From 707643c9caa74db0f5b58334fcbcfdcb79c83aeb Mon Sep 17 00:00:00 2001
From: Bryce Anderson <bl_anderson@apple.com>
Date: Tue, 10 Dec 2024 20:16:29 -0700
Subject: [PATCH] loadbalancer-experimental: Add default policies

Motivation:

Users currently need to understand a lot about the different policies
in order to make a good decision.

Modifications:

Add default connection pool and load balancing policies. This makes it
easy for users to pick the recommended default. This also gives us as
maintainers a place to change the recommended default.
---
 .../loadbalancer/ConnectionPoolPolicies.java    | 17 +++++++++++++----
 .../DefaultLoadBalancerBuilder.java             | 14 ++------------
 .../loadbalancer/LoadBalancingPolicies.java     | 11 +++++++++++
 3 files changed, 26 insertions(+), 16 deletions(-)

diff --git a/servicetalk-loadbalancer-experimental/src/main/java/io/servicetalk/loadbalancer/ConnectionPoolPolicies.java b/servicetalk-loadbalancer-experimental/src/main/java/io/servicetalk/loadbalancer/ConnectionPoolPolicies.java
index c27dfe8430..a1306a4a76 100644
--- a/servicetalk-loadbalancer-experimental/src/main/java/io/servicetalk/loadbalancer/ConnectionPoolPolicies.java
+++ b/servicetalk-loadbalancer-experimental/src/main/java/io/servicetalk/loadbalancer/ConnectionPoolPolicies.java
@@ -25,6 +25,15 @@ private ConnectionPoolPolicies() {
         // no instances
     }
 
+    /**
+     * Get the recommended default {@link ConnectionPoolPolicy}.
+     * @param <C> the concrete type of the {@link LoadBalancedConnection}
+     * @return the recommended default {@link ConnectionPoolPolicy}.
+     */
+    public static <C extends LoadBalancedConnection> ConnectionPoolPolicy<C> defaultPolicy() {
+        return linearSearch();
+    }
+
     /**
      * A connection selection policy that prioritizes a configurable "core" pool.
      * <p>
@@ -44,8 +53,8 @@ private ConnectionPoolPolicies() {
      * @param <C> the concrete type of the {@link LoadBalancedConnection}
      * @return the configured {@link ConnectionPoolPolicy}.
      */
-    public static <C extends LoadBalancedConnection> ConnectionPoolPolicy<C>
-    corePool(final int corePoolSize, final boolean forceCorePool) {
+    public static <C extends LoadBalancedConnection>
+    ConnectionPoolPolicy<C> corePool(final int corePoolSize, final boolean forceCorePool) {
         return CorePoolConnectionSelector.factory(corePoolSize, forceCorePool);
     }
 
@@ -94,8 +103,8 @@ public static <C extends LoadBalancedConnection> ConnectionPoolPolicy<C> linearS
      * @param <C> the concrete type of the {@link LoadBalancedConnection}
      * @return the configured {@link ConnectionPoolPolicy}.
      */
-    public static <C extends LoadBalancedConnection> ConnectionPoolPolicy<C>
-    p2c(int corePoolSize, boolean forceCorePool) {
+    public static <C extends LoadBalancedConnection>
+    ConnectionPoolPolicy<C> p2c(int corePoolSize, boolean forceCorePool) {
         return p2c(DEFAULT_MAX_EFFORT, corePoolSize, forceCorePool);
     }
 
diff --git a/servicetalk-loadbalancer-experimental/src/main/java/io/servicetalk/loadbalancer/DefaultLoadBalancerBuilder.java b/servicetalk-loadbalancer-experimental/src/main/java/io/servicetalk/loadbalancer/DefaultLoadBalancerBuilder.java
index 6a141a7e66..3d17fe8fd5 100644
--- a/servicetalk-loadbalancer-experimental/src/main/java/io/servicetalk/loadbalancer/DefaultLoadBalancerBuilder.java
+++ b/servicetalk-loadbalancer-experimental/src/main/java/io/servicetalk/loadbalancer/DefaultLoadBalancerBuilder.java
@@ -39,8 +39,8 @@ final class DefaultLoadBalancerBuilder<ResolvedAddress, C extends LoadBalancedCo
     private Executor backgroundExecutor;
     @Nullable
     private LoadBalancerObserverFactory loadBalancerObserverFactory;
-    private LoadBalancingPolicy<ResolvedAddress, C> loadBalancingPolicy = defaultLoadBalancingPolicy();
-    private ConnectionPoolPolicy<C> connectionPoolPolicy = defaultConnectionSelectorFactory();
+    private LoadBalancingPolicy<ResolvedAddress, C> loadBalancingPolicy = LoadBalancingPolicies.defaultPolicy();
+    private ConnectionPoolPolicy<C> connectionPoolPolicy = ConnectionPoolPolicies.defaultPolicy();
     private OutlierDetectorConfig outlierDetectorConfig = OutlierDetectorConfig.DEFAULT_CONFIG;
 
     // package private constructor so users must funnel through providers in `LoadBalancers`
@@ -181,14 +181,4 @@ private Executor getExecutor() {
         return backgroundExecutor ==
                 null ? RoundRobinLoadBalancerFactory.SharedExecutor.getInstance() : backgroundExecutor;
     }
-
-    private static <ResolvedAddress, C extends LoadBalancedConnection>
-    LoadBalancingPolicy<ResolvedAddress, C> defaultLoadBalancingPolicy() {
-        return LoadBalancingPolicies.roundRobin().build();
-    }
-
-    private static <C extends LoadBalancedConnection> ConnectionPoolPolicy<C>
-    defaultConnectionSelectorFactory() {
-        return ConnectionPoolPolicies.linearSearch();
-    }
 }
diff --git a/servicetalk-loadbalancer-experimental/src/main/java/io/servicetalk/loadbalancer/LoadBalancingPolicies.java b/servicetalk-loadbalancer-experimental/src/main/java/io/servicetalk/loadbalancer/LoadBalancingPolicies.java
index 260e8008d0..bdcd410853 100644
--- a/servicetalk-loadbalancer-experimental/src/main/java/io/servicetalk/loadbalancer/LoadBalancingPolicies.java
+++ b/servicetalk-loadbalancer-experimental/src/main/java/io/servicetalk/loadbalancer/LoadBalancingPolicies.java
@@ -15,6 +15,8 @@
  */
 package io.servicetalk.loadbalancer;
 
+import io.servicetalk.client.api.LoadBalancedConnection;
+
 /**
  * A collections of factories for constructing a {@link LoadBalancingPolicy}.
  */
@@ -24,6 +26,15 @@ private LoadBalancingPolicies() {
         // no instances.
     }
 
+    /**
+     * Get the recommended default {@link LoadBalancingPolicy}.
+     * @return the recommended default {@link LoadBalancingPolicy}.
+     */
+    public static <ResolvedAddress, C extends LoadBalancedConnection>
+    LoadBalancingPolicy<ResolvedAddress, C> defaultPolicy() {
+        return LoadBalancingPolicies.roundRobin().build();
+    }
+
     /**
      * Builder for the round-robin {@link LoadBalancingPolicy}.
      * Round-robin load balancing is a strategy that maximizes fairness of the request distribution. This comes at the