Topic Queues in Java

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Topic Queues
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Up to this point we have been writing all messages to one big queue and hoping that our consumer is prepared to handle each of these messages But what if we want to handle different types of messages within a single queue differently
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Let me explain what I mean with a simple example Consider the idea of logging We want to log all our messages to a single logging queue That makes sense, because they are all logging messages Normally we want to write all the messages to a log file However, when we get an error- or fatal-level message, we also want to send an email to someone to alert him or her of the problem So how would we do that with a single queue We could write an if statement that parses the log message and determines its level and then decides accordingly, or we could use the concept of a topic Topic queues allow us to effectively write all our messages to a single queue But by adding a routing key to the message, we can then write what is basically a filtered queue on our consumer side When we talked earlier about subscribing to a queue, we built a simple system that published stock prices for a few different stocks and then a consumer that read those prices and printed them to the screen What if we wanted to monitor a particular stock and get an alert if the price should change by more than 5% We could write an if statement, or we could use the power of topic queues Guess which one we will use in this section If you guessed using the if statement approach, then that was a swing and a miss Let s start by making a simple adjustment to the stock publisher code we looked at earlier:
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require 'rubygems' require 'mq' AMQPstart do def price_change pc = rand(1000) / 1000 pc *= -1 if (rand(2) == 1) pc end topic = MQtopic('dow_jones') stocks = {:appl => 1000, :msft => 500} EMadd_periodic_timer(1) do stockseach do |stock, price| price += price_change
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stocks[stock] = price topicpublish(price, :key => "dow_jones#{stock}") end end end
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There is nothing new in this code, except for the line where we call the publish method Notice that we are now passing in a flag called :key with the values dow_jonesappl and dow_jonesmsft Those values allow us to build our custom queue in the consumer code AMQP uses dot notation to associate topics with each other By giving our messages a :key value of something like dow_jonesappl, we are saying that this message and topic, appl, are a subset of the dow_jones topic and should be treated as such Let s look at our consumer code, because that might help further explain what we are talking about:
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require 'rubygems' require 'mq' AMQPstart do topic = MQtopic('dow_jones') previous_price = nil appl_queue = MQqueue('apple') appl_queuebind(topic, :key => 'dow_jonesappl') appl_queuesubscribe do |header, msg| msg = msgto_f previous_price = msg if previous_pricenil diff = msg - previous_price diff *= -1 if diff < 00 per = (diff / previous_price) * 100 if per > 50 puts "! Apple was: #{previous_price} now: #{msg} !" end previous_price = msg end
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all_queue = MQqueue('all stocks') all_queuebind(topic, :key => 'dow_jones*') all_queuesubscribe do |header, msg| puts "#{headerrouting_key}:\t#{Timenow}: #{msg}" end end
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At first glance this code might make our eyes go a bit crossed, but after we start looking at it, it is actually straightforward To kick things off, first we need to get the topic we want to deal with In our case we are interested in the dow_jones topic, so we retrieve it with the following line:
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topic = MQtopic('dow_jones')
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When we have the topic, we create a new queue for apple so that we can monitor its stock and be notified with an alert if its price fluctuates by more than 5% That is done with the following line that you should be familiar with by now:
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appl_queue = MQqueue('apple')
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The next line is where the secret sauce of topics comes in:
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appl_queuebind(topic, :key => 'dow_jonesappl')
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With this line we are binding the dow_jones topic to the apple queue, but we are telling it to feed only the apple queue messages that have a key of dow_jonesappl In essence, messages are now copied to the apple queue for it do with as it pleases The original message stays in the dow_jones queue, waiting for someone else to come and claim it The rest of the code in the apple queue subscribe block just determines whether it is changed by more than 5%, and then it prints a message Moving on, we want to print all our messages Again we create a new queue:
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all_queue = MQqueue('all stocks')
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This time we are calling the queue all stocks We want to bind that queue to the dow jones topic:
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all_queuebind(topic, :key => 'dow_jones*')