The writer module provides some utilties for writing to outside components from within Storm. This includes managing bulk writing. An implemention is included for writing to HDFS in this module. Other writers can be found in their own modules.

Kafka Writer

We have an implementation of a writer which will write batches of messages to Kafka. An interesting aspect of this writer is that it can be configured to allow users to specify a message field which contains the topic for the message.

The configuration for this writer is held in the individual Sensor Configurations:

  • Enrichment under the config element
  • Parsers in the parserConfig element
  • Profiler - Unsupported currently

In each of these, the kafka writer can be configured via a map which has the following elements:

  • kafka.brokerUrl : The broker URL
  • kafka.keySerializer : The key serializer (defaults to StringSerializer)
  • kafka.valueSerializer : The key serializer (defaults to StringSerializer)
  • kafka.zkQuorum : The zookeeper quorum
  • kafka.requiredAcks : Whether to require acks.
  • kafka.topic : The topic to write to
  • kafka.topicField : The field to pull the topic from. If this is specified, then the producer will use this. If it is unspecified, then it will default to the kafka.topic property. If neither are specified, then an error will occur.
  • kafka.producerConfigs : A map of kafka producer configs for advanced customization.

HDFS Writer

The HDFS writer included here expands on what Storm has in several ways. There’s customization in syncing to HDFS, rotation policy, etc. In addition, the writer allows for users to define output paths based on the fields in the provided JSON message. This can be defined using Stellar.

To manage the output path, a base path argument is provided by the Flux file, with the FileNameFormat as follows

    -   id: "fileNameFormat"
        className: "org.apache.storm.hdfs.bolt.format.DefaultFileNameFormat"
            -   name: "withPrefix"
                    - "enrichment-"
            -   name: "withExtension"
                  - ".json"
            -   name: "withPath"
                    - "/apps/metron/"

This means that all output will land in /apps/metron/. With no further adjustment, it will be /apps/metron/<sensor>/. However, by modifying the sensor’s JSON config, it is possible to provide additional pathing based on the the message itself.


  "index": "bro",
  "batchSize": 5,
  "outputPathFunction": "FORMAT('uid-%s', uid)"

will land data in /apps/metron/uid-<uid>/.

For example, if the data contains uid’s 1, 3, and 5, there will be 3 output folders in HDFS:


The Stellar function must return a String, but is not limited to FORMAT functions. Other functions, such as TO_LOWER, TO_UPPER, etc. are all available for use. Typically, it’s preferable to do nontrivial transformations as part of enrichment and simply reference the output here.

If no Stellar function is provided, it will default to putting the sensor in a folder, as above.

A caveat is that the writer will only allow a certain number of files to be created at once. HdfsWriter has a function withMaxOpenFiles allowing this to be set. The default is 500. This can be set in Flux:

    -   id: "hdfsWriter"
        className: "org.apache.metron.writer.hdfs.HdfsWriter"
            -   name: "withFileNameFormat"
                    - ref: "fileNameFormat"
            -   name: "withRotationPolicy"
                    - ref: "hdfsRotationPolicy"
            -   name: "withMaxOpenFiles"
                args: 500