1. MapServer MapCache
(f.k.a mod-geocache)
A Fast Tile-Caching Solution for the
Apache HTTP Server
14 / 09 / 2011 Thomas Bonfort (Terriscope)
Stephen Woodbridge (iMaptools)

2. What’s a tile cache?
Source
Services
Tileset
Grids
requests
Cache

3. Yet Another Tile Cache ?
• Mature / feature-full solutions already out
• High performance needed
• Started out as a small project, to validate the
concept
• Integrated into the MapServer stack for next
release

4. Apache module
• Module = code run by the apache processes
that treat requests
• Native code
• No overhead (e.g. CGI, FastCGI)
• Does not require spawning an interpreter per
concurrent request
• Caveats:
– Thread/process synchronization
– Memory management
– Security

5. Features
• Metatiling
– Cross process/thread locking ensures you
can enable metatiling on an unseeded
tileset
• Image recompression / optimization
– JPEG quality
– PNG compression level
– PNG quantization
– “Mixed” format: PNG+JPEG

6. Features (cont)
• Expiration handling
– HTTP cache controlling headers
– Last-Modified, If-Modified-Since
– Automatically regenerate stale tiles
• Custom error reporting
– Message
– Empty image
– Status code (+optional HTTP header)
• Watermarking

7. Features (cont)
• Dimensions
– Single values
– Intervals
– Regular Expressions
• FeatureInfo forwarding
• FastCGI / CGI option for use outside of apache

8. Services
• TMS
• WMTS
• WMS / WMS-C
• Google Maps XYZ
• VirtualEarth Quadkeys
• KML SuperOverlays

9. “Demo” Interface
• Lists all active services
• Easy way to check configured caches
• Simple OpenLayers slippy-map
• Cut-and-paste definitions for Layers
• http://localhost:8081/mapcache/demo

10. Grids
• Extent
• Projection
• Resolution per level
• Tile size
• Comes preconfigured with popular grids
• Supports grid aliases
• For limited areas, use grid subsets, not your
own grid!

11. Data Sources
• Extensible, anything able to provide an image
for given:
– Width, Height
– Extent
– SRS
– Optionally dimension
• WMS is the only implemented source
– Custom query parameters
– Custom headers

12. Caches
• Extensible mechanism, backend must provide
api to get/set a tile for a given grid, x, y, z
(+dimension)
• Higher level locking mechanism allows on-
demand cache generation
• Backends provide different performance /
manageability tradeoffs
• Currently implemented: filesystem, sqlite,
memcached

13. Disk based caches
• Tilecache hierarchy:
– /tmp/osm/g/17/000/027/304/000/081/334.png
• Custom hierarchy:
– /tmp/{tileset}/{grid}/{x}-{y}-{z}.{ext}
• Support for symlinking blank tiles
• Watch out for filesystem limitations !
– Max files per directory
– Max number of inodes
– Blocksize

14. Sqlite caches
• Store tile data as blobs in sqlite db
• Slower than disk caches, but prevents
filesytem headaches
• Flexible storage options:
– Provided custom schema
– MBTiles schema
– Custom schema, provide your own queries:
• select tile_data from tiles where tile_column=:x and
tile_row=:y and zoom_level=:z");
• insert or replace into
tiles(tile_column,tile_row,zoom_level,tile_data) values
(:x,:y,:z,:data)"

15. Vertical Assembling
Save bandwidth, request a single tiled layer !
&LAYERS=OSM&… &LAYERS=NEXRAD&… &LAYERS=OSM,NEXRAD&…

16. Horizontal Assembling
Support non-tiled clients

17. Tile Assembling
• CPU bound operation: image format (PNG/
JPEG) encoding and decoding
• CPU acceleration (MMX,SSE,…) of pixel
manipulation operations (scaling, blending)
• Configurable resampling
• No reprojection support
• Missing spec for TMS and WMTS support

18. Proxying support
• Transparently add tiling / fast WMS support to
existing services
• Intercepts GetTile / GetMap requests
• Configurable forwarding to other services
based on request parameters

19. Seeder
• Use multiple threads to load the source WMS
• Reseed tiles older than a specified date
• Seed only tiles inside given geometry
– OGR for data access: filter based on SQL
queries, e.g. FIPS_A1=’USA’ ,
pop_density>1000
– GEOS Prepared Geometries for fast
intersection calculation
• Delete mode
• Specify dimension values

20. Benchmarks
• Server: 4-core, 8GB ram, SSD storage
• ab tool used over Gigabit Ethernet
• “warm” filesystem
• All requests on exact same image data

21. TMS requests / sec
Throughput very dependent on image size
Concurrent MapProxy
Geowebcache MapCache
requests wsgi
1 1 280 800 1 780
10 17 000 9 000 30 000
20 18 000 8 000 40 000
100 17 000 7 100 40 000
250 16 500 6 800 40 000

22. SQLite backend, TMS
• MBTiles backend
• 10 threads:
– TileStream: 900
– MapCache: 8300

23. Horizontal assembling
• Workload: ~20 png decompressions + 1 jpg
compression
• 10 threads
• With bilinear resampling
– MapProxy: 29
– MapCache: 43
• With nearest neighbor resampling
– Geowebcache: 35
– MapProxy: 40
– MapCache: 48

24. Vertical assembling
• Workload: 2 png decompressions, 1 jpeg
compression
• 10 threads:
– Geowebcache: unsupported
– MapProxy: 435
– MapCache: 640

25. What’s next?
• MapServer Stack integration
– Docs
– Code sharing (image I/O, build tools)
• Ideas:
– Source assembling at the tileset level
– OGC ServiceExceptions
– HTTP options:
• HTTPS
• Authentication
• POST

26. Want More?
• Soon up to date project page:
http://www.mapserver.org/trunk/mapcache/index.html
• Soon defunct project page:
http://code.google.com/p/mod-geocache/
• Get the code:
– Mapserver trunk svn
– Mapserver 6.2
• Contribute !
• Twitter: @tbonfort