natsdotnet/differences.md

# Go vs .NET NATS Server: Functionality Differences

> Excludes clustering/routes, gateways, leaf nodes, and JetStream.
> Generated 2026-02-22 by comparing `golang/nats-server/server/` against `src/NATS.Server/`.

---

## 1. Core Server Lifecycle

### Server Initialization
| Feature | Go | .NET | Notes |
|---------|:--:|:----:|-------|
| NKey generation (server identity) | Y | Y | Ed25519 key pair via NATS.NKeys at startup |
| System account setup | Y | Y | `$SYS` account created; no event publishing yet (stub) |
| Config file validation on startup | Y | Stub | `-c` flag parsed, `ConfigFile` stored, but no config parser |
| PID file writing | Y | Y | Written on startup, deleted on shutdown |
| Profiling HTTP endpoint (`/debug/pprof`) | Y | Stub | `ProfPort` option exists but endpoint not implemented |
| Ports file output | Y | Y | JSON ports file written to `PortsFileDir` on startup |

### Accept Loop
| Feature | Go | .NET | Notes |
|---------|:--:|:----:|-------|
| Exponential backoff on accept errors | Y | Y | .NET backs off from 10ms to 1s on repeated failures |
| Config reload lock during client creation | Y | N | Go holds `reloadMu` around `createClient` |
| Goroutine/task tracking (WaitGroup) | Y | Y | `Interlocked` counter + drain with 10s timeout on shutdown |
| Callback-based error handling | Y | N | Go uses `errFunc` callback pattern |
| Random/ephemeral port (port=0) | Y | Y | Port resolved after `Bind`+`Listen`, stored in `_options.Port` |

### Shutdown
| Feature | Go | .NET | Notes |
|---------|:--:|:----:|-------|
| Graceful shutdown with `WaitForShutdown()` | Y | Y | Idempotent CAS-guarded `ShutdownAsync()` + blocking `WaitForShutdown()` |
| Close reason tracking per connection | Y | Y | 37-value `ClosedState` enum, CAS-based first-writer-wins `MarkClosed()` |
| Lame duck mode (stop new, drain existing) | Y | Y | `LameDuckShutdownAsync()` with grace period + stagger-close with jitter |
| Wait for accept loop completion | Y | Y | `TaskCompletionSource` signaled in accept loop `finally` |
| Flush pending data before close | Y | Y | `FlushAndCloseAsync()` with best-effort flush, skip-flush for error conditions |

### Signal Handling
| Signal | Go | .NET | Notes |
|--------|:--:|:----:|-------|
| SIGINT (Ctrl+C) | Y | Y | Both handle graceful shutdown |
| SIGTERM | Y | Y | `PosixSignalRegistration` triggers `ShutdownAsync()` |
| SIGUSR1 (reopen logs) | Y | Stub | Signal registered, handler logs "not yet implemented" |
| SIGUSR2 (lame duck mode) | Y | Y | Triggers `LameDuckShutdownAsync()` |
| SIGHUP (config reload) | Y | Stub | Signal registered, handler logs "not yet implemented" |
| Windows Service integration | Y | N | |

---

## 2. Client / Connection Handling

### Concurrency Model
| Feature | Go | .NET | Notes |
|---------|:--:|:----:|-------|
| Separate read + write loops | Y | Y | Channel-based `RunWriteLoopAsync` with `QueueOutbound()` |
| Write coalescing / batch flush | Y | Y | Write loop drains all channel items before single `FlushAsync` |
| Dynamic buffer sizing (512B-64KB) | Y | N | .NET delegates to `System.IO.Pipelines` |
| Output buffer pooling (3-tier) | Y | N | Go pools at 512B, 4KB, 64KB |

### Connection Types
| Type | Go | .NET | Notes |
|------|:--:|:----:|-------|
| CLIENT | Y | Y | |
| ROUTER | Y | N | Excluded per scope |
| GATEWAY | Y | N | Excluded per scope |
| LEAF | Y | N | Excluded per scope |
| SYSTEM (internal) | Y | N | |
| JETSTREAM (internal) | Y | N | |
| ACCOUNT (internal) | Y | N | |
| WebSocket clients | Y | N | |
| MQTT clients | Y | N | |

### Client Features
| Feature | Go | .NET | Notes |
|---------|:--:|:----:|-------|
| Echo suppression (`echo: false`) | Y | Y | .NET checks echo in delivery path (NatsServer.cs:234,253) |
| Verbose mode (`+OK` responses) | Y | Y | Sends `+OK` after CONNECT, SUB, UNSUB, PUB when `verbose:true` |
| No-responders validation | Y | Y | CONNECT rejects `no_responders` without `headers`; 503 HMSG on no match |
| Slow consumer detection | Y | Y | Pending bytes threshold (64MB) + write deadline timeout (10s) |
| Write deadline / timeout policies | Y | Y | `WriteDeadline` option with `CancellationTokenSource.CancelAfter` on flush |
| RTT measurement | Y | N | Go tracks round-trip time per client |
| Per-client trace mode | Y | N | |
| Detailed close reason tracking | Y | Y | 37-value `ClosedState` enum with CAS-based `MarkClosed()` |
| Connection state flags (16 flags) | Y | Y | 7-flag `ClientFlagHolder` with `Interlocked.Or`/`And` |

### Slow Consumer Handling
Go implements a sophisticated slow consumer detection system:
- Tracks `pendingBytes` per client output buffer
- If pending exceeds `maxPending`, enters stall mode (2-5ms waits)
- Total stall capped at 10ms per read cycle
- Closes with `SlowConsumerPendingBytes` or `SlowConsumerWriteDeadline`
- Sets `isSlowConsumer` flag for monitoring

.NET now implements pending bytes tracking and write deadline enforcement via `Channel<ReadOnlyMemory<byte>>`. Key differences from Go: no stall/retry mode (immediate close on threshold exceeded), write deadline via `CancellationTokenSource.CancelAfter` instead of `SetWriteDeadline`. `IsSlowConsumer` flag and server-level `SlowConsumerCount` stats are tracked for monitoring.

### Stats Tracking
| Feature | Go | .NET | Notes |
|---------|:--:|:----:|-------|
| Per-connection atomic stats | Y | Y | .NET uses `Interlocked` for stats access |
| Per-read-cycle stat batching | Y | Y | Local accumulators flushed via `Interlocked.Add` per read cycle |
| Per-account stats | Y | N | |
| Slow consumer counters | Y | Y | `SlowConsumers` and `SlowConsumerClients` incremented on detection |

---

## 3. Protocol Parsing

### Parser Architecture
| Aspect | Go | .NET |
|--------|-----|------|
| Approach | Byte-by-byte state machine (74 states) | Two-phase: line extraction + command dispatch |
| Case handling | Per-state character checks | Bit-mask lowercase normalization (`\| 0x20`) |
| Buffer strategy | Jump-ahead optimization for payloads | Direct size-based reads via Pipe |
| Split-buffer handling | argBuf accumulation with scratch buffer | State variables (`_awaitingPayload`, etc.) |
| Error model | Inline error sending + error return | Exception-based (`ProtocolViolationException`) |
| CRLF in payload | Included in message buffer | Excluded by design |

### Protocol Operations
| Operation | Go | .NET | Notes |
|-----------|:--:|:----:|-------|
| PUB | Y | Y | |
| HPUB (headers) | Y | Y | |
| SUB | Y | Y | |
| UNSUB | Y | Y | |
| CONNECT | Y | Y | |
| INFO | Y | Y | |
| PING / PONG | Y | Y | |
| MSG / HMSG | Y | Y | |
| +OK / -ERR | Y | Y | |
| RS+/RS-/RMSG (routes) | Y | N | Excluded per scope |
| A+/A- (accounts) | Y | N | Excluded per scope |
| LS+/LS-/LMSG (leaf) | Y | N | Excluded per scope |

### Protocol Parsing Gaps
| Feature | Go | .NET | Notes |
|---------|:--:|:----:|-------|
| Multi-client-type command routing | Y | N | Go checks `c.kind` to allow/reject commands |
| Protocol tracing in parser | Y | Y | `TraceInOp()` logs `<<- OP arg` at `LogLevel.Trace` via optional `ILogger` |
| Subject mapping (input→output) | Y | N | Go transforms subjects via mapping rules |
| MIME header parsing | Y | Y | `NatsHeaderParser.Parse()` — status line + key-value headers from `ReadOnlySpan<byte>` |
| Message trace event initialization | Y | N | |

### Protocol Writing
| Aspect | Go | .NET | Notes |
|--------|:--:|:----:|-------|
| INFO serialization | Once at startup | Once at startup | Cached at startup; nonce connections serialize per-connection |
| MSG/HMSG construction | Direct buffer write | Span-based buffer write | `int.TryFormat` + `CopyTo` into rented buffer, no string allocations |
| Pre-encoded constants | Y | Y | Both pre-encode PING/PONG/OK |

---

## 4. Subscriptions & Subject Matching

### Trie Implementation
| Feature | Go | .NET | Notes |
|---------|:--:|:----:|-------|
| Basic trie with `*`/`>` wildcards | Y | Y | Core matching identical |
| Queue group support | Y | Y | |
| Result caching (1024 max) | Y | Y | Same limits |
| `plist` optimization (>256 subs) | Y | N | Go converts high-fanout nodes to array |
| Async cache sweep (background) | Y | N | .NET sweeps inline under write lock |
| Atomic generation ID for invalidation | Y | Y | `Interlocked.Increment` on insert/remove; cached results store generation |
| Cache eviction strategy | Random | First-N | Semantic difference minimal |

### SubList Features
| Feature | Go | .NET | Notes |
|---------|:--:|:----:|-------|
| `Stats()` — comprehensive statistics | Y | Y | Matches, cache hits, inserts, removes tracked via `Interlocked` |
| `HasInterest()` — fast bool check | Y | Y | Walks trie without allocating result list |
| `NumInterest()` — fast count | Y | Y | Counts plain + queue subs without allocation |
| `ReverseMatch()` — pattern→literal query | Y | Y | Finds subscriptions whose wildcards match a literal subject |
| `RemoveBatch()` — efficient bulk removal | Y | Y | Single generation increment for batch; increments `_removes` per sub |
| `All()` — enumerate all subscriptions | Y | Y | Recursive trie walk returning all subscriptions |
| Notification system (interest changes) | Y | N | |
| Local/remote subscription filtering | Y | N | |
| Queue weight expansion (remote subs) | Y | N | |
| `MatchBytes()` — zero-copy byte API | Y | N | |

### Subject Validation
| Feature | Go | .NET | Notes |
|---------|:--:|:----:|-------|
| Basic validation (empty tokens, wildcards) | Y | Y | |
| Literal subject check | Y | Y | |
| UTF-8/null rune validation | Y | Y | `IsValidSubject(string, bool checkRunes)` rejects null bytes |
| Collision detection (`SubjectsCollide`) | Y | Y | Token-by-token wildcard comparison; O(n) via upfront `Split` |
| Token utilities (`tokenAt`, `numTokens`) | Y | Y | `TokenAt` returns `ReadOnlySpan<char>`; `NumTokens` counts separators |
| Stack-allocated token buffer | Y | N | Go uses `[32]string{}` on stack |

### Subscription Lifecycle
| Feature | Go | .NET | Notes |
|---------|:--:|:----:|-------|
| Per-account subscription limit | Y | Y | `Account.IncrementSubscriptions()` returns false when `MaxSubscriptions` exceeded |
| Auto-unsubscribe on max messages | Y | Y | Enforced at delivery; sub removed from trie + client dict when exhausted |
| Subscription routing propagation | Y | N | For clusters |
| Queue weight (`qw`) field | Y | N | For remote queue load balancing |

---

## 5. Authentication & Authorization

### Auth Mechanisms
| Mechanism | Go | .NET | Notes |
|-----------|:--:|:----:|-------|
| Username/password | Y | Y | |
| Token | Y | Y | |
| NKeys (Ed25519) | Y | Y | .NET has framework but integration is basic |
| JWT validation | Y | N | |
| Bcrypt password hashing | Y | Y | .NET supports bcrypt (`$2*` prefix) with constant-time fallback |
| TLS certificate mapping | Y | N | Property exists but no implementation |
| Custom auth interface | Y | N | |
| External auth callout | Y | N | |
| Proxy authentication | Y | N | |
| Bearer tokens | Y | N | |
| User revocation tracking | Y | N | |

### Account System
| Feature | Go | .NET | Notes |
|---------|:--:|:----:|-------|
| Per-account SubList isolation | Y | Y | |
| Multi-account user resolution | Y | Y | `AccountConfig` per account in `NatsOptions.Accounts`; `GetOrCreateAccount` wires limits |
| Account exports/imports | Y | N | |
| Per-account connection limits | Y | Y | `Account.AddClient()` returns false when `MaxConnections` exceeded |
| Per-account subscription limits | Y | Y | `Account.IncrementSubscriptions()` enforced in `ProcessSub()` |
| Account JetStream limits | Y | N | Excluded per scope |

### Permissions
| Feature | Go | .NET | Notes |
|---------|:--:|:----:|-------|
| Publish allow list | Y | Y | |
| Subscribe allow list | Y | Y | |
| Publish deny list | Y | Y | Full enforcement with LRU-cached results |
| Subscribe deny list | Y | Y | Queue-aware deny checking in `IsSubscribeAllowed` |
| Message-level deny filtering | Y | Y | `IsDeliveryAllowed()` checked before MSG send; auto-unsub cleanup on deny |
| Permission caching (128 entries) | Y | Y | `PermissionLruCache` — Dictionary+LinkedList LRU, matching Go's `maxPermCacheSize` |
| Response permissions (reply tracking) | Y | Y | `ResponseTracker` with configurable TTL + max messages; not LRU-cached |
| Auth expiry enforcement | Y | Y | `Task.Delay` timer closes client when JWT/auth expires |
| Permission templates (JWT) | Y | N | e.g., `{{name()}}`, `{{account-tag(...)}}` |

---

## 6. Configuration

### CLI Flags
| Flag | Go | .NET | Notes |
|------|:--:|:----:|-------|
| `-p/--port` | Y | Y | |
| `-a/--addr` | Y | Y | |
| `-n/--name` (ServerName) | Y | Y | |
| `-m/--http_port` (monitoring) | Y | Y | |
| `-c` (config file) | Y | Stub | Flag parsed, stored in `ConfigFile`, no config parser |
| `-D/-V/-DV` (debug/trace) | Y | Y | Sets `Debug`/`Trace` on `NatsOptions`, adjusts Serilog minimum level |
| `--tlscert/--tlskey/--tlscacert` | Y | Y | |
| `--tlsverify` | Y | Y | |
| `--http_base_path` | Y | Y | |
| `--https_port` | Y | Y | |

### Configuration System
| Feature | Go | .NET | Notes |
|---------|:--:|:----:|-------|
| Config file parsing | Y | N | Go has custom `conf` parser with includes |
| Hot reload (SIGHUP) | Y | N | |
| Config change detection | Y | N | Go tracks `inConfig`/`inCmdLine` origins |
| ~450 option fields | Y | ~62 | .NET covers core + debug/trace/logging/limits/tags options |

### Missing Options Categories
- ~~Logging options (file, rotation, syslog, trace levels)~~ — File logging (`-l`), `LogSizeLimit`, Debug/Trace implemented; syslog/color/timestamp not yet
- ~~Advanced limits (MaxSubs, MaxSubTokens, MaxPending, WriteDeadline)~~ — `MaxSubs`, `MaxSubTokens` implemented; MaxPending/WriteDeadline already existed
- ~~Tags/metadata~~ — `Tags` dictionary implemented in `NatsOptions`
- OCSP configuration
- WebSocket/MQTT options
- Operator mode / account resolver

---

## 7. Monitoring

### HTTP Endpoints
| Endpoint | Go | .NET | Notes |
|----------|:--:|:----:|-------|
| `/healthz` | Y | Y | |
| `/varz` | Y | Y | |
| `/connz` | Y | Y | |
| `/` (root listing) | Y | Y | |
| `/routez` | Y | Stub | Returns empty response |
| `/gatewayz` | Y | Stub | Returns empty response |
| `/leafz` | Y | Stub | Returns empty response |
| `/subz` / `/subscriptionsz` | Y | Stub | Returns empty response |
| `/accountz` | Y | Stub | Returns empty response |
| `/accstatz` | Y | Stub | Returns empty response |
| `/jsz` | Y | Stub | Returns empty response |

### Varz Response
| Field Category | Go | .NET | Notes |
|----------------|:--:|:----:|-------|
| Identity (ID, Name, Version) | Y | Y | |
| Network (Host, Port, URLs) | Y | Y | |
| Security (AuthRequired, TLS) | Y | Y | |
| Limits (MaxConn, MaxPayload) | Y | Y | |
| Timing (Start, Now, Uptime) | Y | Y | |
| Runtime (Mem, CPU, Cores) | Y | Y | |
| Connections (current, total) | Y | Y | |
| Messages (in/out msgs/bytes) | Y | Y | |
| SlowConsumer breakdown | Y | N | Go tracks per connection type |
| Cluster/Gateway/Leaf blocks | Y | N | Excluded per scope |
| JetStream block | Y | N | Excluded per scope |
| TLS cert expiry info | Y | N | |

### Connz Response
| Feature | Go | .NET | Notes |
|---------|:--:|:----:|-------|
| Filtering by CID, user, account | Y | Partial | |
| Sorting (11 options) | Y | Y | .NET missing ByStop, ByReason |
| Pagination (offset, limit) | Y | Y | |
| Subscription detail mode | Y | N | |
| TLS peer certificate info | Y | N | |
| JWT/IssuerKey/Tags fields | Y | N | |
| MQTT client ID filtering | Y | N | |
| Proxy info | Y | N | |

---

## 8. TLS

### TLS Modes
| Mode | Go | .NET | Notes |
|------|:--:|:----:|-------|
| No TLS | Y | Y | |
| INFO-first (default NATS) | Y | Y | |
| TLS-first (before INFO) | Y | Y | |
| Mixed/Fallback | Y | Y | |
| TLS-required | Y | Y | |

### TLS Features
| Feature | Go | .NET | Notes |
|---------|:--:|:----:|-------|
| PEM cert/key loading | Y | Y | |
| CA chain validation | Y | Y | |
| Mutual TLS (client certs) | Y | Y | |
| Certificate pinning (SHA256 SPKI) | Y | Y | |
| TLS handshake timeout | Y | Y | |
| TLS rate limiting | Y | Property only | .NET has the option but enforcement is partial |
| First-byte peeking (0x16 detection) | Y | Y | |
| Cert subject→user mapping | Y | N | `TlsMap` property exists, no implementation |
| OCSP stapling | Y | N | |
| Min TLS version control | Y | Y | |

---

## 9. Logging

| Feature | Go | .NET | Notes |
|---------|:--:|:----:|-------|
| Structured logging | Partial | Y | .NET uses Serilog with ILogger<T> |
| File logging with rotation | Y | Y | `-l` flag + `LogSizeLimit` option via Serilog.Sinks.File with `fileSizeLimitBytes` |
| Syslog (local and remote) | Y | N | |
| Log reopening (SIGUSR1) | Y | N | |
| Trace mode (protocol-level) | Y | Y | `-V`/`-DV` flags; parser `TraceInOp()` logs `<<- OP arg` at Trace level |
| Debug mode | Y | Y | `-D`/`-DV` flags lower Serilog minimum to Debug/Verbose |
| Per-subsystem log control | Y | N | |
| Color output on TTY | Y | N | |
| Timestamp format control | Y | N | |

---

## 10. Ping/Pong & Keepalive

| Feature | Go | .NET | Notes |
|---------|:--:|:----:|-------|
| Server-initiated PING | Y | Y | |
| Configurable interval | Y | Y | PingInterval option |
| Max pings out | Y | Y | MaxPingsOut option |
| Stale connection close | Y | Y | |
| RTT-based first PING delay | Y | N | Go delays first PING based on RTT |
| RTT tracking | Y | N | |
| Stale connection watcher | Y | N | Go has dedicated watcher goroutine |

---

## Summary: Critical Gaps for Production Use

### High Priority
1. ~~**Slow consumer detection**~~ — implemented (pending bytes threshold + write deadline)
2. ~~**Write coalescing / batch flush**~~ — implemented (channel-based write loop)

### Medium Priority
3. ~~**Verbose mode**~~ — implemented (`+OK` on CONNECT/SUB/UNSUB/PUB)
4. ~~**Permission deny enforcement at delivery**~~ — implemented (`IsDeliveryAllowed` + auto-unsub cleanup)
5. **Config file parsing** — needed for production deployment (CLI stub exists)
6. **Hot reload** — needed for zero-downtime config changes (SIGHUP stub exists)
7. ~~**File logging with rotation**~~ — implemented (Serilog.Sinks.File with `-l` flag)
8. ~~**No-responders validation**~~ — implemented (CONNECT validation + 503 HMSG)

### Lower Priority
9. **Dynamic buffer sizing** — delegated to Pipe, less optimized for long-lived connections
10. **JWT authentication** — needed for operator mode
11. **TLS certificate mapping** — property exists, not implemented
12. **OCSP support** — certificate revocation checking
13. **Subject mapping** — input→output subject transformation
14. ~~**Protocol tracing**~~ — implemented (`TraceInOp` at `LogLevel.Trace`)
15. ~~**Subscription statistics**~~ — implemented (`Stats()`, `HasInterest()`, `NumInterest()`, etc.)
16. ~~**Per-account limits**~~ — implemented (connection + subscription limits via `AccountConfig`)
17. ~~**Reply subject tracking**~~ — implemented (`ResponseTracker` with TTL + max messages)
18. **Windows Service integration** — needed for Windows deployment