Configuration settings

In this section we discuss the main configuration settings to get kafka-aggregator running. The Configuration class is also documented here and can be used as reference for the configuration settings exposed in the values.yaml when using the Helm chart.

Kafka settings

To configure kafka-aggregator with Kafka, you need to provide the Kafka broker and the schema_registry_url URLs.

Separating source and aggregated schemas

The Confluent Schema Registry is used to manage Avro schemas for the source and aggregation topics.

In a Kafka multi-site set up, usually there’s continuous migration of the Avro schemas from the source Schema Registry to the destination Schema Registry.

kafka-aggregator normally runs on the destination cluster and thus it would register the aggregation topic schemas to the destination Schema Registry. Depending on how replication is configured, you don’t have schema migration back to the source cluster or you might not want to replicate the schemas for the aggregation topics.

Either way, to avoid collisions between schema IDs for schemas created at the source Schema Registry and destination Schema Registry, we recommend deploying a separate Schema Registry to store the schemas for the aggregation topics.

In this case, set the internal_registry_url configuration accordingly. The docker-compose configuration in the kafka-aggregator repository shows how to configure an internal schema registry for kafka-aggregator.

Kafka-aggregator settings

The following configuration settings are specific to the kafka-aggregator application.

Selecting topics to aggregate

kafka-aggregator selects source topics from Kafka using a regular expression topic_regex and exclude source topics listed in the excluded_topics list.

Summary Statistics

The operations configuration setting specifies the summary statistics to be computed for each numerical field in the source topic.

Aggregation window settings

Configure the size of the aggregation using the window_size configuration setting. window_expires specifies the duration to store the data allocated to each window.


Faust allocates at least one message on each aggregation window, if window_size is smaller than the time interval between two consecutive messages Faust will skip that window and no aggregation is computed.

When deciding the size of the aggregation window, an important consideration is the data reduction factor given by R=window_size*f_in/N where f_in is the frequency of the input data stream in Hz and N is the number of summary statistics computed by kafka-aggregator. For example, to get a reduction factor of 100 times in storage for an input data stream of 50Hz the size of the aggregation window must be 10s for N=5.

Also, window_size should be large enough to minimize the standard error associated to the number of messages (sample size) allocated to a window. The smaller the standard error the more precise the computed estimate is. kafka-aggregator stores the sample size in the count field of each aggregated message. That can be used, for example, to compute the standard error of the mean, given by SE=stdev/sqrt(count) where stdev is the sample standard deviation (the square root of the sample variance) computed and stored for each field in the aggregated message.

kafka-aggregator allows to control the minimum sample size to compute statistics by setting the min_sample_size parameter, which by default is min_sample_size=2.


If count is less than min_sample_size there are not enough values in the aggregation window to compute statistics, then kafka-aggregator uses the first value in the window instead.

Special field names

kafka-aggregator excludes by default the field names time, window_size and count. In particular, those fields are added to each aggregated message: time is the midpoint of the aggregation window, window_size is the size of the aggregation window (the window_size configuration setting used at a given time) and count is the sample size as discussed above.

Use the excluded_field_names list to exclude other fields from being aggregated.

Aggregation topic name

By default aggregation topics names are formatted by adding the aggregated suffix to the source topic name {source_topic_name}-aggregated. That can be changed by setting the topic_rename_format parameter in the configuration.

Example module configuration

The kafka-aggregator example module can be used to initialize “example source topics” in Kafka and produce messages for those topics.

source_topic_name_prefix sets the name prefix for the example source topics. The number of topics to create is set by ntopics and the number of fields in each topic is set by nfields. The number of partitions for the example source topics is set by topic_partitions.

The example module also produces messages for the example source topics. The frequency in Hz in which messages are produced is set by frequency and the maximum number of messages produced for each topic is set by max_messages. If max_messages is a number smaller than 1, an indefinite number of messages is produced.

Configuration class

kafkaaggregator.config Module

Configuration definition.


Configuration(broker, registry_url, …) Configuration for kafkaaggregator.