logging: Add ability to deduplicate log messages (#48)

Motivating example is to not excessively log the same http 4xx code over and over from the same ip,
as this doesn't add info and bloats our log volume.

This requires some abuse of logrus to work, but we do this via a formatter which can either emit the original message,
or the empty string to log no message (this is harmless). Later, a seperate goroutine will emit a new log
that logs the original log and how many times it occurred.

We always log the first time we see a given message, then start a timer. If we see the message again at least once in that
time frame, we log at the end of the time frame how many times.
Note this means we can say "repeated once", which is a little confusing but preferable to displaying the original message
unchanged but time delayed, which can be very misleading as there's no indication of the time delay.

log/logging.go

@@ -5,13 +5,19 @@ import (
 	"fmt"
 	"os"
 	"strings"
+	"sync"
+	"time"
 
 	"golang.org/x/net/context"
 
 	log "github.com/Sirupsen/logrus"
 	"github.com/weaveworks/common/user"
 )
 
+const (
+	defaultDedupeInterval = time.Minute
+)
+
 // Setup configures logging output to stderr, sets the log level and sets the formatter.
 func Setup(logLevel string) error {
 	log.SetOutput(os.Stderr)
@@ -24,6 +30,39 @@ func Setup(logLevel string) error {
 	return nil
 }
 
+// SetupDeduplication should be performed after any other logging setup.
+// For all logs less severe or equal to the given log level (but still higher than the logger's configured log level),
+// these logs will be 'deduplicated'. What this means is that, excluding certain special fields like time, multiple
+// identical log entries will be grouped up and a summary message emitted.
+// For example, instead of:
+//     00:00:00 INFO User 123 did xyz
+//     00:00:10 INFO User 123 did xyz
+//     00:00:25 INFO User 123 did xyz
+//     00:00:55 INFO User 123 did xyz
+// you would get:
+//     00:00:00 INFO User 123 did xyz
+//     00:01:00 INFO Repeated 3 times: User 123 did xyz
+// The interval argument controls how long to wait for additional messages to arrive before reporting.
+// Increase it to deduplicate more aggressively, decrease it to lower latency from a log occurring to it appearing.
+// Set it to 0 to pick a sensible default value (recommended).
+// NOTE: For simplicity and efficiency, fields are considered 'equal' if and only if their string representations (%v) are equal.
+func SetupDeduplication(logLevel string, interval time.Duration) error {
+	dedupeLevel, err := log.ParseLevel(logLevel)
+	if err != nil {
+		return fmt.Errorf("Error parsing log level: %v", err)
+	}
+	if interval <= 0 {
+		interval = defaultDedupeInterval
+	}
+
+	// We use a special Formatter to either format the log using the original formatter, or to return ""
+	// so nothing will be written for that event. The repeated entries are later logged along with a field flag
+	// that tells the formatter to ignore the message.
+	stdLogger := log.StandardLogger()
+	stdLogger.Formatter = newDedupeFormatter(stdLogger.Formatter, dedupeLevel, interval)
+	return nil
+}
+
 type textFormatter struct{}
 
 // Based off logrus.TextFormatter, which behaves completely
@@ -35,9 +74,7 @@ func (f *textFormatter) Format(entry *log.Entry) ([]byte, error) {
 	timeStamp := entry.Time.Format("2006/01/02 15:04:05.000000")
 	if len(entry.Data) > 0 {
 		fmt.Fprintf(b, "%s: %s %-44s ", levelText, timeStamp, entry.Message)
-		for k, v := range entry.Data {
-			fmt.Fprintf(b, " %s=%v", k, v)
-		}
+		b.WriteString(fieldsToString(entry.Data))
 	} else {
 		// No padding when there's no fields
 		fmt.Fprintf(b, "%s: %s %s", levelText, timeStamp, entry.Message)
@@ -55,3 +92,93 @@ func (f *textFormatter) Format(entry *log.Entry) ([]byte, error) {
 func With(ctx context.Context) *log.Entry {
 	return log.WithFields(user.LogFields(ctx))
 }
+
+type entryCount struct {
+	entry log.Entry
+	count int
+}
+
+type dedupeFormatter struct {
+	innerFormatter log.Formatter
+	level          log.Level
+	interval       time.Duration
+	seen           map[string]entryCount
+	lock           sync.Mutex
+}
+
+func newDedupeFormatter(innerFormatter log.Formatter, level log.Level, interval time.Duration) *dedupeFormatter {
+	return &dedupeFormatter{
+		innerFormatter: innerFormatter,
+		level:          level,
+		interval:       interval,
+		seen:           map[string]entryCount{},
+	}
+}
+
+func (f *dedupeFormatter) Format(entry *log.Entry) ([]byte, error) {
+	if f.shouldLog(entry) {
+		b, err := f.innerFormatter.Format(entry)
+		return b, err
+	}
+	return []byte{}, nil
+}
+
+func (f *dedupeFormatter) shouldLog(entry *log.Entry) bool {
+	if _, ok := entry.Data["deduplicated"]; ok {
+		// ignore our own logs about deduped messages
+		return true
+	}
+	if entry.Level < f.level {
+		// ignore logs more severe than our level
+		return true
+	}
+	key := fmt.Sprintf("%s %s", entry.Message, fieldsToString(entry.Data))
+	f.lock.Lock()
+	defer f.lock.Unlock()
+	if ec, ok := f.seen[key]; ok {
+		// already seen, increment count and do not log
+		ec.count++
+		f.seen[key] = ec
+		return false
+	}
+	// New message, log it but add it to seen.
+	// We need to copy because the pointer ceases to be valid after we return from Format
+	f.seen[key] = entryCount{entry: *entry}
+	go f.evictEntry(key) // queue to evict later
+	return true
+}
+
+// Wait for interval seconds then evict the entry and send the log
+func (f *dedupeFormatter) evictEntry(key string) {
+	time.Sleep(f.interval)
+	var ec entryCount
+	func() {
+		f.lock.Lock()
+		defer f.lock.Unlock()
+		ec = f.seen[key]
+		delete(f.seen, key)
+	}()
+	if ec.count == 0 {
+		return
+	}
+	entry := log.WithFields(ec.entry.Data).WithField("deduplicated", ec.count)
+	message := fmt.Sprintf("Repeated %d times: %s", ec.count, ec.entry.Message)
+	// There's no way to choose the log level dynamically, so we have to do this hack
+	map[log.Level]func(args ...interface{}){
+		log.PanicLevel: entry.Panic,
+		log.FatalLevel: entry.Fatal,
+		log.ErrorLevel: entry.Error,
+		log.WarnLevel:  entry.Warn,
+		log.InfoLevel:  entry.Info,
+		log.DebugLevel: entry.Debug,
+	}[ec.entry.Level](message)
+}
+
+func fieldsToString(data log.Fields) string {
+	parts := make([]string, 0, len(data))
+	// traversal order here is arbitrary but stable, which is fine for our purposes
+	for k, v := range data {
+		parts = append(parts, fmt.Sprintf("%s=%v", k, v))
+	}
+	return strings.Join(parts, " ")
+}