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Server Side Event Handling

Introduction

In this lab, you will learn to configure events on the server side of a GemFire distributed system.

Concepts you will gain experience with:

  • Creating and registering a CacheLoader

  • Creating and registering a CacheWriter

Estimated completion time: 30 minutes

Preparation

  1. start a locator and one server by using scripts provided in the server-events module, as follows:

    cd server-events/cluster
gfsh run --file=start.gf

  2. In your IDE, navigate to the server-events module. Open the test class ServerEventsTests (in src/test/java under the io.pivotal.bookshop.tests package) and review the tests.

  3. Finally, run the tests (right mouse click on ServerEventsTest → Run As → JUnit Test). You will see that 2 of the 3 tests currently fail.

Creating and Implementing a CacheLoader

The objective of the cache loader is to load data on cache misses on the GemFire server. The load() method is called when the region.get() operation can’t find the value in the cache. The value returned from the cache loader is put into the cache, and returned to the get() operation.

  1. The definition of the BookCacheLoader class is provided in the io.pivotal.bookshop.buslogic package. Open this class file and add the code in the load() method to return a new Book instance. It can either be explicitly generated in this method or by a helper class that you create. Ordinarily this would be loaded from an external data source such as a JDBC data source.

  2. Open the cluster/cluster.xml file and add the necessary configuration to register the BookCacheLoader with the Book region.

  3. Stop and re-start the server. It should now be running with the newly registered BookCacheLoader.

    Note

    To stop the servers, you can use a command similar to the above start command

    gfsh run --file=stop.gf

  4. Re-run the test shouldLoadEntryOnCacheMiss(); it should now pass.

Creating and Implementing a CacheWriter

In this section, you will implement a CacheWriter that performs validation of new entries. If you recall, any new entries that are created will fire the beforeCreate() event method. One of the benefits of having this method called before the actual insert is that we can perform validation to ensure entries meet our desired expectation.

  1. To begin, refer to your ServerEventsTests unit test. The last two tests are designed to assert the correct behavior of your ValidatingCacheWriter implementation.

    Note
    		 In reality, the test shouldAllowEntryWrite() would pass even if there was no CacheWriter registered as you’ve already seen. Its main purpose is to ensure that a correct insert does not generate an error.

  2. Now, open the ValidatingCacheWriter class in the io.pivotal.bookshop.buslogic package. Notice that the beforeCreate() method is left open for you to implement the functionality. Also note that there is a method called validateNewValue() where the logic is performed to determine a valid entry. Take a moment to examine this code. You’ve had some experience already with querying from the client. This is an example of a query being performed on the server. As you can see, a prospective book is considered valid if no other entry exists having the same itemNumber.

  3. Now, return to the beforeCreate() method and implement the appropriate functionality to obtain the correct value, call the validate method and throw a CacheWriterException if the book is considered invalid.

  4. Return to the cluster.xml file and add the necessary configuration to register the ValidatingCacheWriter with the Book region.

  5. Save your work, stop and re-start the server.

  6. Verify you’ve correctly implemented and registered the ValidatingCacheWriter by re-running the ServerEventsTests JUnit test. All tests should now pass.

Congratulations!! You have completed this lab.