Cache only COG header

[dmahr1]

https://cogeotiff.slack.com/archives/C01DE57GLHE/p1603130953009500

Summary

Consider a case where N unique tile requests are made to a single COG. Despite the ENABLE_CACHE environment variable being enabled, all requests would be cache misses. Thus at least 2 * N range requests would need to be made to the COG. But if the COG header were cached separately, then only 1 + N range requests would need to be made.

Details

I plan to incorporate aiocogeo within a traditional tile server middleware that handles regular z/x/y.png requests. These currently read PNG tiles that are stored as a tile pyramid in bucket storage. This dated architecture is space inefficient but very performant. I'm hoping to achieve the space savings of COGs (via YCbCr JPEG compression + GDAL mask bands) without a meaningful increase in latency. One way to eliminate that latency is by caching the header in redis or another fast cache available to many servers. For example:

• Client requests Mercator tile (z, x, y) for cog.tif in cloud bucket storage. Networking layer routes it to COG server 1 (one of many COG servers).
  • COG server 1 checks redis (or another cache) for cog.tif header, but it's not found. CACHE MISS.
  • COG server 1 makes range request to cog.tif header.
  • COG server 1 caches cog.tif header in redis.
  • COG server 1 makes range request for (z, x, y) tile data.
  • COG server 1 performs postprocessing and returns (z, x, y) tile data to client.
• Client requests Mercator tile (z, x + 1, y) for cog.tif in cloud bucket storage. Networking layer routes it to COG server 2.
  • COG server 2 checks redis for cog.tif header and it's found. CACHE HIT.
  • COG server 2 makes range request for (z, x + 1, y) tile data.
  • COG server 2 performs postprocessing and returns (z, x + 1, y) tile data to client.

The two italicized operations for the first request are not necessary for the second request.

[geospatial-jeff]

This was implemented in https://github.com/geospatial-jeff/aiocogeo/pull/90, will be included in the next release!

[dmahr1]

Amazing! You're a hero, @geospatial-jeff. I was planning to play around with this over the holidays but you beat me to it!