When FMMergeableWithSource is enabled, we try minimizing the FlatMapNodes. This may lead to increase in the emit count of spout depending on the tuple fanOut from flatMap operation in the spout. Also, there is no way to have map-side aggregation in some topologies.
We can eliminate these problems by adding aggregation to the spout. This aggregation would add some map-side aggregation ( localized to a spout ) in the absence of FlatMapNode, also we can decrease the emit Count by aggregating it before the emit.
When FMMergeableWithSource is enabled, we try minimizing the FlatMapNodes. This may lead to increase in the emit count of spout depending on the tuple fanOut from flatMap operation in the spout. Also, there is no way to have map-side aggregation in some topologies.
We can eliminate these problems by adding aggregation to the spout. This aggregation would add some map-side aggregation ( localized to a spout ) in the absence of FlatMapNode, also we can decrease the emit Count by aggregating it before the emit.