参考文档:http://blog.csdn.net/lmj623565791/article/details/37620057
工作队列的主要任务是:避免立刻执行资源密集型任务,然后必须等待其完成。相反地,我们进行任务调度:我们把任务封装为消息发送给队列。工作进行在后台运行并不断的从队列中取出任务然后执行。当你运行了多个工作进程时,任务队列中的任务将会被工作进程共享执行。
这样的概念在web应用中极其有用,当在很短的HTTP请求间需要执行复杂的任务。
1、 准备
我们使用Thread.sleep来模拟耗时的任务。我们在发送到队列的消息的末尾添加一定数量的点,每个点代表在工作线程中需要耗时1秒,例如hello…将会需要等待3秒。
发送端:
NewTask.Java
package
com.zhy.rabbit._02_workqueue;
import
java.io.IOException;
import
com.rabbitmq.client.Channel;
import
com.rabbitmq.client.Connection;
import
com.rabbitmq.client.ConnectionFactory;
public
class
NewTask
{
private
final
static
String QUEUE_NAME =
"workqueue"
;
public
static
void
main(String[] args)
throws
IOException
{
ConnectionFactory factory =
new
ConnectionFactory();
factory.setHost(
"localhost"
);
Connection connection = factory.newConnection();
Channel channel = connection.createChannel();
channel.queueDeclare(QUEUE_NAME,
false
,
false
,
false
,
null
);
for
(
int
i =
0
; i <
10
; i++)
{
String dots =
""
;
for
(
int
j =
0
; j <= i; j++)
{
dots +=
"."
;
}
String message =
"helloworld"
+ dots+dots.length();
channel.basicPublish(
""
, QUEUE_NAME,
null
, message.getBytes());
System.out.println(
" [x] Sent '"
+ message +
"'"
);
}
channel.close();
connection.close();
}
}
|
接收端:
Work.java
package
com.zhy.rabbit._02_workqueue;
import
com.rabbitmq.client.Channel;
import
com.rabbitmq.client.Connection;
import
com.rabbitmq.client.ConnectionFactory;
import
com.rabbitmq.client.QueueingConsumer;
public
class
Work
{
private
final
static
String QUEUE_NAME =
"workqueue"
;
public
static
void
main(String[] argv)
throws
java.io.IOException,
java.lang.InterruptedException
{
int
hashCode = Work.
class
.hashCode();
ConnectionFactory factory =
new
ConnectionFactory();
factory.setHost(
"localhost"
);
Connection connection = factory.newConnection();
Channel channel = connection.createChannel();
channel.queueDeclare(QUEUE_NAME,
false
,
false
,
false
,
null
);
System.out.println(hashCode
+
" [*] Waiting for messages. To exit press CTRL+C"
);
QueueingConsumer consumer =
new
QueueingConsumer(channel);
channel.basicConsume(QUEUE_NAME,
true
, consumer);
while
(
true
)
{
QueueingConsumer.Delivery delivery = consumer.nextDelivery();
String message =
new
String(delivery.getBody());
System.out.println(hashCode +
" [x] Received '"
+ message +
"'"
);
doWork(message);
System.out.println(hashCode +
" [x] Done"
);
}
}
/**
* 每个点耗时1s
* @param task
* @throws InterruptedException
*/
private
static
void
doWork(String task)
throws
InterruptedException
{
for
(
char
ch : task.toCharArray())
{
if
(ch ==
'.'
)
Thread.sleep(
1000
);
}
}
}
|
Round-robin 转发
使用任务队列的好处是能够很容易的并行工作。如果我们积压了很多工作,我们仅仅通过增加更多的工作者就可以解决问题,使系统的伸缩性更加容易。
下面我们先运行3个工作者(Work.java)实例,然后运行NewTask.java,3个工作者实例都会得到信息。但是如何分配呢?让我们来看输出结果:[x] Sent 'helloworld.1'
[x] Sent 'helloworld..2'
[x] Sent 'helloworld...3'
[x] Sent 'helloworld....4'
[x] Sent 'helloworld.....5'
[x] Sent 'helloworld......6'
[x] Sent 'helloworld.......7'
[x] Sent 'helloworld........8'
[x] Sent 'helloworld.........9'
[x] Sent 'helloworld..........10'
工作者1:
605645 [*] Waiting for messages. To exit press CTRL+C
605645 [x] Received 'helloworld.1'
605645 [x] Done
605645 [x] Received 'helloworld....4'
605645 [x] Done
605645 [x] Received 'helloworld.......7'
605645 [x] Done
605645 [x] Received 'helloworld..........10'
605645 [x] Done
工作者2:
18019860 [*] Waiting for messages. To exit press CTRL+C
18019860 [x] Received 'helloworld..2'
18019860 [x] Done
18019860 [x] Received 'helloworld.....5'
18019860 [x] Done
18019860 [x] Received 'helloworld........8'
18019860 [x] Done
工作者3:
18019860 [*] Waiting for messages. To exit press CTRL+C
18019860 [x] Received 'helloworld...3'
18019860 [x] Done
18019860 [x] Received 'helloworld......6'
18019860 [x] Done
18019860 [x] Received 'helloworld.........9'
18019860 [x] Done
可以看到,默认的,RabbitMQ会一个一个的发送信息给下一个消费者(consumer),而不考虑每个任务的时长等等,且是一次性分配,并非一个一个分配。平均的每个消费者将会获得相等数量的消息。这样分发消息的方式叫做round-robin。
2、 消息应答(message acknowledgments)
执行一个任务需要花费几秒钟。你可能会担心当一个工作者在执行任务时发生中断。我们上面的代码,一旦RabbItMQ交付了一个信息给消费者,会马上从内存中移除这个信息。在这种情况下,如果杀死正在执行任务的某个工作者,我们会丢失它正在处理的信息。我们也会丢失已经转发给这个工作者且它还未执行的消息。
上面的例子,我们首先开启两个任务,然后执行发送任务的代码(NewTask.java),然后立即关闭第二个任务,结果为:
工作者2:
31054905 [*] Waiting for messages. To exit press CTRL+C
31054905 [x] Received 'helloworld..2'
31054905 [x] Done
31054905 [x] Received 'helloworld....4'
工作者1:
18019860 [*] Waiting for messages. To exit press CTRL+C
18019860 [x] Received 'helloworld.1'
18019860 [x] Done
18019860 [x] Received 'helloworld...3'
18019860 [x] Done
18019860 [x] Received 'helloworld.....5'
18019860 [x] Done
18019860 [x] Received 'helloworld.......7'
18019860 [x] Done
18019860 [x] Received 'helloworld.........9'
18019860 [x] Done
可以看到,第二个工作者至少丢失了6,8,10号任务,且4号任务未完成。
但是,我们不希望丢失任何任务(信息)。当某个工作者(接收者)被杀死时,我们希望将任务传递给另一个工作者。
为了保证消息永远不会丢失,RabbitMQ支持消息应答(message acknowledgments)。消费者发送应答给RabbitMQ,告诉它信息已经被接收和处理,然后RabbitMQ可以自由的进行信息删除。
如果消费者被杀死而没有发送应答,RabbitMQ会认为该信息没有被完全的处理,然后将会重新转发给别的消费者。通过这种方式,你可以确认信息不会被丢失,即使消者偶尔被杀死。
这种机制并没有超时时间这么一说,RabbitMQ只有在消费者连接断开是重新转发此信息。如果消费者处理一个信息需要耗费特别特别长的时间是允许的。
消息应答默认是打开的。上面的代码中我们通过显示的设置autoAsk=true关闭了这种机制。下面我们修改代码(Work.java):
boolean
ack =
false
;
channel.basicConsume(QUEUE_NAME, ack, consumer);
channel.basicAck(delivery.getEnvelope().getDeliveryTag(),
false
);
|
完整修改后的Work.java
package
com.zhy.rabbit._02_workqueue.ack;
import
com.rabbitmq.client.Channel;
import
com.rabbitmq.client.Connection;
import
com.rabbitmq.client.ConnectionFactory;
import
com.rabbitmq.client.QueueingConsumer;
public
class
Work
{
private
final
static
String QUEUE_NAME =
"workqueue"
;
public
static
void
main(String[] argv)
throws
java.io.IOException,
java.lang.InterruptedException
{
int
hashCode = Work.
class
.hashCode();
ConnectionFactory factory =
new
ConnectionFactory();
factory.setHost(
"localhost"
);
Connection connection = factory.newConnection();
Channel channel = connection.createChannel();
channel.queueDeclare(QUEUE_NAME,
false
,
false
,
false
,
null
);
System.out.println(hashCode
+
" [*] Waiting for messages. To exit press CTRL+C"
);
QueueingConsumer consumer =
new
QueueingConsumer(channel);
boolean
ack =
false
;
channel.basicConsume(QUEUE_NAME, ack, consumer);
while
(
true
)
{
QueueingConsumer.Delivery delivery = consumer.nextDelivery();
String message =
new
String(delivery.getBody());
System.out.println(hashCode +
" [x] Received '"
+ message +
"'"
);
doWork(message);
System.out.println(hashCode +
" [x] Done"
);
channel.basicAck(delivery.getEnvelope().getDeliveryTag(),
false
);
}
}
}
|
测试:
我们把消息数量改为5,然后先打开两个消费者(Work.java),然后发送任务(NewTask.java),立即关闭一个消费者,观察输出:
[x] Sent 'helloworld.1'
[x] Sent 'helloworld..2'
[x] Sent 'helloworld...3'
[x] Sent 'helloworld....4'
[x] Sent 'helloworld.....5'
工作者2
18019860 [*] Waiting for messages. To exit press CTRL+C
18019860 [x] Received 'helloworld..2'
18019860 [x] Done
18019860 [x] Received 'helloworld....4'
工作者1
31054905 [*] Waiting for messages. To exit press CTRL+C
31054905 [x] Received 'helloworld.1'
31054905 [x] Done
31054905 [x] Received 'helloworld...3'
31054905 [x] Done
31054905 [x] Received 'helloworld.....5'
31054905 [x] Done
31054905 [x] Received 'helloworld....4'
31054905 [x] Done
可以看到工作者2没有完成的任务4,重新转发给工作者1进行完成了。
3、 消息持久化(Message durability)
我们已经学习了即使消费者被杀死,消息也不会被丢失。但是如果此时RabbitMQ服务被停止,我们的消息仍然会丢失。
当RabbitMQ退出或者异常退出,将会丢失所有的队列和信息,除非你告诉它不要丢失。我们需要做两件事来确保信息不会被丢失:我们需要给所有的队列和消息设置持久化的标志。
第一, 我们需要确认RabbitMQ永远不会丢失我们的队列。为了这样,我们需要声明它为持久化的。
boolean durable = true;
channel.queueDeclare("task_queue", durable, false, false, null);
注:RabbitMQ不允许使用不同的参数重新定义一个队列,所以已经存在的队列,我们无法修改其属性。
第二, 我们需要标识我们的信息为持久化的。通过设置MessageProperties(implements BasicProperties)值为PERSISTENT_TEXT_PLAIN。
channel.basicPublish("", "task_queue",MessageProperties.PERSISTENT_TEXT_PLAIN,message.getBytes());
现在你可以执行一个发送消息的程序,然后关闭服务,再重新启动服务,运行消费者程序做下实验。
4、公平转发(Fair dispatch)
或许会发现,目前的消息转发机制(Round-robin)并非是我们想要的。例如,这样一种情况,对于两个消费者,有一系列的任务,奇数任务特别耗时,而偶数任务却很轻松,这样造成一个消费者一直繁忙,另一个消费者却很快执行完任务后等待。
造成这样的原因是因为RabbitMQ仅仅是当消息到达队列进行转发消息。并不在乎有多少任务消费者并未传递一个应答给RabbitMQ。仅仅盲目转发所有的奇数给一个消费者,偶数给另一个消费者。
为了解决这样的问题,我们可以使用basicQos方法,传递参数为prefetchCount = 1。这样告诉RabbitMQ不要在同一时间给一个消费者超过一条消息。换句话说,只有在消费者空闲的时候会发送下一条信息。
- int prefetchCount = 1;
- channel.basicQos(prefetchCount);
注:如果所有的工作者都处于繁忙状态,你的队列有可能被填充满。你可能会观察队列的使用情况,然后增加工作者,或者使用别的什么策略。
测试:改变发送消息的代码,将消息末尾点数改为6-2个,然后首先开启两个工作者,接着发送消息:
[x] Sent 'helloworld......6'
[x] Sent 'helloworld.....5'
[x] Sent 'helloworld....4'
[x] Sent 'helloworld...3'
[x] Sent 'helloworld..2'
工作者1:
18019860 [*] Waiting for messages. To exit press CTRL+C
18019860 [x] Received 'helloworld......6'
18019860 [x] Done
18019860 [x] Received 'helloworld...3'
18019860 [x] Done
工作者2:
31054905 [*] Waiting for messages. To exit press CTRL+C
31054905 [x] Received 'helloworld.....5'
31054905 [x] Done
31054905 [x] Received 'helloworld....4'
31054905 [x] Done
31054905 [x] Received 'helloworld..2'
31054905 [x] Done
可以看出此时并没有按照之前的Round-robin机制进行转发消息,而是当消费者不忙时进行转发。且这种模式下支持动态增加消费者,因为消息并没有发送出去,动态增加了消费者马上投入工作。而默认的转发机制会造成,即使动态增加了消费者,此时的消息已经分配完毕,无法立即加入工作,即使有很多未完成的任务。
5、完整的代码
NewTask.java
package
com.yhiker.learn.demo.rabbitmq.two;
import
com.rabbitmq.client.Channel;
import
com.rabbitmq.client.Connection;
import
com.rabbitmq.client.ConnectionFactory;
import
com.rabbitmq.client.MessageProperties;
public
class
NewTask {
private
static
final
String TASK_QUEUE_NAME =
"task_queue"
;
public
static
void
main(String[] argv)
throws
Exception {
ConnectionFactory factory =
new
ConnectionFactory();
factory.setHost(
"192.168.1.8"
);
factory.setUsername(
"root"
);
factory.setPassword(
"123456"
);
Connection connection = factory.newConnection();
Channel channel = connection.createChannel();
channel.queueDeclare(TASK_QUEUE_NAME,
true
,
false
,
false
,
null
);
for
(
int
i =
0
; i <
10
; i++) {
String dots =
""
;
for
(
int
j =
0
; j <= i; j++) {
dots +=
"."
;
}
String message =
"helloworld"
+ dots + dots.length();
channel.basicPublish(
""
, TASK_QUEUE_NAME, MessageProperties.PERSISTENT_TEXT_PLAIN,
message.getBytes(
"UTF-8"
));
System.out.println(
" [x] Sent '"
+ message +
"'"
);
}
channel.close();
connection.close();
}
}
|
Work.java
package
com.yhiker.learn.demo.rabbitmq.two;
import
com.rabbitmq.client.*;
import
java.io.IOException;
public
class
Worker {
private
static
final
String TASK_QUEUE_NAME =
"task_queue"
;
public
static
void
main(String[] argv)
throws
Exception {
ConnectionFactory factory =
new
ConnectionFactory();
factory.setHost(
"192.168.1.8"
);
factory.setUsername(
"root"
);
factory.setPassword(
"123456"
);
final
Connection connection = factory.newConnection();
final
Channel channel = connection.createChannel();
channel.queueDeclare(TASK_QUEUE_NAME,
true
,
false
,
false
,
null
);
System.out.println(
" [*] Waiting for messages. To exit press CTRL+C"
);
final
Consumer consumer =
new
DefaultConsumer(channel) {
@Override
public
void
handleDelivery(String consumerTag, Envelope envelope, AMQP.BasicProperties properties,
byte
[] body)
throws
IOException {
String message =
new
String(body,
"UTF-8"
);
System.out.println(
" [x] Received '"
+ message +
"'"
);
try
{
doWork(message);
}
finally
{
System.out.println(
" [x] Done"
);
channel.basicAck(envelope.getDeliveryTag(),
false
);
}
}
};
channel.basicConsume(TASK_QUEUE_NAME,
false
, consumer);
}
private
static
void
doWork(String task) {
for
(
char
ch : task.toCharArray()) {
if
(ch ==
'.'
) {
try
{
Thread.sleep(
1000
);
}
catch
(InterruptedException _ignored) {
Thread.currentThread().interrupt();
}
}
}
}
}
|