docs(archreview #11): correct Critical 1 premise — SBR was already active on master, not NoDowning

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.

docs/Redundancy.md

@@ -154,23 +154,28 @@ Node A lists Node B's `ApplicationUri` and vice-versa. Validated by `DualEndpoin
 
 ## Split-brain
 
-The split-brain resolver is **activated in code** by the typed `ClusterOptions.SplitBrainResolver`
+The split-brain resolver is **active by default**: Akka.Cluster.Hosting's `WithClustering` enables an SBR
-(`KeepOldestOption { DownIfAlone = true }`) set in
+downing provider whenever `ClusterOptions.SplitBrainResolver` is null (it applies
-`ServiceCollectionExtensions.BuildClusterOptions` — this is what registers
+`SplitBrainResolverOption.Default`, which registers `Akka.Cluster.SBR.SplitBrainResolverProvider`), and that
-`Akka.Cluster.SBR.SplitBrainResolverProvider`. **Without that typed registration the `split-brain-resolver`
+provider reads the `split-brain-resolver` HOCON block in `akka.conf`. On top of that,
-HOCON block in `akka.conf` is inert and the cluster runs Akka's default `NoDowning`**, meaning a
+`ServiceCollectionExtensions.BuildClusterOptions` sets the typed `ClusterOptions.SplitBrainResolver`
-hard-crashed node is never downed and neither the cluster singletons nor the `driver` role-leader ever fail
+(`KeepOldestOption { DownIfAlone = true }`) to make the strategy **explicit in code** rather than relying on
-over (a partition would leave both redundancy sides at ServiceLevel 240 indefinitely). The HOCON block is the
+the framework default — it is reinforcing, not the sole activator, and yields the same effective behavior. So
-tuning source only: `active-strategy = keep-oldest`, `stable-after = 15s`, `keep-oldest.down-if-alone = on`,
+the cluster is **not** running `NoDowning`; hard-crashed nodes are downed and both the cluster singletons and
-`failure-detector.threshold = 10.0` (its `active-strategy` + `down-if-alone` must stay consistent with the
+the `driver` role-leader fail over. (Only an *explicit* `NoDowning`, e.g.
-typed option; `stable-after` lives only in HOCON because the typed option can't express it).
+`akka.cluster.downing-provider-class = ""`, would leave both redundancy sides at ServiceLevel 240
+indefinitely.) The HOCON block carries the tuning: `active-strategy = keep-oldest`, `stable-after = 15s`,
+`keep-oldest.down-if-alone = on`, `failure-detector.threshold = 10.0` (its `active-strategy` + `down-if-alone`
+must stay consistent with the typed option; `stable-after` lives only in HOCON because the typed option can't
+express it).
 
 `keep-oldest` is the correct strategy for a 2-node warm-redundancy pair: under a clean partition the oldest
 member (typically the long-running primary) stays up and the smaller (or younger) side downs itself within
 ~`stable-after` seconds; `down-if-alone` downs a node that loses its only peer. `keep-majority`/`static-quorum`
 are wrong for two nodes (no majority in a 1-1 split). The `RedundancyStateActor` on the surviving partition
-re-computes from the post-partition `Cluster.State`, and a hard-crashed (not gracefully stopped) node now
+re-computes from the post-partition `Cluster.State`, and a hard-crashed (not gracefully stopped) node
-triggers the same failover.
+triggers the same failover — verified by `HardKillFailoverTests` (its negative control confirms failover
+survives removing the typed option, and only breaks under an explicit `NoDowning`).
 
 There is no operator-driven role swap during a partition. Failover is what the cluster does automatically.
 

src/Core/ZB.MOM.WW.OtOpcUa.Cluster/Resources/akka.conf

@@ -37,12 +37,13 @@ akka {
         roles = []
         min-nr-of-members = 1
 
-        # Split-brain resolver. This HOCON block is the TUNING source only — it does NOT
+        # Split-brain resolver (arch-review 03/S1). This block IS active: Akka.Cluster.Hosting's
-        # activate the resolver. Activation happens in code via the typed
+        # WithClustering enables an SBR downing provider by default (it applies
-        # ClusterOptions.SplitBrainResolver (KeepOldestOption { DownIfAlone = true }) set in
+        # SplitBrainResolverOption.Default when ClusterOptions.SplitBrainResolver is null), and that
-        # ServiceCollectionExtensions.BuildClusterOptions; without that the cluster runs Akka's
+        # provider reads this block. ServiceCollectionExtensions.BuildClusterOptions additionally sets
-        # default NoDowning and hard-crashed nodes never fail over (arch-review 03/S1).
+        # the typed KeepOldestOption { DownIfAlone = true } to make the strategy EXPLICIT in code —
-        # active-strategy + keep-oldest.down-if-alone below MUST match the typed option.
+        # its active-strategy + keep-oldest.down-if-alone MUST match this block. (The cluster is NOT
+        # NoDowning; only an explicit downing-provider-class = "" would disable failover.)
         # stable-after must stay >= failure-detector.acceptable-heartbeat-pause (10s) + margin.
         split-brain-resolver {
             active-strategy = "keep-oldest"

src/Core/ZB.MOM.WW.OtOpcUa.Cluster/ServiceCollectionExtensions.cs

@@ -84,16 +84,19 @@ public static class ServiceCollectionExtensions
     }
 
     /// <summary>
-    /// Builds the <see cref="ClusterOptions"/> for the fused-host cluster, including the
+    /// Builds the <see cref="ClusterOptions"/> for the fused-host cluster, setting the split-brain-resolver
-    /// <b>activating</b> split-brain-resolver registration.
+    /// strategy <b>explicitly in code</b>.
     ///
-    /// The <c>split-brain-resolver</c> HOCON block in <c>Resources/akka.conf</c> is inert on its own —
+    /// <b>Activation note (arch-review 03/S1, corrected):</b> Akka.Cluster.Hosting's <c>WithClustering</c>
-    /// Akka.NET only runs the resolver when a downing provider is registered, otherwise the cluster
+    /// already enables an SBR downing provider <b>by default</b> — when <see cref="ClusterOptions.SplitBrainResolver"/>
-    /// falls back to <b>NoDowning</b> and a hard-crashed node is never downed (singletons and the
+    /// is <c>null</c> it applies <c>SplitBrainResolverOption.Default</c>, which registers
-    /// <c>driver</c> role-leader never fail over, and a partition leaves both redundancy sides at
+    /// <c>Akka.Cluster.SBR.SplitBrainResolverProvider</c> and reads the <c>split-brain-resolver</c> HOCON block in
-    /// ServiceLevel 240 forever). Setting <see cref="ClusterOptions.SplitBrainResolver"/> is what
+    /// <c>Resources/akka.conf</c>. So the cluster was <b>not</b> running <c>NoDowning</c> before this option was
-    /// activates <c>Akka.Cluster.SBR.SplitBrainResolverProvider</c> under the hood — this is the piece
+    /// set: the pre-existing akka.conf <c>keep-oldest</c> block was already active, and hard-crashed nodes already
-    /// that was missing (arch-review 03/S1).
+    /// failed over. Setting this typed option makes the strategy <b>explicit in code</b> (independent of the
+    /// framework default) rather than being the sole activator — it is reinforcing/belt-and-suspenders and
+    /// produces the same effective behavior. (Only an <i>explicit</i> <c>NoDowning</c>, e.g.
+    /// <c>downing-provider-class = ""</c>, would disable failover.)
     ///
     /// <see cref="KeepOldestOption"/> with <c>DownIfAlone=true</c> mirrors the HOCON intent and is the
     /// correct strategy for a 2-node warm-redundancy pair: on an even split the oldest member (typically
@@ -103,7 +106,7 @@ public static class ServiceCollectionExtensions
     /// typed option cannot express it (it must stay ≥ <c>failure-detector.acceptable-heartbeat-pause</c>).
     /// </summary>
     /// <param name="options">The bound cluster options carrying seed nodes and roles.</param>
-    /// <returns>The cluster options with seed nodes, roles, and the activated split-brain resolver.</returns>
+    /// <returns>The cluster options with seed nodes, roles, and the explicit split-brain-resolver strategy.</returns>
     public static ClusterOptions BuildClusterOptions(AkkaClusterOptions options)
     {
         return new ClusterOptions

tests/Core/ZB.MOM.WW.OtOpcUa.Cluster.Tests/SplitBrainResolverActivationTests.cs

@@ -6,12 +6,15 @@ using Xunit;
 namespace ZB.MOM.WW.OtOpcUa.Cluster.Tests;
 
 /// <summary>
-/// Guards the split-brain-resolver ACTIVATION (arch-review 03/S1). The <c>split-brain-resolver</c>
+/// Guards that the split-brain-resolver strategy stays set <b>explicitly in code</b> (arch-review 03/S1,
-/// HOCON block is inert on its own; the resolver only runs when the typed
+/// premise corrected — see #11). Akka.Cluster.Hosting already enables an SBR downing provider by default
-/// <see cref="ClusterOptions.SplitBrainResolver"/> is set. These tests assert
+/// (it applies <c>SplitBrainResolverOption.Default</c> when <see cref="ClusterOptions.SplitBrainResolver"/>
-/// <see cref="ServiceCollectionExtensions.BuildClusterOptions"/> registers it, so a future refactor
+/// is null, which reads the akka.conf <c>keep-oldest</c> block), so the cluster is not NoDowning even
-/// cannot silently drop back to Akka's default NoDowning (the exact "built-but-never-wired" failure
+/// without the typed option. These tests assert
-/// mode this finding described).
+/// <see cref="ServiceCollectionExtensions.BuildClusterOptions"/> sets the typed
+/// <c>KeepOldestOption { DownIfAlone = true }</c> so the strategy is explicit in code rather than relying on
+/// the framework default — a future refactor cannot silently drop that explicitness. (Failover behaviour is
+/// separately verified by <c>HardKillFailoverTests</c>, which passes regardless of activation path.)
 /// </summary>
 public sealed class SplitBrainResolverActivationTests
 {
@@ -21,14 +24,14 @@ public sealed class SplitBrainResolverActivationTests
         Roles = new[] { "admin", "driver" },
     };
 
-    /// <summary>Verifies the cluster options carry a split-brain resolver (provider gets activated).</summary>
+    /// <summary>Verifies the cluster options carry an explicit split-brain resolver strategy.</summary>
     [Fact]
-    public void BuildClusterOptions_activates_a_split_brain_resolver()
+    public void BuildClusterOptions_sets_an_explicit_split_brain_resolver()
     {
         var options = ServiceCollectionExtensions.BuildClusterOptions(SampleOptions());
 
         options.SplitBrainResolver.ShouldNotBeNull(
-            "SplitBrainResolver must be set — without it the cluster runs default NoDowning and never fails over on a hard crash.");
+            "SplitBrainResolver must be set explicitly in code — keeps the strategy independent of Akka.Cluster.Hosting's default provider rather than relying on it.");
     }
 
     /// <summary>Verifies the resolver is keep-oldest with down-if-alone (correct for a 2-node pair).</summary>