Status

Current state: Under discussion

Discussion thread: here [Change the link from the KIP proposal email archive to your own email thread]

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Motivation

Single Message Transformations (SMTs) in Kafka Connect provide a convenient, code-free way to modify records from source connectors before they get sent to a Kafka topic. In some more complex use cases it can be desired to apply SMTs dependent on some aspect of the record being processed.

For example, with Debezium there are topics which represent a schema change and topics which represent a data change, and users might want to apply transformations selectively, based on the topic type. SMTs cannot currently do this, since they're applied to all records produced by a source connector, irrespective of their intended topic. This problem would be solved if it was possible to apply an SMT according to the name of the topic (See KAFKA-7052 for further details).

This KIP proposes a way to only apply a particular transformation if the resource matches some condition. The condition is defined by a new interface and the implementations for common conditions will be provided. Connector authors and users will also be able to provide their own condition implementations for special cases, but this is not expected to be a common need.

A new Filter SMT will also be implemented. This will filter records which do, or do not, satisfy a given condition. This is advantageous for users who do not want to incur the storage costs of consuming everything from a source connector, for example. Instead they can chose to ingest only the records of interest. Likewise for sink connectors it will enable exporting a subset of data without needing to resort to a Kafka Streams application to filter it first.

Public Interfaces

A new Predicate interface will be added in the new org.apache.kafka.connect.transforms.predicates package.

/**
* A condition on ConnectRecords.
* Implementations of this interface can be used for filtering records and conditionally applying Transformations.
* Implementations must be public and have a public constructor with no parameters.
*/
public interface Predicate<R extends ConnectRecord<R>> extends Configurable, AutoCloseable {

    /** 
     * Configuration specification for this predicate. 
     */
    ConfigDef config();

    /**
     * Returns whether the given record satisfies this predicate.
     */
    boolean test(R record);

    @Override
    void close();
}

All transformations will gain new implicit configuration parameters which will be consumed by the connect runtime and not passed to the Transformation.configure() method.

A new Filter SMT will be added to enable record filtering.

Proposed Changes

Predicates

The Predicate interface is described above. The interface will be a worker plug-in, loaded in the same way as other worker plug-ins such as converters, connectors, and REST extensions. This would include aliasing behaviour allowing users to specify predicates using their simple class names as long as no two predicate plug-ins with the same simple name are available on the worker.

In order to apply a transformation conditionally, all transformations will implicitly support  a String predicate configuration parameter, which names a particular predicate.

To negate the result of a predicate, all transformations will implicitly support a boolean negate configuration parameter, which defaults to false.

In addition to the Predicate interface described above, this KIP will provide the following implementations:

TopicNameMatches

test() will return true when the ConnectRecord.topic() (i.e. it's name) matches a given Java regular expression pattern.

HasHeaderKey

test() will return true when the ConnectRecord.headers() has 1 or more headers with a given key.

RecordIsTombstone

test() will return true when the ConnectRecord represents a tombstone (i.e. has a null value). This predicate has no configuration parameters.

Conditionally applying an SMT

When a Transformation is configured with the new predicate parameter its application will happen conditionally. The value of the predicate parameter will be the name of a predicate defined under the predicates prefix. Configuration for the predicate will come from all other configuration parameters starting with the same predicates... analogous to how the transformations in a transformation chain are configured. These will be supplied to the Predicate.configure(Map) method.

If during processing the predicate throws an exception this will be handled in the same way as errors in transformations.

Consider the following example of a transformation chain with a single conditionally applied ExtractField$Key SMT:

transforms=t2
transforms.t2.predicate=has-my-prefix
transforms.t2.negate=true
transforms.t2.type=org.apache.kafka.connect.transforms.ExtractField$Key
transforms.t2.field=c1
predicates=has-my-prefix
predicates.has-my-prefix.type=org.apache.kafka.connect.predicates.TopicNameMatch
predicates.has-my-prefix.pattern=my-prefix-.*

The transform t2 is only evaluated when the predicate has-my-prefix is false (the negate parameter). That predicate is configured by the keys with prefix predicates.has-my-prefix. The predicate class is org.apache.kafka.connect.predicates.TopicNameMatch and it's pattern parameter has the value my-prefix-.* . Thus the SMT will be applied only to records where the topic name does not  start with my-prefix-.

The benefit of defining the predicate separately from the transform is it makes it easier to apply the same predicate to multiple transforms, or to have one set of transforms predicated on one predicate and another set of transforms predicated on that predicates negation.

The Filter SMT

A new Filter transformation will be added in the existing org.apache.kafka.connect.transforms package. This will return null from apply(ConnectRecord). This is not of much use on its own, but is intended to applied conditionally as described above. This will allow messages to be filtered according to the predicate.

Consider the following example of a transformation chain with a single Filter SMT:

transforms=filter
transforms.filter.type=org.apache.kafka.connect.transforms.Filter
transforms.filter.predicate=foo-or-bar
predicates=foo-or-bar
predicates.foo-or-bar.type=org.apache.kafka.connect.transforms.predicates.TopicNameMatch
predicates.foo-or-bar.pattern=foo|bar

The predicate class is org.apache.kafka.connect.predicates.TopicNameMatch and it takes a single configuration parameter, pattern. Records having a topic name "foo" or "bar" match the predicate, so the filter SMT will be evaluated, will return null and therefore those records are filtered out.

Compatibility, Deprecation, and Migration Plan

Users will need to perform a rolling upgrade of a distributed connect cluster before they can start using the new Filter SMT or conditional SMTs.

Adding the new implicit predicate and negate parameters to transformations means that any existing transformation which already took config parameters of these names would not be configurable (i.e. the implicit parameters will mask the transformation parameters of the same name). Similarly existing connectors might have a configuration parameters prefixed by predicates, which would be masked by the new top-level parameter. The analogous situation arose when support for SMTs was originally added in KIP-66.

Rejected Alternatives

• Numerous alternative ways to configure conditional SMTs which reduced or removed the possibility of collision with existing connectors were considered. They were more verbose and difficult to understand.