目的
确保给定客户端访问的服务资源不能超过指定的限制。
解释
真实世界例子
一个年轻的人类和一个年老的侏儒走进一家酒吧。他们开始向酒保点啤酒。酒保立刻判定这个年轻人不应该在短时间摄入过量啤酒，并且如果年轻人没有花费足够的时间，酒保将不在为他服务。而这样对老人而言，服务的速率可以提高。
通俗的说
节流模式用于对资源的访问进行速率限制。
维基百科说
控制应用程序实例、单个用户或整个服务使用的资源的消耗。这可以使系统继续运行并满足服务水平协议，即使需求的增加对资源造成了极端的负载。
程序示例
BarCustomer 类提供了 Bartender API的客户端. CallsCount 跟踪每一个 BarCustomer的请求。
java
public class BarCustomer {
@Getter
private final String name;
@Getter
private final int allowedCallsPerSecond;
public BarCustomer(String name, int allowedCallsPerSecond, CallsCount callsCount) {
if (allowedCallsPerSecond < 0) {
throw new InvalidParameterException("Number of calls less than 0 not allowed");
}
this.name = name;
this.allowedCallsPerSecond = allowedCallsPerSecond;
callsCount.addTenant(name);
}
}
@Slf4j
public final class CallsCount {
private final Map<String, AtomicLong> tenantCallsCount = new ConcurrentHashMap<>();
public void addTenant(String tenantName) {
tenantCallsCount.putIfAbsent(tenantName, new AtomicLong(0));
}
public void incrementCount(String tenantName) {
tenantCallsCount.get(tenantName).incrementAndGet();
}
public long getCount(String tenantName) {
return tenantCallsCount.get(tenantName).get();
}
public void reset() {
tenantCallsCount.replaceAll((k, v) -> new AtomicLong(0));
LOGGER.info("reset counters");
}
}接下来，引入客户正在请求的服务。为了追踪请求数，一个节流器使用计时器。
java
public interface Throttler {
void start();
}
public class ThrottleTimerImpl implements Throttler {
private final int throttlePeriod;
private final CallsCount callsCount;
public ThrottleTimerImpl(int throttlePeriod, CallsCount callsCount) {
this.throttlePeriod = throttlePeriod;
this.callsCount = callsCount;
}
@Override
public void start() {
new Timer(true).schedule(new TimerTask() {
@Override
public void run() {
callsCount.reset();
}
}, 0, throttlePeriod);
}
}Bartender 为 BarCustomer们提供 orderDrink 服务。 顾客可能不知道啤酒的供应率是受其外表限制的。
java
class Bartender {
private static final Logger LOGGER = LoggerFactory.getLogger(Bartender.class);
private final CallsCount callsCount;
public Bartender(Throttler timer, CallsCount callsCount) {
this.callsCount = callsCount;
timer.start();
}
public int orderDrink(BarCustomer barCustomer) {
var tenantName = barCustomer.getName();
var count = callsCount.getCount(tenantName);
if (count >= barCustomer.getAllowedCallsPerSecond()) {
LOGGER.error("I'm sorry {}, you've had enough for today!", tenantName);
return -1;
}
callsCount.incrementCount(tenantName);
LOGGER.debug("Serving beer to {} : [{} consumed] ", barCustomer.getName(), count+1);
return getRandomCustomerId();
}
private int getRandomCustomerId() {
return ThreadLocalRandom.current().nextInt(1, 10000);
}
}现在可以看到实际的完整示例。BarCustomer中年轻人每秒限请求两次，年老的侏儒则每秒限请求4次。
java
public static void main(String[] args) {
var callsCount = new CallsCount();
var human = new BarCustomer("young human", 2, callsCount);
var dwarf = new BarCustomer("dwarf soldier", 4, callsCount);
var executorService = Executors.newFixedThreadPool(2);
executorService.execute(() -> makeServiceCalls(human, callsCount));
executorService.execute(() -> makeServiceCalls(dwarf, callsCount));
executorService.shutdown();
try {
executorService.awaitTermination(10, TimeUnit.SECONDS);
} catch (InterruptedException e) {
LOGGER.error("Executor service terminated: {}", e.getMessage());
}
}
private static void makeServiceCalls(BarCustomer barCustomer, CallsCount callsCount) {
var timer = new ThrottleTimerImpl(1000, callsCount);
var service = new Bartender(timer, callsCount);
// Sleep is introduced to keep the output in check and easy to view and analyze the results.
IntStream.range(0, 50).forEach(i -> {
service.orderDrink(barCustomer);
try {
Thread.sleep(100);
} catch (InterruptedException e) {
LOGGER.error("Thread interrupted: {}", e.getMessage());
}
});
}程序输出：
18:46:36.218 [Timer-0] INFO com.iluwatar.throttling.CallsCount - reset counters
18:46:36.218 [Timer-1] INFO com.iluwatar.throttling.CallsCount - reset counters
18:46:36.242 [pool-1-thread-2] DEBUG com.iluwatar.throttling.Bartender - Serving beer to dwarf soldier : [1 consumed]
18:46:36.242 [pool-1-thread-1] DEBUG com.iluwatar.throttling.Bartender - Serving beer to young human : [1 consumed]
18:46:36.342 [pool-1-thread-2] DEBUG com.iluwatar.throttling.Bartender - Serving beer to dwarf soldier : [2 consumed]
18:46:36.342 [pool-1-thread-1] DEBUG com.iluwatar.throttling.Bartender - Serving beer to young human : [2 consumed]
18:46:36.443 [pool-1-thread-1] ERROR com.iluwatar.throttling.Bartender - I'm sorry young human, you've had enough for today!
18:46:36.443 [pool-1-thread-2] DEBUG com.iluwatar.throttling.Bartender - Serving beer to dwarf soldier : [3 consumed]
18:46:36.544 [pool-1-thread-1] ERROR com.iluwatar.throttling.Bartender - I'm sorry young human, you've had enough for today!
18:46:36.544 [pool-1-thread-2] DEBUG com.iluwatar.throttling.Bartender - Serving beer to dwarf soldier : [4 consumed]
18:46:36.645 [pool-1-thread-2] ERROR com.iluwatar.throttling.Bartender - I'm sorry dwarf soldier, you've had enough for today!
18:46:36.645 [pool-1-thread-1] ERROR com.iluwatar.throttling.Bartender - I'm sorry young human, you've had enough for today!
18:46:36.745 [pool-1-thread-1] ERROR com.iluwatar.throttling.Bartender - I'm sorry young human, you've had enough for today!
18:46:36.745 [pool-1-thread-2] ERROR com.iluwatar.throttling.Bartender - I'm sorry dwarf soldier, you've had enough for today!
18:46:36.846 [pool-1-thread-1] ERROR com.iluwatar.throttling.Bartender - I'm sorry young human, you've had enough for today!
18:46:36.846 [pool-1-thread-2] ERROR com.iluwatar.throttling.Bartender - I'm sorry dwarf soldier, you've had enough for today!
18:46:36.947 [pool-1-thread-2] ERROR com.iluwatar.throttling.Bartender - I'm sorry dwarf soldier, you've had enough for today!
18:46:36.947 [pool-1-thread-1] ERROR com.iluwatar.throttling.Bartender - I'm sorry young human, you've had enough for today!
18:46:37.048 [pool-1-thread-2] ERROR com.iluwatar.throttling.Bartender - I'm sorry dwarf soldier, you've had enough for today!
18:46:37.048 [pool-1-thread-1] ERROR com.iluwatar.throttling.Bartender - I'm sorry young human, you've had enough for today!
18:46:37.148 [pool-1-thread-1] ERROR com.iluwatar.throttling.Bartender - I'm sorry young human, you've had enough for today!
18:46:37.148 [pool-1-thread-2] ERROR com.iluwatar.throttling.Bartender - I'm sorry dwarf soldier, you've had enough for today!类图
适用性
节流模式应采用:
- 当需要限制业务访问时，不会对业务性能产生较大影响。
- 当多个客户端正在使用相同的服务资源时，必须根据每个客户端的使用量进行限制。