fix(auditlog): site audit DB onto the data volume; required path + soft flush

Closes WP1.2 of the arch-review remediation plan (finding #2, High):
SqliteAuditWriterOptions.DatabasePath defaulted to CWD-relative "auditlog.db",
which on the docker rig resolves onto the container's ephemeral overlayfs
(not the mounted /app/data volume), silently discarding the pending audit
forward-state backlog on every recreate; nothing in docker/ or docker-env2/
overrode it; FlushIntervalMs was validated but never read by the writer loop
(one commit per event even at trickle rate); and no PRAGMA synchronous was
set (SQLite's FULL default fsyncs every commit).

- DatabasePath now has no default (mirrors ZB.MOM.WW.LocalDb's LocalDbOptions.Path)
  and is required pre-host for Site nodes only, via a new StartupValidator raw-config
  check (top-level "AuditLog:SiteWriter:DatabasePath", NOT nested under ScadaBridge:
  AddAuditLog binds that section off the configuration root). SqliteAuditWriterOptionsValidator
  deliberately does NOT check DatabasePath itself, because AddAuditLog runs its
  ValidateOnStart on both Central and Site composition roots but only Site nodes
  ever resolve the writer — checking it there would fail Central's boot too.
- All 8 site-node appsettings under docker/ and docker-env2/ now set
  AuditLog:SiteWriter:DatabasePath to /app/data/auditlog.db (mounted volume,
  survives container recreate, same convention as LocalDb:Path); the local-dev
  base appsettings.Site.json sets ./data/auditlog.db to match.
- The writer loop now honors FlushIntervalMs: after draining the immediately
  available burst, it keeps the transaction open (bounded by FlushIntervalMs
  from the first event) waiting for more trickle-rate events before committing,
  instead of flushing (and fsyncing) per event.
- PRAGMA synchronous = NORMAL on the write connection — audit is best-effort by
  design (CLAUDE.md: "Audit-write failure NEVER aborts the user-facing action"),
  so NORMAL's narrower power-loss window is an acceptable trade for far fewer
  fsyncs; WAL mode still guarantees no corruption.
- Tests: StartupValidator site-required/blank/central-exempt cases; writer
  trickle-load single-transaction coalescing + beyond-interval separate-transaction
  regression (new FlushCountForTests seam); options-validator doc updates reflecting
  the moved responsibility. Full suite runs green: AuditLog.Tests 368/368,
  Host.Tests 480/480.

One-time migration note: the existing container-local auditlog.db (wherever it
landed under CWD) is abandoned by this change, not migrated — already-forwarded
rows are safe centrally (AuditLog is the durable copy), and any still-Pending
rows on the abandoned path are lost once. This is the exact bug being fixed, not
a new loss: those rows were already living outside the mounted volume and would
not have survived the next container recreate regardless. Cross-reference
docs/known-issues/2026-07-20-cached-telemetry-drain-hot-loop.md, which this
placement bug caused.

docker-env2/site-x-node-a/appsettings.Site.json

@@ -78,5 +78,16 @@
   // Replication is opt-in and configured separately; absent = local-only.
   "LocalDb": {
     "Path": "/app/data/site-localdb.db"
+  },
+  // arch-review remediation WP1.2: the site hot-path audit writer's SQLite file has no
+  // default path (mirrors LocalDb:Path above) and StartupValidator now requires it for
+  // Site nodes — an unset value used to fall back to a bare "auditlog.db" resolved
+  // relative to CWD, i.e. the container's ephemeral overlayfs, so the pending audit
+  // backlog was silently discarded on every recreate. On the mounted /app/data volume,
+  // same as LocalDb:Path, so it survives container recreate.
+  "AuditLog": {
+    "SiteWriter": {
+      "DatabasePath": "/app/data/auditlog.db"
+    }
   }
 }

docker-env2/site-x-node-b/appsettings.Site.json

@@ -78,5 +78,16 @@
   // Replication is opt-in and configured separately; absent = local-only.
   "LocalDb": {
     "Path": "/app/data/site-localdb.db"
+  },
+  // arch-review remediation WP1.2: the site hot-path audit writer's SQLite file has no
+  // default path (mirrors LocalDb:Path above) and StartupValidator now requires it for
+  // Site nodes — an unset value used to fall back to a bare "auditlog.db" resolved
+  // relative to CWD, i.e. the container's ephemeral overlayfs, so the pending audit
+  // backlog was silently discarded on every recreate. On the mounted /app/data volume,
+  // same as LocalDb:Path, so it survives container recreate.
+  "AuditLog": {
+    "SiteWriter": {
+      "DatabasePath": "/app/data/auditlog.db"
+    }
   }
 }

docker/site-a-node-a/appsettings.Site.json

@@ -120,5 +120,16 @@
       "MaxOplogRows": 250000,
       "MaxOplogAge": "2.00:00:00"
     }
+  },
+  // arch-review remediation WP1.2: the site hot-path audit writer's SQLite file has no
+  // default path (mirrors LocalDb:Path above) and StartupValidator now requires it for
+  // Site nodes — an unset value used to fall back to a bare "auditlog.db" resolved
+  // relative to CWD, i.e. the container's ephemeral overlayfs, so the pending audit
+  // backlog was silently discarded on every recreate. On the mounted /app/data volume,
+  // same as LocalDb:Path, so it survives container recreate.
+  "AuditLog": {
+    "SiteWriter": {
+      "DatabasePath": "/app/data/auditlog.db"
+    }
   }
 }

docker/site-a-node-b/appsettings.Site.json

@@ -113,5 +113,16 @@
       "MaxOplogRows": 250000,
       "MaxOplogAge": "2.00:00:00"
     }
+  },
+  // arch-review remediation WP1.2: the site hot-path audit writer's SQLite file has no
+  // default path (mirrors LocalDb:Path above) and StartupValidator now requires it for
+  // Site nodes — an unset value used to fall back to a bare "auditlog.db" resolved
+  // relative to CWD, i.e. the container's ephemeral overlayfs, so the pending audit
+  // backlog was silently discarded on every recreate. On the mounted /app/data volume,
+  // same as LocalDb:Path, so it survives container recreate.
+  "AuditLog": {
+    "SiteWriter": {
+      "DatabasePath": "/app/data/auditlog.db"
+    }
   }
 }

docker/site-b-node-a/appsettings.Site.json

@@ -86,5 +86,16 @@
   // Replication is opt-in and configured separately; absent = local-only.
   "LocalDb": {
     "Path": "/app/data/site-localdb.db"
+  },
+  // arch-review remediation WP1.2: the site hot-path audit writer's SQLite file has no
+  // default path (mirrors LocalDb:Path above) and StartupValidator now requires it for
+  // Site nodes — an unset value used to fall back to a bare "auditlog.db" resolved
+  // relative to CWD, i.e. the container's ephemeral overlayfs, so the pending audit
+  // backlog was silently discarded on every recreate. On the mounted /app/data volume,
+  // same as LocalDb:Path, so it survives container recreate.
+  "AuditLog": {
+    "SiteWriter": {
+      "DatabasePath": "/app/data/auditlog.db"
+    }
   }
 }

docker/site-b-node-b/appsettings.Site.json

@@ -86,5 +86,16 @@
   // Replication is opt-in and configured separately; absent = local-only.
   "LocalDb": {
     "Path": "/app/data/site-localdb.db"
+  },
+  // arch-review remediation WP1.2: the site hot-path audit writer's SQLite file has no
+  // default path (mirrors LocalDb:Path above) and StartupValidator now requires it for
+  // Site nodes — an unset value used to fall back to a bare "auditlog.db" resolved
+  // relative to CWD, i.e. the container's ephemeral overlayfs, so the pending audit
+  // backlog was silently discarded on every recreate. On the mounted /app/data volume,
+  // same as LocalDb:Path, so it survives container recreate.
+  "AuditLog": {
+    "SiteWriter": {
+      "DatabasePath": "/app/data/auditlog.db"
+    }
   }
 }

docker/site-c-node-a/appsettings.Site.json

@@ -86,5 +86,16 @@
   // Replication is opt-in and configured separately; absent = local-only.
   "LocalDb": {
     "Path": "/app/data/site-localdb.db"
+  },
+  // arch-review remediation WP1.2: the site hot-path audit writer's SQLite file has no
+  // default path (mirrors LocalDb:Path above) and StartupValidator now requires it for
+  // Site nodes — an unset value used to fall back to a bare "auditlog.db" resolved
+  // relative to CWD, i.e. the container's ephemeral overlayfs, so the pending audit
+  // backlog was silently discarded on every recreate. On the mounted /app/data volume,
+  // same as LocalDb:Path, so it survives container recreate.
+  "AuditLog": {
+    "SiteWriter": {
+      "DatabasePath": "/app/data/auditlog.db"
+    }
   }
 }

docker/site-c-node-b/appsettings.Site.json

@@ -86,5 +86,16 @@
   // Replication is opt-in and configured separately; absent = local-only.
   "LocalDb": {
     "Path": "/app/data/site-localdb.db"
+  },
+  // arch-review remediation WP1.2: the site hot-path audit writer's SQLite file has no
+  // default path (mirrors LocalDb:Path above) and StartupValidator now requires it for
+  // Site nodes — an unset value used to fall back to a bare "auditlog.db" resolved
+  // relative to CWD, i.e. the container's ephemeral overlayfs, so the pending audit
+  // backlog was silently discarded on every recreate. On the mounted /app/data volume,
+  // same as LocalDb:Path, so it survives container recreate.
+  "AuditLog": {
+    "SiteWriter": {
+      "DatabasePath": "/app/data/auditlog.db"
+    }
   }
 }

src/ZB.MOM.WW.ScadaBridge.AuditLog/Site/SqliteAuditWriter.cs

@@ -174,6 +174,24 @@ public class SqliteAuditWriter : IAuditWriter, ISiteAuditQueue, IAsyncDisposable
             pragmaCmd.ExecuteNonQuery();
         }
 
+        // synchronous=NORMAL (arch-review remediation WP1.2): SQLite's FULL default
+        // fsyncs on every transaction commit, which is the dominant cost of the
+        // per-event write path this same change coalesces via FlushIntervalMs — FULL
+        // would still fsync once per FLUSH even after that fix. NORMAL fsyncs at
+        // fewer, WAL-checkpoint-aligned points instead of every commit; the window
+        // this opens is a handful of the most recent commits lost on OS crash / power
+        // loss (WAL mode itself still guarantees no *corruption*, only a possible
+        // short rollback on unclean restart). That trade is acceptable here because
+        // audit writes are explicitly best-effort by design (CLAUDE.md: "Audit-write
+        // failure NEVER aborts the user-facing action — audit is best-effort, the
+        // action's own success/failure path is authoritative") — this is the same
+        // durability class as a dropped audit row, not a new risk class.
+        using (var pragmaCmd = _connection.CreateCommand())
+        {
+            pragmaCmd.CommandText = "PRAGMA synchronous = NORMAL";
+            pragmaCmd.ExecuteNonQuery();
+        }
+
         // Enable FK enforcement on the WRITE connection. PRAGMA foreign_keys is
         // a per-connection, per-session setting in SQLite — it is NOT persisted
         // in the database file, so every new connection that may INSERT into
@@ -306,20 +324,67 @@ public class SqliteAuditWriter : IAuditWriter, ISiteAuditQueue, IAsyncDisposable
         {
             batch.Clear();
             batch.Add(first);
+            var deadline = DateTime.UtcNow.AddMilliseconds(_options.FlushIntervalMs);
 
             // Pull additional ready events up to BatchSize. TryRead is non-
             // blocking and lets us amortise the transaction overhead across a
-            // burst of concurrent enqueues.
-            while (batch.Count < _options.BatchSize &&
-                   _writeQueue.Reader.TryRead(out var next))
+            // burst of concurrent enqueues. Once the immediately-available
+            // burst is drained, keep the transaction open for up to
+            // FlushIntervalMs waiting for MORE events to trickle in — this is
+            // the soft-flush coalescing arch-review remediation WP1.2 adds:
+            // without it, a low-rate trickle of writes (one script call every
+            // few ms, well under BatchSize) fsyncs once PER EVENT, because the
+            // writer loop reaches an empty channel and flushes immediately.
+            // FlushIntervalMs bounds the worst-case added latency any single
+            // event can see from this coalescing.
+            while (batch.Count < _options.BatchSize)
             {
-                batch.Add(next);
+                if (_writeQueue.Reader.TryRead(out var next))
+                {
+                    batch.Add(next);
+                    continue;
+                }
+
+                var remaining = deadline - DateTime.UtcNow;
+                if (remaining <= TimeSpan.Zero)
+                {
+                    break;
+                }
+
+                var waitToRead = _writeQueue.Reader.WaitToReadAsync().AsTask();
+                var completed = await Task.WhenAny(
+                    waitToRead, Task.Delay(remaining)).ConfigureAwait(false);
+
+                if (completed != waitToRead)
+                {
+                    // Deadline elapsed with nothing new — flush what we have.
+                    break;
+                }
+
+                // WaitToReadAsync resolving false means the channel completed
+                // (Dispose) with nothing left to read; resolving true means at
+                // least one item is available, so the top of the loop's
+                // TryRead will pick it up.
+                if (!await waitToRead.ConfigureAwait(false))
+                {
+                    break;
+                }
             }
 
             FlushBatch(batch);
         }
     }
 
+    /// <summary>
+    /// Total number of <see cref="FlushBatch"/> transactions committed since
+    /// construction. Test-only observability seam (arch-review remediation
+    /// WP1.2) for asserting that FlushIntervalMs coalesces a trickle of
+    /// near-simultaneous writes into a single transaction rather than one
+    /// commit per event; production code never reads this.
+    /// </summary>
+    internal int FlushCountForTests => _flushCountForTests;
+    private int _flushCountForTests;
+
     private void FlushBatch(IReadOnlyList<PendingAuditEvent> batch)
     {
         lock (_writeLock)
@@ -436,6 +501,7 @@ public class SqliteAuditWriter : IAuditWriter, ISiteAuditQueue, IAsyncDisposable
                 }
 
                 transaction.Commit();
+                _flushCountForTests++;
             }
             catch (Exception ex)
             {

src/ZB.MOM.WW.ScadaBridge.AuditLog/Site/SqliteAuditWriterOptions.cs

@@ -9,8 +9,23 @@ namespace ZB.MOM.WW.ScadaBridge.AuditLog.Site;
 /// </summary>
 public sealed class SqliteAuditWriterOptions
 {
-    /// <summary>SQLite database path (or in-memory URI for tests).</summary>
-    public string DatabasePath { get; set; } = "auditlog.db";
+    /// <summary>
+    /// SQLite database path (or in-memory URI for tests). No default —
+    /// mirrors <c>LocalDb:Path</c> (<c>ZB.MOM.WW.LocalDb.LocalDbOptions.Path</c>): a
+    /// bare filename here would resolve relative to the process CWD, which on the
+    /// docker rig is the container's ephemeral overlayfs, not the mounted
+    /// <c>/app/data</c> volume — the file (and its pending forward-state backlog)
+    /// would be silently discarded on every container recreate. Site nodes MUST set
+    /// this explicitly (e.g. <c>/app/data/auditlog.db</c>); <c>StartupValidator</c>
+    /// in ZB.MOM.WW.ScadaBridge.Host enforces the requirement pre-host for Site nodes only,
+    /// the same way it enforces <c>Communication:GrpcPsk</c>. Central composition roots also
+    /// bind this options type (<c>AddAuditLog</c> is shared between roles) but never resolve
+    /// <see cref="SqliteAuditWriter"/>, so the empty default is inert there and deliberately
+    /// left unvalidated by <see cref="SqliteAuditWriterOptionsValidator"/> (which only checks
+    /// the role-agnostic channel/batch/flush knobs) — enforcing it there too would fail
+    /// central's boot via <c>ValidateOnStart</c>, since <c>AddAuditLog</c> runs on both roles.
+    /// </summary>
+    public string DatabasePath { get; set; } = "";
 
     /// <summary>
     /// Capacity of the bounded write queue. Set high enough that ordinary

src/ZB.MOM.WW.ScadaBridge.AuditLog/Site/SqliteAuditWriterOptionsValidator.cs

@@ -6,19 +6,25 @@ namespace ZB.MOM.WW.ScadaBridge.AuditLog.Site;
 /// Validates <see cref="SqliteAuditWriterOptions"/> at host startup
 /// (arch-review 08 round 2 NF4). The channel/batch/flush knobs feed the
 /// background writer task; a zero anywhere stalls it (nothing drains, nothing
-/// flushes), and an empty <c>DatabasePath</c> leaves the SQLite writer with no
-/// backing store. <see cref="SqliteAuditWriterOptions.BacklogPollIntervalSeconds"/>
+/// flushes). <see cref="SqliteAuditWriterOptions.BacklogPollIntervalSeconds"/>
 /// is intentionally NOT validated — a non-positive value has a documented
 /// fall-back-to-30s contract in <c>SiteAuditBacklogReporter</c>.
 /// </summary>
+/// <remarks>
+/// <see cref="SqliteAuditWriterOptions.DatabasePath"/> is deliberately NOT
+/// validated here (arch-review remediation WP1.2). <c>AddAuditLog</c> runs on
+/// BOTH Central and Site composition roots and binds this options type with
+/// <c>ValidateOnStart</c> either way, but only Site nodes ever resolve
+/// <see cref="SqliteAuditWriter"/> — requiring a non-empty path in this
+/// role-agnostic validator would fail Central's boot too. The requirement is
+/// instead enforced pre-host, Site-role-only, by <c>StartupValidator</c> in
+/// ZB.MOM.WW.ScadaBridge.Host (mirrors how it gates <c>Communication:GrpcPsk</c>).
+/// </remarks>
 public sealed class SqliteAuditWriterOptionsValidator : OptionsValidatorBase<SqliteAuditWriterOptions>
 {
     /// <inheritdoc />
     protected override void Validate(ValidationBuilder builder, SqliteAuditWriterOptions options)
     {
-        builder.RequireThat(!string.IsNullOrWhiteSpace(options.DatabasePath),
-            $"AuditLog:SiteWriter:{nameof(SqliteAuditWriterOptions.DatabasePath)} must be a non-empty path.");
-
         builder.RequireThat(options.ChannelCapacity > 0,
             $"AuditLog:SiteWriter:{nameof(SqliteAuditWriterOptions.ChannelCapacity)} " +
             $"({options.ChannelCapacity}) must be > 0.");

src/ZB.MOM.WW.ScadaBridge.Host/StartupValidator.cs

@@ -159,6 +159,28 @@ public static class StartupValidator
                     + "endpoint under ScadaBridge:Communication:CentralGrpcEndpoints "
                     + "(e.g. http://scadabridge-central-a:8083). Central nodes leave it empty.");
 
+                // The site hot-path audit writer's SQLite file (arch-review remediation
+                // WP1.2). SqliteAuditWriterOptions.DatabasePath has no default (mirrors
+                // LocalDb:Path) — an unset value would previously fall back to a bare
+                // "auditlog.db" resolved relative to the process CWD, which on the docker
+                // rig is the container's ephemeral overlayfs, not the mounted /app/data
+                // volume: the file, and every pending (not-yet-forwarded) audit row in it,
+                // was silently discarded on every container recreate. AddAuditLog binds
+                // this options type on BOTH roles (SqliteAuditWriterOptionsValidator
+                // deliberately does not check DatabasePath there, so Central's boot is
+                // unaffected — see that validator's remarks), so the Site-only requirement
+                // lives here, the same way GrpcPsk is gated just above. NOTE: unlike every
+                // other key in this method, AuditLog:SiteWriter is a TOP-LEVEL config
+                // section (AddAuditLog binds "AuditLog:SiteWriter" off the configuration
+                // root, not "ScadaBridge:AuditLog:SiteWriter") — no ScadaBridge: prefix.
+                p.Require("AuditLog:SiteWriter:DatabasePath",
+                    value => !string.IsNullOrWhiteSpace(value),
+                    "is required for Site nodes: the SQLite hot-path audit writer has no "
+                    + "default path (mirrors LocalDb:Path) — an unset value would silently "
+                    + "resolve to a CWD-relative file on the container's ephemeral overlayfs "
+                    + "and lose the pending audit backlog on every redeploy. Point it at the "
+                    + "mounted data volume, e.g. /app/data/auditlog.db.");
+
                 // ScadaBridge:Database:SiteDbPath was required here until LocalDb
                 // Phase 2. The site's tables now live in the consolidated LocalDb
                 // database (LocalDb:Path, which SiteServiceRegistration requires),

src/ZB.MOM.WW.ScadaBridge.Host/appsettings.Site.json

@@ -80,5 +80,14 @@
   // fails to boot, so every site config must set it.
   "LocalDb": {
     "Path": "./data/site-localdb.db"
+  },
+  // arch-review remediation WP1.2: the site hot-path audit writer's SQLite file has no
+  // default path (mirrors LocalDb:Path above) - AuditLog:SiteWriter:DatabasePath is
+  // REQUIRED and validated pre-host on Site nodes (StartupValidator), the same way
+  // LocalDb:Path is, so every site config must set it.
+  "AuditLog": {
+    "SiteWriter": {
+      "DatabasePath": "./data/auditlog.db"
+    }
   }
 }

tests/ZB.MOM.WW.ScadaBridge.AuditLog.Tests/Site/SqliteAuditWriterOptionsValidatorTests.cs

@@ -5,30 +5,30 @@ namespace ZB.MOM.WW.ScadaBridge.AuditLog.Tests.Site;
 /// <summary>
 /// Eager startup validation for <see cref="SqliteAuditWriterOptions"/>
 /// (arch-review 08 round 2 NF4). The site hot-path writer's channel/batch/flush
-/// knobs must be positive or the background writer task cannot make progress;
-/// an empty <c>DatabasePath</c> would leave the SQLite writer with nowhere to go.
+/// knobs must be positive or the background writer task cannot make progress.
 /// <c>BacklogPollIntervalSeconds</c> is deliberately NOT validated — a
 /// non-positive value has a documented fall-back-to-30s contract.
 /// </summary>
+/// <remarks>
+/// <c>DatabasePath</c> is intentionally NOT covered here (arch-review remediation
+/// WP1.2): it defaults to <c>""</c> (no CWD-relative default — mirrors
+/// <c>LocalDb:Path</c>), but this validator runs on both Central and Site
+/// composition roots via <c>ValidateOnStart</c>, and only Site nodes ever resolve
+/// the writer. The Site-only requirement is enforced pre-host by
+/// <c>StartupValidator</c> in ZB.MOM.WW.ScadaBridge.Host — see
+/// <c>StartupValidatorTests.SiteWithoutAuditLogDatabasePath_FailsValidation</c>.
+/// </remarks>
 public class SqliteAuditWriterOptionsValidatorTests
 {
     [Fact]
     public void DefaultOptions_AreValid()
     {
+        // Includes the empty DatabasePath default — this validator does not check
+        // it (see class remarks); only StartupValidator does, and only for Site nodes.
         var validator = new SqliteAuditWriterOptionsValidator();
         Assert.True(validator.Validate(null, new SqliteAuditWriterOptions()).Succeeded);
     }
 
-    [Fact]
-    public void EmptyDatabasePath_Fails()
-    {
-        var validator = new SqliteAuditWriterOptionsValidator();
-        var result = validator.Validate(null, new SqliteAuditWriterOptions { DatabasePath = "" });
-        Assert.False(result.Succeeded);
-        Assert.Contains(result.Failures!,
-            f => f.Contains(nameof(SqliteAuditWriterOptions.DatabasePath), StringComparison.Ordinal));
-    }
-
     [Fact]
     public void ZeroChannelCapacity_Fails()
     {

tests/ZB.MOM.WW.ScadaBridge.AuditLog.Tests/Site/SqliteAuditWriterWriteTests.cs

@@ -24,7 +24,8 @@ public class SqliteAuditWriterWriteTests
     private static (SqliteAuditWriter writer, string dataSource) CreateWriter(
         string testName,
         int? channelCapacity = null,
-        INodeIdentityProvider? nodeIdentity = null)
+        INodeIdentityProvider? nodeIdentity = null,
+        int? flushIntervalMs = null)
     {
         var dataSource = $"file:{testName}-{Guid.NewGuid():N}?mode=memory&cache=shared";
         var opts = new SqliteAuditWriterOptions { DatabasePath = dataSource };
@@ -32,6 +33,10 @@ public class SqliteAuditWriterWriteTests
         {
             opts.ChannelCapacity = cap;
         }
+        if (flushIntervalMs is int flush)
+        {
+            opts.FlushIntervalMs = flush;
+        }
 
         // Default identity provider returns null — existing tests pre-date
         // SourceNode stamping and have no expectation about it. New stamping
@@ -181,6 +186,69 @@ public class SqliteAuditWriterWriteTests
         Assert.Equal(1000, Convert.ToInt64(sidecarCmd.ExecuteScalar()));
     }
 
+    // ----- FlushIntervalMs soft-flush coalescing (arch-review remediation WP1.2) ----- //
+
+    /// <summary>
+    /// A trickle of events — well under <see cref="SqliteAuditWriterOptions.BatchSize"/>,
+    /// spaced closer together than <see cref="SqliteAuditWriterOptions.FlushIntervalMs"/> —
+    /// must coalesce into ONE transaction rather than fsyncing once per event. This is the
+    /// bug WP1.2 fixes: <c>FlushIntervalMs</c> was validated but never read by the writer
+    /// loop, so every trickle-rate write incurred its own commit (and, under SQLite's
+    /// default synchronous=FULL, its own fsync).
+    /// </summary>
+    [Fact]
+    public async Task WriteAsync_TrickleLoad_CoalescesIntoOneTransaction()
+    {
+        var (writer, dataSource) = CreateWriter(
+            nameof(WriteAsync_TrickleLoad_CoalescesIntoOneTransaction),
+            flushIntervalMs: 500);
+        await using var _ = writer;
+
+        const int count = 5;
+        var writeTasks = new List<Task>(count);
+        for (int i = 0; i < count; i++)
+        {
+            writeTasks.Add(writer.WriteAsync(NewEvent()));
+            await Task.Delay(15); // spaced well under the 500ms flush window
+        }
+        await Task.WhenAll(writeTasks);
+
+        using var connection = OpenVerifierConnection(dataSource);
+        using var cmd = connection.CreateCommand();
+        cmd.CommandText = "SELECT COUNT(*) FROM audit_event;";
+        Assert.Equal(count, Convert.ToInt64(cmd.ExecuteScalar()));
+
+        // The defining assertion: all 5 trickled-in writes landed in a single
+        // FlushBatch transaction, not up to 5 separate commits.
+        Assert.Equal(1, writer.FlushCountForTests);
+    }
+
+    /// <summary>
+    /// The flip side: two events spaced FURTHER apart than
+    /// <see cref="SqliteAuditWriterOptions.FlushIntervalMs"/> must NOT be held open
+    /// waiting for a partner — the deadline bounds worst-case added latency, it does
+    /// not turn into an unbounded debounce.
+    /// </summary>
+    [Fact]
+    public async Task WriteAsync_EventsSpacedBeyondFlushInterval_FlushSeparately()
+    {
+        var (writer, dataSource) = CreateWriter(
+            nameof(WriteAsync_EventsSpacedBeyondFlushInterval_FlushSeparately),
+            flushIntervalMs: 50);
+        await using var _ = writer;
+
+        await writer.WriteAsync(NewEvent());
+        await Task.Delay(300); // well beyond the 50ms flush window
+        await writer.WriteAsync(NewEvent());
+
+        using var connection = OpenVerifierConnection(dataSource);
+        using var cmd = connection.CreateCommand();
+        cmd.CommandText = "SELECT COUNT(*) FROM audit_event;";
+        Assert.Equal(2, Convert.ToInt64(cmd.ExecuteScalar()));
+
+        Assert.Equal(2, writer.FlushCountForTests);
+    }
+
     [Fact]
     public async Task WriteAsync_DuplicateEventId_FirstWriteWins_NoException()
     {

tests/ZB.MOM.WW.ScadaBridge.Host.Tests/SiteHealthEndpointTests.cs

@@ -27,10 +27,12 @@ public class SiteHealthEndpointTests : IDisposable
     private readonly List<IDisposable> _disposables = new();
     private readonly Dictionary<string, string?> _previousEnv = new(StringComparer.Ordinal);
     private readonly string _tempDbPath;
+    private readonly string _tempAuditDbPath;
 
     public SiteHealthEndpointTests()
     {
         _tempDbPath = Path.Combine(Path.GetTempPath(), $"scadabridge_health_ep_{Guid.NewGuid()}.db");
+        _tempAuditDbPath = Path.Combine(Path.GetTempPath(), $"scadabridge_health_ep_audit_{Guid.NewGuid()}.db");
 
         // Whole-key env overrides, the sanctioned path: supplying GrpcPsk concretely makes the
         // pre-host secrets expander skip appsettings.Site.json's ${secret:SB-GRPC-PSK-site-1}
@@ -50,6 +52,11 @@ public class SiteHealthEndpointTests : IDisposable
         SetEnv("ScadaBridge__Cluster__SeedNodes__1", "akka.tcp://scadabridge@localhost:18085");
         SetEnv("ScadaBridge__Communication__GrpcPsk", "test-psk-0123456789");
         SetEnv("LocalDb__Path", _tempDbPath);
+        // arch-review remediation WP1.2: SqliteAuditWriterOptions.DatabasePath has no default
+        // and StartupValidator now requires it for Site nodes — same reason LocalDb__Path is
+        // overridden above (appsettings.Site.json's own default is CWD-relative and would
+        // otherwise litter the test working directory when SiteAuditBacklogReporter probes it).
+        SetEnv("AuditLog__SiteWriter__DatabasePath", _tempAuditDbPath);
     }
 
     private void SetEnv(string key, string? value)
@@ -71,6 +78,7 @@ public class SiteHealthEndpointTests : IDisposable
         }
 
         try { File.Delete(_tempDbPath); } catch { /* best effort */ }
+        try { File.Delete(_tempAuditDbPath); } catch { /* best effort */ }
 
         GC.SuppressFinalize(this);
     }

tests/ZB.MOM.WW.ScadaBridge.Host.Tests/StartupValidatorTests.cs

@@ -44,6 +44,10 @@ public class StartupValidatorTests
         // Phase 4: gRPC (CentralControlService) is the only site→central transport, so a Site
         // node must list at least one central gRPC endpoint to dial (no Akka fallback remains).
         ["ScadaBridge:Communication:CentralGrpcEndpoints:0"] = "http://central-a:8083",
+        // arch-review remediation WP1.2: the site hot-path audit writer's SQLite file has no
+        // default path (mirrors LocalDb:Path) — required so it lands on the mounted data volume,
+        // not the container's ephemeral overlayfs.
+        ["AuditLog:SiteWriter:DatabasePath"] = "/app/data/auditlog.db",
     };
 
     [Fact]
@@ -124,6 +128,43 @@ public class StartupValidatorTests
         Assert.Null(Record.Exception(() => StartupValidator.Validate(config)));
     }
 
+    [Fact]
+    public void SiteWithoutAuditLogDatabasePath_FailsValidation()
+    {
+        // arch-review remediation WP1.2: SqliteAuditWriterOptions.DatabasePath has no default
+        // (mirrors LocalDb:Path). An unset value used to fall back to a bare "auditlog.db"
+        // resolved relative to CWD — on the docker rig, the container's ephemeral overlayfs —
+        // so the node booted, looked healthy, and lost its pending audit backlog on every
+        // recreate. Fail fast at boot instead.
+        var values = ValidSiteConfig();
+        values.Remove("AuditLog:SiteWriter:DatabasePath");
+        var config = BuildConfig(values);
+
+        var ex = Assert.Throws<InvalidOperationException>(() => StartupValidator.Validate(config));
+        Assert.Contains("DatabasePath", ex.Message);
+    }
+
+    [Fact]
+    public void SiteWithBlankAuditLogDatabasePath_FailsValidation()
+    {
+        var values = ValidSiteConfig();
+        values["AuditLog:SiteWriter:DatabasePath"] = "   ";
+        var config = BuildConfig(values);
+
+        var ex = Assert.Throws<InvalidOperationException>(() => StartupValidator.Validate(config));
+        Assert.Contains("DatabasePath", ex.Message);
+    }
+
+    [Fact]
+    public void CentralWithoutAuditLogDatabasePath_PassesValidation()
+    {
+        // AddAuditLog binds SqliteAuditWriterOptions on both roles, but only Site nodes ever
+        // resolve the writer — the requirement must not fire for Central.
+        var config = BuildConfig(ValidCentralConfig());
+
+        Assert.Null(Record.Exception(() => StartupValidator.Validate(config)));
+    }
+
     [Fact]
     public void MissingRole_FailsValidation()
     {