Callbacks and Observers in Java

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observers << ring_servernotify('take', [:callback_service, nil], 60) observers << ring_servernotify('delete', [:callback_service, nil], 60) threads = [] observerseach do |observer| threads << Threadnew do observereach do |event| puts eventinspect end end end threadseach {|t| tjoin}
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This code might seem a bit daunting at first, but actually it s fairly straightforward and simple Let s look at just a piece of it first:
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ring_servernotify('write', [:callback_service, nil], 60)
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All we are doing with this line of code is telling the RingServer to give us a NotifyTemplateEntry object for the write action matching the Tuple template [:callback_service, nil], and we want to stop listening after 60 seconds If we want to listen for callbacks indefinitely, we can either pass in nil for the last parameter or not pass in the parameter I have chosen 60 seconds to demonstrate something a little later The NotifyTemplateEntry object we receive from the RingServer can then be used to process our callback events In our code we are creating three different callbacks The first two we have already discussed and should be familiar to you by now The third, however, is a fairly new topic We will discuss it a bit more in the section Renewing Rinda Services, but what we are doing with the delete callback is asking for a notification whenever a Tuple has been explicitly deleted by another service or when the RingServer deletes the Tuple because it has expired All that is left is to create a separate Thread for each of our observers and then just call the each method on the observer and print the event we receive
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When our client runs, it takes out the Tuple, increments the number, and then writes the Tuple back to the RingServer:
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["take", [:callback_service, 0]] ["write", [:callback_service, 1]]
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As you can see, the event we receive from our callback is an Array containing two parameters The first parameter is the type of notification we are receiving For example, in our first event, we receive a take callback The second parameter contains the actual Tuple that triggered the callback This is useful if we want to do something such as logging the event, as we just did Thirty seconds after we receive our first callbacks, the RingServer automatically expires the Tuple so that the delete callback is triggered:
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["delete", [:callback_service, 1]]
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Another 30 seconds after that, our callbacks expire, and we receive a close notification for each of them:
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["close"] ["close"] ["close"]
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We have not previously mentioned the close callback, but its use should be fairly obvious Because we specified that we want our callback hooks active for only 60 seconds when they expire, we get a callback telling us that This event could then be used to do everything from shutting down the program to writing to a log to simply reregistering itself
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In Summary
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This section was a brief introduction to the ideas and concepts behind callbacks and observers We used those ideas and implement callbacks for a simple service, and you saw how each of them responded over time and what their response properties looked like If you would like to know more about the observer design pattern, and design patterns in general, I recommend that you read Russ Olsen s great book on design patterns, Design Patterns in Ruby (Addison-Wesley Professional, 2007) It
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is a wonderful book that is easy to read and follow along with; it should be required reading for every Ruby developer The author does a wonderful job explaining the pattern in great detail, a subject that is much deeper than I have time to explain here
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In 1, when we discussed DRb, we talked about security Because Rinda is built on top of the DRb libraries, we are already pretty much armed with all there is to know about security when using Rinda Let s do a quick review of some of those points, and I ll point out where Rinda may differ slightly First, let s review this code sample again:
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require 'drb' # !!! UNSAFE CODE DO NOT RUN !!! ro = DRbObject::new_with_uri("druby://yourservercom:8989") class << ro undef :instance_eval end roinstance_eval("`rm -rf *`")
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If you remember, this block of code has the unfortunate side effect of deleting the entire file system on the server it gets executed on This is still true If you do not set your $SAFE level when using Rinda, your server will be unsecure! I cannot stress that point enough Make sure you are at least running at $SAFE level 1 Please review the Security section of 1 if you still do not understand this point
Access Control Lists (ACLs)
Our knowledge of ACLs serves us well in Rinda, because there is no change in how we do things in that area In Rinda ACLs are even more useful than in straight DRb applications In DRb you have to know exactly where the application you are looking for is Using the premise of security through obscurity, you can say that only applications that you want to access the service will know where to find it Of course, this is a bad idea for many reasons, but this book is not here to talk about network, or application, security at that level All I will say on that subject is, please don t use this