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+# Core Server Lifecycle — Design
+
+Implements all gaps from section 1 of `differences.md` (Core Server Lifecycle).
+
+Reference: `golang/nats-server/server/server.go`, `client.go`, `signal.go`
+
+## Components
+
+### 1. ClosedState Enum & Close Reason Tracking
+
+New file `src/NATS.Server/ClosedState.cs` — full Go enum (37 values from `client.go:188-228`).
+
+- `NatsClient` gets `CloseReason` property, `MarkClosed(ClosedState)` method
+- Close reason set in `RunAsync` finally blocks based on exception type
+- Error-related reasons (ReadError, WriteError, TLSHandshakeError) skip flush on close
+- `NatsServer.RemoveClient` logs close reason via structured logging
+
+### 2. Accept Loop Exponential Backoff
+
+Port Go's `acceptError` pattern from `server.go:4607-4627`.
+
+- Constants: `AcceptMinSleep = 10ms`, `AcceptMaxSleep = 1s`
+- On `SocketException`: sleep `tmpDelay`, double it, cap at 1s
+- On success: reset to 10ms
+- During sleep: check `_quitCts` to abort if shutting down
+- Non-temporary errors break the loop
+
+### 3. Ephemeral Port (port=0)
+
+After `_listener.Bind()` + `Listen()`, resolve actual port:
+
+```csharp
+if (_options.Port == 0)
+{
+    var actualPort = ((IPEndPoint)_listener.LocalEndPoint!).Port;
+    _options.Port = actualPort;
+    _serverInfo.Port = actualPort;
+}
+```
+
+Add public `Port` property on `NatsServer` exposing the resolved port.
+
+### 4. Graceful Shutdown with WaitForShutdown
+
+New fields on `NatsServer`:
+- `_shutdown` (volatile bool)
+- `_shutdownComplete` (TaskCompletionSource)
+- `_quitCts` (CancellationTokenSource) — internal shutdown signal
+
+`ShutdownAsync()` sequence:
+1. Guard: if already shutting down, return
+2. Set `_shutdown = true`, cancel `_quitCts`
+3. Close `_listener` (stops accept loop)
+4. Close all client connections with `ServerShutdown` reason
+5. Wait for active client tasks to drain
+6. Stop monitor server
+7. Signal `_shutdownComplete`
+
+`WaitForShutdown()`: blocks on `_shutdownComplete.Task`.
+
+`Dispose()`: calls `ShutdownAsync` synchronously if not already shut down.
+
+### 5. Task Tracking
+
+Track active client tasks for clean shutdown:
+- `_activeClientCount` (int, Interlocked)
+- `_allClientsExited` (TaskCompletionSource, signaled when count hits 0 during shutdown)
+- Increment in `AcceptClientAsync`, decrement in `RunClientAsync` finally block
+- `ShutdownAsync` waits on `_allClientsExited` with timeout
+
+### 6. Flush Pending Data Before Close
+
+`NatsClient.FlushAndCloseAsync(bool minimalFlush)`:
+- If not skip-flush reason: flush stream with 100ms write deadline
+- Close socket
+
+`MarkClosed(ClosedState)` sets skip-flush flag for: ReadError, WriteError, SlowConsumerPendingBytes, SlowConsumerWriteDeadline, TLSHandshakeError.
+
+### 7. Lame Duck Mode
+
+New options: `LameDuckDuration` (default 2min), `LameDuckGracePeriod` (default 10s).
+
+`LameDuckShutdownAsync()`:
+1. Set `_lameDuckMode = true`
+2. Close listener (stop new connections)
+3. Wait `LameDuckGracePeriod` (10s default) for clients to drain naturally
+4. Stagger-close remaining clients over `LameDuckDuration - GracePeriod`
+   - Sleep interval = remaining duration / client count (min 1ms, max 1s)
+   - Randomize slightly to avoid reconnect storms
+5. Call `ShutdownAsync()` for final cleanup
+
+Accept loop: on error, if `_lameDuckMode`, exit cleanly.
+
+### 8. PID File & Ports File
+
+New options: `PidFile` (string?), `PortsFileDir` (string?).
+
+PID file: `File.WriteAllText(pidFile, Process.GetCurrentProcess().Id.ToString())`
+Ports file: JSON with `{ "client": port, "monitor": monitorPort }` written to `{dir}/{exe}_{pid}.ports`
+
+Written at startup, deleted at shutdown.
+
+### 9. Signal Handling
+
+In `Program.cs`, use `PosixSignalRegistration` (.NET 6+):
+
+- `SIGTERM` → `server.ShutdownAsync()` then exit
+- `SIGUSR2` → `server.LameDuckShutdownAsync()`
+- `SIGUSR1` → log "log reopen not yet supported"
+- `SIGHUP` → log "config reload not yet supported"
+
+Keep existing Ctrl+C handler (SIGINT).
+
+### 10. Server Identity NKey (Stub)
+
+Generate Ed25519 key pair at construction. Store as `ServerNKey` (public) and `_serverSeed` (private). Not used in protocol yet — placeholder for future cluster identity.
+
+### 11. System Account (Stub)
+
+Create `$SYS` account in `_accounts` at construction. Expose as `SystemAccount` property. No internal subscriptions yet.
+
+### 12. Config File & Profiling (Stubs)
+
+- `NatsOptions.ConfigFile` — if set, log warning "config file parsing not yet supported"
+- `NatsOptions.ProfPort` — if set, log warning "profiling endpoint not yet supported"
+- `Program.cs`: add `-c` CLI flag
+
+## Testing
+
+- Accept loop backoff: mock socket that throws N times, verify delays
+- Ephemeral port: start server with port=0, verify resolved port > 0
+- Graceful shutdown: start server, connect clients, call ShutdownAsync, verify all disconnected
+- WaitForShutdown: verify it blocks until shutdown completes
+- Close reason tracking: verify correct ClosedState for auth timeout, max connections, stale connection
+- Lame duck mode: start server, connect clients, trigger lame duck, verify staggered closure
+- PID file: start server with PidFile option, verify file contents, verify deleted on shutdown
+- Ports file: start server with PortsFileDir, verify JSON contents
+- Flush before close: verify data is flushed before socket close during shutdown
+- System account: verify $SYS account exists after construction