OpenStreetMap
Load testing vector tiles
As part of bringing the new vector tile servers into production, I had to benchmark their performance. Since there’s a cache in front of the servers, it’s challenging to benchmark them accurately. Although we’ve never had a heavy load on the vector tile servers, we’ve been running raster tile servers for years.
All tile requests on the standard layer are logged, and from those logs, I can generate a list of tiles to benchmark the vector tile servers. The logs are stored as Parquet files, which I query using Amazon Athena, a hosted Presto database.
Vector tiles and raster tiles typically have different scales at the same zoom level. To convert raster tile requests to equivalent vector tile requests, I divide the x and y coordinates by 2 and decrease the zoom level by 1. I also skip zoom 0 raster tile requests to simplify the process, as these don’t affect performance since zoom 0 is always cached.
The OSMF shortbread tiles have a maximum zoom of 14. Lower scales (higher zoom levels) are achieved by overzooming on the client side. Requests from zoom 1 to 15 should have their zoom level lowered by 1. Requests from zoom 16 to 19 need their zoom level decreased by the difference between their level and 14. I divide the x and y coordinates by 2 the appropriate number of times to match the new zoom level.
Filtering to have only cache misses gets me a request list on the backend servers.
SELECT
CASE WHEN z > 15 THEN 14 ELSE z - 1 END AS v_z,
bitwise_right_shift(x, CASE WHEN z > 15 THEN z-14 ELSE 1 END) AS v_x,
bitwise_right_shift(x, CASE WHEN z > 15 THEN z-14 ELSE 1 END) AS v_y
z, x, y
FROM fastly_success_logs_v1
WHERE year=2025 AND month=5 AND day = 1 AND hour = 1
AND z >= 1
AND cachehit = 'MISS';
Unfortunately, this is the wrong list.
