AggregatorSupplier serves as a marker for what the
ValuesSourceRegistry holds to construct aggregator instances.
The aggregators for each aggregation should all share a signature, and that signature should be used to create an AggregatorSupplier for
that aggregation. Alternatively, if an existing supplier has a matching signature, please re-use that.
In many cases, this can be a simple wrapper over the aggregator constructor. If that is sufficient, please just use a reference to the
constructor. Implementing a supplier (typically as a lambda) should only be necessary when factors besides the
ValuesSourceType
are necessary for selecting the correct aggregator implementation. This happens in terms for example where we make decisions based on
the availability of global ordinals.
The suppliers are responsible for any casting of
ValuesSource that needs to happen. They must accept a base
ValuesSource
instance. The suppliers may perform additional logic to configure the aggregator as needed, such as in
TermsAggregatorFactory deciding the execution mode.
There is ongoing work to normalize aggregator constructor signatures, and thus reduce the number of AggregatorSupplier interfaces.