Add counter for exposing scanner metrics

README.md

@@ -104,37 +104,59 @@ To leverage the DDB checkpoint we'll also need to create a table:
 
 ## Options
 
-The consumer allows the following optional overrides:
+The consumer allows the following optional overrides.
 
-* Kinesis Client
-* Logger
+### Client
+
+Override the Kinesis client if there is any special config needed:
 
 ```go
-// new kinesis client
-svc := kinesis.New(session.New(aws.NewConfig()))
+// client
+client := kinesis.New(session.New(aws.NewConfig()))
 
-// new consumer with custom client
+// consumer
 c, err := consumer.New(
 	consumer,
 	streamName,
-	consumer.WithClient(svc),
+	consumer.WithClient(client),
 )
 ```
 
-## Logging
+### Metrics
 
-The package defaults to `ioutil.Discard` which will silence log output. This can be overridden with the preferred logging strategy:
+Add optional counter for exposing counts for checkpoints and records processed:
 
 ```go
-func main() {
-	// ...
+// counter
+counter := expvar.NewMap("counters")
 
+// consumer
+c, err := consumer.New(checkpoint, appName, streamName,
+	consumer.WithCounter(counter),
+)
+```
+
+The [expvar package](https://golang.org/pkg/expvar/) will display consumer counts:
+
+```
+"counters": {
+    "checkpoints": 3,
+    "records": 13005
+},
+```
+
+### Logging
+
+The package defaults to `ioutil.Discard` so swallow all logs. This can be customized with the preferred logging strategy:
+
+```go
 // logger
 logger := log.New(os.Stdout, "consumer-example: ", log.LstdFlags)
 
 // consumer
-	c, err := consumer.New(checkpoint, appName, streamName, consumer.WithLogger(logger))
-}
+c, err := consumer.New(checkpoint, appName, streamName,
+	consumer.WithLogger(logger),
+)
 ```
 
 ## Contributing

consumer.go

@@ -15,6 +15,15 @@ import (
 
 type Record = kinesis.Record
 
+// Counter is used for exposing basic metrics from the scanner
+type Counter interface {
+	Add(string, int64)
+}
+
+type noopCounter struct{}
+
+func (n noopCounter) Add(string, int64) {}
+
 // Option is used to override defaults when creating a new Consumer
 type Option func(*Consumer) error
 
@@ -34,6 +43,14 @@ func WithLogger(logger *log.Logger) Option {
 	}
 }
 
+// WithCounter overrides the default counter
+func WithCounter(counter Counter) Option {
+	return func(c *Consumer) error {
+		c.counter = counter
+		return nil
+	}
+}
+
 // New creates a kinesis consumer with default settings. Use Option to override
 // any of the optional attributes.
 func New(checkpoint checkpoint.Checkpoint, app, stream string, opts ...Option) (*Consumer, error) {
@@ -64,7 +81,7 @@ func New(checkpoint checkpoint.Checkpoint, app, stream string, opts ...Option) (
 
 	// provide default logger
 	if c.logger == nil {
-		c.logger = log.New(ioutil.Discard, "kinesis-consumer: ", log.LstdFlags)
+		c.logger = log.New(ioutil.Discard, "", log.LstdFlags)
 	}
 
 	// provide a default kinesis client
@@ -72,6 +89,11 @@ func New(checkpoint checkpoint.Checkpoint, app, stream string, opts ...Option) (
 		c.client = kinesis.New(session.New(aws.NewConfig()))
 	}
 
+	// provide default no-op counter
+	if c.counter == nil {
+		c.counter = &noopCounter{}
+	}
+
 	return c, nil
 }
 
@@ -82,6 +104,7 @@ type Consumer struct {
 	client     *kinesis.Kinesis
 	logger     *log.Logger
 	checkpoint checkpoint.Checkpoint
+	counter    Counter
 }
 
 // Scan scans each of the shards of the stream, calls the callback
@@ -168,7 +191,9 @@ loop:
 					}
 				}
 
-				c.logger.Println("checkpointing", shardID, len(resp.Records))
+				c.counter.Add("records", int64(len(resp.Records)))
+				c.counter.Add("checkpoints", 1)
+
 				if err := c.checkpoint.Set(shardID, lastSeqNum); err != nil {
 					c.logger.Printf("set checkpoint error: %v", err)
 				}

examples/consumer/main.go

@@ -2,15 +2,30 @@ package main
 
 import (
 	"context"
+	"expvar"
 	"flag"
 	"fmt"
 	"log"
+	"net"
+	"net/http"
 	"os"
 
 	consumer "github.com/harlow/kinesis-consumer"
 	checkpoint "github.com/harlow/kinesis-consumer/checkpoint/redis"
 )
 
+// kick off a server for exposing metrics
+func init() {
+	sock, err := net.Listen("tcp", "localhost:8080")
+	if err != nil {
+		log.Fatalf("net listen error: %v", err)
+	}
+	go func() {
+		fmt.Println("Metrics available at http://localhost:8080/debug/vars")
+		http.Serve(sock, nil)
+	}()
+}
+
 func main() {
 	var (
 		app    = flag.String("app", "", "App name")
@@ -18,8 +33,10 @@ func main() {
 	)
 	flag.Parse()
 
-	// logger
-	logger := log.New(os.Stdout, "consumer-example: ", log.LstdFlags)
+	var (
+		counter = expvar.NewMap("counters")
+		logger  = log.New(os.Stdout, "consumer-example: ", log.LstdFlags)
+	)
 
 	// checkpoint
 	ck, err := checkpoint.New(*app, *stream)
@@ -28,12 +45,15 @@ func main() {
 	}
 
 	// consumer
-	c, err := consumer.New(ck, *app, *stream, consumer.WithLogger(logger))
+	c, err := consumer.New(ck, *app, *stream,
+		consumer.WithLogger(logger),
+		consumer.WithCounter(counter),
+	)
 	if err != nil {
 		log.Fatalf("consumer error: %v", err)
 	}
 
-	// scan stream
+	// start scan
 	err = c.Scan(context.TODO(), func(r *consumer.Record) bool {
 		fmt.Println(string(r.Data))
 		return true // continue scanning