56 lines
2.3 KiB
Markdown
56 lines
2.3 KiB
Markdown
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# ADR-003: Exported Logger Interface
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**Status:** Accepted
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**Date:** 2026-03-18
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## Context
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Global ADR-001 establishes that app-facing modules define a local `Logger` interface
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satisfied by `logz.Logger` so that libraries do not import `logz` directly. For this
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duck-typing pattern to work, `logz` must export its `Logger` interface with a stable
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method set that other packages can replicate locally.
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If `Logger` were unexported, or if `New` returned `*slogLogger` (the concrete type),
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consumers would need to import `logz` just to name the type in a parameter or field
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declaration, coupling every module to the logging library.
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## Decision
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`Logger` is an exported interface in the `logz` package:
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```go
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type Logger interface {
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Debug(msg string, args ...any)
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Info(msg string, args ...any)
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Warn(msg string, args ...any)
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Error(msg string, err error, args ...any)
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With(args ...any) Logger
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WithContext(ctx context.Context) Logger
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}
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```
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`New(opts Options) Logger` returns the interface, not `*slogLogger`. The concrete
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type is unexported. `With` returns `Logger` (interface), not `*slogLogger` — this
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is enforced by the return type in the interface definition and by the compliance test.
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Modules that accept a logger define a local interface with the subset of methods they
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use (typically `Info`, `Warn`, `Error`). `logz.Logger` satisfies any such local
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interface because it is a superset.
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The `Error` signature deliberately differs from `slog`'s: `Error(msg string, err
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error, args ...any)`. The error is a first-class parameter rather than a key-value
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attribute, enabling automatic enrichment from duck-typed interfaces (`ErrorCode`,
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`ErrorContext`) without any extra caller code.
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## Consequences
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- Tier 1+ modules can accept a logger via a local `Logger` interface without
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importing `logz`. They are decoupled from the logging backend.
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- `launcher`, which imports `logz` directly (it is the bootstrap layer), uses
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`logz.Logger` as the concrete type in its `New` signature.
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- Adding methods to `logz.Logger` is a breaking change for all callers that replicate
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the interface locally. Any new method must be evaluated carefully and accompanied
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by a version bump.
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- The `Error(msg, err, args...)` convention is not interchangeable with `slog`'s
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`Error(msg, args...)`. Adapters must account for this signature difference.
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