co.cask.cdap.common.discovery.AbstractEndpointStrategy.java Source code

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Here is the source code for co.cask.cdap.common.discovery.AbstractEndpointStrategy.java

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/*
 * Copyright  2015 Cask Data, Inc.
 *
 * Licensed 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 co.cask.cdap.common.discovery;

import com.google.common.util.concurrent.SettableFuture;
import org.apache.twill.common.Cancellable;
import org.apache.twill.common.Threads;
import org.apache.twill.discovery.Discoverable;
import org.apache.twill.discovery.ServiceDiscovered;

import java.util.concurrent.TimeUnit;

/**
 * An abstract {@link EndpointStrategy} that helps implementation of any strategy.
 * It provides a default implementation for the {@link #pick(long, TimeUnit)} method.
 */
public abstract class AbstractEndpointStrategy implements EndpointStrategy {

    protected final ServiceDiscovered serviceDiscovered;

    /**
     * Constructs an instance with the given {@link ServiceDiscovered}.
     */
    protected AbstractEndpointStrategy(ServiceDiscovered serviceDiscovered) {
        this.serviceDiscovered = serviceDiscovered;
    }

    @Override
    public Discoverable pick(long timeout, TimeUnit timeoutUnit) {
        Discoverable discoverable = pick();
        if (discoverable != null) {
            return discoverable;
        }
        final SettableFuture<Discoverable> future = SettableFuture.create();
        Cancellable cancellable = serviceDiscovered.watchChanges(new ServiceDiscovered.ChangeListener() {
            @Override
            public void onChange(ServiceDiscovered serviceDiscovered) {
                // The serviceDiscovered provided is the same as the one in the field, hence ok to just call pick().
                Discoverable discoverable = pick();
                if (discoverable != null) {
                    future.set(discoverable);
                }
            }
        }, Threads.SAME_THREAD_EXECUTOR);
        try {
            return future.get(timeout, timeoutUnit);
        } catch (Exception e) {
            return null;
        } finally {
            cancellable.cancel();
        }
    }
}