# Batch 21 Events + MsgTrace Design

**Date:** 2026-02-27  
**Batch:** 21 (`Events + MsgTrace`)  
**Scope:** Design only. No implementation in this document.

## Problem

Batch 21 contains high-fanout eventing and tracing behavior used by system account messaging, server stats, admin requests, and distributed message tracing.

Current tracker scope from `batch show 21`:
- Features: `118` (all `deferred`)
- Tests: `9` (all `deferred`)
- Dependencies: batches `18`, `19`
- Go sources: `server/events.go`, `server/msgtrace.go`

## Context Findings

- Existing .NET code already has many event and trace DTO/types in:
  - `dotnet/src/ZB.MOM.NatsNet.Server/Events/EventTypes.cs`
  - `dotnet/src/ZB.MOM.NatsNet.Server/MessageTrace/MsgTraceTypes.cs`
- Runtime behavior methods mapped in Batch 21 are largely missing from `NatsServer` and `ClientConnection`.
- Existing backlog tests for Events/MsgTrace include placeholder-style assertions and must be replaced with behavior-valid tests for mapped Batch 21 IDs.
- `test show` mapping confirms target class/methods for all 9 tests:
  - Events: 6 methods in `EventsHandlerTests`
  - MsgTrace: 2 methods in `MessageTracerTests`
  - Concurrency: 1 method in `ConcurrencyTests1`

## Assumptions

- Batch 21 work begins only after dependencies (batches 18 and 19) are complete enough to compile and run related tests.
- We preserve existing project structure and naming patterns (`NatsServer.*.cs`, `ClientConnection.*.cs`, `ImplBacklog/*.Impltests.cs`).
- No new integration infrastructure is introduced in this batch; infra-blocked items remain deferred with explicit reasons.

## Approaches

### Approach A: Monolithic implementation in existing large files

Implement all methods directly in `NatsServer.cs`, `ClientConnection.cs`, `ClientTypes.cs`, and `NatsServerTypes.cs`.

Trade-offs:
- Pros: Minimal file creation.
- Cons: Very high merge conflict risk, poor reviewability, difficult verification per feature group.

### Approach B (Recommended): Partial-file segmentation by runtime domain

Add focused partial/runtime files for events and msgtrace behavior while leaving DTO/type files intact.

Trade-offs:
- Pros: Enables clear feature-group boundaries, easier per-group build/test loops, lower risk of accidental regressions.
- Cons: Requires a few new files and up-front structure decisions.

### Approach C: Test-only first, then backfill features

Start by rewriting the 9 tests to force implementation behavior.

Trade-offs:
- Pros: Fast feedback for mapped tests.
- Cons: Batch has 118 features and only 9 mapped tests; test-first alone leaves large behavior surface unvalidated.

## Recommended Design

Use **Approach B**.

### Code organization

Implement runtime behavior in small, targeted files:
- `dotnet/src/ZB.MOM.NatsNet.Server/NatsServer.Events.cs` (core eventing/send loops)
- `dotnet/src/ZB.MOM.NatsNet.Server/NatsServer.Events.System.cs` (system subscriptions/requests)
- `dotnet/src/ZB.MOM.NatsNet.Server/NatsServer.Events.Admin.cs` (reload/kick/ldm/debug/OCSP event paths)
- `dotnet/src/ZB.MOM.NatsNet.Server/ClientConnection.MsgTrace.cs` (client-side trace enable/init helpers)
- `dotnet/src/ZB.MOM.NatsNet.Server/MessageTrace/MsgTraceRuntime.cs` (runtime trace mutation/send behavior)
- plus targeted edits in:
  - `dotnet/src/ZB.MOM.NatsNet.Server/NatsServerTypes.cs` (ServerInfo capability helpers)
  - `dotnet/src/ZB.MOM.NatsNet.Server/ClientTypes.cs` (ClientInfo helper projections)
  - `dotnet/src/ZB.MOM.NatsNet.Server/Accounts/Account.cs` (`Account.statz`)

### Behavior domains

- Domain 1: Capability and metadata helpers (`ServerInfo`, `ClientInfo`, hashing helpers).
- Domain 2: Internal system message send/receive loops and event state lifecycle.
- Domain 3: Remote server/account tracking and statsz/advisory publication.
- Domain 4: System subscription wiring and request handlers (connsz/statsz/idz/nsubs/reload/kick/ldm).
- Domain 5: OCSP advisory events and misc utility wrappers.
- Domain 6: Message trace runtime (trace enablement, header extraction, event aggregation, publish path).

### Test design

Replace mapped placeholder tests with behavior checks that assert:
- System subscription registration and unsubscribe behavior.
- Connection update timer/sweep behavior under local/remote account changes.
- Remote latency update validation and bad payload handling.
- MsgTrace connection-name normalization and trace-header parsing correctness.
- No-race JetStream compact scenario behavior for mapped test ID 2412.

### Execution model

Port features in 6 groups (<=20 IDs each), then tests in 2 waves, with strict per-feature verification and anti-stub gates.

## Risks and Mitigations

- Risk: Placeholder tests can pass while behavior is wrong.
  - Mitigation: Mandatory anti-stub checks and per-test evidence before status updates.
- Risk: Large eventing surface can regress unrelated server behavior.
  - Mitigation: Build gate after each feature group + full unit-test checkpoint between tasks.
- Risk: Some tests require runtime topology not available in unit test harness.
  - Mitigation: keep deferred with explicit blocker reason; do not stub.

## Success Criteria

- All 118 Batch 21 feature IDs moved through `stub -> complete -> verified` only with build + test evidence.
- All 9 Batch 21 test IDs either `verified` with real assertions or `deferred` with explicit blocker reason.
- No new stubs (`NotImplementedException`, empty bodies, TODO placeholders) in touched feature/test files.
- Batch 21 can be completed with `batch complete 21` once all IDs satisfy status requirements.