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This makes it impossible to write pure unit tests for the function code that mock the cache. 

See

Functions invoked with onRegion always have to cast the FunctionContext to RegionFunctionContext

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public void execute(FunctionContext context) {
    RegionFunctionContext ctx = (RegionFunctionContext) context;
 

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Jira
showSummary false server ASF JIRA columns key,summary,type,created,updated,due,assignee,reporter,priority,status,resolution serverId 5aa69414-a9e9-3523-82ec-879b028fb15b key GEODE-394

for more details

Getting the local data set of a function is not intuitive, uses static functions that expect concrete objects

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We can also get rid of FunctionAdapter in favor of using default methods on Function. 

See

Suggested API changes

Instead of a single Function interface, there should be multiple interfaces for different types of functions. TODO - Do we really need VoidFunction (equivalent to hasResult() == false)The function interface should return a value. For functions that don't return a value, they can return null.

interface RegionFunction<T> {
  public T execute(RegionFunctionContext context)
}

interface Function<T> {
  public T execute(FunctionContext context)
}

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