From 118ce9cb1622fcfc6de98d3acfea56e4ed111eec Mon Sep 17 00:00:00 2001
From: zhangdong <493738387@qq.com>
Date: Thu, 16 Feb 2023 10:31:24 +0800
Subject: [PATCH] [Enhance](ComputeNode) change logic of
 BeSelectionPolicy.getCandidateBackends (#16737)

The previous logic is how many cn can be returned at most. Instead,
if the number of cn is less than expectBeNum, need to use mix to fill in,
until the number of cn equals with expectBeNum or mix nodes are also used up
---
 .../doris/planner/external/BackendPolicy.java    |  2 +-
 .../apache/doris/system/BeSelectionPolicy.java   | 16 ++++++----------
 .../doris/system/SystemInfoServiceTest.java      | 10 +++++-----
 3 files changed, 12 insertions(+), 16 deletions(-)

diff --git a/fe/fe-core/src/main/java/org/apache/doris/planner/external/BackendPolicy.java b/fe/fe-core/src/main/java/org/apache/doris/planner/external/BackendPolicy.java
index cdc436a83a..a7a7ba8cc3 100644
--- a/fe/fe-core/src/main/java/org/apache/doris/planner/external/BackendPolicy.java
+++ b/fe/fe-core/src/main/java/org/apache/doris/planner/external/BackendPolicy.java
@@ -58,7 +58,7 @@ public class BackendPolicy {
                 .needLoadAvailable()
                 .addTags(tags)
                 .preferComputeNode()
-                .assignCandidateNum(Config.backend_num_for_federation)
+                .assignExpectBeNum(Config.backend_num_for_federation)
                 .build();
         backends.addAll(policy.getCandidateBackends(Env.getCurrentSystemInfo().getIdToBackend().values()));
         if (backends.isEmpty()) {
diff --git a/fe/fe-core/src/main/java/org/apache/doris/system/BeSelectionPolicy.java b/fe/fe-core/src/main/java/org/apache/doris/system/BeSelectionPolicy.java
index 6c95f6ecfb..acd38f14bd 100644
--- a/fe/fe-core/src/main/java/org/apache/doris/system/BeSelectionPolicy.java
+++ b/fe/fe-core/src/main/java/org/apache/doris/system/BeSelectionPolicy.java
@@ -47,7 +47,7 @@ public class BeSelectionPolicy {
     public boolean allowOnSameHost = false;
 
     public boolean preferComputeNode = false;
-    public int candidateNum = Integer.MAX_VALUE;
+    public int expectBeNum = 0;
 
     private BeSelectionPolicy() {
 
@@ -105,8 +105,8 @@ public class BeSelectionPolicy {
             return this;
         }
 
-        public Builder assignCandidateNum(int candidateNum) {
-            policy.candidateNum = candidateNum;
+        public Builder assignExpectBeNum(int expectBeNum) {
+            policy.expectBeNum = expectBeNum;
             return this;
         }
 
@@ -141,25 +141,21 @@ public class BeSelectionPolicy {
 
     public List<Backend> getCandidateBackends(ImmutableCollection<Backend> backends) {
         List<Backend> filterBackends = backends.stream().filter(this::isMatch).collect(Collectors.toList());
-        Collections.shuffle(filterBackends);
         List<Backend> candidates = new ArrayList<>();
         if (preferComputeNode) {
             int num = 0;
             // pick compute node first
             for (Backend backend : filterBackends) {
                 if (backend.isComputeNode()) {
-                    if (num >= candidateNum) {
-                        break;
-                    }
                     candidates.add(backend);
                     num++;
                 }
             }
             // fill with some mix node.
-            if (num < candidateNum) {
+            if (num < expectBeNum) {
                 for (Backend backend : filterBackends) {
                     if (backend.isMixNode()) {
-                        if (num >= candidateNum) {
+                        if (num >= expectBeNum) {
                             break;
                         }
                         candidates.add(backend);
@@ -170,7 +166,7 @@ public class BeSelectionPolicy {
         } else {
             candidates.addAll(filterBackends);
         }
-
+        Collections.shuffle(candidates);
         return candidates;
     }
 
diff --git a/fe/fe-core/src/test/java/org/apache/doris/system/SystemInfoServiceTest.java b/fe/fe-core/src/test/java/org/apache/doris/system/SystemInfoServiceTest.java
index 546449b79c..0afa019310 100644
--- a/fe/fe-core/src/test/java/org/apache/doris/system/SystemInfoServiceTest.java
+++ b/fe/fe-core/src/test/java/org/apache/doris/system/SystemInfoServiceTest.java
@@ -239,11 +239,11 @@ public class SystemInfoServiceTest {
         Assert.assertEquals(1, infoService.selectBackendIdsByPolicy(policy02, 1).size());
 
         BeSelectionPolicy policy03 = new BeSelectionPolicy.Builder().addTags(Sets.newHashSet(taga))
-                .setStorageMedium(TStorageMedium.HDD).preferComputeNode().assignCandidateNum(0).build();
-        Assert.assertEquals(0, infoService.selectBackendIdsByPolicy(policy03, 1).size());
+                .setStorageMedium(TStorageMedium.HDD).preferComputeNode().assignExpectBeNum(0).build();
+        Assert.assertEquals(1, infoService.selectBackendIdsByPolicy(policy03, 1).size());
 
         BeSelectionPolicy policy04 = new BeSelectionPolicy.Builder().addTags(Sets.newHashSet(taga))
-                .setStorageMedium(TStorageMedium.HDD).preferComputeNode().assignCandidateNum(1).build();
+                .setStorageMedium(TStorageMedium.HDD).preferComputeNode().assignExpectBeNum(1).build();
         Assert.assertEquals(1, infoService.selectBackendIdsByPolicy(policy04, 1).size());
 
         // one compute node and two mix node
@@ -264,11 +264,11 @@ public class SystemInfoServiceTest {
         Assert.assertEquals(0, infoService.selectBackendIdsByPolicy(policy05, 3).size());
 
         BeSelectionPolicy policy06 = new BeSelectionPolicy.Builder().addTags(Sets.newHashSet(taga))
-                .setStorageMedium(TStorageMedium.HDD).preferComputeNode().assignCandidateNum(2).build();
+                .setStorageMedium(TStorageMedium.HDD).preferComputeNode().assignExpectBeNum(2).build();
         Assert.assertEquals(2, infoService.selectBackendIdsByPolicy(policy06, 2).size());
 
         BeSelectionPolicy policy07 = new BeSelectionPolicy.Builder().addTags(Sets.newHashSet(taga))
-                .setStorageMedium(TStorageMedium.HDD).preferComputeNode().assignCandidateNum(3).build();
+                .setStorageMedium(TStorageMedium.HDD).preferComputeNode().assignExpectBeNum(3).build();
         Assert.assertEquals(3, infoService.selectBackendIdsByPolicy(policy07, 3).size());
     }