The systemd journal stores messages with structured metadata in journal files. That makes service output searchable without first inventing a parser for every application’s text format. Keeping those messages for a useful period is a separate requirement. Storage location, size policy, rotation, and forwarding determine which evidence remains available when an incident is investigated later. ...
A useful journal query selects the records that answer a specific incident question. Unit identity, invocation, time, priority, and application fields provide different dimensions. Combining them correctly matters more than finding a familiar word somewhere in the machine’s history. journald can attach process metadata, but it does not automatically collect every file an application writes or infer the business meaning of each message. ...
A log_format that resembles JSON is not necessarily a reliable structured event format. nginx substitutes variable values into the text, and those values can contain quotes, backslashes, control characters, or no data at all. Correct string escaping and correct value types are separate requirements. Testing emitted records with a real parser catches failures that a visually convincing configuration or successful nginx -t cannot establish. ...
Structured logging assigns stable meaning to fields instead of asking each query to infer everything from message text. A record might contain an event name, request identifier, numeric duration, severity, and a human-readable message. Free text remains useful and can also be searched or parsed. The practical gain is that routine questions can use a documented schema whose meaning survives changes to explanatory wording. ...
An nginx access log records request information when logging runs at the end of processing. The selected fields can distinguish what the client requested, which URI ultimately handled it, what response was sent, and what happened during upstream attempts. Keep these observations separate: one status, duration, or path cannot explain the whole request. ...
A wall-clock label such as 2026-11-01 01:30 does not identify one instant by itself. Its interpretation depends on a timezone and sometimes on which occurrence was intended during a clock transition. Conversely, an explicitly defined Unix timestamp already carries epoch and unit semantics without displaying a zone name. Decide which kind of value your system is storing before choosing a format. ...
Logrotate evaluates configuration when invoked, rotates eligible logs, and applies the selected retention and compression actions. A timer or another scheduler determines when that evaluation happens. The tool does not continuously cap file size, and renaming a log does not automatically redirect an application’s open descriptor. A working setup combines evaluation frequency, file policy, and the writer’s actual reopening behavior. ...
Metrics and logs are different representations of observed behavior, not competing definitions of truth. A metric usually summarizes measurements under a set of dimensions. A log record describes an event and can carry detailed context. Both can answer aggregate questions if their coverage and semantics support the calculation. Neither becomes authoritative merely because its dashboard looks cleaner or its records contain more fields. ...