klog

klog is a logging library for the Go programming language. Developed by Google and widely used in the Kubernetes project, it originated as a fork of glog and provides leveled, runtime-configurable logging for Go applications. The package emphasizes simplicity and speed, offering a familiar set of logging functions and optional file-backed logging.

Its core feature is a verbosity mechanism accessed via V(n). Calls like klog.Infof, klog.Warningf, and klog.Errorf log messages at fixed levels, while klog.V(n).Infof allows conditional logging based on the current verbosity level. Logging output can be directed to standard error, to log files, or to both, through flags such as --logtostderr, --log_dir, and --alsologtostderr. It retains file and line information for messages and prefixes logs with timestamps.

Versions and usage: The library exists in two main flavors, v1 and v2, with v2 under the import path k8s.io/klog/v2. Many projects initialize flags with klog.InitFlags(nil) and then call flag.Parse() before logging. Typical usage includes klog.Infof("message"), klog.Warningf("message"), and klog.Errorf("error: %v", err). Verbose logs are enabled by setting the global verbosity level with V(n). At program end, klog.Flush() writes any buffered logs.

See also: Kubernetes, glog, logging library.