Greg Hogan created FLINK-4257:
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Summary: Handle delegating algorithm change of class
Key: FLINK-4257
URL: https://issues.apache.org/jira/browse/FLINK-4257
Project: Flink
Issue Type: Bug
Components: Gelly
Affects Versions: 1.1.0
Reporter: Greg Hogan
Assignee: Greg Hogan
Fix For: 1.1.0
A class created by {{ProxyFactory}} can intercept and reinterpret method calls
using its {{MethodHandler}}, but is restricted in that
* the type of the proxy class cannot be changed
* method return types must be honored
We have algorithms such as {{VertexDegree}} and {{TriangleListing}} that change
return type depending on configuration, even between single and dual input
functions. This can be problematic, e.g. in {{OperatorTranslation}} where we
test {{dataSet instanceof SingleInputOperator}} or {{dataSet instanceof
TwoInputOperator}}.
Even simply changing operator can be problematic, e.g.
{{MapOperator.translateToDataFlow}} returns {{MapOperatorBase}} whereas
{{ReduceOperator.translateToDataFlow}} returns {{SingleInputOperator}}.
I see two ways to solve these issues. By adding a simple {{NoOpOperator}} that
is skipped over during {{OperatorTranslation}} we could wrap all algorithm
output and always be proxying the same class.
Alternatively, making changes only within Gelly we can append a "no-op"
pass-through {{MapFunction}} to any algorithm output which is not a
{{SingleInputOperator}}. And {{Delegate can also walk the superclass hierarchy
such we are always proxying {{SingleInputOperator}}.
There is one additional issue. When we call {{DataSet.output}} the delegate's
{{MethodHandler}} must reinterpret this call to add itself to the list of sinks.
As part of this issue I will also add manual tests to Gelly for the library
algorithms which do not have integration tests.
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