OSLog / os.Logger Defaults

When to invoke

• Starting a new Apple-platform project and picking a logging library.
• Writing the first Logger declaration.
• Deciding the default for privacy interpolation.
• User asks "OSLog vs SwiftLog vs CocoaLumberjack", ".private vs .public how to pick".

Default decisions

• Use Apple's built-in `os.Logger` (import os); do not pull in any third-party logging library.
• Naming conventions:
• subsystem = bundle ID (e.g. com.example.myapp)
• category = module name (aligned with the SwiftPM target name)
• Privacy interpolation is type-dependent, not "all private": dynamic strings and complex objects default to .private; integer, floating-point, and Boolean values default to .public. Any identifying numeric value (player ID, user ID, serial number) must be marked .private explicitly.

import os

extension Logger {
    static let engine = Logger(subsystem: "com.example.myapp", category: "Engine")
}

Logger.engine.info("user \(userId, privacy: .public) loaded puzzle \(puzzleId, privacy: .private)")
//                                    ^^^^^^^ explicit public       ^^^^^^^ explicit private —
//                                                                    identifying values need this
//                                                                    even when the type (Int, Bool)
//                                                                    would otherwise default public

Rationale

• Native integration with Console.app / Instruments / the unified logging system; zero dependencies.
• Friendly to Swift 6 actor / Sendable.
• Native privacy interpolation; .private values are redacted whenever no debugger is attached (see "What .private actually means" below).
• No third-party SDK pulled in → no extra entries in PrivacyInfo.xcprivacy, consistent with the "no third-party tracking" stance.

What .private actually means (easily misunderstood)

Practice observed — none of the four official pages in `references/official-docs.md` state the debugger-attached or `OSLogStore` behavior below; this is derived from testing, not documented.

• .private content is redacted wherever no debugger is attached — this includes a TestFlight user viewing their own Console.app, not only "in someone else's sysdiagnose after release."
• When the local Xcode debugger is attached to a running process, private values are still visible.
• `OSLogStore` does not bypass redaction: in a TestFlight or production build, OSLogStore reading its own process still sees <private> in place of redacted values — only a process that Xcode itself launched gets unredacted output. .private is redaction, not encryption; never log raw PII even under .private.

Deviation considerations

• Cross-platform shared logger interface (Linux / Android target): use swift-log (apple/swift-log) as a facade; on Apple platforms, back it with a third-party OSLog handler package (apple/swift-log's own distribution ships no OSLog backend) so the interface stays platform-neutral.
• Need remote log aggregation: pair with telemetry-facade-pattern's fan-out sink rather than replacing OSLog directly.
• A third-party crash reporter requires its own logger: usually avoidable; if not, keep its use scoped to that SDK.

Verification checklist

• No import Logging / import CocoaLumberjack / import Sentry or other third-party logging.
• Each module has its own Logger extension with a category aligned to the module name.
• Every .public annotation can be explained as non-privacy-violating (e.g. non-PII, build hash).
• PII / player IDs / tokens are always .private (or not logged at all).

Related skills

• telemetry-facade-pattern: where OSLogSink sits within the facade.
• apple-three-piece-analytics: OSLog is an Apple-only path, in the same "no third-party" stance as ASC / MetricKit / GC.
• apple-public-repo-security: .private corresponds to sysdiagnose redaction, but is still visible under a debugger — the safety reasoning for the public repo relies on this semantics.
• ios-performance-engineering: OSSignposter / os_signpost intervals and Instruments profiling; this skill stops at Logger.
• Official sources: when verifying or updating a factual or version-sensitive claim, read references/official-docs.md.