HOCON inert / NoDowning / never fails over' framing in:
- ServiceCollectionExtensions.BuildClusterOptions XML comment
- akka.conf split-brain-resolver comment
- docs/Redundancy.md Split-brain section
- SplitBrainResolverActivationTests summary + assertion message (+ method rename)
No code revert (the typed option is correct belt-and-suspenders). Cluster.Tests 29/29.
Surfaced by the #9 hard-kill failover negative control.
The split-brain-resolver HOCON block in akka.conf was inert — nothing
registered a downing provider, so the cluster ran Akka's default NoDowning:
a hard-crashed node was never downed, cluster singletons + the driver
role-leader never failed over, and a partition left both redundancy sides
at ServiceLevel 240 indefinitely.
Activate the resolver via the typed ClusterOptions.SplitBrainResolver
(KeepOldestOption { DownIfAlone = true }) in a new testable
BuildClusterOptions helper. keep-oldest + down-if-alone is correct for a
2-node warm-redundancy pair. HOCON keeps stable-after (typed option can't
express it) + a cross-reference comment. Deterministic unit guard added;
end-to-end verified the resolved ActorSystem config carries the provider
and stable-after survives the HOCON/Hosting merge. Redundancy.md corrected.
Hard-kill failover integration test tracked as a follow-up (effort-M harness).
DriverHostActor/DriverInstanceActor and cluster events log via Akka's ILoggingAdapter, which had no Serilog bridge — under the Windows service host the default StandardOutLogger output is discarded, making the driver-role actor graph invisible (this masked the data-plane diagnosis).
- Add Akka.Logger.Serilog 1.5.60 (deps satisfied by Akka 1.5.62 / Serilog 4.3.1).
- WithOtOpcUaClusterBootstrap: ConfigureLoggers(DebugLevel; ClearLoggers(); AddLogger<SerilogLogger>()) — Akka.Hosting owns logger setup, so HOCON akka.loggers alone is not honored.
- Program.cs: set static Serilog.Log.Logger from the DI root logger after Build() (AddZbSerilog registers the MEL provider but not the static logger, which the Akka SerilogLogger and the startup banner both need).
Code review at HEAD 7286d320. Cluster-001 (SeedFromCurrentState reads from one
snapshot), Cluster-003 (HoconLoader double-dispose), Cluster-004 (stale akka.conf
header), Cluster-005 (ServiceLevelCalculator tests added to Cluster.Tests). Cluster-002
deferred (no production caller).
Joseph Doherty