Optimize per_route config pre-processing for filters.

[qiwzhang]

Description:

This is referring to the new per_route_config added in envoyproxy/data-plane-api#605 (comment)

Specifically to optimize the pre-processing of per-route config. Here is the proposal

like to clarify how per_route_config_sha can be used.

A filter needs to preprocess the per_route_config.

obj = pre_process(pre_route_config);

Such pre-processing can be slow, it should NOT be performed for each request. One way to optimize it is to save the preprocessed "obj" in some global config. When a request comes, the filter just needs to detect if per_route_config has been changed. Without "sha" map, the filter needs to serialize the proto and sha it to detect the config change. If per-route config is big, these two steps can be slow too.

Another option is to add a new function in FilterFactory

void setRouteConfig(string unique_route_id, Struct config) PURE;

This function will be called when RDS receives new route tables, This will give filter chance to preprocess per_route config and save the pre-processed obj into its global map (not pre-request).

as global_map[ route_id ] = obj;

During request, obj can be retrieved as

obj = global_map[ filter_callback->route()->unique_route_id() ];

This is better than first approach, the pre-process is done in the config thread, not at worker thread.

Agreed that, such optimization can be done in the later phase.

[rshriram]

@mattklein123 we can close this..