[GitHub] [druid] abhishekagarwal87 commented on a change in pull request #11495: Select broker based on query context parameter `brokerService`

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abhishekagarwal87 commented on a change in pull request #11495:
URL: https://github.com/apache/druid/pull/11495#discussion_r677140980



##########
File path: server/src/main/java/org/apache/druid/server/router/QueryContextTieredBrokerSelectorStrategy.java
##########
@@ -0,0 +1,97 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.druid.server.router;
+
+import com.fasterxml.jackson.annotation.JsonCreator;
+import com.fasterxml.jackson.annotation.JsonProperty;
+import com.google.common.annotations.VisibleForTesting;
+import com.google.common.base.Optional;
+import org.apache.commons.lang.StringUtils;
+import org.apache.druid.java.util.common.logger.Logger;
+import org.apache.druid.query.Query;
+import org.apache.druid.query.QueryContexts;
+
+import javax.annotation.Nullable;
+
+/**
+ * Implementation of {@link TieredBrokerSelectorStrategy} which uses the parameter
+ * {@link QueryContexts#BROKER_SERVICE_NAME} in the Query context to select the
+ * Broker Service.
+ * <p>
+ * If the {@link #fallbackBrokerService} is set to a valid Broker Service Name,
+ * then all queries that do not specify a valid value for
+ * {@link QueryContexts#BROKER_SERVICE_NAME} would be directed to the
+ * {@code #fallbackBrokerService}. Note that the {@code fallbackBrokerService}
+ * can be different from the {@link TieredBrokerConfig#getDefaultBrokerServiceName()}.
+ */
+public class QueryContextTieredBrokerSelectorStrategy implements TieredBrokerSelectorStrategy
+{
+  private static final Logger log = new Logger(QueryContextTieredBrokerSelectorStrategy.class);
+
+  private final String fallbackBrokerService;
+
+  @JsonCreator
+  public QueryContextTieredBrokerSelectorStrategy(
+      @JsonProperty("fallbackBrokerService") @Nullable String fallbackBrokerService
+  )
+  {
+    this.fallbackBrokerService = fallbackBrokerService;
+  }
+
+  @Override
+  public Optional<String> getBrokerServiceName(TieredBrokerConfig tierConfig, Query query)
+  {
+    try {
+      final String contextBrokerService = QueryContexts.getBrokerServiceName(query);
+
+      if (isValidBrokerService(contextBrokerService, tierConfig)) {
+        // If the broker service in the query context is valid, use that
+        return Optional.of(contextBrokerService);
+      } else if (isValidBrokerService(fallbackBrokerService, tierConfig)) {

Review comment:
       could we perform this check in the constructor itself? your class would have Optional<String> broker that would either be Optional.absent() or Optional.of(fallbackBrokerService). you can return the field as it is. 

##########
File path: docs/design/router.md
##########
@@ -109,6 +109,19 @@ Including this strategy means all timeBoundary queries are always routed to the
 
 Queries with a priority set to less than minPriority are routed to the lowest priority Broker. Queries with priority set to greater than maxPriority are routed to the highest priority Broker. By default, minPriority is 0 and maxPriority is 1. Using these default values, if a query with priority 0 (the default query priority is 0) is sent, the query skips the priority selection logic.
 
+#### queryContext
+
+This strategy reads the parameter `brokerService` from the query context and routes the query accordingly. If no valid `brokerService` is specified in the query context, the field `fallbackBrokerService` is used if set to a valid non-null value.

Review comment:
       can we call `fallbackBrokerService` `defaultBrokerService` instead? Latter looks more common in druid codebase. 




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