external/storm-kafka/README.md

Storm Kafka
====================

Provides core Storm and Trident spout implementations for consuming data from Apache Kafka 0.8.x.

##Spouts
We support both Trident and core Storm spouts. For both spout implementations, we use a BrokerHost interface that
tracks Kafka broker host to partition mapping and kafkaConfig that controls some Kafka related parameters.

###BrokerHosts
In order to initialize your Kafka spout/emitter you need to construct an instance of the marker interface BrokerHosts.
Currently, we support the following two implementations:

####ZkHosts
ZkHosts is what you should use if you want to dynamically track Kafka broker to partition mapping. This class uses
Kafka's ZooKeeper entries to track brokerHost -> partition mapping. You can instantiate an object by calling
```java
    public ZkHosts(String brokerZkStr, String brokerZkPath)
    public ZkHosts(String brokerZkStr)
```
Where brokerZkStr is just ip:port (e.g. localhost:2181). brokerZkPath is the root directory under which all the topics and
partition information is stored. By default this is /brokers which is what the default Kafka implementation uses.

By default, the broker-partition mapping is refreshed every 60 seconds from ZooKeeper. If you want to change it, you
should set host.refreshFreqSecs to your chosen value.

####StaticHosts
This is an alternative implementation where broker -> partition information is static. In order to construct an instance
of this class, you need to first construct an instance of GlobalPartitionInformation.

```java
    Broker brokerForPartition0 = new Broker("localhost");//localhost:9092
    Broker brokerForPartition1 = new Broker("localhost", 9092);//localhost:9092 but we specified the port explicitly
    Broker brokerForPartition2 = new Broker("localhost:9092");//localhost:9092 specified as one string.
    GlobalPartitionInformation partitionInfo = new GlobalPartitionInformation();
    partitionInfo.addPartition(0, brokerForPartition0);//mapping from partition 0 to brokerForPartition0
    partitionInfo.addPartition(1, brokerForPartition1);//mapping from partition 1 to brokerForPartition1
    partitionInfo.addPartition(2, brokerForPartition2);//mapping from partition 2 to brokerForPartition2
    StaticHosts hosts = new StaticHosts(partitionInfo);
```

###KafkaConfig
The second thing needed for constructing a kafkaSpout is an instance of KafkaConfig.
```java
    public KafkaConfig(BrokerHosts hosts, String topic)
    public KafkaConfig(BrokerHosts hosts, String topic, String clientId)
```

The BrokerHosts can be any implementation of BrokerHosts interface as described above. The topic is name of Kafka topic.
The optional ClientId is used as a part of the ZooKeeper path where the spout's current consumption offset is stored.

There are 2 extensions of KafkaConfig currently in use.

Spoutconfig is an extension of KafkaConfig that supports additional fields with ZooKeeper connection info and for controlling
behavior specific to KafkaSpout. The Zkroot will be used as root to store your consumer's offset. The id should uniquely
identify your spout.
```java
public SpoutConfig(BrokerHosts hosts, String topic, String zkRoot, String id);
```
In addition to these parameters, SpoutConfig contains the following fields that control how KafkaSpout behaves:
```java
    // setting for how often to save the current Kafka offset to ZooKeeper
    public long stateUpdateIntervalMs = 2000;

    // Retry strategy for failed messages
    public String failedMsgRetryManagerClass = ExponentialBackoffMsgRetryManager.class.getName();

    // Exponential back-off retry settings.  These are used by ExponentialBackoffMsgRetryManager for retrying messages after a bolt
    // calls OutputCollector.fail(). These come into effect only if ExponentialBackoffMsgRetryManager is being used.
    // Initial delay between successive retries
    public long retryInitialDelayMs = 0;
    public double retryDelayMultiplier = 1.0;
    
    // Maximum delay between successive retries    
    public long retryDelayMaxMs = 60 * 1000;
    // Failed message will be retried infinitely if retryLimit is less than zero. 
    public int retryLimit = -1;     

```
Core KafkaSpout only accepts an instance of SpoutConfig.

TridentKafkaConfig is another extension of KafkaConfig.
TridentKafkaEmitter only accepts TridentKafkaConfig.

The KafkaConfig class also has bunch of public variables that controls your application's behavior. Here are defaults:
```java
    public int fetchSizeBytes = 1024 * 1024;
    public int socketTimeoutMs = 10000;
    public int fetchMaxWait = 10000;
    public int bufferSizeBytes = 1024 * 1024;
    public MultiScheme scheme = new RawMultiScheme();
    public boolean ignoreZkOffsets = false;
    public long startOffsetTime = kafka.api.OffsetRequest.EarliestTime();
    public long maxOffsetBehind = Long.MAX_VALUE;
    public boolean useStartOffsetTimeIfOffsetOutOfRange = true;
    public int metricsTimeBucketSizeInSecs = 60;
```

Most of them are self explanatory except MultiScheme.
###MultiScheme
MultiScheme is an interface that dictates how the ByteBuffer consumed from Kafka gets transformed into a storm tuple. It
also controls the naming of your output field.

```java
  public Iterable<List<Object>> deserialize(ByteBuffer ser);
  public Fields getOutputFields();
```

The default `RawMultiScheme` just takes the `ByteBuffer` and returns a tuple with the ByteBuffer converted to a `byte[]`. The name of the outputField is "bytes". There are alternative implementations like `SchemeAsMultiScheme` and `KeyValueSchemeAsMultiScheme` which can convert the `ByteBuffer` to `String`.

There is also an extension of `SchemeAsMultiScheme`, `MessageMetadataSchemeAsMultiScheme`,
which has an additional deserialize method that accepts the message `ByteBuffer` in addition to the `Partition` and `offset` associated with the message.

```java
public Iterable<List<Object>> deserializeMessageWithMetadata(ByteBuffer message, Partition partition, long offset)

```

This is useful for auditing/replaying messages from arbitrary points on a Kafka topic, saving the partition and offset of each message of a discrete stream instead of persisting the entire message.

###Failed message retry
FailedMsgRetryManager is an interface which defines the retry strategy for a failed message. Default implementation is ExponentialBackoffMsgRetryManager which retries with exponential delays
between consecutive retries. To use a custom implementation, set SpoutConfig.failedMsgRetryManagerClass to the full classname
of implementation. Here is the interface 

```java
    // Spout initialization can go here. This can be called multiple times during lifecycle of a worker. 
    void prepare(SpoutConfig spoutConfig, Map stormConf);

    // Message corresponding to offset has failed. This method is called only if retryFurther returns true for offset.
    void failed(Long offset);

    // Message corresponding to offset has been acked.  
    void acked(Long offset);

    // Message corresponding to the offset, has been re-emitted and under transit.
    void retryStarted(Long offset);

    /**
     * The offset of message, which is to be re-emitted. Spout will fetch messages starting from this offset
     * and resend them, except completed messages.
     */
    Long nextFailedMessageToRetry();

    /**
     * @return True if the message corresponding to the offset should be emitted NOW. False otherwise.
     */
    boolean shouldReEmitMsg(Long offset);

    /**
     * Spout will clean up the state for this offset if false is returned. If retryFurther is set to true,
     * spout will called failed(offset) in next call and acked(offset) otherwise 
     */
    boolean retryFurther(Long offset);

    /**
     * Clear any offsets before kafkaOffset. These offsets are no longer available in kafka.
     */
    Set<Long> clearOffsetsBefore(Long kafkaOffset);
``` 

#### Version incompatibility
In Storm versions prior to 1.0, the MultiScheme methods accepted a `byte[]` instead of `ByteBuffer`. The `MultScheme` and the related
Scheme apis were changed in version 1.0 to accept a ByteBuffer instead of a byte[].

This means that pre 1.0 kafka spouts will not work with Storm versions 1.0 and higher. While running topologies in Storm version 1.0
and higher, it must be ensured that the storm-kafka version is at least 1.0. Pre 1.0 shaded topology jars that bundles
storm-kafka classes must be rebuilt with storm-kafka version 1.0 for running in clusters with storm 1.0 and higher.

### Examples

#### Core Spout

```java
BrokerHosts hosts = new ZkHosts(zkConnString);
SpoutConfig spoutConfig = new SpoutConfig(hosts, topicName, "/" + topicName, UUID.randomUUID().toString());
spoutConfig.scheme = new SchemeAsMultiScheme(new StringScheme());
KafkaSpout kafkaSpout = new KafkaSpout(spoutConfig);
```

#### Trident Spout
```java
TridentTopology topology = new TridentTopology();
BrokerHosts zk = new ZkHosts("localhost");
TridentKafkaConfig spoutConf = new TridentKafkaConfig(zk, "test-topic");
spoutConf.scheme = new SchemeAsMultiScheme(new StringScheme());
OpaqueTridentKafkaSpout spout = new OpaqueTridentKafkaSpout(spoutConf);
```


### How KafkaSpout stores offsets of a Kafka topic and recovers in case of failures

As shown in the above KafkaConfig properties, you can control from where in the Kafka topic the spout begins to read by
setting `KafkaConfig.startOffsetTime` as follows:

1. `kafka.api.OffsetRequest.EarliestTime()`:  read from the beginning of the topic (i.e. from the oldest messages onwards)
2. `kafka.api.OffsetRequest.LatestTime()`: read from the end of the topic (i.e. any new messsages that are being written to the topic)
3. A Unix timestamp aka seconds since the epoch (e.g. via `System.currentTimeMillis()`):
   see [How do I accurately get offsets of messages for a certain timestamp using OffsetRequest?](https://cwiki.apache.org/confluence/display/KAFKA/FAQ#FAQ-HowdoIaccuratelygetoffsetsofmessagesforacertaintimestampusingOffsetRequest?) in the Kafka FAQ

As the topology runs the Kafka spout keeps track of the offsets it has read and emitted by storing state information
under the ZooKeeper path `SpoutConfig.zkRoot+ "/" + SpoutConfig.id`.  In the case of failures it recovers from the last
written offset in ZooKeeper.

> **Important:**  When re-deploying a topology make sure that the settings for `SpoutConfig.zkRoot` and `SpoutConfig.id`
> were not modified, otherwise the spout will not be able to read its previous consumer state information (i.e. the
> offsets) from ZooKeeper -- which may lead to unexpected behavior and/or to data loss, depending on your use case.

This means that when a topology has run once the setting `KafkaConfig.startOffsetTime` will not have an effect for
subsequent runs of the topology because now the topology will rely on the consumer state information (offsets) in
ZooKeeper to determine from where it should begin (more precisely: resume) reading.
If you want to force the spout to ignore any consumer state information stored in ZooKeeper, then you should
set the parameter `KafkaConfig.ignoreZkOffsets` to `true`.  If `true`, the spout will always begin reading from the
offset defined by `KafkaConfig.startOffsetTime` as described above.


## Using storm-kafka with different versions of Scala

Storm-kafka's Kafka dependency is defined as `provided` scope in maven, meaning it will not be pulled in
as a transitive dependency. This allows you to use a version of Kafka built against a specific Scala version.

When building a project with storm-kafka, you must explicitly add the Kafka dependency. For example, to
use Kafka 0.8.1.1 built against Scala 2.10, you would use the following dependency in your `pom.xml`:

```xml
        <dependency>
            <groupId>org.apache.kafka</groupId>
            <artifactId>kafka_2.10</artifactId>
            <version>0.8.1.1</version>
            <exclusions>
                <exclusion>
                    <groupId>org.apache.zookeeper</groupId>
                    <artifactId>zookeeper</artifactId>
                </exclusion>
                <exclusion>
                    <groupId>log4j</groupId>
                    <artifactId>log4j</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
```

Note that the ZooKeeper and log4j dependencies are excluded to prevent version conflicts with Storm's dependencies.

##Writing to Kafka as part of your topology
You can create an instance of org.apache.storm.kafka.bolt.KafkaBolt and attach it as a component to your topology or if you
are using trident you can use org.apache.storm.kafka.trident.TridentState, org.apache.storm.kafka.trident.TridentStateFactory and
org.apache.storm.kafka.trident.TridentKafkaUpdater.

You need to provide implementation of following 2 interfaces

###TupleToKafkaMapper and TridentTupleToKafkaMapper
These interfaces have 2 methods defined:

```java
    K getKeyFromTuple(Tuple/TridentTuple tuple);
    V getMessageFromTuple(Tuple/TridentTuple tuple);
```

As the name suggests, these methods are called to map a tuple to Kafka key and Kafka message. If you just want one field
as key and one field as value, then you can use the provided FieldNameBasedTupleToKafkaMapper.java
implementation. In the KafkaBolt, the implementation always looks for a field with field name "key" and "message" if you
use the default constructor to construct FieldNameBasedTupleToKafkaMapper for backward compatibility
reasons. Alternatively you could also specify a different key and message field by using the non default constructor.
In the TridentKafkaState you must specify what is the field name for key and message as there is no default constructor.
These should be specified while constructing and instance of FieldNameBasedTupleToKafkaMapper.

###KafkaTopicSelector and trident KafkaTopicSelector
This interface has only one method
```java
public interface KafkaTopicSelector {
    String getTopics(Tuple/TridentTuple tuple);
}
```
The implementation of this interface should return the topic to which the tuple's key/message mapping needs to be published
You can return a null and the message will be ignored. If you have one static topic name then you can use
DefaultTopicSelector.java and set the name of the topic in the constructor.
`FieldNameTopicSelector` and `FieldIndexTopicSelector` use to support decided which topic should to push message from tuple.
User could specify the field name or field index in tuple ,selector will use this value as topic name which to publish message.
When the topic name not found , `KafkaBolt` will write messages into default topic .
Please make sure the default topic have created .

### Specifying Kafka producer properties
You can provide all the produce properties in your Storm topology by calling `KafkaBolt.withProducerProperties()` and `TridentKafkaStateFactory.withProducerProperties()`. Please see  http://kafka.apache.org/documentation.html#newproducerconfigs
Section "Important configuration properties for the producer" for more details.

###Using wildcard kafka topic match
You can do a wildcard topic match by adding the following config
```
     Config config = new Config();
     config.put("kafka.topic.wildcard.match",true);

```

After this you can specify a wildcard topic for matching e.g. clickstream.*.log.  This will match all streams matching clickstream.my.log, clickstream.cart.log etc


###Putting it all together

For the bolt :
```java
        TopologyBuilder builder = new TopologyBuilder();

        Fields fields = new Fields("key", "message");
        FixedBatchSpout spout = new FixedBatchSpout(fields, 4,
                    new Values("storm", "1"),
                    new Values("trident", "1"),
                    new Values("needs", "1"),
                    new Values("javadoc", "1")
        );
        spout.setCycle(true);
        builder.setSpout("spout", spout, 5);
        //set producer properties.
        Properties props = new Properties();
        props.put("bootstrap.servers", "localhost:9092");
        props.put("acks", "1");
        props.put("key.serializer", "org.apache.kafka.common.serialization.StringSerializer");
        props.put("value.serializer", "org.apache.kafka.common.serialization.StringSerializer");

        KafkaBolt bolt = new KafkaBolt()
                .withProducerProperties(props)
                .withTopicSelector(new DefaultTopicSelector("test"))
                .withTupleToKafkaMapper(new FieldNameBasedTupleToKafkaMapper());
        builder.setBolt("forwardToKafka", bolt, 8).shuffleGrouping("spout");

        Config conf = new Config();

        StormSubmitter.submitTopology("kafkaboltTest", conf, builder.createTopology());
```

For Trident:

```java
        Fields fields = new Fields("word", "count");
        FixedBatchSpout spout = new FixedBatchSpout(fields, 4,
                new Values("storm", "1"),
                new Values("trident", "1"),
                new Values("needs", "1"),
                new Values("javadoc", "1")
        );
        spout.setCycle(true);

        TridentTopology topology = new TridentTopology();
        Stream stream = topology.newStream("spout1", spout);

        //set producer properties.
        Properties props = new Properties();
        props.put("bootstrap.servers", "localhost:9092");
        props.put("acks", "1");
        props.put("key.serializer", "org.apache.kafka.common.serialization.StringSerializer");
        props.put("value.serializer", "org.apache.kafka.common.serialization.StringSerializer");

        TridentKafkaStateFactory stateFactory = new TridentKafkaStateFactory()
                .withProducerProperties(props)
                .withKafkaTopicSelector(new DefaultTopicSelector("test"))
                .withTridentTupleToKafkaMapper(new FieldNameBasedTupleToKafkaMapper("word", "count"));
        stream.partitionPersist(stateFactory, fields, new TridentKafkaUpdater(), new Fields());

        Config conf = new Config();
        StormSubmitter.submitTopology("kafkaTridentTest", conf, topology.build());
```

## Committer Sponsors

 * P. Taylor Goetz ([ptgoetz@apache.org](mailto:ptgoetz@apache.org))