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loadbalancer-experimental: add provider for enabling DefaultLoadBalancer (#2900)
Motivation: We want to make it easy for users to enable DefaultLoadBalancer for specific clients and then manipulate it's behavior via system properties so they don't require rebuilding apps to test. Modifications: Add a new package that includes a SingleAddressHttpClientBuilderProvider which enables users to enable DefaultLoadBalancer for clients based on the address used, or all clients if desired.
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‎servicetalk-loadbalancer-experimental-provider/README.adoc

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= DefaultLoadBalancer Providers
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This package provides providers for enabling the DefaultLoadBalancer via system properties to allow for easy
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experimentation that doesn't require a recompilation of the application.
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> WARNING: this package is only for experimentation and will be removed in the future.
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== Enabling DefaultLoadBalancer via System Properties
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=== Dynamically Loading the DefaultHttpLoadBalancerProvider
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This package uses the standard providers pattern. To enable the provider you need to both include this package as
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part of your application bundle and also include a file in the resources as follows:
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```
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resources/META-INF/services/io.servicetalk.http.api.HttpProviders$SingleAddressHttpClientBuilderProvider
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```
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The contents of this must contain the line
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```
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io.servicetalk.loadbalancer.experimental.DefaultHttpLoadBalancerProvider
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```
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=== Targeting Clients for Which to Enable DefaultLoadBalancer
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The `DefaultHttpLoadBalancerProvider` supports enabling the load balancer either for all clients or only a set of
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specific clients. Enabling the load balancer for all clients can be done by setting the following system property:
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```
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io.servicetalk.loadbalancer.experimental.clientsEnabledFor=all
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```
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The experimental load balancer can also be enabled for only a subset of clients. This can be done via setting the
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system property to a comma separated list:
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```
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io.servicetalk.loadbalancer.experimental.clientsEnabledFor=service1,service2
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```
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The specific names will depend on how the client is built. If the client is built using a `HostAndPort`, the names are
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only the host component. If the client is built using some other unresolved address form then the string representation
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of that is used.
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=== Customizing Name Extraction
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The provider depends on the service name for selecting which client to use. If you're using a custom naming system
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the default implementation may not be able to decode the unresolved address type to the appropriate name. Custom naming
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schemes can be supported by extending the `DefaultHttpLoadBalancerProvider` and overriding the `clientNameFromAddress`
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method. Then this custom provider can be added to the service load list as described in the section above.
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=== All Supported Properties
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All system properties contain the prefix "io.servicetalk.loadbalancer.experimental.". A comprehensive list of the
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supported properties can be found in the `DefaultLoadBalancerProviderConfig` class for reference.

‎servicetalk-loadbalancer-experimental-provider/build.gradle

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/*
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* Copyright © 2024 Apple Inc. and the ServiceTalk project authors
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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apply plugin: "io.servicetalk.servicetalk-gradle-plugin-internal-library"
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dependencies {
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implementation platform(project(":servicetalk-dependencies"))
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testImplementation enforcedPlatform("org.junit:junit-bom:$junit5Version")
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api project(":servicetalk-client-api")
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api project(":servicetalk-concurrent-api")
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implementation project(":servicetalk-annotations")
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implementation project(":servicetalk-loadbalancer")
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implementation project(":servicetalk-loadbalancer-experimental")
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implementation project(":servicetalk-http-api")
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implementation project(":servicetalk-http-netty")
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implementation project(":servicetalk-utils-internal")
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implementation "com.google.code.findbugs:jsr305"
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implementation "org.slf4j:slf4j-api"
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}

‎servicetalk-loadbalancer-experimental-provider/src/main/java/io/servicetalk/loadbalancer/experimental/DefaultHttpLoadBalancerProvider.java

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/*
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* Copyright © 2024 Apple Inc. and the ServiceTalk project authors
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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package io.servicetalk.loadbalancer.experimental;
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import io.servicetalk.client.api.LoadBalancerFactory;
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import io.servicetalk.http.api.DelegatingSingleAddressHttpClientBuilder;
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import io.servicetalk.http.api.FilterableStreamingHttpLoadBalancedConnection;
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import io.servicetalk.http.api.HttpLoadBalancerFactory;
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import io.servicetalk.http.api.HttpProviders;
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import io.servicetalk.http.api.SingleAddressHttpClientBuilder;
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import io.servicetalk.http.netty.DefaultHttpLoadBalancerFactory;
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import io.servicetalk.loadbalancer.LoadBalancers;
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import io.servicetalk.transport.api.HostAndPort;
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import org.slf4j.Logger;
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import org.slf4j.LoggerFactory;
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import static java.util.Objects.requireNonNull;
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/**
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* A client builder provider that supports enabling the new `DefaultLoadBalancer` in applications via property flags.
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* See the packages README.md for more details.
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*/
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public class DefaultHttpLoadBalancerProvider implements HttpProviders.SingleAddressHttpClientBuilderProvider {
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private static final Logger LOGGER = LoggerFactory.getLogger(DefaultHttpLoadBalancerProvider.class);
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private final DefaultLoadBalancerProviderConfig config;
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public DefaultHttpLoadBalancerProvider() {
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this(DefaultLoadBalancerProviderConfig.INSTANCE);
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}
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// exposed for testing
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DefaultHttpLoadBalancerProvider(final DefaultLoadBalancerProviderConfig config) {
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this.config = requireNonNull(config, "config");
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}
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@Override
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public final <U, R> SingleAddressHttpClientBuilder<U, R> newBuilder(U address,
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SingleAddressHttpClientBuilder<U, R> builder) {
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final String serviceName = clientNameFromAddress(address);
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if (config.enabledForServiceName(serviceName)) {
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try {
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HttpLoadBalancerFactory<R> loadBalancerFactory = DefaultHttpLoadBalancerFactory.Builder.<R>from(
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defaultLoadBalancer(serviceName)).build();
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builder = builder.loadBalancerFactory(loadBalancerFactory);
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return new LoadBalancerIgnoringBuilder(builder, serviceName);
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} catch (Throwable ex) {
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LOGGER.warn("Failed to enabled DefaultLoadBalancer for client to address {}.", address, ex);
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}
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}
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return builder;
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}
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private <R> LoadBalancerFactory<R, FilterableStreamingHttpLoadBalancedConnection> defaultLoadBalancer(
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String serviceName) {
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return LoadBalancers.<R, FilterableStreamingHttpLoadBalancedConnection>
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builder("experimental-load-balancer")
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.loadBalancerObserver(new DefaultLoadBalancerObserver(serviceName))
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// set up the new features.
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.outlierDetectorConfig(config.outlierDetectorConfig())
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.loadBalancingPolicy(config.getLoadBalancingPolicy())
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.build();
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}
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/**
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* Extract the service name from the address object.
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* Note: this is a protected method to allow overriding for custom address types.
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* @param <U> the unresolved type of the address.
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* @param address the address from which to extract the service name.
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* @return the String representation of the provided address.
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*/
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protected <U> String clientNameFromAddress(U address) {
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String serviceName;
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if (address instanceof HostAndPort) {
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serviceName = ((HostAndPort) address).hostName();
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} else if (address instanceof String) {
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serviceName = (String) address;
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} else {
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LOGGER.warn("Unknown service address type={} was provided, "
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+ "default 'toString()' will be used as serviceName", address.getClass());
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serviceName = address.toString();
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}
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return serviceName;
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}
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private static final class LoadBalancerIgnoringBuilder<U, R>
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extends DelegatingSingleAddressHttpClientBuilder<U, R> {
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private final String serviceName;
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LoadBalancerIgnoringBuilder(final SingleAddressHttpClientBuilder<U, R> delegate, final String serviceName) {
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super(delegate);
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this.serviceName = serviceName;
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}
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@Override
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public SingleAddressHttpClientBuilder<U, R> loadBalancerFactory(
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HttpLoadBalancerFactory<R> loadBalancerFactory) {
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LOGGER.info("Ignoring http load balancer factory of type {} for client to {} which has " +
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"DefaultLoadBalancer enabled.", loadBalancerFactory.getClass(), serviceName);
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return this;
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}
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}
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}

‎servicetalk-loadbalancer-experimental-provider/src/main/java/io/servicetalk/loadbalancer/experimental/DefaultLoadBalancerObserver.java

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/*
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* Copyright © 2024 Apple Inc. and the ServiceTalk project authors
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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package io.servicetalk.loadbalancer.experimental;
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import io.servicetalk.client.api.NoActiveHostException;
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import io.servicetalk.client.api.ServiceDiscovererEvent;
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import io.servicetalk.loadbalancer.LoadBalancerObserver;
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import org.slf4j.Logger;
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import org.slf4j.LoggerFactory;
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import java.util.Collection;
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import javax.annotation.Nullable;
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import static java.util.Objects.requireNonNull;
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final class DefaultLoadBalancerObserver implements LoadBalancerObserver {
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private static final Logger LOGGER = LoggerFactory.getLogger(DefaultLoadBalancerObserver.class);
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private final String clientName;
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DefaultLoadBalancerObserver(final String clientName) {
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this.clientName = requireNonNull(clientName, "clientName");
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}
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@Override
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public HostObserver hostObserver(Object resolvedAddress) {
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return new HostObserverImpl(resolvedAddress);
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}
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@Override
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public void onNoHostsAvailable() {
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LOGGER.debug("{}- onNoHostsAvailable()", clientName);
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}
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@Override
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public void onServiceDiscoveryEvent(Collection<? extends ServiceDiscovererEvent<?>> events, int oldHostSetSize,
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int newHostSetSize) {
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LOGGER.debug("{}- onServiceDiscoveryEvent(events: {}, oldHostSetSize: {}, newHostSetSize: {})",
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clientName, events, oldHostSetSize, newHostSetSize);
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}
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@Override
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public void onNoActiveHostsAvailable(int hostSetSize, NoActiveHostException exception) {
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LOGGER.debug("{}- No active hosts available. Host set size: {}.", clientName, hostSetSize, exception);
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}
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private final class HostObserverImpl implements HostObserver {
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private final Object resolvedAddress;
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HostObserverImpl(final Object resolvedAddress) {
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this.resolvedAddress = resolvedAddress;
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}
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@Override
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public void onHostMarkedExpired(int connectionCount) {
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LOGGER.debug("{}:{}- onHostMarkedExpired(connectionCount: {})",
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clientName, resolvedAddress, connectionCount);
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}
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@Override
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public void onActiveHostRemoved(int connectionCount) {
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LOGGER.debug("{}:{}- onActiveHostRemoved(connectionCount: {})",
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clientName, resolvedAddress, connectionCount);
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}
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@Override
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public void onExpiredHostRevived(int connectionCount) {
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LOGGER.debug("{}:{}- onExpiredHostRevived(connectionCount: {})",
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clientName, resolvedAddress, connectionCount);
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}
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@Override
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public void onExpiredHostRemoved(int connectionCount) {
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LOGGER.debug("{}:{}- onExpiredHostRemoved(connectionCount: {})",
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clientName, resolvedAddress, connectionCount);
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}
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@Override
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public void onHostMarkedUnhealthy(@Nullable Throwable cause) {
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LOGGER.debug("{}:{}- onHostMarkedUnhealthy(ex)", clientName, resolvedAddress, cause);
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}
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@Override
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public void onHostRevived() {
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LOGGER.debug("{}:{}- onHostRevived()", clientName, resolvedAddress);
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}
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}
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}

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