# Internode msgpack/JSON Convergence Runbook

Operational runbook for retiring the redundant JSON compatibility fields on internode
gRPC metadata RPCs (grpc-optimization **P2-1**). This is a **cross-version** change: it
proceeds strictly by observation-gated stages, never in one step.

## Background

Internode RPCs dual-encode each metadata value as **both**:

- a msgpack binary field (`*_bin`, e.g. `file_info_bin`), and
- a JSON compatibility string (e.g. `file_info`).

Decoders prefer the `_bin` payload and fall back to the JSON string only when `_bin` is
empty (`decode_msgpack_or_json`). The dual-write costs bandwidth and CPU. Before the JSON
fields can be dropped, the fallback branch must be proven **unused** in production —
otherwise a rolling upgrade with mixed node versions could read an emptied field.

## The observation metric (already shipped)

```
rustfs_system_network_internode_msgpack_json_fallback_total{direction, message}
rustfs_system_network_internode_msgpack_json_decode_error_total{direction, message, codec}
```

Incremented whenever a decode falls back to the JSON field because the msgpack payload was
absent.

- `direction="request"` — a server decoding a peer's request (`node_service/disk.rs`).
- `direction="response"` — a client decoding a peer's response (`cluster/rpc/remote_disk.rs`),
  including the list-level `ReadMultiple` / `BatchReadVersion` fallbacks.
- `message` — the value name, e.g. `FileInfo`, `RawFileInfo`, `ReadMultipleResp`.
- `codec` — the failed codec for decode errors: `msgpack` for corrupt non-empty `_bin`, or `json` for corrupt legacy fallback JSON.

## Stage 0 — Observe (current stage)

Ship the current release (which contains the counter) and let it run for **at least one
full release window** across the whole fleet. The counter must stay at **zero**.

Confirm zero across the observation window (adjust `[30d]` to the window length):

```promql
sum by (direction, message) (
  increase(rustfs_system_network_internode_msgpack_json_fallback_total[30d])
)
```

Every series must be `0`. A non-zero value means some peer is still emitting an empty
`_bin` (an old node, or a message whose sender does not fill `_bin`) — investigate the
`{direction, message}` label before proceeding.

Decode errors must also stay at zero across the observation window:

```promql
sum by (direction, message, codec) (
  increase(rustfs_system_network_internode_msgpack_json_decode_error_total[30d])
)
```

A non-zero `codec="msgpack"` series means a peer sent corrupt or incompatible `_bin` bytes; it must fail closed and block convergence. A non-zero `codec="json"` series means the legacy fallback field was corrupt or semantically incompatible; it also blocks convergence and rollback confidence.

Standing alert (keep enabled through all stages):

```yaml
- alert: InternodeMsgpackJsonFallback
  expr: sum by (direction, message) (increase(rustfs_system_network_internode_msgpack_json_fallback_total[15m])) > 0
  for: 5m
  labels: { severity: warning }
  annotations:
    summary: "Internode RPC fell back to JSON decode ({{ $labels.direction }}/{{ $labels.message }})"
    description: "A peer sent an empty msgpack _bin payload. Do NOT advance msgpack-only convergence while this fires."
- alert: InternodeMsgpackJsonDecodeError
  expr: sum by (direction, message, codec) (increase(rustfs_system_network_internode_msgpack_json_decode_error_total[15m])) > 0
  for: 5m
  labels: { severity: warning }
  annotations:
    summary: "Internode RPC msgpack/JSON decode failed ({{ $labels.direction }}/{{ $labels.message }}/{{ $labels.codec }})"
    description: "A peer sent an undecodable msgpack or JSON compatibility payload. Do NOT advance msgpack-only convergence while this fires."
```

## Field → peer-decoder audit

The send-side change (Stage 1) may only empty a JSON field whose **peer decodes `_bin`
first**. The following mapping is verified against the current code.

### Convergence-ready (peer decodes `_bin` first)

| Direction | Message / field | Peer decoder |
|---|---|---|
| request | `WriteMetadata.file_info` | `FileInfo` (node_service/disk.rs) |
| request | `UpdateMetadata.file_info` | `FileInfo` |
| request | `UpdateMetadata.opts` | `UpdateMetadataOpts` |
| request | `RenameData.file_info` | `FileInfo` |
| request | `ReadMultiple.read_multiple_req` | `ReadMultipleReq` |
| request | `BatchReadVersion.batch_read_version_req` | `BatchReadVersionReq` |
| request | `Read*.opts` | `ReadOptions` |
| response | `ReadVersion.file_info` | `FileInfo` (cluster/rpc/remote_disk.rs) |
| response | `ReadXL.raw_file_info` | `RawFileInfo` |
| response | `RenameData.rename_data_resp` | `RenameDataResp` |
| response | `ReadMultiple` resp list | per-item + list fallback |
| response | `BatchReadVersion` resp list | per-item + list fallback |

### `_bin` support added THIS release — converge after their own window

The `DeleteVersion`/`DeleteVersions` protos had **no `_bin` fields**. They gained additive
`*_bin` fields plus bin-first server decoders in this release, and the client now dual-writes
them. They are **kept out** of the msgpack-only set (always dual-write) until their own
fallback counter has read zero across a window with the new decoders fully deployed.

| Direction | Message / field | Status |
|---|---|---|
| request | `DeleteVersion.file_info` (`FileInfo`) | `_bin` added; dual-write; converge after window |
| request | `DeleteVersion.opts` (`DeleteOptions`) | `_bin` added; dual-write; converge after window |
| request | `DeleteVersions.versions` (`FileInfoVersions`) | `_bin` added; dual-write; converge after window |
| request | `DeleteVersions.opts` (`DeleteOptions`) | `_bin` added; dual-write; converge after window |

> Any `*_bin` proto field not in the tables above must be mapped to a confirmed `_bin`-first
> peer decoder before it is added to the convergence set.

### Still JSON-only (no `_bin` field)

| Direction | Message / field | Note |
|---|---|---|
| response | `DeleteVersion.raw_file_info` | proto has no `_bin`; needs an additive proto field before it can converge. |

## Stage 1 — Stop writing JSON (env-gated, after Stage 0 reads zero)

The send-side lever is **implemented** and requires two default-off env flags:

- `RUSTFS_INTERNODE_RPC_MSGPACK_ONLY=true`
- `RUSTFS_INTERNODE_RPC_MSGPACK_ONLY_FLEET_CONFIRMED=true`

The first flag only requests msgpack-only. The second flag is the explicit proof gate that the fleet has passed the release-window fallback, capability, and rollback checks. If only `RUSTFS_INTERNODE_RPC_MSGPACK_ONLY=true` is set, RustFS keeps dual-writing JSON compatibility fields so old JSON-only peers remain compatible. When both flags are enabled, the convergence-ready fields above send only `_bin` and leave the JSON string empty; the `_bin` payload is always sent and decoders keep the JSON read fallback unchanged. The delete fields are excluded (dual-write) per the section above.

Only enable it **after** Stage 0 has read zero for a full window across the fleet:

1. Ship with the flag **off** (no behavior change).
2. Rehearse the `request-only` gate with `RUSTFS_INTERNODE_RPC_MSGPACK_ONLY=true` and `RUSTFS_INTERNODE_RPC_MSGPACK_ONLY_FLEET_CONFIRMED=false`. This must keep writing JSON compatibility fields and is safe with unsupported/old peers; any fallback or decode-error increment still blocks convergence.
3. Enable it on one canary node with both `RUSTFS_INTERNODE_RPC_MSGPACK_ONLY=true` and `RUSTFS_INTERNODE_RPC_MSGPACK_ONLY_FLEET_CONFIRMED=true`, then restart that node only and watch the fallback and decode-error counters for a soak period while real internode traffic is present. If they stay zero, enable fleet-wide.
4. **Rollback:** set either `RUSTFS_INTERNODE_RPC_MSGPACK_ONLY=false` or `RUSTFS_INTERNODE_RPC_MSGPACK_ONLY_FLEET_CONFIRMED=false` (or unset either flag) and restart. No
   wire-format was broken in this stage, so rollback is immediate and safe.

The benchmark helper pins these operator states as dry-run phases:

```bash
scripts/run_internode_grpc_ab_bench.sh --stage p2 --phase before --dry-run
scripts/run_internode_grpc_ab_bench.sh --stage p2 --phase request-only --dry-run
scripts/run_internode_grpc_ab_bench.sh --stage p2 --phase canary --dry-run
scripts/run_internode_grpc_ab_bench.sh --stage p2 --phase after --dry-run
scripts/run_internode_grpc_ab_bench.sh --stage p2 --phase rollback --dry-run
```

## Stage 2 — Remove the proto JSON fields (next release, N+1)

Only after Stage 1 has been stable with the flag on for a full window and the counter is
still zero.

1. Mark the retired text fields `reserved` in `crates/protos/src/node.proto` (never reuse
   the field numbers) and delete the JSON read-fallback branches; codec becomes msgpack-only.
2. This is a hard wire-format change — it requires the mixed-version upgrade rehearsal
   (four-node scripts) to pass, and it cannot be rolled back by env alone.

## Rollback matrix

| Stage | Wire-format broken? | Rollback |
|---|---|---|
| 0 Observe | no | n/a (metric only) |
| 1 msgpack-only send | no | unset either msgpack-only env flag + restart |
| 2 remove fields | yes | redeploy prior release; field numbers stay `reserved` |

## Related

- Codec + counter implementation: commit `feat(internode): P2 msgpack/JSON codec observability + encode buffer presizing`.
- Decoders: `decode_msgpack_or_json` in `crates/ecstore/src/cluster/rpc/remote_disk.rs` (client)
  and `rustfs/src/storage/rpc/node_service/disk.rs` (server).
