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7f4fb34182
It is better developing vmctl tool in VictoriaMetrics repository, so it could be released together with the rest of vmutils tools such as vmalert, vmagent, vmbackup, vmrestore and vmauth.
117 lines
5.2 KiB
Go
117 lines
5.2 KiB
Go
// Package log provides a structured logger.
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//
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// Structured logging produces logs easily consumed later by humans or
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// machines. Humans might be interested in debugging errors, or tracing
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// specific requests. Machines might be interested in counting interesting
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// events, or aggregating information for off-line processing. In both cases,
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// it is important that the log messages are structured and actionable.
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// Package log is designed to encourage both of these best practices.
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//
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// Basic Usage
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//
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// The fundamental interface is Logger. Loggers create log events from
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// key/value data. The Logger interface has a single method, Log, which
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// accepts a sequence of alternating key/value pairs, which this package names
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// keyvals.
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//
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// type Logger interface {
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// Log(keyvals ...interface{}) error
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// }
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//
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// Here is an example of a function using a Logger to create log events.
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//
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// func RunTask(task Task, logger log.Logger) string {
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// logger.Log("taskID", task.ID, "event", "starting task")
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// ...
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// logger.Log("taskID", task.ID, "event", "task complete")
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// }
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//
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// The keys in the above example are "taskID" and "event". The values are
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// task.ID, "starting task", and "task complete". Every key is followed
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// immediately by its value.
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//
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// Keys are usually plain strings. Values may be any type that has a sensible
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// encoding in the chosen log format. With structured logging it is a good
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// idea to log simple values without formatting them. This practice allows
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// the chosen logger to encode values in the most appropriate way.
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//
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// Contextual Loggers
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//
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// A contextual logger stores keyvals that it includes in all log events.
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// Building appropriate contextual loggers reduces repetition and aids
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// consistency in the resulting log output. With and WithPrefix add context to
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// a logger. We can use With to improve the RunTask example.
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//
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// func RunTask(task Task, logger log.Logger) string {
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// logger = log.With(logger, "taskID", task.ID)
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// logger.Log("event", "starting task")
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// ...
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// taskHelper(task.Cmd, logger)
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// ...
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// logger.Log("event", "task complete")
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// }
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//
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// The improved version emits the same log events as the original for the
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// first and last calls to Log. Passing the contextual logger to taskHelper
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// enables each log event created by taskHelper to include the task.ID even
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// though taskHelper does not have access to that value. Using contextual
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// loggers this way simplifies producing log output that enables tracing the
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// life cycle of individual tasks. (See the Contextual example for the full
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// code of the above snippet.)
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//
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// Dynamic Contextual Values
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//
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// A Valuer function stored in a contextual logger generates a new value each
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// time an event is logged. The Valuer example demonstrates how this feature
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// works.
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//
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// Valuers provide the basis for consistently logging timestamps and source
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// code location. The log package defines several valuers for that purpose.
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// See Timestamp, DefaultTimestamp, DefaultTimestampUTC, Caller, and
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// DefaultCaller. A common logger initialization sequence that ensures all log
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// entries contain a timestamp and source location looks like this:
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//
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// logger := log.NewLogfmtLogger(log.NewSyncWriter(os.Stdout))
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// logger = log.With(logger, "ts", log.DefaultTimestampUTC, "caller", log.DefaultCaller)
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//
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// Concurrent Safety
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//
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// Applications with multiple goroutines want each log event written to the
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// same logger to remain separate from other log events. Package log provides
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// two simple solutions for concurrent safe logging.
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//
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// NewSyncWriter wraps an io.Writer and serializes each call to its Write
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// method. Using a SyncWriter has the benefit that the smallest practical
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// portion of the logging logic is performed within a mutex, but it requires
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// the formatting Logger to make only one call to Write per log event.
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//
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// NewSyncLogger wraps any Logger and serializes each call to its Log method.
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// Using a SyncLogger has the benefit that it guarantees each log event is
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// handled atomically within the wrapped logger, but it typically serializes
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// both the formatting and output logic. Use a SyncLogger if the formatting
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// logger may perform multiple writes per log event.
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//
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// Error Handling
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//
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// This package relies on the practice of wrapping or decorating loggers with
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// other loggers to provide composable pieces of functionality. It also means
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// that Logger.Log must return an error because some
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// implementations—especially those that output log data to an io.Writer—may
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// encounter errors that cannot be handled locally. This in turn means that
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// Loggers that wrap other loggers should return errors from the wrapped
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// logger up the stack.
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//
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// Fortunately, the decorator pattern also provides a way to avoid the
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// necessity to check for errors every time an application calls Logger.Log.
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// An application required to panic whenever its Logger encounters
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// an error could initialize its logger as follows.
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//
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// fmtlogger := log.NewLogfmtLogger(log.NewSyncWriter(os.Stdout))
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// logger := log.LoggerFunc(func(keyvals ...interface{}) error {
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// if err := fmtlogger.Log(keyvals...); err != nil {
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// panic(err)
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// }
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// return nil
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// })
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package log
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