Asynchronous Communication in Java

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Asynchronous Communication
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Although a key advantage of synchronous communication is that a requestor receives a reply as part of the process, this feature can actually be viewed as a major disadvantage Not only can there be a long latency involved in a reply (which may lead to confusion or time-out), but any time spent waiting for a reply is less efficient than not waiting for it Also, in many cases a reply is either not needed or not needed immediately The mode of communication in which a request receives a reply at some later time (or receives no reply at all) is referred to as asynchronous It helps to visualize the difference between synchronous and asynchronous communication to understand why waiting for a reply can be considered a disadvantage Suppose that company A and company B have established a partnership where B resells some of A's products They develop a B2B-style notification system where A uses software to automatically refigure and determine prices on its products (it can do this monthly, for example) and notifies B about such changes Figure 6-3 shows how A and B might accomplish their goals by communicating synchronously It assumes that each computation takes one second and disregards any network latency Figure 6-3 Synchronous price update processing
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We see that company A needs 1 second to calculate the price of a hammer and then stalls for 1 second while communicating the price to B This is because B requires 1 second to update that price in its own database After receiving a reply from B that it has updated the hammer price, A determines the new price on a wrench, which again requires 1 second Notice that there is an arrow at the top of the figure this lets A know that its request was fulfilled The total time to process these price updates is at least 4 seconds because the communication is synchronous and so A needs to receive a reply from B Figure 6-4 shows how the same process might occur asynchronously Since company A doesn't require any reply from B (perhaps later it will want to know that B received the price changes, but this isn't immediately necessary), it can calculate the price of a hammer, communicate it to some queue that B can access without waiting for a reply Instead, it can go on to calculate the price for the wrench On its own time, B will eventually visit the queue and process the information A has left for it there Instead of wasting a cycle waiting for B to do its work, A moves right along to the next task, which makes it 50 percent more productive in this example! It completes its work in 2 seconds instead of 4 Notice that B isn't slowed down because A was waiting on its first update It's only active for 2 seconds as well, not the original 3
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Figure 6-4 Asynchronous price update processing
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Performance improvement from asynchronous communication varies, of course, because it depends on the network latency, the time B requires to do its work, and the amount of work A has to complete It also has a lot to do with the application itself and its business requirements For example, why can't A simply do all of its work first and then send all of the updated product prices to B at once Also, is it really necessary for B to commit the change to its database before acknowledging A Can't it simply hold the data in some temporary memory, respond to A, and then commit the data at some later time There are many ways to construct asynchronous communication solutions, but all of them use some sort of queuing approach This is fundamental to asynchrony: If the consumer is unavailable, the producer needs some way to log his request so that he can continue with another activity The queue represents this logical log file For example, Figure 6-4 shows organization A using software to communicate requests at some future time to a queue that's accessible by software run by organization B Without a queue, there's no place to temporarily store requests while they await processing Thus, behavior has to be synchronous or the data will be lost Messaging Systems Probably the most popular form of asynchronous communication today is the one associated with messaging systems Not to be confused with e-mail, messaging allows applications from different organizations or companies to integrate easily and efficiently Most of these systems are based on an asynchronous communication model In addition, messaging systems provide a consistent interface that doesn't force either side to learn new proprietary APIs or change existing ones There's no need for two companies to worry about what language their application(s) are written in and on what operating systems they're deployed Just like HTTP and the JVM, messaging systems implicitly define a region of platform
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independence so that, instead of worrying about the ugly details of software integration between disparate machines and programming languages, companies can focus on the more interesting and pressing business problems related to the data they exchange Messaging systems are so named because they rephrase client/server communication between systems in the form of text messages As in the HTTP protocol, normalizing communication between the two parties in this way is necessary to achieve platform independence And like HTTP, there's a nice side effect to messaging: It's far easier to debug than a binary, possibly proprietary, protocol This makes it a more attractive approach to application integration than more cryptic methods such as Electronic Data Interchange (EDI) What's more, message-based integration schemes are perfectly positioned to work on top of HTTP, since this protocol is itself message-based This allows them to
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Leverage the network optimizations of HTTP (such as persistent connections) Leverage security features (such as SSL) Avoid many of the firewall issues that can hamper non-HTTP approaches
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One distinction between normal synchronous communication and asynchronous messaging is that the former is associated with the client/server model, whereas the latter is typically associated with the peer-to-peer model In client/server systems, one party requests information and one party provides it In peer-to-peer systems, both parties produce and consume information Still as I said earlier, the difference between these two approaches is smaller than it sounds Even under a messaging scheme, there's going to be a provider and a consumer The difference is that the consumer doesn't have to request the information in order to get it; rather, the information is subscribed to so that it can be received if and when it's published by the provider
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