Copyright 2012 Netflix, 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.
/**
 * Copyright 2012 Netflix, 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 com.netflix.hystrix;

import rx.Observable;

import com.netflix.hystrix.HystrixCommandProperties.ExecutionIsolationStrategy;
import com.netflix.hystrix.strategy.executionhook.HystrixCommandExecutionHook;
import com.netflix.hystrix.strategy.properties.HystrixPropertiesStrategy;
import com.netflix.hystrix.strategy.eventnotifier.HystrixEventNotifier;

Used to wrap code that will execute potentially risky functionality (typically meaning a service call over the network)
with fault and latency tolerance, statistics and performance metrics capture, circuit breaker and bulkhead functionality.
This command should be used for a purely non-blocking call pattern. The caller of this command will be subscribed to the Observable returned by the run() method.
Type parameters:
<R> – 
           the return type
@ThreadSafe
/**
 * Used to wrap code that will execute potentially risky functionality (typically meaning a service call over the network)
 * with fault and latency tolerance, statistics and performance metrics capture, circuit breaker and bulkhead functionality.
 * This command should be used for a purely non-blocking call pattern. The caller of this command will be subscribed to the Observable<R> returned by the run() method.
 * 
 * @param <R>
 *            the return type
 * 
 * @ThreadSafe
 */
public abstract class HystrixObservableCommand<R> extends AbstractCommand<R> implements HystrixObservable<R>, HystrixInvokableInfo<R> {

    
Construct a HystrixObservableCommand with defined HystrixCommandGroupKey. 
 The HystrixCommandKey will be derived from the implementing class name. 
Params:
group –  HystrixCommandGroupKey used to group together multiple HystrixObservableCommand objects. 
 The HystrixCommandGroupKey is used to represent a common relationship between commands. For example, a library or team name, the system all related commands interace with, common business purpose etc./**
     * Construct a {@link HystrixObservableCommand} with defined {@link HystrixCommandGroupKey}.
     * <p>
     * The {@link HystrixCommandKey} will be derived from the implementing class name.
     * 
     * @param group
     *            {@link HystrixCommandGroupKey} used to group together multiple {@link HystrixObservableCommand} objects.
     *            <p>
     *            The {@link HystrixCommandGroupKey} is used to represent a common relationship between commands. For example, a library or team name, the system all related commands interace with,
     *            common business purpose etc.
     */
    protected HystrixObservableCommand(HystrixCommandGroupKey group) {
        // use 'null' to specify use the default
        this(new Setter(group));
    }

    
Overridden to true so that all onNext emissions are captured
Returns:
if onNext events should be reported on This affects HystrixRequestLog, and HystrixEventNotifier currently. Metrics/Hooks later/**
     *
     * Overridden to true so that all onNext emissions are captured
     *
     * @return if onNext events should be reported on
     * This affects {@link HystrixRequestLog}, and {@link HystrixEventNotifier} currently.  Metrics/Hooks later
     */
    @Override
    protected boolean shouldOutputOnNextEvents() {
        return true;
    }

    @Override
    protected String getFallbackMethodName() {
        return "resumeWithFallback";
    }

    @Override
    protected boolean isFallbackUserDefined() {
        Boolean containsFromMap = commandContainsFallback.get(commandKey);
        if (containsFromMap != null) {
            return containsFromMap;
        } else {
            Boolean toInsertIntoMap;
            try {
                getClass().getDeclaredMethod("resumeWithFallback");
                toInsertIntoMap = true;
            } catch (NoSuchMethodException nsme) {
                toInsertIntoMap = false;
            }
            commandContainsFallback.put(commandKey, toInsertIntoMap);
            return toInsertIntoMap;
        }
    }

    @Override
    protected boolean commandIsScalar() {
        return false;
    }

    
Construct a HystrixObservableCommand with defined Setter that allows injecting property and strategy overrides and other optional arguments. 
 NOTE: The HystrixCommandKey is used to associate a HystrixObservableCommand with HystrixCircuitBreaker, HystrixCommandMetrics and other objects. 
 Do not create multiple HystrixObservableCommand implementations with the same HystrixCommandKey but different injected default properties as the first instantiated will win. 
 Properties passed in via Setter.andCommandPropertiesDefaults are cached for the given HystrixCommandKey for the life of the JVM or until Hystrix.reset() is called. Dynamic properties allow runtime changes. Read more on the Hystrix Wiki.
Params:
setter – 
           Fluent interface for constructor arguments/**
     * Construct a {@link HystrixObservableCommand} with defined {@link Setter} that allows injecting property and strategy overrides and other optional arguments.
     * <p>
     * NOTE: The {@link HystrixCommandKey} is used to associate a {@link HystrixObservableCommand} with {@link HystrixCircuitBreaker}, {@link HystrixCommandMetrics} and other objects.
     * <p>
     * Do not create multiple {@link HystrixObservableCommand} implementations with the same {@link HystrixCommandKey} but different injected default properties as the first instantiated will win.
     * <p>
     * Properties passed in via {@link Setter#andCommandPropertiesDefaults} are cached for the given {@link HystrixCommandKey} for the life of the JVM
     * or until {@link Hystrix#reset()} is called. Dynamic properties allow runtime changes. Read more on the <a href="https://github.com/Netflix/Hystrix/wiki/Configuration">Hystrix Wiki</a>.
     * 
     * @param setter
     *            Fluent interface for constructor arguments
     */
    protected HystrixObservableCommand(Setter setter) {
        // use 'null' to specify use the default
        this(setter.groupKey, setter.commandKey, setter.threadPoolKey, null, null, setter.commandPropertiesDefaults, setter.threadPoolPropertiesDefaults, null, null, null, null, null);
    }

    
Allow constructing a HystrixObservableCommand with injection of most aspects of its functionality. 

Some of these never have a legitimate reason for injection except in unit testing.

Most of the args will revert to a valid default if 'null' is passed in.
/**
     * Allow constructing a {@link HystrixObservableCommand} with injection of most aspects of its functionality.
     * <p>
     * Some of these never have a legitimate reason for injection except in unit testing.
     * <p>
     * Most of the args will revert to a valid default if 'null' is passed in.
     */
    HystrixObservableCommand(HystrixCommandGroupKey group, HystrixCommandKey key, HystrixThreadPoolKey threadPoolKey, HystrixCircuitBreaker circuitBreaker, HystrixThreadPool threadPool,
            HystrixCommandProperties.Setter commandPropertiesDefaults, HystrixThreadPoolProperties.Setter threadPoolPropertiesDefaults,
            HystrixCommandMetrics metrics, TryableSemaphore fallbackSemaphore, TryableSemaphore executionSemaphore,
            HystrixPropertiesStrategy propertiesStrategy, HystrixCommandExecutionHook executionHook) {
        super(group, key, threadPoolKey, circuitBreaker, threadPool, commandPropertiesDefaults, threadPoolPropertiesDefaults, metrics, fallbackSemaphore, executionSemaphore, propertiesStrategy, executionHook);
    }

    
Fluent interface for arguments to the HystrixObservableCommand constructor. 

The required arguments are set via the 'with' factory method and optional arguments via the 'and' chained methods.

Example:
 
 Setter.withGroupKey(HystrixCommandGroupKey.Factory.asKey("GroupName"))
.andCommandKey(HystrixCommandKey.Factory.asKey("CommandName"))
.andEventNotifier(notifier);
 
@NotThreadSafe
/**
     * Fluent interface for arguments to the {@link HystrixObservableCommand} constructor.
     * <p>
     * The required arguments are set via the 'with' factory method and optional arguments via the 'and' chained methods.
     * <p>
     * Example:
     * <pre> {@code
     *  Setter.withGroupKey(HystrixCommandGroupKey.Factory.asKey("GroupName"))
                .andCommandKey(HystrixCommandKey.Factory.asKey("CommandName"))
                .andEventNotifier(notifier);
     * } </pre>
     * 
     * @NotThreadSafe
     */
    final public static class Setter {

        protected final HystrixCommandGroupKey groupKey;
        protected HystrixCommandKey commandKey;
        protected HystrixThreadPoolKey threadPoolKey;
        protected HystrixCommandProperties.Setter commandPropertiesDefaults;
        protected HystrixThreadPoolProperties.Setter threadPoolPropertiesDefaults;

        
Setter factory method containing required values.

All optional arguments can be set via the chained methods.
Params:
groupKey –  HystrixCommandGroupKey used to group together multiple HystrixObservableCommand objects. 
 The HystrixCommandGroupKey is used to represent a common relationship between commands. For example, a library or team name, the system all related commands interace with, common business purpose etc./**
         * Setter factory method containing required values.
         * <p>
         * All optional arguments can be set via the chained methods.
         * 
         * @param groupKey
         *            {@link HystrixCommandGroupKey} used to group together multiple {@link HystrixObservableCommand} objects.
         *            <p>
         *            The {@link HystrixCommandGroupKey} is used to represent a common relationship between commands. For example, a library or team name, the system all related commands interace
         *            with,
         *            common business purpose etc.
         */
        protected Setter(HystrixCommandGroupKey groupKey) {
            this.groupKey = groupKey;

            // default to using SEMAPHORE for ObservableCommand
            commandPropertiesDefaults = setDefaults(HystrixCommandProperties.Setter());
        }

        
Setter factory method with required values.

All optional arguments can be set via the chained methods.
Params:
groupKey –  HystrixCommandGroupKey used to group together multiple HystrixObservableCommand objects. 
 The HystrixCommandGroupKey is used to represent a common relationship between commands. For example, a library or team name, the system all related commands interace with, common business purpose etc./**
         * Setter factory method with required values.
         * <p>
         * All optional arguments can be set via the chained methods.
         * 
         * @param groupKey
         *            {@link HystrixCommandGroupKey} used to group together multiple {@link HystrixObservableCommand} objects.
         *            <p>
         *            The {@link HystrixCommandGroupKey} is used to represent a common relationship between commands. For example, a library or team name, the system all related commands interace
         *            with,
         *            common business purpose etc.
         */
        public static Setter withGroupKey(HystrixCommandGroupKey groupKey) {
            return new Setter(groupKey);
        }

        
Params:
commandKey –  HystrixCommandKey used to identify a HystrixObservableCommand instance for statistics, circuit-breaker, properties, etc. 

           By default this will be derived from the instance class name.
           
 NOTE: Every unique HystrixCommandKey will result in new instances of HystrixCircuitBreaker, HystrixCommandMetrics and HystrixCommandProperties. Thus, the number of variants should be kept to a finite and reasonable number to avoid high-memory usage or memory leacks. 

           Hundreds of keys is fine, tens of thousands is probably not.
Returns:
Setter for fluent interface via method chaining/**
         * @param commandKey
         *            {@link HystrixCommandKey} used to identify a {@link HystrixObservableCommand} instance for statistics, circuit-breaker, properties, etc.
         *            <p>
         *            By default this will be derived from the instance class name.
         *            <p>
         *            NOTE: Every unique {@link HystrixCommandKey} will result in new instances of {@link HystrixCircuitBreaker}, {@link HystrixCommandMetrics} and {@link HystrixCommandProperties}.
         *            Thus,
         *            the number of variants should be kept to a finite and reasonable number to avoid high-memory usage or memory leacks.
         *            <p>
         *            Hundreds of keys is fine, tens of thousands is probably not.
         * @return Setter for fluent interface via method chaining
         */
        public Setter andCommandKey(HystrixCommandKey commandKey) {
            this.commandKey = commandKey;
            return this;
        }

        
Optional
Params:
commandPropertiesDefaults –  Setter with property overrides for this specific instance of HystrixObservableCommand. 
 See the HystrixPropertiesStrategy JavaDocs for more information on properties and order of precedence.
Returns:
Setter for fluent interface via method chaining/**
         * Optional
         * 
         * @param commandPropertiesDefaults
         *            {@link HystrixCommandProperties.Setter} with property overrides for this specific instance of {@link HystrixObservableCommand}.
         *            <p>
         *            See the {@link HystrixPropertiesStrategy} JavaDocs for more information on properties and order of precedence.
         * @return Setter for fluent interface via method chaining
         */
        public Setter andCommandPropertiesDefaults(HystrixCommandProperties.Setter commandPropertiesDefaults) {
            this.commandPropertiesDefaults = setDefaults(commandPropertiesDefaults);
            return this;
        }

        private HystrixCommandProperties.Setter setDefaults(HystrixCommandProperties.Setter commandPropertiesDefaults) {
            if (commandPropertiesDefaults.getExecutionIsolationStrategy() == null) {
                // default to using SEMAPHORE for ObservableCommand if the user didn't set it
                commandPropertiesDefaults.withExecutionIsolationStrategy(ExecutionIsolationStrategy.SEMAPHORE);
            }
            return commandPropertiesDefaults;
        }

    }

    
Implement this method with code to be executed when AbstractCommand.observe() or AbstractCommand.toObservable() are invoked. 
Returns:
R response type/**
     * Implement this method with code to be executed when {@link #observe()} or {@link #toObservable()} are invoked.
     * 
     * @return R response type
     */
    protected abstract Observable<R> construct();

    
If AbstractCommand.observe() or AbstractCommand.toObservable() fails in any way then this method will be invoked to provide an opportunity to return a fallback response. 

This should do work that does not require network transport to produce.

In other words, this should be a static or cached result that can immediately be returned upon failure.
 If network traffic is wanted for fallback (such as going to MemCache) then the fallback implementation should invoke another HystrixObservableCommand instance that protects against that network access and possibly has another level of fallback that does not involve network access. 

DEFAULT BEHAVIOR: It throws UnsupportedOperationException.
Returns:
R or UnsupportedOperationException if not implemented/**
     * If {@link #observe()} or {@link #toObservable()} fails in any way then this method will be invoked to provide an opportunity to return a fallback response.
     * <p>
     * This should do work that does not require network transport to produce.
     * <p>
     * In other words, this should be a static or cached result that can immediately be returned upon failure.
     * <p>
     * If network traffic is wanted for fallback (such as going to MemCache) then the fallback implementation should invoke another {@link HystrixObservableCommand} instance that protects against
     * that network
     * access and possibly has another level of fallback that does not involve network access.
     * <p>
     * DEFAULT BEHAVIOR: It throws UnsupportedOperationException.
     * 
     * @return R or UnsupportedOperationException if not implemented
     */
    protected Observable<R> resumeWithFallback() {
        return Observable.error(new UnsupportedOperationException("No fallback available."));
    }

    @Override
    final protected Observable<R> getExecutionObservable() {
        return construct();
    }
    
    @Override
    final protected Observable<R> getFallbackObservable() {
        return resumeWithFallback();
    }
}