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_total{direction, message, codec}
rustfs_system_network_internode_msgpack_json_decode_error_total{direction, message, codec}

The decode counter increments after a successful msgpack or JSON compatibility decode. The fallback counter increments whenever a decode falls back to the JSON field because the msgpack payload was absent. The decode-error counter increments whenever either codec fails to decode.

• 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 these counters) and let it run for at least one full release window across the whole fleet. The fallback and decode-error counters must stay at zero.

First confirm each expected message/direction has real traffic in the observation window:

sum by (direction, message, codec) (
  increase(rustfs_system_network_internode_msgpack_json_decode_total[30d])
)

For every convergence-ready message/direction below, the codec="msgpack" series must be non-zero before a zero fallback result is meaningful. Missing series, zero traffic, counter reset, or scrape gaps make the gate inconclusive rather than passed.

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

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:

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):

- 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)

_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.

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)

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:

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

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).