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Status

Current state: Accpted

Discussion thread:https://lists.apache.org/thread/ogo7ntmj8srdcko2h86vvd9djjsjfvcj 

Vote thread: https://lists.apache.org/thread/q4kn2g6tmc837ph2zvff40pgpmgzok3d

JIRA: KAFKA-15629 - Getting issue details... STATUS

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Motivation

In the current IQv2 code, there are noticeable differences when interfacing with kv-store  and ts-kv-store . Notably, the return type V  acts as a simple value for kv-store  but evolves into ValueAndTimestamp<V>  for ts-kv-store , which presents type safety issues in the API.

public <V> void shouldHandleKeyQuery(
            final Integer key,
            final Function<V, Integer> valueExtactor,
            final Integer expectedValue) {
        ...
	  	final KeyQuery<Integer, V> query = KeyQuery.withKey(key);
	    ...
	  	final StateQueryRequest<V> request =
                inStore(STORE_NAME)
                        .withQuery(query)
                        .withPartitions(mkSet(0, 1))
                        .withPositionBound(PositionBound.at(INPUT_POSITION));
		...
        final StateQueryResult<V> result =
                IntegrationTestUtils.iqv2WaitForResult(kafkaStreams, request);
        ...
        final QueryResult<V> queryResult = result.getOnlyPartitionResult();
        ...
        final V result1 = queryResult.getResult();
        final Integer integer = valueExtactor.apply(result1);
        assertThat(integer, is(expectedValue));
       ...
    } 

Before the introduction of TimestampedKeyQuery , when using KeyQuery , we obtained the result using the following code:

final V result1 = queryResult.getResult();

This meant that the returned result could be of two potential data types: plain V  or ValueAndTimestamp<V> . This was a source of inconsistency. For instance, querying a kv-store  with KeyQuery  would return a V  type, but querying a ts-kv-store  would yield a ValueAndTimestamp<V> . This behavior is unintuitive and potentially confusing for developers.

To ensure consistency, we suggest that KeyQuery always return the plain V type, enhancing the predictability of the mentioned code. Likewise, RangeQuery should uniformly return the plain V KeyValueIterator.

For those requiring timestamped values from a ts-kv-store , we recommend introducing a new query type: TimestampedKeyQuery . This new query will specifically target ts-kv-stores  and will return ValueAndTimestamp<V> . Furthermore, to complement this, TimestampedRangeQuery  should be introduced to query ranges in ts-kv-stores , ensuring that the returned value always includes timestamps.

public final class TimestampedKeyQuery<K, V> implements Query<ValueAndTimestamp<V>>

public final class TimestampedRangeQuery<K, V> implements Query<KeyValueIterator<K, ValueAndTimestamp<V>>>

Why introduce TimestampedKeyQuery  and TimestampedRangeQuery ? The primary motivation behind this is to ensure type safety and foster a clear distinction in our API. They bridge the difference between simple key-value stores and those integrated with timestamps, offering a more robust and intuitive querying mechanism.

Proposed Changes

Within the current IQv2 codebase, there have been distinct interactions between plain-kv-store and ts-kv-store. These differences, especially in return types, have raised concerns over type safety within the API.

To address these challenges and streamline the querying experience, we have decided to refine our approach and introduce two specialized query types: TimestampedKeyQuery  and TimestampedRangeQuery .

TimestampedKeyQuery : This query type will consistently return ValueAndTimestamp<V> , ensuring that there's a clear and predictable return type associated with timestamped key-value stores.

@Evolving
public final class TimestampedKeyQuery<K, V> implements Query<ValueAndTimestamp<V>> {

...

    /**
     * Creates a query that will retrieve the record identified by {@code key} if it exists
     * (or {@code null} otherwise).
     * @param key The key to retrieve
     * @param <K> The type of the key
     * @param <V> The type of the value that will be retrieved
     */
    public static <K, V> TimestampedKeyQuery<K, V> withKey(final K key) 

    /**
     * Specifies that the cache should be skipped during query evaluation. This means, that the query will always
     * get forwarded to the underlying store.
     */
    public TimestampedKeyQuery<K, V> skipCache() 

    /**
     * The key that was specified for this query.
     */
    public K key() 

    /**
     * The flag whether to skip the cache or not during query evaluation.
     */
    public boolean isSkipCache() 

TimestampedRangeQuery : Tailored for ranges with timestamps, this query will return a KeyValueIterator<K, ValueAndTimestamp<V>>

According to KIP-968, this KIP introduces the public enum ResultOrder to determine whether keys are sorted in ascending or descending  or unordered order. Order is based on the serialized byte[] of the keys, not the 'logical' key order. employs the withDescendingKeys() and withAscendingKeys() methods to specify that the keys should be ordered in descending or ascending or unordered sequence, and the resultOrder() method to retrieve the value of enum value in  ResultOrder. I've incorporated these variables and methods into the TimestampedRangeQuery  class and modified some method inputs. As a result, we can now use withDescendingKeys() to obtain results in reverse order and use withAscendingKeys to obtain the result in ascending order. 

@Evolving
public final class TimestampedRangeQuery<K, V> implements Query<KeyValueIterator<K, ValueAndTimestamp<V>>> {

    ...

    /**
     * Interactive range query using a lower and upper bound to filter the keys returned.
     * @param lower The key that specifies the lower bound of the range
     * @param upper The key that specifies the upper bound of the range
     * @param <K> The key type
     * @param <V> The value type
     */
    public static <K, V> TimestampedRangeQuery<K, V> withRange(final K lower, final K upper) 
 

	/**
     * Determines if the serialized byte[] of the keys in ascending or descending or unordered order.
     * Order is based on the serialized byte[] of the keys, not the 'logical' key order.
     * @return return the order of return records base on the serialized byte[] of the keys (can be unordered, or in ascending, or in descending order).
     */
    public ResultOrder resultOrder() {
        return order;
    }

    /**
     * Set the query to return the serialized byte[] of the keys in descending order.
     * Order is based on the serialized byte[] of the keys, not the 'logical' key order.
     * @return a new RangeQuery instance with descending flag set.
     */
    public TimestampedRangeQuery<K, V> withDescendingKeys() {
        return new TimestampedRangeQuery<>(this.lower, this.upper, ResultOrder.DESCENDING);
    }

    /**
     * Set the query to return the serialized byte[] of the keys in Ascending order.
     * Order is based on the serialized byte[] of the keys, not the 'logical' key order.
     * @return a new RangeQuery instance with ascending flag set.
     */
    public TimestampedRangeQuery<K, V> withAscendingKeys() {
        return new TimestampedRangeQuery<>(this.lower, this.upper, ResultOrder.ASCENDING);
    }
   
}

According to KIP-968, we introduce a public enum ResultOrder.

ResultOrder enum
It helps with specifying the order of the returned results by the query.

package org.apache.kafka.streams.query;
  
public enum ResultOrder {
    ANY,
    ASCENDING,
    DESCENDING
}

Compatibility, Deprecation, and Migration Plan

• Changing the semantics of existing KeyQuery and RangeQuery  is a breaking change. However, both classes are marked as @Evolving` and thus a breaking change in a minor release is allowed without a deprecation period. Given that IQv2 is not yet widely adopted, we believe it’s cleaner to make this breaking change right away.
• Adding new query types does not imply any compatibility concerns.

Test Plan

To ensure the robustness and accuracy of our new query types, TimestampedKeyQuery  and TimestampedRangeQuery , it's essential to have thorough test coverage. With that in mind, we propose the creation of two specific test methods:

shouldHandleTimestampedKeyQuery : This test method will validate the functionality of TimestampedKeyQuery , ensuring it consistently returns ValueAndTimestamp<V>  as expected.

shouldHandleTimestampedRangeQuery : This method is tailored to verify the TimestampedRangeQuery , ensuring that it correctly returns a KeyValueIterator<K, ValueAndTimestamp<V>> .

We will focus on conducting a detailed test for shouldHandleTimestampedRangeQuery .

Rejected Alternatives

The alternative would be to deprecate the existing KeyQuery  and RangeQuery  and add new query types that always return plain value. However, it seems to introduce unnecessary “deprecation noise”, and it would be hard to find good names for these newly added query types. Making a semantics change on the existing queries allows us to keep the existing names.