RFC: Integrate Aggregation Layers with Attribute Manager (#3808)

dev-docs/RFCs/v8.0/grid-aggregation-attributemanager.md

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+# RFC: Integrate Aggregation Layers with Attribute Manager
+
+* Authors: Ravi Akkenapally
+* Date: October 22, 2019
+* Status: **Draft**
+* POC: https://github.com/uber/deck.gl/pull/3777
+
+> I am currently experimenting with all required integrations with `Layer` and `AttributeManager` and it is likely some of these proposed solutions could change.
+
+
+## Overview
+
+`GPUGridAggregator` first introduced in 6.0 has been updated to support CPU and GPU Grid aggregation, multiple coordinate systems and aggregation of multiple weights. Several layers in `@deck.gl/aggregation-modules`, `ScreenGridLayer`, `GPUGridLayer` and `ContourLayer` use `GPUGridAggregator` to perform aggregation to a grid.
+
+Since 6.0, several new features/improvements have happened in other parts of deck.gl, like `extensions` (data filtering, `fp64`  support), `prop types` and `AttributeManager` (like binary data support and optimizations like using const attributes, etc).
+
+
+## Problem
+
+All new features mentioned above are tied to the `Layer` and `AttributeManager`. In all aggregation layers, aggregation is performed as offline rendering (not part of `Layer.model` and `Layer.draw()`), and `GPUGridAggregator` performs this by building attributes and `Model` object outside of `AttributeManager` and `Layer` lifecycle methods, hence these new features are not available for aggregation layers.
+
+
+## Proposal
+
+Introduce new composite layer `AggregationLayer`, which owns `GPUGridAggregator`, intercepts required layer lifecycle methods and also setups all attributes using `AttributeManager`. And all of the layers that use `GPUGridAggregator`, must be a `Composite Layer` and extend from `AggregationLayer` to achieve aggregation.
+
+### AggregationLayer
+
+#### Attributes setup
+
+`AttributeManager` is only available for `Layer` and it's subclasses and not available for a `CompositeLayer`. We can create an instance of `AttributeManger` in `AggregationLayer` and provide it to all aggregation layers which can be used for all offline renderings performed for aggregation.
+
+#### Model setup
+
+When an `extension` is enabled, new `attributes`, `uniforms` and shader `modules` need to used in the `Model` construction. These new parameters can be queried in `AggregationLayer` and used to create `Model` object for `GPUGridAggregator`.
+
+When `extension` parameters change, `Layer` will gather new uniforms from corresponding `module` object. We can perform this task in `AggregationLayer` and update `GPUGridAggregator`'s' `Model` object.
+
+#### State tracking
+
+`GPUGridAggregator` maintains its own state and performs least required actions when any state is changed. Given, now we have to integrate with `Layer` lifecycle methods, some of this code may have to be moved from `GPUGridAggregator` into `AggregationLayer` and/or add new methods to `GPUGridAggregator` for state updates.
+
+
+## Conclusion
+
+Integrating `GPUGridAggregator` with `AttributeManager` and `Layer` lifecycle methods, enable extension like `data-filtering` and all current and future features of `AttributeManager`.