Allow logging formatter to be configured

This changeset simply adds hooks into the configuration system to support
multiple different kinds of output formats. These formatters are provided by
logrus and include options such as "text" and "json". The configuraiton
documentation has been updated accordingly.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

cmd/registry/config.yml

@@ -1,5 +1,6 @@
 version: 0.1
-loglevel: debug
+log:
+  level: debug
 storage:
     filesystem:
         rootdirectory: /tmp/registry-dev

cmd/registry/main.go

@@ -9,6 +9,7 @@ import (
 	"os"
 
 	log "github.com/Sirupsen/logrus"
+	"github.com/Sirupsen/logrus/formatters/logstash"
 	"github.com/bugsnag/bugsnag-go"
 	"github.com/docker/distribution/configuration"
 	ctxu "github.com/docker/distribution/context"
@@ -49,7 +50,10 @@ func main() {
 		fatalf("configuration error: %v", err)
 	}
 
-	log.SetLevel(logLevel(config.Loglevel))
+	if err := configureLogging(config); err != nil {
+		fatalf("error configuring logger: %v", err)
+	}
+
 	ctx = context.WithValue(ctx, "version", version.Version)
 	ctx = ctxu.WithLogger(ctx, ctxu.GetLogger(ctx, "version"))
 
@@ -111,16 +115,6 @@ func resolveConfiguration() (*configuration.Configuration, error) {
 	return config, nil
 }
 
-func logLevel(level configuration.Loglevel) log.Level {
-	l, err := log.ParseLevel(string(level))
-	if err != nil {
-		log.Warnf("error parsing level %q: %v", level, err)
-		l = log.InfoLevel
-	}
-
-	return l
-}
-
 func configureReporting(app *handlers.App) http.Handler {
 	var handler http.Handler = app
 
@@ -157,6 +151,48 @@ func configureReporting(app *handlers.App) http.Handler {
 	return handler
 }
 
+// configureLogging the default logger using the configuration. Must be called
+// before creating any contextual loggers.
+func configureLogging(config *configuration.Configuration) error {
+	if config.Log.Level == "" && config.Log.Formatter == "" {
+		// If no config for logging is set, fallback to deprecated "Loglevel".
+		log.SetLevel(logLevel(config.Loglevel))
+		return nil
+	}
+
+	log.SetLevel(logLevel(config.Log.Level))
+
+	switch config.Log.Formatter {
+	case "json":
+		log.SetFormatter(&log.JSONFormatter{})
+	case "text":
+		log.SetFormatter(&log.TextFormatter{})
+	case "logstash":
+		log.SetFormatter(&logstash.LogstashFormatter{Type: "registry"})
+	default:
+		// just let the library use default on empty string.
+		if config.Log.Formatter != "" {
+			return fmt.Errorf("unsupported logging formatter: %q", config.Log.Formatter)
+		}
+	}
+
+	if config.Log.Formatter != "" {
+		log.Debugf("using %q logging formatter", config.Log.Formatter)
+	}
+
+	return nil
+}
+
+func logLevel(level configuration.Loglevel) log.Level {
+	l, err := log.ParseLevel(string(level))
+	if err != nil {
+		l = log.InfoLevel
+		log.Warnf("error parsing level %q: %v, using %q	", level, err, l)
+	}
+
+	return l
+}
+
 // debugServer starts the debug server with pprof, expvar among other
 // endpoints. The addr should not be exposed externally. For most of these to
 // work, tls cannot be enabled on the endpoint, so it is generally separate.

configuration/configuration.go

@@ -16,7 +16,19 @@ type Configuration struct {
 	// Version is the version which defines the format of the rest of the configuration
 	Version Version `yaml:"version"`
 
-	// Loglevel is the level at which registry operations are logged
+	// Log supports setting various parameters related to the logging
+	// subsystem.
+	Log struct {
+		// Level is the granularity at which registry operations are logged.
+		Level Loglevel `yaml:"level"`
+
+		// Formatter overrides the default formatter with another. Options
+		// include "text", "json" and "logstash".
+		Formatter string `yaml:"formatter"`
+	}
+
+	// Loglevel is the level at which registry operations are logged. This is
+	// deprecated. Please use Log.Level in the future.
 	Loglevel Loglevel `yaml:"loglevel"`
 
 	// Storage is the configuration for the registry's storage driver

doc/configuration.md

@@ -4,7 +4,10 @@ Below is a comprehensive example of all possible configuration options for the r
 
 ```yaml
 version: 0.1
-loglevel: debug
+log:
+	level: debug
+	formatter: text
+loglevel: debug # deprecated: use "log"
 storage:
 	filesystem:
 		rootdirectory: /tmp/registry
@@ -87,18 +90,34 @@ The version option is **required** and indicates the version of the configuratio
 
 N.B. The version of the registry software may be found at [/version/version.go](https://github.com/docker/distribution/blob/master/version/version.go)
 
+## log
+
+The log subsection configures the behavior of the logging system. The logging
+system outputs everything to stdout. The granularity and format of the log
+messages can be adjusted with this configuration section.
+
+```yaml
+log:
+	level: debug
+	formatter: text
+```
+
+- level: **Optional** - Sets the sensitivity of logging output. Permitted
+  values are `error`, `warn`, `info` and `debug`. The default is `info`.
+- formatter: **Optional** - This selects the format of logging output, which
+  mostly affects how keyed attributes for a log line are encoded. Options are
+  "text", "json" or "logstash". The default is "text".
+
 ## loglevel
 
+> **DEPRECATED:** Please use [log](#log) instead.
+
 ```yaml
 loglevel: debug
 ```
 
-The loglevel option is **required** and sets the sensitivity of logging output. Permitted values are:
-
-- ```error```
-- ```warn```
-- ```info```
-- ```debug```
+Permitted values are `error`, `warn`, `info` and `debug`. The default is
+`info`.
 
 ## storage