In Example 3, we create a simple client module for our dictionary service. The problem with that client was that it did not monitor the dynamic availability of the dictionary service, thus an error would occur if the dictionary service disappeared while the client was using it. In this example we create a client for the dictionary service that monitors the dynamic availability of the dictionary service. The result is a more robust client.
The functionality of the new dictionary client is essentially the same as the old client, it reads words from standard input and checks for their existence in the dictionary service. Our module uses its module context to register itself as a service event listener; monitoring service events allows the module to monitor the dynamic availability of the dictionary service. Our client uses the first dictionary service it finds. The source code for our module is as follows in a file called
The client listens for service events indicating the arrival or departure of dictionary services. If a new dictionary service arrives, the module will start using that service if and only if it currently does not have a dictionary service. If an existing dictionary service disappears, the module will check to see if the disappearing service is the one it is using; if it is it stops using it and tries to query for another dictionary service, otherwise it ignores the event.
As in Example 3, we must link our module to the
After running the
usCoreExamplesDriver executable, and loading the event listener module, we can use the
l dictionaryclient2 command to load our robust dictionary client module:
CppMicroServices-debug> bin/usCoreExamplesDriver > l eventlistener Starting to listen for service events. > l dictionaryclient2 Ex1: Service of type IDictionaryService/1.0 registered. Enter a blank line to exit. Enter word:
The above command loads the module and its dependencies (the
dictionaryservice module) in a single step. When we load the module, it will use the main thread to prompt us for words. Enter one word at a time to check the words and enter a blank line to stop checking words. To reload the module, we must use the
s command to get the module identifier number for the module and first use the
u <id> command to unload the module, then the
l <id> command to re-load it. To test the dictionary service, enter any of the words in the dictionary (e.g., "welcome", "to", "the", "micro", "services", "tutorial") or any word not in the dictionary.
Since this client monitors the dynamic availability of the dictionary service, it is robust in the face of sudden departures of the the dictionary service. Further, when a dictionary service arrives, it automatically gets the service if it needs it and continues to function. These capabilities are a little difficult to demonstrate since we are using a simple single-threaded approach, but in a multi-threaded or GUI-oriented application this robustness is very useful.
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