Share parsed tiles across maps

[lucaswoj]

When one page contains many maps which use the similar styles to show the same geographic region we can improve performance by sharing parsed tiles between maps.
#1. Create Globally Unique Tile Identifier

Tiles can be uniquely identified by a hash of their

• coordinate
• source's URL or data
• style layer's id (to accommodate the paintVertexArrays)
• style layer's type
• style layer's layout properties
• style layer's paint properties which are property-functions

I'm not sure how exactly to handle ref families. The easiest solution is to have an entry in the tile cache per layer, not per ref family. Every entry within the same ref family would still point to the same Tile object.

This could replace the existing Tile#id property
#2. Create Shared Tile Cache

The simplest way to implement this is to replace the SourceCache#_cache instance property with a static SourceCache.cache property, keyed by the globally unique tile identifier ☝️. We will need to tweak the cache's retention policy to suit.

We may need to implement this cache as a locking cache to prevent simultaneous requests for the same time from spawning multiple requests.

cc @anandthakker

[lucaswoj]

Related to #899

[anandthakker]

👍

@lucaswoj thinking about this in the context of custom sources, it probably makes sense to implement 1 in a way that allows the Style and the Source to share responsibility for generating a tile's unique id.

[anandthakker]

Hmm -- @lucaswoj right now, for a given coord, the source cache holds one Tile per source--not per style layer. It seems to me like the "tiles" you're describing above are really closer to Buckets.

It almost seems like instead of caching Tiles, we could cache individual Buckets--keyed off of a hash like you described above--and then use them to construct Tiles on the fly... except that Tiles also have stuff like:

• collisionBoxArray
• collisionTile
• symbolInstancesArray
• symbolQuadsArray
• featureIndex

I don't yet know enough about how these work to guess whether they would make such a strategy infeasible. What do you think?

[lucaswoj]

@anandthakker Ah. Good thinking. We'll need to think a little bigger in terms of the architectural changes needed to support this feature (#2432 would do the trick)

[andrewharvey]

see also https://stackoverflow.com/questions/47891132/create-several-mapbox-gl-maps-that-show-different-markers-using-single-map-insta/

[mourner]

Closing as stale — not enough interest to pursue this.