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Wombat

Kafka Topic Mirroring

Kafka-flavoured Wombat, a favourite here in the Wisdom, is a quick-and-easy recipe you can whip up in minutes. This cookbook will illustrate how to use Wombat for consuming and publishing events to Kafka, with the goal of mirroring Kafka topics while preserving partition mappings.

For example, the diagram below shows partition preservation of some process where wombat consumes an event from Partition 2 of Topic A and maps it to Partition 2 of Topic B:

  Topic A                                                                  Topic B
+-----------------------------+                                          +-----------------------------+
|                             |                                          |                             |
|    +--------------------+   |                                          |   +--------------------+    |
| P1 |                    |   |                                          |   |                    | P1 |
|    +--------------------+   |                                          |   +--------------------+    |
|                             |       +--------------------------+       |                             |
|    +--------------------+   |       |                          |       |   +--------------------+    |
| P2 |                    |---------->|          wombat          |---------->|                    | P2 |
|    +--------------------+   |       |                          |       |   +--------------------+    |
|                             |       +--------------------------+       |                             |
|    +--------------------+   |                                          |   +--------------------+    |
| P3 |                    |   |                                          |   |                    | P3 |
|    +--------------------+   |                                          |   +--------------------+    |
|                             |                                          |                             |
+-----------------------------+                                          +-----------------------------+

Consuming Events

To start consuming data, we can use the kafka_franz input component. Here, we will read in all new events from the foo and bar topics.

input:
  kafka_franz:
    consumer_group: wombat_bridge_consumer
    seed_brokers: [ TODO ]
    topics: [ foo, bar ]

Publishing Events

We can use the kafka_franz output component for publishing messages to a topic. As you’ll see, this component is incredibly flexible, with several fields supporting string interpolation for dynamic value setting.

Let’s route all events received from foo and bar to some existing topics named output-foo and output-bar, respectively.

Fortunately, Wombat makes this straightforward as the kafka_franz input component attaches useful metadata to each message, including the source event’s kafka_key, kafka_topic, and kafka_partition.

Using string interpolation, we can then extract the original topic name from the kafka_topic metadata field, prepend the output- prefix, and pass this as output to the topic field — dynamically setting the topic destinations.

output:
  kafka_franz:
    seed_brokers: [ TODO ]
    topic: 'output-${! metadata("kafka_topic") }'

Recall from earlier that we also wanted to preserve our partition mapping when writing to new topics. Again, we can use metadata to retrieve the original partition of each message in the source topic. We’ll use the kafka_partition metadata field in conjunction with setting partitioner to manual — overriding any other fancy partitioning algorithm in favour of preserving our initial mapping. Combining again with string interpolation, we get the following:

output:
  kafka_franz:
    seed_brokers: [ TODO ]
    topic: 'output-${! metadata("kafka_topic") }'
    partition: ${! metadata("kafka_partition") }
    partitioner: manual

Voilà! The above config:

  • Consumes events from the foo and bar topics
  • Routes the output destination of events from foo to output-foo and from bar to output-bar using the kafka_topic metadata field
  • Explicitly sets the message partition to that of the source message using the metadata field kafka_partition

For completeness, we can also route all consumed events back to their original source topic and partition.

output:
  kafka_franz:
    seed_brokers: [ TODO ]
    topic: ${! metadata("kafka_topic") }
    partition: ${! metadata("kafka_partition") }
    partitioner: manual

Regular Expression Matching

We begin by consuming from 2 topics: foobar and foobaz.

input:
  kafka_franz:
    consumer_group: wombat_bridge_consumer
    seed_brokers: [ TODO ]
    topics: [ foobar, foobaz ]

Notice that both topics share a common prefix of foo. It’s easy to imagine a large or variable amount of topics needing to be consumed by the input. Luckily, we have tools for that as the kafka_franz input also has regular expression matching capabilities.

Include your topics pattern as regex and include regexp_topics: true so that listed topics are interpreted as regex.

input:
  kafka_franz:
    consumer_group: wombat_bridge_consumer
    seed_brokers: [ TODO ]
    topics: [ foo.* ]
    regexp_topics: true

Now Wombat will consume events from all topics with the prefix foo.

Final Words

Wow, you’re a natural, aren’t you?

In this cookbook, we’ve explored how to use Wombat to mirror Kafka topics while preserving partition mappings. We’ve covered:

  • Consuming events from Kafka topics
  • Publishing events to dynamically determined topics
  • Preserving partition information when writing to new topics
  • Regular expressions for matching and consuming from many topics

Happy streaming!