music_library/backlog/docs/doc-3 - Force-production-logs-to-single-line-format.md

id, title, type, created_date, updated_date

id	title	type	created_date	updated_date
doc-3	Force production logs to single-line format	other	2026-05-03 13:53	2026-05-04 06:55

Research: Force production logs to single-line format

Current state

Config (config/config.exs):

config :logger, :default_formatter,
  format: "$time $metadata[$level] $message\n",
  metadata: [:request_id]

Override (config/prod.exs):

config :logger, level: :info

Log sources in prod:

Source	Produces	Problem
Phoenix.Logger (telemetry)	GET /health + Sent 200 in Xms	Two separate lines for one request — can't reverse
Phoenix.Logger (telemetry)	CONNECTED TO Phoenix.LiveView.Socket...	One event with embedded \n — 4+ lines
Custom Logger.info/error/...	Varies	Some may be multi-line
OTP / stack traces	Crash reports	Multi-line

Phoenix.Logger is auto-attached by Phoenix.Endpoint and handles telemetry events including [:phoenix, :endpoint, :start], [:phoenix, :endpoint, :stop], and [:phoenix, :socket_connected].

Chosen approach: Option 2 (comprehensive)

1. Disable Phoenix.Logger — config :phoenix, :logger, false in prod
2. Add Logster v2 — Logster.attach_phoenix_logger() for single-line HTTP request logging (logfmt format)
3. Custom telemetry handler — for [:phoenix, :socket_connected] LiveView handshakes (if not covered by Logster)
4. Custom Logger.Formatter — safety net replacing \n → \\n in all remaining messages
5. Code audit — fix multi-line Logger calls in project code

Why Logster v2 over v1

v2 (~> 2.0.0-rc.5) uses Logster.attach_phoenix_logger() in application.ex instead of replacing a plug in the endpoint. This is the modern approach that works with Phoenix's telemetry-based logging architecture. It outputs logfmt (key=value) format by default, merging the GET /path and Sent 200 lines into one.

Architecture impact

Touchpoint	Change
config/prod.exs	Add config :phoenix, :logger, false + Logster config + formatter config
lib/music_library/application.ex	Add conditional Logster.attach_phoenix_logger()
lib/music_library_web/telemetry_log_handler.ex	New module — handles socket_connected telemetry
lib/music_library/logger/single_line_formatter.ex	New module — Logger.Formatter format function
mix.exs	Add {:logster, "~> 2.0.0-rc.5"}
Docs	Update docs/production-infrastructure.md with logging config

Performance

• Logster: one telemetry handler invocation per request, logfmt formatting is string interpolation — negligible overhead
• Custom formatter: one String.replace/3 per log event, Logger already truncates messages — negligible
• No CPU or memory concerns at the log volume of a personal app

Dependencies

• logster ~> 2.0.0-rc.5 (1.9M downloads, MIT license, active maintenance)

Cost

• None — no paid services involved

---

Questions resolved

1. Newline style: Replace \n with \\n (escaped, traceable)
2. Scope: Prod-only; dev keeps current multi-line format for readability
3. Module naming: MusicLibrary.Logger.SingleLineFormatter and MusicLibraryWeb.TelemetryLogHandler