JIRA:
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Discussion:
Motivation:
Whenever a call is made to get a particular key from a Kafka Streams instance, currently it returns a store wrapper that contains a list of the stores for all the running and restoring/replica(with KIP-535: Allow state stores to serve stale reads during rebalance) on the instance via StreamThreadStateStoreProvider#stores().
When serving queries (like `get(key)` or `range(from, to)`), the wrapper actually iterates over all the underlying stores and issues the same query on each one. This is quite inefficient, and more importantly, it disallows some capabilities that KIP-535 intended to provide.
KIP-535 introduced two discovery mechanism so that users could implement a query routing layer, the ability to find out the partition for a specific key, and the ability to find out the locations and freshness of each replica of each partition of a store. Further, it introduced one key mechanism of a resilient query fetch layer, the ability to serve queries from hot-standby replicas and not just running active ones.
What is implicit is that the query routing layer would select an instance from which to fetch each partition of a store that the query spans, and then fan out to execute sub-queries against each such partition on the selected instances. However, the current store() API disallows this last step. Callers are only able to query all partitions on the local instance, not one specific partition.
Here's an example of how this is a drawback:
Imagine we have a cluster with two instances (A and B), and a store S with two partitions (0 and 1). Imagine further that store S has one active and one standby replica configured. Say, instance A hosts (0-active and 1-standby) and instance B hosts (1-active and 0-standby). Now, suppose the query routing layer wants to query the standby replica (so as not to compete with active processing). This arrangement is currently impossible. What would happen instead is that both instance A and B would return results from both partition 0 and 1, and the query router would have to de-duplicate the results. Plus, it would not achieve the objective to avoid competing with active processing.
To fill this gap, this KIP proposes to allow querying a specific partition of a store, while still preserving the ability to query all local partitions. This would also reduce latencies while querying a particular key from an instance, as it will fetch the key only from the specific store partition where it belongs which would be very helpful in instances containing multiple partitions.
Public Interfaces:
- Adding new Class StoreQueryParams.java to provide user options to the QueryableStoreProvider layer to understand what kind of stores a user wants. It would currently include whether a user is okay with serving stale data and if user already knows what is the partition of the store a user is looking at. Since store name and partition would be a unique combination, a taskId can be generated from this information to return the store for that particular task.
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package org.apache.kafka.streams;
import org.apache.kafka.streams.state.QueryableStoreType;
import java.util.Objects;
/**
* Represents all the query options that a user can provide to state what kind of stores it is expecting. The options would be whether a user would want to enable/disable stale stores* or whether it knows the list of partitions that it specifically wants to fetch. If this information is not provided the default behavior is to fetch the stores for all the partitions available on that instance* for that particular store name.
* It contains a partition, which for a point queries can be populated from the KeyQueryMetadata.
*/
public class StoreQueryParams<T> {
private Integer partition;
private boolean includeStaleStores;
private final String storeName;
private final QueryableStoreType<T> queryableStoreType;
private StoreQueryParams(final String storeName, final QueryableStoreType<T> queryableStoreType) {
this.storeName = storeName;
this.queryableStoreType = queryableStoreType;
}
public static <T> StoreQueryParams<T> fromNameAndType(final String storeName, final QueryableStoreType<T> queryableStoreType) {
return new StoreQueryParams(storeName, queryableStoreType);
}
/**
* Get the partition to be used to fetch list of Queryable store from QueryableStoreProvider.
* If the function returns null, it would mean that no specific partition has been requested so all the local partitions
* for the store will be returned.
*
* @return Integer partition
*/
public Integer getPartition() {
return partition;
}
/**
* Get the flag includeStaleStores. If true, include standbys and recovering stores along with running stores.
*
* @return boolean includeStaleStores
*/
public boolean isIncludeStaleStores() {
return includeStaleStores;
}
/**
* Get the {@link StoreQueryParams} with stale(standby, restoring) stores added via fetching the stores.
*
* @param partition The specific integer partition to be fetched from the stores list by using {@link StoreQueryParams}.
*
* @return String storeName
*/
public StoreQueryParams<T> withPartition(final Integer partition) {
this.partition = partition;
return this;
}
/**
* Get the {@link StoreQueryParams} with stale(standby, restoring) stores added via fetching the stores.
*
* @return String storeName
*/
public StoreQueryParams<T> withIncludeStaleStores() {
this.includeStaleStores = true;
return this;
}
/**
* Get the store name for which key is queried by the user.
*
* @return String storeName
*/
public String getStoreName() {
return storeName;
}
/**
* Get the queryable store type for which key is queried by the user.
*
* @return QueryableStoreType queryableStoreType
*/
public QueryableStoreType<T> getQueryableStoreType() {
return queryableStoreType;
}
@Override
public boolean equals(final Object obj) {
if (!(obj instanceof StoreQueryParams)) {
return false;
}
final StoreQueryParams storeQueryParams = (StoreQueryParams) obj;
return Objects.equals(storeQueryParams.partition, partition)
&& Objects.equals(storeQueryParams.includeStaleStores, includeStaleStores)
&& Objects.equals(storeQueryParams.storeName, storeName)
&& Objects.equals(storeQueryParams.queryableStoreType, queryableStoreType);
}
@Override
public String toString() {
return "StoreQueryParams {" +
"partition=" + partition +
", includeStaleStores=" + includeStaleStores +
", storeName=" + storeName +
", queryableStoreType=" + queryableStoreType +
'}';
}
@Override
public int hashCode() {
return Objects.hash(partition, includeStaleStores, storeName, queryableStoreType);
}
}
- Deprecating the KafkaStreams#store(final String storeName, final QueryableStoreType<T> queryableStoreType, final boolean includeStaleStores) in favour of the funtion mentioned below.
/**
* Get a facade wrapping the local {@link StateStore} instances with the provided {@link StoreQueryParams}.
* StoreQueryParams need required parameters to be set, which are {@code storeName} and if
* type is accepted by the provided {@link QueryableStoreType#accepts(StateStore) queryableStoreType}.
* The optional parameters to the StoreQueryParams include {@code partition} and {@code includeStaleStores}.
* The returned object can be used to query the {@link StateStore} instances.
*
* @param storeQueryParams If StoreQueryParams.fromNameAndType(storeName, queryableStoreType).withPartition(int partition) is used, it allow queries on the specific partition irrespective if it is a standby
* or a restoring replicas in addition to active ones.
* If StoreQueryParams.fromNameAndType(storeName, queryableStoreType).withIncludeStaleStores() is used, it allow queries on standbys and restoring replicas in addition to active ones for all the local partitions on the instance.
* If StoreQueryParams.fromNameAndType(storeName, queryableStoreType).withIncludeStaleStores().withPartition(int partition), it allow queries on the specific partition irrespective if it is a standby
* or a restoring replicas in addition to active ones..
* By default, if just storeQueryParams is used, it returns all the local partitions for the store which are in running state.
* @param <T> return type
* @return A facade wrapping the local {@link StateStore} instances
* @throws InvalidStateStoreException if Kafka Streams is (re-)initializing or a store with {@code storeName} and
* {@code queryableStoreType} doesn't exist
*/
public <T> T store(final StoreQueryParams<T> storeQueryParams) {
validateIsRunningOrRebalancing();
return queryableStoreProvider.getStore(storeQueryParams);
}
Proposed Changes:
- Add a new public class StoreQueryParams.java to set options for what kind of stores a user wants.
- Create a taskId from the combination of store name and partition provided by the user.
- In StreamThreadStateStoreProvider.java return only the stores for the task requested by the user and also check the condition to return only running stores or standby/recovering stores as well.
Compatibility, Deprecation, and Migration Plan:
- KafkaStreams#store(final String storeName, final QueryableStoreType<T> queryableStoreType, final boolean includeStaleStores) will be changed to the one mentioned in the Public Interfaces changes. Since the mentioned function is not released yet in any version, no deprecation is required.
Rejected Alternatives:
- Overload the QueryableStoreProvider#getStore() and StreamThreadStateStoreProvider#stores() with new parameters to pass a list of partitions along with the currently passed flag includeStaleStores.