Testing and Thumbnail previews

As part of the testing process I now generate samples of all base and overlay styles for a predefined area, measuring the time it takes to render (with all three result formats linked) and creating a list of thumbnail PNGs on an overview page, linking to the actual full size PNGs.

Next step: integrate the thumbnails into the style selection part of the “new map” form in some clever way …

Experimental: Using PostgreSQL Query Cache

Rendering PDF, PNG and SVG output makes Mapnik execute the same set of queries three times in a row. This has two effects:

  • rendering takes longer as queries are re-executed
  • output of the different formats may differ as data may have changed in between

By using the PostgreSQL Query Cache with a cache time of 10 minutes rendering the additional formats takes less time as query results can be reused, this also guarantees that all three formats are rendered using the same data.

On the other hand re-issuing a render request within less than ten minutes will not show changed data. This shouldn’t be much of an issue though as right now OSM changes are only processed every 15 minutes anyway.

Database maintenance

I’m going to do an OSM database reimport this weekend (Feb 25-26).

The web frontend will be online most of the time, so existing maps can still be retrieved, and new rendering jobs can be created, but the rendering daemon will be offline for at least all of Saturday, probably most of Sunday, too.

So don’t expect any new jobs entered over the weekend to be rendered before Monday, Feb. 27th, and the database may still be a bit behind for a few more days until things have fully caught up with minutely diffs again.

Continue reading “Database maintenance”