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Posted to commits@druid.apache.org by "jtuglu1 (via GitHub)" <gi...@apache.org> on 2025/09/04 06:12:37 UTC

[PR] Add stopTaskCountPercent field to LagBasedAutoscaler (druid)

jtuglu1 opened a new pull request, #18480:
URL: https://github.com/apache/druid/pull/18480

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   Allows the maximum number of stopped tasks to scale proportional to the task autoscaler. This is useful in cases where scaling up/down a lot can mess up the (over/under) # of tasks being marked for publishing.
   
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Re: [PR] Add stopTaskCountPercent field to LagBasedAutoscaler (druid)

Posted by "jtuglu1 (via GitHub)" <gi...@apache.org>.
jtuglu1 commented on code in PR #18480:
URL: https://github.com/apache/druid/pull/18480#discussion_r2325443828


##########
indexing-service/src/main/java/org/apache/druid/indexing/seekablestream/supervisor/autoscaler/LagBasedAutoScalerConfig.java:
##########
@@ -100,6 +103,9 @@ public LagBasedAutoScalerConfig(
     this.scaleOutStep = scaleOutStep != null ? scaleOutStep : 2;
     this.minTriggerScaleActionFrequencyMillis = minTriggerScaleActionFrequencyMillis
         != null ? minTriggerScaleActionFrequencyMillis : 600000;
+
+    Preconditions.checkArgument(stopTaskCountPercent == null || (stopTaskCountPercent > 0.0 && stopTaskCountPercent <= 1.0), "0.0 < stopTaskCountPercent <= 1.0");

Review Comment:
   I don't really like the idea of using an int field for this kind of thing. I'll switch the name to ratio.



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Re: [PR] Add stopTaskCountPercent field to LagBasedAutoscaler (druid)

Posted by "jtuglu1 (via GitHub)" <gi...@apache.org>.
jtuglu1 commented on code in PR #18480:
URL: https://github.com/apache/druid/pull/18480#discussion_r2328458319


##########
indexing-service/src/main/java/org/apache/druid/indexing/seekablestream/supervisor/autoscaler/LagBasedAutoScalerConfig.java:
##########
@@ -49,6 +50,7 @@ public class LagBasedAutoScalerConfig implements AutoScalerConfig
   private final boolean enableTaskAutoScaler;
   private final long minTriggerScaleActionFrequencyMillis;
   private final AggregateFunction lagAggregate;
+  @Nullable private Double stopTaskCountPercent;

Review Comment:
   > I just meant we should drop the @Nullable from @Nullable private Double stopTaskCountPercent
   
   I personally like to mark these things as `@Nullable` or `Optional<T>` as it makes it clear when working the raw value that the value could be null (not just return value of the accessor method). In that case `startTaskCount` annotation should also be changed. We should have a clear standard on these things (like [this](https://google.github.io/styleguide/javaguide.html#s4.8.5-annotations)) so style doesn't differ based on who is reviewing.



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Re: [PR] Add stopTaskCountPercent field to LagBasedAutoscaler (druid)

Posted by "jtuglu1 (via GitHub)" <gi...@apache.org>.
jtuglu1 commented on code in PR #18480:
URL: https://github.com/apache/druid/pull/18480#discussion_r2325443828


##########
indexing-service/src/main/java/org/apache/druid/indexing/seekablestream/supervisor/autoscaler/LagBasedAutoScalerConfig.java:
##########
@@ -100,6 +103,9 @@ public LagBasedAutoScalerConfig(
     this.scaleOutStep = scaleOutStep != null ? scaleOutStep : 2;
     this.minTriggerScaleActionFrequencyMillis = minTriggerScaleActionFrequencyMillis
         != null ? minTriggerScaleActionFrequencyMillis : 600000;
+
+    Preconditions.checkArgument(stopTaskCountPercent == null || (stopTaskCountPercent > 0.0 && stopTaskCountPercent <= 1.0), "0.0 < stopTaskCountPercent <= 1.0");

Review Comment:
   Value range is (0, 1.]. I don't really like the idea of using an int field for this kind of thing. I'll switch the name to ratio.



##########
indexing-service/src/main/java/org/apache/druid/indexing/seekablestream/supervisor/autoscaler/LagBasedAutoScalerConfig.java:
##########
@@ -100,6 +103,9 @@ public LagBasedAutoScalerConfig(
     this.scaleOutStep = scaleOutStep != null ? scaleOutStep : 2;
     this.minTriggerScaleActionFrequencyMillis = minTriggerScaleActionFrequencyMillis
         != null ? minTriggerScaleActionFrequencyMillis : 600000;
+
+    Preconditions.checkArgument(stopTaskCountPercent == null || (stopTaskCountPercent > 0.0 && stopTaskCountPercent <= 1.0), "0.0 < stopTaskCountPercent <= 1.0");

Review Comment:
   Value range is (0, 1.0]. I don't really like the idea of using an int field for this kind of thing. I'll switch the name to ratio.



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Re: [PR] Add stopTaskCountPercent field to LagBasedAutoscaler (druid)

Posted by "jtuglu1 (via GitHub)" <gi...@apache.org>.
jtuglu1 commented on code in PR #18480:
URL: https://github.com/apache/druid/pull/18480#discussion_r2328458319


##########
indexing-service/src/main/java/org/apache/druid/indexing/seekablestream/supervisor/autoscaler/LagBasedAutoScalerConfig.java:
##########
@@ -49,6 +50,7 @@ public class LagBasedAutoScalerConfig implements AutoScalerConfig
   private final boolean enableTaskAutoScaler;
   private final long minTriggerScaleActionFrequencyMillis;
   private final AggregateFunction lagAggregate;
+  @Nullable private Double stopTaskCountPercent;

Review Comment:
   > I just meant we should drop the @Nullable from @Nullable private Double stopTaskCountPercent
   
   I personally like to mark these things as `@Nullable` as it makes it clear when working the raw value that the value could be null (not just the accessor method). In that case `startTaskCount` annotation should also be changed. We should have a clear standard on these things (like [this](https://google.github.io/styleguide/javaguide.html) so style doesn't differ based on who is reviewing.



##########
indexing-service/src/main/java/org/apache/druid/indexing/seekablestream/supervisor/autoscaler/LagBasedAutoScalerConfig.java:
##########
@@ -49,6 +50,7 @@ public class LagBasedAutoScalerConfig implements AutoScalerConfig
   private final boolean enableTaskAutoScaler;
   private final long minTriggerScaleActionFrequencyMillis;
   private final AggregateFunction lagAggregate;
+  @Nullable private Double stopTaskCountPercent;

Review Comment:
   > I just meant we should drop the @Nullable from @Nullable private Double stopTaskCountPercent
   
   I personally like to mark these things as `@Nullable` as it makes it clear when working the raw value that the value could be null (not just the accessor method). In that case `startTaskCount` annotation should also be changed. We should have a clear standard on these things (like [this](https://google.github.io/styleguide/javaguide.html)) so style doesn't differ based on who is reviewing.



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Re: [PR] Add stopTaskCountPercent field to LagBasedAutoscaler (druid)

Posted by "jtuglu1 (via GitHub)" <gi...@apache.org>.
jtuglu1 commented on code in PR #18480:
URL: https://github.com/apache/druid/pull/18480#discussion_r2328458319


##########
indexing-service/src/main/java/org/apache/druid/indexing/seekablestream/supervisor/autoscaler/LagBasedAutoScalerConfig.java:
##########
@@ -49,6 +50,7 @@ public class LagBasedAutoScalerConfig implements AutoScalerConfig
   private final boolean enableTaskAutoScaler;
   private final long minTriggerScaleActionFrequencyMillis;
   private final AggregateFunction lagAggregate;
+  @Nullable private Double stopTaskCountPercent;

Review Comment:
   > I just meant we should drop the @Nullable from @Nullable private Double stopTaskCountPercent
   
   I personally like to mark these things as `@Nullable` as it makes it clear when working the raw value that the value could be null (not just return value of the accessor method). In that case `startTaskCount` annotation should also be changed. We should have a clear standard on these things (like [this](https://google.github.io/styleguide/javaguide.html#s4.8.5-annotations)) so style doesn't differ based on who is reviewing.



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Re: [PR] Add stopTaskCountPercent field to LagBasedAutoscaler (druid)

Posted by "jtuglu1 (via GitHub)" <gi...@apache.org>.
jtuglu1 commented on code in PR #18480:
URL: https://github.com/apache/druid/pull/18480#discussion_r2325449655


##########
indexing-service/src/main/java/org/apache/druid/indexing/seekablestream/supervisor/autoscaler/LagBasedAutoScalerConfig.java:
##########
@@ -212,6 +218,15 @@ public AggregateFunction getLagAggregate()
     return lagAggregate;
   }
 
+  @Override
+  @JsonProperty
+  @Nullable
+  @JsonInclude(JsonInclude.Include.NON_NULL)

Review Comment:
   Hmmm. Are we sure we want to do that? This would exclude ALL fields that are `null`. In the case where there's some malformed input we may not include it in the spec at all (not visible to the user). I'd prefer to reserve this for fields which make sense to be nullable.



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Re: [PR] Add stopTaskCountPercent field to LagBasedAutoscaler (druid)

Posted by "jtuglu1 (via GitHub)" <gi...@apache.org>.
jtuglu1 commented on code in PR #18480:
URL: https://github.com/apache/druid/pull/18480#discussion_r2328458319


##########
indexing-service/src/main/java/org/apache/druid/indexing/seekablestream/supervisor/autoscaler/LagBasedAutoScalerConfig.java:
##########
@@ -49,6 +50,7 @@ public class LagBasedAutoScalerConfig implements AutoScalerConfig
   private final boolean enableTaskAutoScaler;
   private final long minTriggerScaleActionFrequencyMillis;
   private final AggregateFunction lagAggregate;
+  @Nullable private Double stopTaskCountPercent;

Review Comment:
   > I just meant we should drop the @Nullable from @Nullable private Double stopTaskCountPercent
   
   In that case `startTaskCount` annotation should also be changed. We should have a clear standard on these things so style doesn't differ based on who is reviewing.



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Re: [PR] Add stopTaskCountPercent field to LagBasedAutoscaler (druid)

Posted by "jtuglu1 (via GitHub)" <gi...@apache.org>.
jtuglu1 commented on code in PR #18480:
URL: https://github.com/apache/druid/pull/18480#discussion_r2325449655


##########
indexing-service/src/main/java/org/apache/druid/indexing/seekablestream/supervisor/autoscaler/LagBasedAutoScalerConfig.java:
##########
@@ -212,6 +218,15 @@ public AggregateFunction getLagAggregate()
     return lagAggregate;
   }
 
+  @Override
+  @JsonProperty
+  @Nullable
+  @JsonInclude(JsonInclude.Include.NON_NULL)

Review Comment:
   Hmmm. Are we sure we want to do that? This would exclude ALL fields that are `null`. In the case where there's some malformed input we may not include it in the spec at all (not visible to the user). I'd prefer to reserve this for fields which make sense to be nullable, and nullable fields tend to bloat the config specs.



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Re: [PR] Add stopTaskCountPercent field to LagBasedAutoscaler (druid)

Posted by "jtuglu1 (via GitHub)" <gi...@apache.org>.
jtuglu1 commented on code in PR #18480:
URL: https://github.com/apache/druid/pull/18480#discussion_r2328457981


##########
indexing-service/src/main/java/org/apache/druid/indexing/seekablestream/supervisor/autoscaler/LagBasedAutoScalerConfig.java:
##########
@@ -212,6 +218,15 @@ public AggregateFunction getLagAggregate()
     return lagAggregate;
   }
 
+  @Override
+  @JsonProperty
+  @Nullable
+  @JsonInclude(JsonInclude.Include.NON_NULL)

Review Comment:
   > I don't quite follow. A non-nullable field would never be null anyway.
   Of course. I think I was just trying to point out that we should have a standard for leaving null (but optional) fields in a config, versus keeping them out.
   > For anyone reading the json, it doesn't make a real difference if the field is absent or if it is assigned "field": null.
   I think it does. I believe what Gian was implying was that for non-required fields, having null values in the config looks weird/adds bloat.



##########
indexing-service/src/main/java/org/apache/druid/indexing/seekablestream/supervisor/autoscaler/LagBasedAutoScalerConfig.java:
##########
@@ -212,6 +218,15 @@ public AggregateFunction getLagAggregate()
     return lagAggregate;
   }
 
+  @Override
+  @JsonProperty
+  @Nullable
+  @JsonInclude(JsonInclude.Include.NON_NULL)

Review Comment:
   > I don't quite follow. A non-nullable field would never be null anyway.
   
   Of course. I think I was just trying to point out that we should have a standard for leaving null (but optional) fields in a config, versus keeping them out.
   
   > For anyone reading the json, it doesn't make a real difference if the field is absent or if it is assigned "field": null.
   
   I think it does. I believe what Gian was implying was that for non-required fields, having null values in the config looks weird/adds bloat.



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Re: [PR] Add stopTaskCountPercent field to LagBasedAutoscaler (druid)

Posted by "maytasm (via GitHub)" <gi...@apache.org>.
maytasm commented on code in PR #18480:
URL: https://github.com/apache/druid/pull/18480#discussion_r2329077025


##########
docs/ingestion/supervisor.md:
##########
@@ -80,6 +80,7 @@ The following table outlines the configuration properties for `autoScalerConfig`
 |`taskCountStart`|Optional config to specify the number of ingestion tasks to start with. When you enable the autoscaler, Druid ignores the value of `taskCount` in `ioConfig` and, if specified, starts with the `taskCountStart` number of tasks. Otherwise, defaults to `taskCountMin`.|No|`taskCountMin`|
 |`minTriggerScaleActionFrequencyMillis`|The minimum time interval between two scale actions.| No|600000|
 |`autoScalerStrategy`|The algorithm of autoscaler. Druid only supports the `lagBased` strategy. See [Autoscaler strategy](#autoscaler-strategy) for more information.|No|`lagBased`|
+|`stopTaskCountRatio`|A variable version of `ioConfig.stopTaskCount` with a valid range of (0.0, 1.0]. Allows the maximum number of stoppable tasks in steady state to be proportional to the number of tasks currently running.|No||

Review Comment:
   nit: might worth mentioning that when stopTaskCountRatio is set, it takes precedence over stopTaskCount 



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Re: [PR] Add stopTaskCountPercent field to LagBasedAutoscaler (druid)

Posted by "kfaraz (via GitHub)" <gi...@apache.org>.
kfaraz commented on code in PR #18480:
URL: https://github.com/apache/druid/pull/18480#discussion_r2328497511


##########
indexing-service/src/main/java/org/apache/druid/indexing/seekablestream/supervisor/autoscaler/LagBasedAutoScalerConfig.java:
##########
@@ -212,6 +218,15 @@ public AggregateFunction getLagAggregate()
     return lagAggregate;
   }
 
+  @Override
+  @JsonProperty
+  @Nullable
+  @JsonInclude(JsonInclude.Include.NON_NULL)

Review Comment:
   > Of course. I think I was just trying to point out that we should have a standard for leaving null (but optional) fields in a config, versus keeping them out.
   
   Yeah, that would be nice. I think this could be a config on the object mapper itself.
   
   > I think it does.
   
   Oh, I must be missing something then. The only fields that would have ever have a null value are nullable (i.e. non required) fields. Required fields should always be validated in the constructor or assigned a default value if null. So the serialized JSON will always have a non-null value for required fields. A malformed input should fail during deserializing itself.
   
   That said, it is really fine either way. It is only a style choice. 🙂 
   And it seems that in the entirety of Druid code, this annotation is never used on the class level.
   So we can just stick to that for the time being.



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Re: [PR] Add stopTaskCountPercent field to LagBasedAutoscaler (druid)

Posted by "kfaraz (via GitHub)" <gi...@apache.org>.
kfaraz commented on code in PR #18480:
URL: https://github.com/apache/druid/pull/18480#discussion_r2326393726


##########
indexing-service/src/main/java/org/apache/druid/indexing/seekablestream/supervisor/autoscaler/LagBasedAutoScalerConfig.java:
##########
@@ -212,6 +218,15 @@ public AggregateFunction getLagAggregate()
     return lagAggregate;
   }
 
+  @Override
+  @JsonProperty
+  @Nullable
+  @JsonInclude(JsonInclude.Include.NON_NULL)

Review Comment:
   > I'd prefer to reserve this for fields which make sense to be nullable
   
   I don't quite follow. A non-nullable field would never be null anyway.
   The constructor would either assign a default value to it or throw an exception.
   For anyone reading the json, it doesn't make a real difference if the field is absent or if it is assigned `"field": null`.



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Re: [PR] Add stopTaskCountPercent field to LagBasedAutoscaler (druid)

Posted by "jtuglu1 (via GitHub)" <gi...@apache.org>.
jtuglu1 commented on code in PR #18480:
URL: https://github.com/apache/druid/pull/18480#discussion_r2328458319


##########
indexing-service/src/main/java/org/apache/druid/indexing/seekablestream/supervisor/autoscaler/LagBasedAutoScalerConfig.java:
##########
@@ -49,6 +50,7 @@ public class LagBasedAutoScalerConfig implements AutoScalerConfig
   private final boolean enableTaskAutoScaler;
   private final long minTriggerScaleActionFrequencyMillis;
   private final AggregateFunction lagAggregate;
+  @Nullable private Double stopTaskCountPercent;

Review Comment:
   > I just meant we should drop the @Nullable from @Nullable private Double stopTaskCountPercent
   
   I personally like to mark these things as `@Nullable` as it makes it clear when working the raw value that the value could be null (not just the accessor method). In that case `startTaskCount` annotation should also be changed. We should have a clear standard on these things so style doesn't differ based on who is reviewing.



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Re: [PR] Add stopTaskCountPercent field to LagBasedAutoscaler (druid)

Posted by "kfaraz (via GitHub)" <gi...@apache.org>.
kfaraz commented on code in PR #18480:
URL: https://github.com/apache/druid/pull/18480#discussion_r2326395289


##########
indexing-service/src/main/java/org/apache/druid/indexing/seekablestream/supervisor/autoscaler/LagBasedAutoScalerConfig.java:
##########
@@ -49,6 +50,7 @@ public class LagBasedAutoScalerConfig implements AutoScalerConfig
   private final boolean enableTaskAutoScaler;
   private final long minTriggerScaleActionFrequencyMillis;
   private final AggregateFunction lagAggregate;
+  @Nullable private Double stopTaskCountPercent;

Review Comment:
   Yes, the pattern suggested in that comment makes sense.
   
   I just meant we should drop the `@Nullable` from `@Nullable private Double stopTaskCountPercent`
   since none of the other fields have it and it doesn't serve any real purpose.
   It is important to have `@Nullable` in the public APIs like the getter and the constructor arg.



##########
indexing-service/src/main/java/org/apache/druid/indexing/seekablestream/supervisor/autoscaler/LagBasedAutoScalerConfig.java:
##########
@@ -49,6 +50,7 @@ public class LagBasedAutoScalerConfig implements AutoScalerConfig
   private final boolean enableTaskAutoScaler;
   private final long minTriggerScaleActionFrequencyMillis;
   private final AggregateFunction lagAggregate;
+  @Nullable private Double stopTaskCountPercent;

Review Comment:
   Also, should this field be `final`?



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Re: [PR] Add stopTaskCountPercent field to LagBasedAutoscaler (druid)

Posted by "jtuglu1 (via GitHub)" <gi...@apache.org>.
jtuglu1 merged PR #18480:
URL: https://github.com/apache/druid/pull/18480


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Re: [PR] Add stopTaskCountPercent field to LagBasedAutoscaler (druid)

Posted by "jtuglu1 (via GitHub)" <gi...@apache.org>.
jtuglu1 commented on code in PR #18480:
URL: https://github.com/apache/druid/pull/18480#discussion_r2325455228


##########
indexing-service/src/main/java/org/apache/druid/indexing/seekablestream/supervisor/autoscaler/LagBasedAutoScalerConfig.java:
##########
@@ -49,6 +50,7 @@ public class LagBasedAutoScalerConfig implements AutoScalerConfig
   private final boolean enableTaskAutoScaler;
   private final long minTriggerScaleActionFrequencyMillis;
   private final AggregateFunction lagAggregate;
+  @Nullable private Double stopTaskCountPercent;

Review Comment:
   I'm following a pattern @gianm had [suggested](https://github.com/apache/druid/pull/17900#discussion_r2047872048) a while back.



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Re: [PR] Add stopTaskCountPercent field to LagBasedAutoscaler (druid)

Posted by "jtuglu1 (via GitHub)" <gi...@apache.org>.
jtuglu1 commented on code in PR #18480:
URL: https://github.com/apache/druid/pull/18480#discussion_r2328458319


##########
indexing-service/src/main/java/org/apache/druid/indexing/seekablestream/supervisor/autoscaler/LagBasedAutoScalerConfig.java:
##########
@@ -49,6 +50,7 @@ public class LagBasedAutoScalerConfig implements AutoScalerConfig
   private final boolean enableTaskAutoScaler;
   private final long minTriggerScaleActionFrequencyMillis;
   private final AggregateFunction lagAggregate;
+  @Nullable private Double stopTaskCountPercent;

Review Comment:
   > I just meant we should drop the @Nullable from @Nullable private Double stopTaskCountPercent
   
   I personally like to mark these things as `@Nullable` as it makes it clear when working the raw value that the value could be null (not just the accessor method). In that case `startTaskCount` annotation should also be changed. We should have a clear standard on these things (like [this](https://google.github.io/styleguide/javaguide.html) so style doesn't differ based on who is reviewing.



##########
indexing-service/src/main/java/org/apache/druid/indexing/seekablestream/supervisor/autoscaler/LagBasedAutoScalerConfig.java:
##########
@@ -49,6 +50,7 @@ public class LagBasedAutoScalerConfig implements AutoScalerConfig
   private final boolean enableTaskAutoScaler;
   private final long minTriggerScaleActionFrequencyMillis;
   private final AggregateFunction lagAggregate;
+  @Nullable private Double stopTaskCountPercent;

Review Comment:
   > I just meant we should drop the @Nullable from @Nullable private Double stopTaskCountPercent
   
   I personally like to mark these things as `@Nullable` as it makes it clear when working the raw value that the value could be null (not just the accessor method). In that case `startTaskCount` annotation should also be changed. We should have a clear standard on these things (like [this])(https://google.github.io/styleguide/javaguide.html) so style doesn't differ based on who is reviewing.



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Re: [PR] Add stopTaskCountPercent field to LagBasedAutoscaler (druid)

Posted by "jtuglu1 (via GitHub)" <gi...@apache.org>.
jtuglu1 commented on code in PR #18480:
URL: https://github.com/apache/druid/pull/18480#discussion_r2328458319


##########
indexing-service/src/main/java/org/apache/druid/indexing/seekablestream/supervisor/autoscaler/LagBasedAutoScalerConfig.java:
##########
@@ -49,6 +50,7 @@ public class LagBasedAutoScalerConfig implements AutoScalerConfig
   private final boolean enableTaskAutoScaler;
   private final long minTriggerScaleActionFrequencyMillis;
   private final AggregateFunction lagAggregate;
+  @Nullable private Double stopTaskCountPercent;

Review Comment:
   > I just meant we should drop the @Nullable from @Nullable private Double stopTaskCountPercent
   
   I personally like to mark these things as `@Nullable` as it makes it clear when working the raw value that the value could be null (not just the accessor method). In that case `startTaskCount` annotation should also be changed. We should have a clear standard on these things (like [this](https://google.github.io/styleguide/javaguide.html#s4.8.5-annotations)) so style doesn't differ based on who is reviewing.



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Re: [PR] Add stopTaskCountPercent field to LagBasedAutoscaler (druid)

Posted by "kfaraz (via GitHub)" <gi...@apache.org>.
kfaraz commented on code in PR #18480:
URL: https://github.com/apache/druid/pull/18480#discussion_r2328495981


##########
indexing-service/src/main/java/org/apache/druid/indexing/seekablestream/supervisor/autoscaler/LagBasedAutoScalerConfig.java:
##########
@@ -49,6 +50,7 @@ public class LagBasedAutoScalerConfig implements AutoScalerConfig
   private final boolean enableTaskAutoScaler;
   private final long minTriggerScaleActionFrequencyMillis;
   private final AggregateFunction lagAggregate;
+  @Nullable private Double stopTaskCountPercent;

Review Comment:
   Yeah, that's fair. It would be nice to have a common standard across the codebase.



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Re: [PR] Add stopTaskCountPercent field to LagBasedAutoscaler (druid)

Posted by "kfaraz (via GitHub)" <gi...@apache.org>.
kfaraz commented on code in PR #18480:
URL: https://github.com/apache/druid/pull/18480#discussion_r2324294101


##########
indexing-service/src/main/java/org/apache/druid/indexing/seekablestream/supervisor/autoscaler/LagBasedAutoScalerConfig.java:
##########
@@ -212,6 +218,15 @@ public AggregateFunction getLagAggregate()
     return lagAggregate;
   }
 
+  @Override
+  @JsonProperty
+  @Nullable
+  @JsonInclude(JsonInclude.Include.NON_NULL)

Review Comment:
   This can be moved to the class level itself.



##########
indexing-service/src/main/java/org/apache/druid/indexing/seekablestream/supervisor/autoscaler/LagBasedAutoScalerConfig.java:
##########
@@ -100,6 +103,9 @@ public LagBasedAutoScalerConfig(
     this.scaleOutStep = scaleOutStep != null ? scaleOutStep : 2;
     this.minTriggerScaleActionFrequencyMillis = minTriggerScaleActionFrequencyMillis
         != null ? minTriggerScaleActionFrequencyMillis : 600000;
+
+    Preconditions.checkArgument(stopTaskCountPercent == null || (stopTaskCountPercent > 0.0 && stopTaskCountPercent <= 1.0), "0.0 < stopTaskCountPercent <= 1.0");

Review Comment:
   If the value range is [0, 1], we should call the field `stopTaskCountRatio`.
   Alternatively, if we want to keep a percentage in the range [0, 100], we should use an int field instead.



##########
indexing-service/src/test/java/org/apache/druid/indexing/overlord/supervisor/autoscaler/LagBasedAutoScalerConfigTest.java:
##########
@@ -0,0 +1,294 @@
+/*
+ * 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.indexing.overlord.supervisor.autoscaler;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import org.apache.druid.indexing.seekablestream.supervisor.autoscaler.LagBasedAutoScalerConfig;
+import org.junit.Assert;
+import org.junit.Test;
+
+public class LagBasedAutoScalerConfigTest

Review Comment:
   Thanks for adding these tests!



##########
indexing-service/src/main/java/org/apache/druid/indexing/seekablestream/supervisor/autoscaler/LagBasedAutoScalerConfig.java:
##########
@@ -49,6 +50,7 @@ public class LagBasedAutoScalerConfig implements AutoScalerConfig
   private final boolean enableTaskAutoScaler;
   private final long minTriggerScaleActionFrequencyMillis;
   private final AggregateFunction lagAggregate;
+  @Nullable private Double stopTaskCountPercent;

Review Comment:
   Nit: I don't think we need to mark this as nullable too. It should be enough to mark just the getter as nullable.



##########
docs/ingestion/supervisor.md:
##########
@@ -80,6 +80,7 @@ The following table outlines the configuration properties for `autoScalerConfig`
 |`taskCountStart`|Optional config to specify the number of ingestion tasks to start with. When you enable the autoscaler, Druid ignores the value of `taskCount` in `ioConfig` and, if specified, starts with the `taskCountStart` number of tasks. Otherwise, defaults to `taskCountMin`.|No|`taskCountMin`|
 |`minTriggerScaleActionFrequencyMillis`|The minimum time interval between two scale actions.| No|600000|
 |`autoScalerStrategy`|The algorithm of autoscaler. Druid only supports the `lagBased` strategy. See [Autoscaler strategy](#autoscaler-strategy) for more information.|No|`lagBased`|
+|`stopTaskCountPercent`|A variable version of `ioConfig.stopTaskCount`. Allows the maximum number of stoppable tasks in steady state to be proportional to the # of tasks running.|No||

Review Comment:
   ```suggestion
   |`stopTaskCountPercent`|A variable version of `ioConfig.stopTaskCount`. Allows the maximum number of stoppable tasks in steady state to be proportional to the number of tasks currently running.|No||
   ```



##########
indexing-service/src/test/java/org/apache/druid/indexing/overlord/supervisor/autoscaler/LagBasedAutoScalerConfigTest.java:
##########
@@ -0,0 +1,294 @@
+/*
+ * 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.indexing.overlord.supervisor.autoscaler;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import org.apache.druid.indexing.seekablestream.supervisor.autoscaler.LagBasedAutoScalerConfig;
+import org.junit.Assert;
+import org.junit.Test;
+
+public class LagBasedAutoScalerConfigTest
+{
+  private static final ObjectMapper OBJECT_MAPPER = new ObjectMapper();
+
+  @Test
+  public void testSerdeWithDefaults() throws Exception
+  {
+    LagBasedAutoScalerConfig config = OBJECT_MAPPER.readValue(
+        OBJECT_MAPPER.writeValueAsString(
+            OBJECT_MAPPER.readValue(
+                "{}",
+                LagBasedAutoScalerConfig.class
+            )

Review Comment:
   Just this part should be enough to verify the default values.
   We need not perform a repeated deserialize-serialize-deserialize.



##########
indexing-service/src/test/java/org/apache/druid/indexing/seekablestream/supervisor/SeekableStreamSupervisorStateTest.java:
##########
@@ -2689,6 +2690,55 @@ public Duration getEmissionDuration()
     replayAll();
   }
 
+  @Test
+  public void testMaxAllowedStopsWithStopTaskCountPercent()
+  {
+    LagAggregator lagAggregator = createMock(LagAggregator.class);
+    AutoScalerConfig autoScalerConfig = new LagBasedAutoScalerConfig(

Review Comment:
   Looking at the various test cases, I think it might make sense to add a `Builder` for the `LagBasedAutoScalerConfig` class. Not a blocker for this PR, though.



##########
indexing-service/src/test/java/org/apache/druid/indexing/overlord/supervisor/autoscaler/LagBasedAutoScalerConfigTest.java:
##########
@@ -0,0 +1,294 @@
+/*
+ * 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.indexing.overlord.supervisor.autoscaler;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import org.apache.druid.indexing.seekablestream.supervisor.autoscaler.LagBasedAutoScalerConfig;
+import org.junit.Assert;
+import org.junit.Test;
+
+public class LagBasedAutoScalerConfigTest
+{
+  private static final ObjectMapper OBJECT_MAPPER = new ObjectMapper();
+
+  @Test
+  public void testSerdeWithDefaults() throws Exception
+  {
+    LagBasedAutoScalerConfig config = OBJECT_MAPPER.readValue(
+        OBJECT_MAPPER.writeValueAsString(
+            OBJECT_MAPPER.readValue(
+                "{}",
+                LagBasedAutoScalerConfig.class
+            )

Review Comment:
   We can have one separate test which verifies that the object obtained on serialize and then deserialize is the same as the original.



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