consumer

Golang Kinesis Consumer

Kinesis consumer applications written in Go. This library is intended to be a lightweight wrapper around the Kinesis API to read records, save checkpoints (with swappable backends), and gracefully recover from service timeouts/errors.

Alternate serverless options:

• Kinesis to Firehose can be used to archive data directly to S3, Redshift, or Elasticsearch without running a consumer application.

• Process Kinesis Streams with Golang and AWS Lambda for serverless processing and checkpoint management.

Installation

Get the package source:

$ go get github.com/harlow/kinesis-consumer

Overview

The consumer leverages a handler func that accepts a Kinesis record. The Scan method will consume all shards concurrently and call the callback func as it receives records from the stream.

Important 1: The Scan func will also poll the stream to check for new shards, it will automatcially start consuming new shards added to the stream.

Important 2: The default Log, Counter, and Checkpoint are no-op which means no logs, counts, or checkpoints will be emitted when scanning the stream. See the options below to override these defaults.

import(
	// ...

	consumer "github.com/harlow/kinesis-consumer"
)

func main() {
	var stream = flag.String("stream", "", "Stream name")
	flag.Parse()

	// consumer
	c, err := consumer.New(*stream)
	if err != nil {
		log.Fatalf("consumer error: %v", err)
	}

	// start scan
	err = c.Scan(context.TODO(), func(r *consumer.Record) error {
		fmt.Println(string(r.Data))
		return nil // continue scanning
	})
	if err != nil {
		log.Fatalf("scan error: %v", err)
	}

	// Note: If you need to aggregate based on a specific shard
	// the `ScanShard` function should be used instead.
}

ScanFunc

ScanFunc is the type of the function called for each message read from the stream. The record argument contains the original record returned from the AWS Kinesis library.

type ScanFunc func(r *Record) error

If an error is returned, scanning stops. The sole exception is when the function returns the special value SkipCheckpoint.

// continue scanning
return nil

// continue scanning, skip checkpoint
return consumer.SkipCheckpoint

// stop scanning, return error
return errors.New("my error, exit all scans")

Use context cancel to signal the scan to exit without error. For example if we wanted to gracefulloy exit the scan on interrupt.

// trap SIGINT, wait to trigger shutdown
signals := make(chan os.Signal, 1)
signal.Notify(signals, os.Interrupt)

// context with cancel
ctx, cancel := context.WithCancel(context.Background())

go func() {
	<-signals
	cancel() // call cancellation
}()

err := c.Scan(ctx, func(r *consumer.Record) error {
	fmt.Println(string(r.Data))
	return nil // continue scanning
})

Checkpoint

To record the progress of the consumer in the stream we use a checkpoint to store the last sequence number the consumer has read from a particular shard. The boolean value SkipCheckpoint of consumer.ScanError determines if checkpoint will be activated. ScanError is returned by the record processing callback.

This will allow consumers to re-launch and pick up at the position in the stream where they left off.

The uniq identifier for a consumer is [appName, streamName, shardID]

Note: The default checkpoint is no-op. Which means the scan will not persist any state and the consumer will start from the beginning of the stream each time it is re-started.

To persist scan progress choose one of the following checkpoints:

Redis Checkpoint

The Redis checkpoint requries App Name, and Stream Name:

import checkpoint "github.com/harlow/kinesis-consumer/checkpoint/redis"

// redis checkpoint
ck, err := checkpoint.New(appName)
if err != nil {
	log.Fatalf("new checkpoint error: %v", err)
}

DynamoDB Checkpoint

The DynamoDB checkpoint requires Table Name, App Name, and Stream Name:

import checkpoint "github.com/harlow/kinesis-consumer/checkpoint/ddb"

// ddb checkpoint
ck, err := checkpoint.New(appName, tableName)
if err != nil {
	log.Fatalf("new checkpoint error: %v", err)
}

// Override the Kinesis if any needs on session (e.g. assume role)
myDynamoDbClient := dynamodb.New(session.New(aws.NewConfig()))

// For versions of AWS sdk that fixed config being picked up properly, the example of
// setting region should work.
//    myDynamoDbClient := dynamodb.New(session.New(aws.NewConfig()), &aws.Config{
//        Region: aws.String("us-west-2"),
//    })

ck, err := checkpoint.New(*app, *table, checkpoint.WithDynamoClient(myDynamoDbClient))
if err != nil {
  log.Fatalf("new checkpoint error: %v", err)
}

// Or we can provide your own Retryer to customize what triggers a retry inside checkpoint
// See code in examples
// ck, err := checkpoint.New(*app, *table, checkpoint.WithDynamoClient(myDynamoDbClient), checkpoint.WithRetryer(&MyRetryer{}))

To leverage the DDB checkpoint we'll also need to create a table:

Partition key: namespace
Sort key: shard_id

Postgres Checkpoint

The Postgres checkpoint requires Table Name, App Name, Stream Name and ConnectionString:

import checkpoint "github.com/harlow/kinesis-consumer/checkpoint/postgres"

// postgres checkpoint
ck, err := checkpoint.New(app, table, connStr)
if err != nil {
  log.Fatalf("new checkpoint error: %v", err)
}

To leverage the Postgres checkpoint we'll also need to create a table:

CREATE TABLE kinesis_consumer (
	namespace text NOT NULL,
	shard_id text NOT NULL,
	sequence_number numeric NOT NULL,
	CONSTRAINT kinesis_consumer_pk PRIMARY KEY (namespace, shard_id)
);

The table name has to be the same that you specify when creating the checkpoint. The primary key composed by namespace and shard_id is mandatory in order to the checkpoint run without issues and also to ensure data integrity.

Mysql Checkpoint

The Mysql checkpoint requires Table Name, App Name, Stream Name and ConnectionString (just like the Postgres checkpoint!):

import checkpoint "github.com/harlow/kinesis-consumer/checkpoint/mysql"

// mysql checkpoint
ck, err := checkpoint.New(app, table, connStr)
if err != nil {
  log.Fatalf("new checkpoint error: %v", err)
}

To leverage the Mysql checkpoint we'll also need to create a table:

CREATE TABLE kinesis_consumer (
	namespace varchar(255) NOT NULL,
	shard_id varchar(255) NOT NULL,
	sequence_number numeric(65,0) NOT NULL,
	CONSTRAINT kinesis_consumer_pk PRIMARY KEY (namespace, shard_id)
);

The table name has to be the same that you specify when creating the checkpoint. The primary key composed by namespace and shard_id is mandatory in order to the checkpoint run without issues and also to ensure data integrity.

Options

The consumer allows the following optional overrides.

Kinesis Client

Override the Kinesis client if there is any special config needed:

// client
client := kinesis.New(session.NewSession(aws.NewConfig()))

// consumer
c, err := consumer.New(streamName, consumer.WithClient(client))

Metrics

Add optional counter for exposing counts for checkpoints and records processed:

// counter
counter := expvar.NewMap("counters")

// consumer
c, err := consumer.New(streamName, consumer.WithCounter(counter))

The expvar package will display consumer counts:

"counters": {
    "checkpoints": 3,
    "records": 13005
},

Consumer starting point

Kinesis allows consumers to specify where on the stream they'd like to start consuming from. The default in this library is LATEST (Start reading just after the most recent record in the shard).

This can be adjusted by using the WithShardIteratorType option in the library:

// override starting place on stream to use TRIM_HORIZON
c, err := consumer.New(
  *stream,
  consumer.WithShardIteratorType(kinesis.ShardIteratorTypeTrimHorizon)
)

See AWS Docs for more options.

Logging

Logging supports the basic built-in logging library or use thrid party external one, so long as it implements the Logger interface.

For example, to use the builtin logging package, we wrap it with myLogger structure.

// A myLogger provides a minimalistic logger satisfying the Logger interface.
type myLogger struct {
	logger *log.Logger
}

// Log logs the parameters to the stdlib logger. See log.Println.
func (l *myLogger) Log(args ...interface{}) {
	l.logger.Println(args...)
}

The package defaults to ioutil.Discard so swallow all logs. This can be customized with the preferred logging strategy:

// logger
logger := &myLogger{
	logger: log.New(os.Stdout, "consumer-example: ", log.LstdFlags),
}

// consumer
c, err := consumer.New(streamName, consumer.WithLogger(logger))

To use a more complicated logging library, e.g. apex log

type myLogger struct {
	logger *log.Logger
}

func (l *myLogger) Log(args ...interface{}) {
	l.logger.Infof("producer", args...)
}

func main() {
	log := &myLogger{
		logger: alog.Logger{
			Handler: text.New(os.Stderr),
			Level:   alog.DebugLevel,
		},
	}

Contributing

Please see CONTRIBUTING.md for more information. Thank you, contributors!

License

Copyright (c) 2015 Harlow Ward. It is free software, and may be redistributed under the terms specified in the LICENSE file.

www.hward.com  ·  GitHub @harlow  ·  Twitter @harlow_ward