ID Conversion in Java

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ID Conversion
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Now that we have looked at a couple of different ID converters, we can create our own custom ID converter Why would we want to do that Perhaps we want to have some sort of signature in the ID so that we can tell by looking at it where it originated Perhaps we are using a database as our object store on the server, and we want to use the primary key for the database row as the ID Or perhaps we just want to be different Whatever the motivation, it is easy to create a custom ID converter In our custom ID converter, we just want to append HW: to the object_id for our objects so that we know that those objects came from the HelloWorldServer Here is what our ID converter would look like:
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class CustomIdConv def to_id(obj) "HW:#{objobject_id}" end def to_obj(ref) ObjectSpace_id2ref(refgsub(/^HW:/, '')to_i) end end
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For an ID converter to work in DRb, it needs to respond to two methods
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to_obj and to_id Looking at to_id first, we can see that it takes an object Our to_id method returns a new String that simply concatenates HW: to the object_id, which returns something like HW:12345 Our to_obj is a little more
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complex, but it is still relatively straightforward The method takes a reference ID, HW:12345, and strips HW:, leaving us with 12345 We have to convert the 12345 String to an Integer so that we can pass it to the _id2ref method on ObjectSpace The _id2ref method on ObjectSpace retrieves the referenced object from the proper memory location in the Ruby VM and returns it As you saw earlier, when we talked about DRb::TimerIdConv, installing a new ID converter is simple We can install our custom ID converter like so:
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DRbinstall_id_conv(CustomIdConvnew)
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1 Distributed Ruby (DRb)
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Now, if we were to run our client, we would see something like the following print to the screen:
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#<DRb::DRbObject:0x822bc @uri="druby://127001:61676", @ref="HW:264770">
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Using Multiple ID Converters
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Earlier I mentioned that when you install a converter using the DRbinstall_ id_conv method, you tell DRb that all services in that Ruby VM are now to use that particular ID converter What happens if you want to use multiple ID converters The solution is quite simple If you remember when we talked about setting up a service to work over SSL, you learned that the start_service method takes a third parameter, a Hash of configuration parameters One parameter in the list of available parameters is :idconv, which tells DRb to use a particular ID converter for that service and that service alone:
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DRbstart_service("druby://127001:61676", HelloWorldServernew, {:idconv => CustomIdConvnew}) DRbstart_service("druby://127001:61677", HelloWorldServernew)
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Here we have set up two instances of our HelloWorldServer The first instance, on port 61676, will use our CustomIdConv class The second instance, on port 61677, will use the default ID converter, DRb::DRbIdConv Let s write a client so that we can see the different ID converters in practice:
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require 'drb' s1 = DRbObjectnew_with_uri("druby://127001:61676") s2 = DRbObjectnew_with_uri("druby://127001:61677") puts s1say_helloinspect puts s2say_helloinspect
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When we run our client, we should see something like the following printed:
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#<DRb::DRbObject:0x820dc @uri="druby://127001:61676", @ref="HW:264380"> #<DRb::DRbObject:0x818d0 @uri="druby://127001:61677", @ref=262660>
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As you can see, our first Hello object has a reference of "HW:264380", the format used by our CustomIdConv class The second Hello object has a simpler reference of 262660 Because the reference on the second Hello object is not prepended with HW:, we know that it is not using our CustomIdConv class, but rather the default DRb::DRbIdConv ID converter
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In Summary
In this section we talked about a couple of built-in ID converters the default ID converter, DRb::DRbIdConv, and the timer-based converter, DRb::TimerIdConv We discussed the possibility of referenced objects no longer being available because they have been garbage-collected, and a couple of potential solutions You learned how to install a new ID converter for all the services in a particular Ruby VM, as well as how to use a custom converter for a particular service Finally, you created your own custom ID converter