Allow empty tiered replicants map for load rules

docs/operations/rule-configuration.md

@@ -109,7 +109,16 @@ In the web console you can use the up and down arrows on the right side of the i
 
 Load rules define how Druid assigns segments to [historical process tiers](./mixed-workloads.md#historical-tiering), and how many replicas of a segment exist in each tier.
 
-If you have a single tier, Druid automatically names the tier `_default` and loads all segments onto it. If you define an additional tier, you must define a load rule to specify which segments to load on that tier. Until you define a load rule, your new tier remains empty.
+If you have a single tier, Druid automatically names the tier `_default`. If you define an additional tier, you must define a load rule to specify which segments to load on that tier. Until you define a load rule, your new tier remains empty.
+
+All load rules can have these properties:
+
+|Property|Description|Required|Default value|
+|---------|-----------|---------|-------------|
+| `tieredReplicants`| Map from tier names to the respective number of segment replicas to be loaded on those tiers. The number of replicas for each tier must be either 0 or a positive integer.| No | When `useDefaultTierForNull` is `true`, the default value is `{"_default_tier": 2}` i.e. 2 replicas to be loaded on the `_default_tier`.<br/><br/>When `useDefaultTierForNull` is `false`, the default value is `{}` i.e. no replicas to be loaded on any tier. |
+|`useDefaultTierForNull`|Determines the default value of `tieredReplicants` if it is not specified or set to `null`.| No | `true`|
+
+Specific types of load rules discussed below may have other properties too.
 
 ### Forever load rule
 
@@ -130,6 +139,7 @@ The following example places one replica of each segment on a custom tier named
 Set the following property:
 
 - `tieredReplicants`: a map of tier names to the number of segment replicas for that tier.
+- `useDefaultTierForNull`: This parameter determines the default value of `tieredReplicants` and only has an effect if the field is not present. The default value of `useDefaultTierForNull` is true.
 
 ### Period load rule
 
@@ -158,6 +168,7 @@ Set the following properties:
 
   You can use this property to load segments with future start and end dates, where "future" is relative to the time when the Coordinator evaluates data against the rule. Defaults to `true`.
 - `tieredReplicants`: a map of tier names to the number of segment replicas for that tier.
+- `useDefaultTierForNull`: This parameter determines the default value of `tieredReplicants` and only has an effect if the field is not present. The default value of `useDefaultTierForNull` is true.
 
 ### Interval load rule
 
@@ -180,6 +191,7 @@ Set the following properties:
 
 - `interval`: the load interval specified as an [ISO 8601](https://en.wikipedia.org/wiki/ISO_8601) range encoded as a string.
 - `tieredReplicants`: a map of tier names to the number of segment replicas for that tier.
+- `useDefaultTierForNull`: This parameter determines the default value of `tieredReplicants` and only has an effect if the field is not present. The default value of `useDefaultTierForNull` is true.
 
 ## Drop rules
 

server/src/main/java/org/apache/druid/metadata/SQLMetadataRuleManager.java

@@ -96,7 +96,8 @@ public Void withHandle(Handle handle) throws Exception
                       ImmutableMap.of(
                           DruidServer.DEFAULT_TIER,
                           DruidServer.DEFAULT_NUM_REPLICANTS
-                      )
+                      ),
+                      null
                   )
               );
               final String version = DateTimes.nowUtc().toString();

server/src/main/java/org/apache/druid/server/coordinator/rules/ForeverLoadRule.java

@@ -21,30 +21,24 @@
 
 import com.fasterxml.jackson.annotation.JsonCreator;
 import com.fasterxml.jackson.annotation.JsonProperty;
-import com.google.common.collect.ImmutableMap;
-import org.apache.druid.client.DruidServer;
 import org.apache.druid.timeline.DataSegment;
 import org.joda.time.DateTime;
 import org.joda.time.Interval;
 
+import javax.annotation.Nullable;
 import java.util.Map;
-import java.util.Objects;
 
 /**
  */
 public class ForeverLoadRule extends LoadRule
 {
-  private final Map<String, Integer> tieredReplicants;
-
   @JsonCreator
   public ForeverLoadRule(
-      @JsonProperty("tieredReplicants") Map<String, Integer> tieredReplicants
+      @JsonProperty("tieredReplicants") Map<String, Integer> tieredReplicants,
+      @JsonProperty("useDefaultTierForNull") @Nullable Boolean useDefaultTierForNull
   )
   {
-    this.tieredReplicants = tieredReplicants == null
-                            ? ImmutableMap.of(DruidServer.DEFAULT_TIER, DruidServer.DEFAULT_NUM_REPLICANTS)
-                            : tieredReplicants;
-    validateTieredReplicants(this.tieredReplicants);
+    super(tieredReplicants, useDefaultTierForNull);
   }
 
   @Override
@@ -54,20 +48,6 @@ public String getType()
     return "loadForever";
   }
 
-  @Override
-  @JsonProperty
-  public Map<String, Integer> getTieredReplicants()
-  {
-    return tieredReplicants;
-  }
-
-  @Override
-  public int getNumReplicants(String tier)
-  {
-    Integer retVal = tieredReplicants.get(tier);
-    return (retVal == null) ? 0 : retVal;
-  }
-
   @Override
   public boolean appliesTo(DataSegment segment, DateTime referenceTimestamp)
   {
@@ -80,22 +60,4 @@ public boolean appliesTo(Interval interval, DateTime referenceTimestamp)
     return true;
   }
 
-  @Override
-  public boolean equals(Object o)
-  {
-    if (this == o) {
-      return true;
-    }
-    if (o == null || getClass() != o.getClass()) {
-      return false;
-    }
-    ForeverLoadRule that = (ForeverLoadRule) o;
-    return Objects.equals(tieredReplicants, that.tieredReplicants);
-  }
-
-  @Override
-  public int hashCode()
-  {
-    return Objects.hash(tieredReplicants);
-  }
 }

server/src/main/java/org/apache/druid/server/coordinator/rules/IntervalLoadRule.java

@@ -21,14 +21,14 @@
 
 import com.fasterxml.jackson.annotation.JsonCreator;
 import com.fasterxml.jackson.annotation.JsonProperty;
-import com.google.common.collect.ImmutableMap;
-import org.apache.druid.client.DruidServer;
 import org.apache.druid.java.util.common.logger.Logger;
 import org.apache.druid.timeline.DataSegment;
 import org.joda.time.DateTime;
 import org.joda.time.Interval;
 
+import javax.annotation.Nullable;
 import java.util.Map;
+import java.util.Objects;
 
 /**
  */
@@ -37,16 +37,15 @@ public class IntervalLoadRule extends LoadRule
   private static final Logger log = new Logger(IntervalLoadRule.class);
 
   private final Interval interval;
-  private final Map<String, Integer> tieredReplicants;
 
   @JsonCreator
   public IntervalLoadRule(
       @JsonProperty("interval") Interval interval,
-      @JsonProperty("tieredReplicants") Map<String, Integer> tieredReplicants
+      @JsonProperty("tieredReplicants") Map<String, Integer> tieredReplicants,
+      @JsonProperty("useDefaultTierForNull") @Nullable Boolean useDefaultTierForNull
   )
   {
-    this.tieredReplicants = tieredReplicants == null ? ImmutableMap.of(DruidServer.DEFAULT_TIER, DruidServer.DEFAULT_NUM_REPLICANTS) : tieredReplicants;
-    validateTieredReplicants(this.tieredReplicants);
+    super(tieredReplicants, useDefaultTierForNull);
     this.interval = interval;
   }
 
@@ -57,20 +56,6 @@ public String getType()
     return "loadByInterval";
   }
 
-  @Override
-  @JsonProperty
-  public Map<String, Integer> getTieredReplicants()
-  {
-    return tieredReplicants;
-  }
-
-  @Override
-  public int getNumReplicants(String tier)
-  {
-    final Integer retVal = tieredReplicants.get(tier);
-    return retVal == null ? 0 : retVal;
-  }
-
   @JsonProperty
   public Interval getInterval()
   {
@@ -98,24 +83,16 @@ public boolean equals(Object o)
     if (o == null || getClass() != o.getClass()) {
       return false;
     }
-
-    IntervalLoadRule that = (IntervalLoadRule) o;
-
-    if (interval != null ? !interval.equals(that.interval) : that.interval != null) {
-      return false;
-    }
-    if (tieredReplicants != null ? !tieredReplicants.equals(that.tieredReplicants) : that.tieredReplicants != null) {
+    if (!super.equals(o)) {
       return false;
     }
-
-    return true;
+    IntervalLoadRule that = (IntervalLoadRule) o;
+    return Objects.equals(interval, that.interval);
   }
 
   @Override
   public int hashCode()
   {
-    int result = interval != null ? interval.hashCode() : 0;
-    result = 31 * result + (tieredReplicants != null ? tieredReplicants.hashCode() : 0);
-    return result;
+    return Objects.hash(super.hashCode(), interval);
   }
 }

server/src/main/java/org/apache/druid/server/coordinator/rules/LoadRule.java

@@ -19,39 +19,106 @@
 
 package org.apache.druid.server.coordinator.rules;
 
-import org.apache.druid.java.util.common.IAE;
+import com.fasterxml.jackson.annotation.JsonProperty;
+import com.google.common.collect.ImmutableMap;
+import org.apache.druid.client.DruidServer;
+import org.apache.druid.common.config.Configs;
+import org.apache.druid.error.InvalidInput;
 import org.apache.druid.timeline.DataSegment;
 
 import java.util.Map;
+import java.util.Objects;
 
 /**
  * LoadRules indicate the number of replicants a segment should have in a given tier.
  */
 public abstract class LoadRule implements Rule
 {
+  private final Map<String, Integer> tieredReplicants;
+  /**
+   * Used to determing the default value if tieredReplicants is null in {@link #handleNullTieredReplicants}.
+   */
+  private final boolean useDefaultTierForNull;
+
+  protected LoadRule(Map<String, Integer> tieredReplicants, Boolean useDefaultTierForNull)
+  {
+    this.useDefaultTierForNull = Configs.valueOrDefault(useDefaultTierForNull, true);
+    this.tieredReplicants = handleNullTieredReplicants(tieredReplicants, this.useDefaultTierForNull);
+    validateTieredReplicants(this.tieredReplicants);
+  }
+
+  @JsonProperty
+  public Map<String, Integer> getTieredReplicants()
+  {
+    return tieredReplicants;
+  }
+
+  @JsonProperty
+  public boolean useDefaultTierForNull()
+  {
+    return useDefaultTierForNull;
+  }
+
   @Override
   public void run(DataSegment segment, SegmentActionHandler handler)
   {
     handler.replicateSegment(segment, getTieredReplicants());
   }
 
-  protected static void validateTieredReplicants(final Map<String, Integer> tieredReplicants)
+  /**
+   * Returns the given {@code tieredReplicants} map unchanged if it is non-null (including empty).
+   * Returns the following default values if the given map is null.
+   * <ul>
+   * <li>If {@code useDefaultTierForNull} is true, returns a singleton map from {@link DruidServer#DEFAULT_TIER} to {@link DruidServer#DEFAULT_NUM_REPLICANTS}.</li>
+   * <li>If {@code useDefaultTierForNull} is false, returns an empty map. This causes segments to have a replication factor of 0 and not get assigned to any historical.</li>
+   * </ul>
+   */
+  private static Map<String, Integer> handleNullTieredReplicants(final Map<String, Integer> tieredReplicants, boolean useDefaultTierForNull)
   {
-    if (tieredReplicants.size() == 0) {
-      throw new IAE("A rule with empty tiered replicants is invalid");
+    if (useDefaultTierForNull) {
+      return Configs.valueOrDefault(tieredReplicants, ImmutableMap.of(DruidServer.DEFAULT_TIER, DruidServer.DEFAULT_NUM_REPLICANTS));
+    } else {
+      return Configs.valueOrDefault(tieredReplicants, ImmutableMap.of());
     }
+  }
+
+  private static void validateTieredReplicants(final Map<String, Integer> tieredReplicants)
+  {
     for (Map.Entry<String, Integer> entry : tieredReplicants.entrySet()) {
       if (entry.getValue() == null) {
-        throw new IAE("Replicant value cannot be empty");
+        throw InvalidInput.exception("Invalid number of replicas for tier [%s]. Value must not be null.", entry.getKey());
       }
       if (entry.getValue() < 0) {
-        throw new IAE("Replicant value [%d] is less than 0, which is not allowed", entry.getValue());
+        throw InvalidInput.exception("Invalid number of replicas for tier [%s]. Value [%d] must be positive.", entry.getKey(), entry.getValue());
       }
     }
   }
 
-  public abstract Map<String, Integer> getTieredReplicants();
+  public int getNumReplicants(String tier)
+  {
+    Integer retVal = getTieredReplicants().get(tier);
+    return (retVal == null) ? 0 : retVal;
+  }
 
-  public abstract int getNumReplicants(String tier);
+  @Override
+  public boolean equals(Object o)
+  {
+    if (this == o) {
+      return true;
+    }
+    if (o == null || getClass() != o.getClass()) {
+      return false;
+    }
+    LoadRule loadRule = (LoadRule) o;
+    return useDefaultTierForNull == loadRule.useDefaultTierForNull && Objects.equals(
+        tieredReplicants,
+        loadRule.tieredReplicants
+    );
+  }
 
+  @Override
+  public int hashCode()
+  {
+    return Objects.hash(tieredReplicants, useDefaultTierForNull);
+  }
 }