# AGENTS.md

This file provides guidance to Codex (Codex.ai/code) when working with code in this repository.

## Project Overview

This project ports the [NATS server](https://github.com/nats-io/nats-server) from Go to .NET 10 / C#. The Go reference implementation lives in `golang/nats-server/`. The .NET port lives at the repository root.

NATS is a high-performance publish-subscribe messaging system. It supports wildcards (`*` single token, `>` multi-token), queue groups for load balancing, request-reply, clustering (full-mesh routes, gateways, leaf nodes), and persistent streaming via JetStream.

## Build & Test Commands

The solution file is `NatsDotNet.slnx`.

```bash
# Build the solution
dotnet build

# Run all tests
dotnet test

# Run tests with verbose output
dotnet test -v normal

# Run a single test project
dotnet test tests/NATS.Server.Tests

# Run a specific test project
dotnet test tests/NATS.Server.Core.Tests
dotnet test tests/NATS.Server.JetStream.Tests

# Run a specific test by name
dotnet test tests/NATS.Server.Core.Tests --filter "FullyQualifiedName~TestName"

# Run the NATS server (default port 4222)
dotnet run --project src/NATS.Server.Host

# Run the NATS server on a custom port
dotnet run --project src/NATS.Server.Host -- -p 14222

# Clean and rebuild
dotnet clean && dotnet build
```

## .NET Project Structure

```
NatsDotNet.slnx                           # Solution file
src/
  NATS.Server/                            # Core server library
    NatsServer.cs                         # Server: listener, accept loop, shutdown
    NatsClient.cs                         # Per-connection client: read/write loops, sub tracking
    NatsOptions.cs                        # Server configuration (port, host, etc.)
    Protocol/
      NatsParser.cs                       # Protocol state machine (PUB, SUB, UNSUB, etc.)
      NatsProtocol.cs                     # Wire-level protocol writing (INFO, MSG, PING/PONG)
    Subscriptions/
      SubjectMatch.cs                     # Subject validation and wildcard matching
      SubList.cs                          # Trie-based subscription list with caching
      SubListResult.cs                    # Match result container (plain subs + queue groups)
      Subscription.cs                     # Subscription model (subject, sid, queue, client)
  NATS.Server.Host/                       # Executable host app
    Program.cs                            # Entry point, CLI arg parsing (-p port)
tests/
  NATS.Server.TestUtilities/              # Shared helpers, fixtures, parity tools (class library)
  NATS.Server.Core.Tests/                 # Client, server, parser, config, subscriptions, protocol
  NATS.Server.Auth.Tests/                 # Auth, accounts, permissions, JWT, NKeys
  NATS.Server.JetStream.Tests/            # JetStream API, streams, consumers, storage, cluster
  NATS.Server.Raft.Tests/                 # RAFT consensus
  NATS.Server.Clustering.Tests/           # Routes, cluster topology, inter-server protocol
  NATS.Server.Gateways.Tests/             # Gateway connections, interest modes
  NATS.Server.LeafNodes.Tests/            # Leaf node connections, hub-spoke
  NATS.Server.Mqtt.Tests/                 # MQTT protocol bridge
  NATS.Server.Monitoring.Tests/           # Monitor endpoints, events, system events
  NATS.Server.Transport.Tests/            # WebSocket, TLS, OCSP, IO
  NATS.E2E.Tests/                         # End-to-end tests using NATS.Client.Core NuGet
```

## Go Reference Commands

```bash
# Build the Go reference server
cd golang/nats-server && go build

# Run Go tests for a specific area
cd golang/nats-server && go test -v -run TestName ./server/ -count=1 -timeout=30m

# Run all Go server tests (slow, ~30min)
cd golang/nats-server && go test -v ./server/ -count=1 -timeout=30m
```

## Architecture: NATS Server (Reference)

The Go source in `golang/nats-server/server/` is the authoritative reference. Key files by subsystem:

### Core Message Path
- **`server.go`** — Server struct, startup lifecycle (`NewServer` → `Run` → `WaitForShutdown`), listener management
- **`client.go`** (6700 lines) — Connection handling, `readLoop`/`writeLoop` goroutines, per-client subscription tracking, dynamic buffer sizing (512→65536 bytes), client types: `CLIENT`, `ROUTER`, `GATEWAY`, `LEAF`, `SYSTEM`
- **`parser.go`** — Protocol state machine. Text protocol: `PUB`, `SUB`, `UNSUB`, `CONNECT`, `INFO`, `PING/PONG`, `MSG`. Extended: `HPUB/HMSG` (headers), `RPUB/RMSG` (routes). Control line limit: 4096 bytes. Default max payload: 1MB.
- **`sublist.go`** — Trie-based subject matcher with wildcard support. Nodes have `psubs` (plain), `qsubs` (queue groups), special pointers for `*` and `>` wildcards. Results are cached with atomic generation IDs for invalidation.

### Authentication & Accounts
- **`auth.go`** — Auth mechanisms: username/password, token, NKeys (Ed25519), JWT, external auth callout, LDAP
- **`accounts.go`** (137KB) — Multi-tenant account isolation. Each account has its own `Sublist`, client set, and subject namespace. Supports exports/imports between accounts, service latency tracking.
- **`jwt.go`**, **`nkey.go`** — JWT claims parsing and NKey validation

### Clustering
- **`route.go`** — Full-mesh cluster routes. Route pooling (default 3 connections per peer). Account-specific dedicated routes. Protocol: `RS+`/`RS-` for subscribe propagation, `RMSG` for routed messages.
- **`gateway.go`** (103KB) — Inter-cluster bridges. Interest-only mode optimizes traffic. Reply subject mapping (`_GR_.` prefix) avoids cross-cluster conflicts.
- **`leafnode.go`** — Hub-and-spoke topology for edge deployments. Only subscribed subjects shared with hub. Loop detection via `$LDS.` prefix.

### JetStream (Persistence)
- **`jetstream.go`** — Orchestration, API subject handlers (`$JS.API.*`)
- **`stream.go`** (8000 lines) — Stream lifecycle, retention policies (Limits, Interest, WorkQueue), subject transforms, mirroring/sourcing
- **`consumer.go`** — Stateful readers. Push vs pull delivery. Ack policies: None, All, Explicit. Redelivery tracking, priority groups.
- **`filestore.go`** (337KB) — Block-based persistent storage with S2 compression, encryption (ChaCha20/AES-GCM), indexing
- **`memstore.go`** — In-memory storage with hash-wheel TTL expiration
- **`raft.go`** — RAFT consensus for clustered JetStream. Meta-cluster for metadata, per-stream/consumer RAFT groups.

### Configuration & Monitoring
- **`opts.go`** — CLI flags + config file loading. CLI overrides config. Supports hot reload on signal.
- **`monitor.go`** — HTTP endpoints: `/varz`, `/connz`, `/routez`, `/gatewayz`, `/jsz`, `/healthz`
- **`conf/`** — Config file parser (custom format with includes)

### Internal Data Structures
- **`server/avl/`** — AVL tree for sparse sequence sets (ack tracking)
- **`server/stree/`** — Subject tree for per-subject state in streams
- **`server/gsl/`** — Generic subject list, optimized trie
- **`server/thw/`** — Time hash wheel for efficient TTL expiration

## Key Porting Considerations

**Concurrency model:** Go uses goroutines (one per connection readLoop + writeLoop). Map to async/await with `Task`-based I/O. Use `Channel<T>` or `Pipe` for producer-consumer patterns where Go uses channels.

**Locking:** Go `sync.RWMutex` maps to `ReaderWriterLockSlim`. Go `sync.Map` maps to `ConcurrentDictionary`. Go `atomic` operations map to `Interlocked` or `volatile`.

**Subject matching:** The `Sublist` trie is performance-critical. Every published message triggers a `Match()` call. Cache invalidation uses atomic generation counters.

**Protocol parsing:** The parser is a byte-by-byte state machine. In .NET, use `System.IO.Pipelines` for zero-copy parsing with `ReadOnlySequence<byte>`.

**Buffer management:** Go uses `[]byte` slices with pooling. Map to `ArrayPool<byte>` and `Memory<T>`/`Span<T>`.

**Compression:** NATS uses S2 (Snappy variant) for route/gateway compression. Use an equivalent .NET S2 library or IronSnappy.

**Ports:** Client=4222, Cluster=6222, Monitoring=8222, Leaf=5222, Gateway=7222.

## Message Flow Summary

```
Client PUB → parser → permission check → Sublist.Match() →
  ├─ Local subscribers: MSG to each (queue subs: pick one per group)
  ├─ Cluster routes: RMSG to peers (who deliver to their locals)
  ├─ Gateways: forward to interested remote clusters
  └─ JetStream: if subject matches a stream, store + deliver to consumers
```

## NuGet Package Management

This solution uses **Central Package Management (CPM)** via `Directory.Packages.props` at the repo root. All package versions are defined centrally there.

- In `.csproj` files, use `<PackageReference Include="Foo" />` **without** a `Version` attribute
- To add a new package: add a `<PackageVersion>` entry in `Directory.Packages.props`, then reference it without version in the project's csproj
- To update a version: change it only in `Directory.Packages.props` — all projects pick it up automatically
- Never specify `Version` on `<PackageReference>` in individual csproj files

## Logging

Use **Microsoft.Extensions.Logging** (`ILogger<T>`) for all logging throughout the server. Wire up **Serilog** as the logging provider in the host application.

- Inject `ILogger<T>` via constructor in all components (NatsServer, NatsClient, etc.)
- Use **Serilog.Context.LogContext** to push contextual properties (client ID, remote endpoint, subscription subject) so they appear on all log entries within that scope
- Use structured logging with message templates: `logger.LogInformation("Client {ClientId} subscribed to {Subject}", id, subject)` — never string interpolation
- Log levels: `Trace` for protocol bytes, `Debug` for per-message flow, `Information` for lifecycle events (connect/disconnect), `Warning` for protocol violations, `Error` for unexpected failures

## Testing

- **xUnit 3** for test framework
- **Shouldly** for assertions — use `value.ShouldBe(expected)`, `action.ShouldThrow<T>()`, etc. Do NOT use `Assert.*` from xUnit
- **NSubstitute** for mocking/substitution when needed
- Do **NOT** use FluentAssertions or Moq — these are explicitly excluded
- Test project uses global `using Shouldly;`

## Porting Guidelines

- Use modern .NET 10 / C# 14 best practices (primary constructors, collection expressions, `field` keyword where stable, file-scoped namespaces, raw string literals, etc.)
- Prefer `readonly record struct` for small value types over mutable structs
- Use `required` properties and `init` setters for initialization-only state
- Use pattern matching and switch expressions where they improve clarity
- Prefer `System.Text.Json` source generators for JSON serialization
- Use `ValueTask` where appropriate for hot-path async methods

## Agent Model Guidance

- **Sonnet** (`model: "sonnet"`) — use for simpler implementation tasks: straightforward file modifications, adding packages, converting assertions, boilerplate code
- **Opus** (default) — use for complex tasks, architectural decisions, design work, tricky protocol logic, and code review
- **Parallel subagents** — use where tasks are independent and don't touch the same files (e.g., converting test files in parallel, adding packages while updating docs)

## Documentation

Follow the documentation rules in [`documentation_rules.md`](documentation_rules.md) for all project documentation. Key points:

- Documentation lives in `Documentation/` with component subfolders (Protocol, Subscriptions, Server, Configuration, Operations)
- Use `PascalCase.md` file names, always specify language on code blocks, use real code snippets (not invented examples)
- Update documentation when code changes — see the trigger rules and component map in the rules file
- Technical and direct tone, explain "why" not just "what", present tense

## Conventions

- Reference the Go implementation file and line when porting a subsystem
- Maintain protocol compatibility — the .NET server must interoperate with existing NATS clients and Go servers in a cluster
- Use the same configuration file format as the Go server (parsed by `conf/` package)
- Match the Go server's monitoring JSON response shapes for tooling compatibility