Error Sentinel Convention

Milvus error handling has two distinct layers. This document describes the rules that separate them, the rationale, and an audit of where the codebase currently violates them. Companions: error_handling_guide.md (day-to-day how-to) and error_handling_casebook.md (real positive/negative examples of the mistakes that survive review).

The two layers

1. Typed merr (wire-protocol errors)

Defined in pkg/util/merr/errors.go (ErrCollectionNotFound, ErrParameterInvalid, etc.). These carry a numeric error code that is serialized into commonpb.Status{ErrorCode, Reason} and shipped to the client over gRPC. They are the only thing a client (or another Milvus component on the receiving end of an RPC) sees.

Creation: merr.WrapErrXxxMsg(...) / merr.WrapErrXxxErr(cause, ...) (origination only — never to add context to a typed merr that already exists, see merr.Wrap).

2. Internal sentinels (single-process control flow)

Created with errors.New(...) at package scope (e.g. errIgnoredAlterAlias, errReleaseCollectionNotLoaded, errNodeNotEnough). These are signaling vocabulary inside a single Go process: a callee tells its caller "this is an idempotent no-op" or "the queue is empty" so the caller can branch / retry / ignore. They are not part of the wire protocol.

The catcher is always errors.Is(err, sentinelX) at some boundary in the calling stack, and the boundary either:

• translates to merr.Success() (idempotency: e.g. drop something that doesn't exist → success), or
• translates to a typed merr merr.WrapErrXxxMsg(...) (e.g. user already exists → WrapErrParameterInvalid).

The hard invariant

Any internal sentinel must be errors.Is-caught and translated to a typed merr (or Success) before crossing any gRPC handler boundary — client-facing or component-to-component.

Why "any gRPC boundary, not just client-facing": gRPC serializes errors to commonpb.Status{ErrorCode, Reason}. The Go-level pointer identity that errors.New sentinels rely on does not survive the wire. The peer's merr.Error(status) reconstructs a typed merr from the numeric code; the sentinel chain is gone forever. So a sentinel that escapes an internal coord RPC is just as broken as one that escapes a user-facing RPC — only quieter, because no customer sees the resulting Code=65535 (unexpected).

What breaks the invariant

The errors.Is chain survives return err, errors.Wrap(err, "...") / merr.Wrap(err, "...") (cockroachdb thin wrap), and merr.WrapErrServiceInternalErr(err, "...") — milvusError.Unwrap() returns the inner error, so errors.Is(outer, innerSentinel) stays true through any of them. It is destroyed only by:

• Putting the cause in a format argument instead of the chain: merr.WrapErrXxxMsg("...: %s", err), or the %w mistake (WrapErr*Msg formats with fmt.Sprintf, which does not honor %w and renders %!w(...)). The inner error reaches the message text but is unreachable via Unwrap(), so errors.Is(outer, innerSentinel) returns false.
• Any custom wrapper that doesn't implement Unwrap().

Do not confuse this with the separate code/retriability rule. merr.Wrap(err, ...) and merr.WrapErrXxxErr(err, ...) both keep errors.Is intact, but they differ in what the result reports at the boundary:

• merr.Wrap(err, ...) preserves the inner's Code() and IsRetryable — the chain still resolves to the inner's *milvusError.
• merr.WrapErrXxxErr(err, ...) reports the outer sentinel's code and retriability: As() resolves to ErrServiceInternal (Code 5, non-retriable), masking the inner's classification.

So there are two distinct rules, often conflated:

1. To keep errors.Is working: never stuff the cause into a format string; pass it as the error argument.
2. To add context without changing the classification (preserve the inner code + retriability): use merr.Wrap, not merr.WrapErr*Err. Reserve merr.WrapErrXxxErr for when you intend to assert a new classification (e.g. this genuinely is a service-internal error). (See also feedback rule on merr.Wrap vs merr.WrapErr*Err.)

Naming convention

The convention has two layers, matched to the two error categories:

Wire-protocol layer (typed merr) — uppercase Err* in pkg/util/merr only

All errors that may cross any gRPC boundary (client-facing or component-to-component) must be *merr.milvusError defined in pkg/util/merr/errors.go. They have:

• A numeric code passed to newMilvusError(...); uniqueness is enforced by the init-time code registry (defining a second sentinel on an occupied code panics at package init, since milvusError.Is matches by code alone)
• A var ErrXxx = newMilvusError(...) declaration in pkg/util/merr/errors.go
• An exported WrapErrXxxMsg / WrapErrXxxErr helper

If an error needs to be visible to the wire, it lives here. No exceptions.

Every sentinel also carries an Input-vs-System classification (who is to blame) that drives Status.Retriable, the fail_input/fail_system metric labels, lb_policy failover and retry.Do; see "Input vs System: who is to blame?" in error_handling_guide.md.

Code-range partition

Codes are allocated in families. Before adding a sentinel, scan the registry (grep -nE "= newMilvusError\(" pkg/util/merr/errors.go) and place the new code inside its family's range — the init-time registry panics on a duplicate, but it cannot tell you that 1305 belongs to the MQ family. Don't open a new range for a one-off; most "new" errors fit an existing family or an existing sentinel (check before inventing: both ErrSegcore and ErrMqInternal were nearly re-invented during the standardization work).

65535 ((1<<16)-1) is errUnexpected — the wire fallback for errors that carry no merr code. It is reserved; never originate it deliberately. The 2000–2099 segcore range is owned by the cgo conversion table: go through merr.SegcoreError (pkg/util/merr/utils.go), which consults the code/retriability table in pkg/util/merr/segcore.go — don't hand-pick numbers in the range (casebook Pattern 7).

milvusError.Is matches by code — two consequences

1. One code, one sentinel. Two sentinels sharing a code would be errors.Is-equal; the init-time registry panic exists to make that unrepresentable.
2. Promoting an internal errors.New sentinel to a merr widens every guard. A bare sentinel matches by pointer identity; a merr matches by code. After a conversion, errors.Is(err, thatSentinel) matches any error with the same code — silently. Before converting, run grep -rn "errors.Is(.*<sentinelName>" and audit every hit (casebook Pattern 6 documents the data-loss-class near-miss this rule comes from).

Internal-sentinel layer — lowercase errXxx, same-package only

Internal sentinels live in internal/... packages, are created with errors.New(...), and are lowercase / unexported. The rule:

A var err* = errors.New(...) declared in internal/... may only be referenced inside the same Go package. Cross-package consumers must not see it.

This makes Go visibility do the enforcement: if you need a signal across package boundaries, you either (a) lift it into the wire layer as a typed merr, or (b) redesign the API so the signal flows via a return value (e.g. (ignored bool, err error)), not via the error type.

Example (current code, after the 04-coord cleanup):

// errFull / errNoSuchElement are INTERNAL sentinels: caught by errors.Is
// inside the compaction inspector / scheduler loop and never serialized
// across any gRPC boundary.
var (
    errFull          = errors.New("compaction queue is full")
    errNoSuchElement = errors.New("compaction queue has no element")
)

Cross-package idempotency: use a return-value flag, not an exported sentinel

The previous code exported meta.ErrResourceGroupOperationIgnored so that the parent package querycoordv2 could catch it via errors.Is and translate to merr.Success(). This was an exported Err* in internal/... — visually indistinguishable from a merr.ErrXxx typed wire error, easy to misuse.

The current code instead encodes the signal in the return value:

// meta/resource_manager.go
func (rm *ResourceManager) CheckIfResourceGroupAddable(...) (ignored bool, err error) {
    if proto.Equal(rm.groups[rgName].GetConfig(), cfg) {
        return true, nil   // idempotent no-op
    }
    ...
}

// querycoordv2/ddl_callbacks_alter_resource_group.go (broadcaster)
func (s *Server) broadcastCreateResourceGroup(...) (ignored bool, err error) {
    if ignored, err := s.meta.CheckIfResourceGroupAddable(...); err != nil || ignored {
        return ignored, err
    }
    ...
}

// querycoordv2/services.go (RPC handler)
ignored, err := s.broadcastCreateResourceGroup(ctx, req)
if err != nil { return merr.Status(err), nil }
if ignored { return merr.Success(), nil }

No sentinel crosses the package boundary; the signal travels via a structured return value. This is the preferred pattern for any new cross-package idempotency case.

Current state (audit done on err-std-04-coord branch, 2026-05-19)

Across internal/{datacoord,rootcoord,querycoordv2} there are 28 errors.New(...) sentinels. Their fates:

Cleanup status

1. ✅ Done — errEmptyUsername / errEmptyRoleName / errEmptyPrivilegeGroupName at the origin sites in internal/rootcoord/meta_table.go now emit merr.WrapErrParameterInvalidMsg("username is empty") etc. directly; the bare sentinels no longer exist.
2. ✅ Done — errTypeNotFound at the catcher in internal/querycoordv2/ops_services.go:87,101 is now wrapped as merr.WrapErrParameterInvalidMsg("invalid checker type %d: %v", req.CheckerID, err).
3. Not done — delete VerifyResponse and its three dead-code sentinels (errNilResponse, errNilStatusResponse, errUnknownResponseType).

Future linter ideas

Three candidates, in order of how cheap they are to implement and how hard the enforcement is. Tier 1.5's return form is now implemented (see below); Tier 1 and Tier 2 remain a design queue.

Tier 1 — exported-sentinel ban (1 hour to write)

The simplest rule: internal/... packages may not declare exported var Err\w+ = errors.New(...). Scan all internal/... *.go files, fail CI if any match. Two paths to fix a violation:

1. Lowercase it (var errXxx = errors.New(...)) — only callable inside the same package. If the lint fails because a cross-package caller needs the signal, see fix 2.
2. Refactor the API so the signal travels via a return value (e.g. add ignored bool to the return tuple) and delete the sentinel.

This makes Go visibility itself the enforcement mechanism: anything that needs to look like merr.ErrXxx to a reviewer can only exist in pkg/util/merr. Internal sentinels stay quietly lowercase in their owning package.

Lowercase sentinels (var errXxx = errors.New(...)) inside internal/... are still encouraged to carry an INTERNAL: ... doc comment for reviewer context, but it's not enforced — the visibility rule already prevents the worst-case (Err* collision with merr.ErrXxx).

Tier 1.5 — bare-usage ban (1 hour grep, half-day AST for 100% precision)

Status — return form implemented. The return form of this ban is now enforced by a gocritic/ruleguard rule (rawmerrerror in rules.go), run under make verifiers: it rejects return errors.New / fmt.Errorf / errors.Errorf from function bodies (package-level sentinels, cmd/, tests/, codegen and the walimpls harness exempt). The day-to-day guide is error_handling_guide.md. The no-exceptions form below (local :=, panic(...), function argument) is not covered: ruleguard's DSL cannot match "a call anywhere in a function body but not in a ValueSpec", so the full ban still needs the AST-based Tier 2 linter.

Hardest enforcement, no exceptions. internal/... packages may not use errors.New(...) or errors.Errorf(...) inline inside a function body. The only legal site for these calls is a package-level var <Name> = errors.New(...) (sentinel declaration).

Allow/deny matrix:

Why no exceptions (even for "local break signal" / "log-only" / "panic-bound" cases that look harmless today):

• Today's local var can be hoisted to package-level by tomorrow's refactor and silently start crossing boundaries.
• A linter with exceptions needs AST-level wire-reachability analysis (expensive); a no-exception linter is one grep.
• Forces authors to use the right primitive instead of errors.New as a universal escape hatch:
  • break signal from a callback → define a type doneSignal struct{} that implements error and use errors.As. Intent is now in the type, not in a string-keyed sentinel.
  • "unreachable" assertion → just panic(...). If caller already does if err != nil { panic(err) }, fold it into the callee.
  • input validation / config validation → merr.WrapErrParameterInvalidMsg(...) or status.NewInvalidArgument(...) depending on layer.

Implementation: grep version covers ~95% true violations in ~1 hour. AST version (go/analysis) covers the edge cases (e.g. init() body assigning to a package var) but needs ~half a day. Start with grep, upgrade if false-positive rate exceeds 5%.

# grep skeleton
grep -rnE 'errors\.(New|Errorf)\(' internal/ --include='*.go' \
  | grep -v _test.go \
  | grep -vE ':[0-9]+:\s*(var\s+)?[A-Za-z_]+\s*=\s*errors\.(New|Errorf)' \
  | grep -vE ':[0-9]+:\s*[A-Za-z_]+\s+(\w+\s+)?=\s*errors\.(New|Errorf)'
# Any remaining line = violation

A //nolint:err-bare escape valve with a required justification comment handles the genuine outliers (a few init() patterns, embedded errors.Mark usage, etc.).

Tier 2 — escape-path linter (~1 day, go/analysis based)

For every gRPC handler method (anything matching the internal/{rootcoord,datacoord,querycoordv2}/services.go,root_coord.go,*_handler.go pattern, return type (*proto.XxxResponse, error) or (*commonpb.Status, error)), trace the err-return data-flow. Any error that:

• transitively originates from an INTERNAL:-tagged sentinel, and
• reaches a return Status{Code: merr.Status(err)} or return err without passing through an errors.Is(err, internalSentinelX) { ... } branch,

is a violation. Report file:line of the leak.

This catches the actual invariant violation (the 3 RBAC empty sentinels would have been flagged), not just naming hygiene. Requires AST analysis; worth doing if the cost of one more silent Code=1 to a client is high.

Tier 3 (no longer necessary if Tier 2 is in place) — wrap-rule linter

Scan merr\.WrapErr[A-Za-z]+Err\(err, (note: first arg err, not a fresh string-only origination) and require the cause err to not itself be a typed merr. This is what feedback_merr_wrap_rule already enforces by convention; Tier 2 would catch the symptom (sentinel escape) as a side effect.

Related rules

• feedback_merr_wrap_rule (this repo's collaboration memory): "Add context to an existing err with merr.Wrap / merr.Wrapf, never with merr.WrapErr*Err — the latter masks the inner typed code and retriability (the errors.Is chain itself is preserved via Unwrap())."
• project_errstd_autogen_defects: three systematic defects in the auto-generated errors.Wrap → merr conversion in this branch series; defects #2 and #3 are direct consequences of violating the rules in this document.