What's a Tiling Server

• Precomputed 256x256 images for fast access to static data
• Grid (projection, extent, resolutions)
• Source (WMS, MapServer, GDAL)
• Cache
• Tileset
• Service

What's MapCache?

• Tiling Library with Apache, FastCGI, Nginx and Node frontends
• Versatile
  • Multiple Cache Backends
  • Multiple Protocols
  • Tile Management Features: seeding, recompression, interpolation
• Native code: Fast and Efficient
• Ease deployment: Demo interface

History

• Started as an independant project in 2010
• Integrated under MapServer governance in 2012

Protocols

• XYZ etc...
• KML
• WMS

Standard Tile Addressing

• TMS
• WMTS (RestFull + KVP)
• VirtualEarth QuadKeys
• MapGuide
• Google x,y,z

WMS

• WMS-Tiled
• Responds to untiled requests by assembling tiles
• Can mirror a full WMS service: layer-by-layer, layer lists
• Forwards GetFeatureInfo requests

(OGC) Proxy

• Intercepts requests that can be served from cached tiles
• Forwards other requests to other upstream server(s)
• Use Case

Miscellaneous

• Caching HTTP Headers
• "Auto" Expiry of old tiles
• Error Reporting
  • Error Message
  • Empty Image
  • Status Code
• Meta-Tiling
• Watermarking
• Max-Cached-Zoom: Upsample lower level tiles

Image Operations

• Image data can be recompressed/optimized before being stored into a tile
  • Avoid double JPEG compression, i.e. request PNG from source WMS
  • Be agressive on compression when storing a tile (e.g. quantized PNG with max compression)
  • CPU time for dynamic recompressions can be lowered, at the cost of more bandwidth

Image Formats

• PNG
  • Compression level
  • Quantization (color reduction to 8bit palette)
• JPEG
  • Compression level
  • RGB/YCbCr
• Mixed (JPEG + PNG)
  • Use for raster coverages on limited areas
  • PNG for tiles with transparency
  • JPEG otherwise

Handling Empty/Uniform Tiles

• Strategy dependent on the cache backend
• • Symbolic linking
  • Reconstruct Paletted 1bit PNG from uniform color
  • For fully seeded caches, treat absent tile as fully transparent

Grids

• Multiple Grids per Tileset (e.g. WGS84 + Mercator)
• Handles Grid Aliasing
  • EPSG:3857/EPSG:900913
  • Non Standard Codes
• Grid Subsets
  • min/max resolution
  • Restricted extent (with tolerance!)

Caches

• Backend able to store tile data for given x,y,z
• Extensible: Exists(), Get(), Put(), Delete()
• Some backend specific hacks: symlinking, blank detection

Disk Cache

• Mainly for dev, testing, or small tilesets
• Pros
  • Simple, just provide a base directory
  • Fast
  • Supports blank tile detection
  • (Re-)Use custom layouts: TileCache, ArcGis, Template
• Cons
  • Difficult to manage large number of files
  • Hits File System Limits
  • Wasted space due to File Sytem block size

Sqlite Cache

• Stores tile data as blobs in an sqlite file
• Pros
  • Single File
  • Extensible to support any schema (MBtiles implemented)
  • Efficient Space (blank tile detection possible)
• Cons
  • Some tweaking needed for very large caches (more than 1 Tb)
  • Concurrent Insertions

MemCache

• Ideal for Temporary Data: Forecasts, Sensors, ...
• Pros
  • Easily distributed
  • Automatic pruning
• Cons
  • Limited Storage

(Geo)TIFF Cache

• Specialized storage for satellite imagery basemaps
• JPEG tiles are read directly from a constrained (Geo)TIFF file hierarchy
• Configurable number of tiles per TIFF file
• Templated access to file name
• Pros
  • Efficient on disk usage (TIFF container)
  • TIFF files can be shared with other applications
• Cons
  • Limited to JPEG data (no transparency)
  • No Concurrent writing to a single TIFF file
  • Updates/deletion of individual tiles does not reclaim storage space
• Use-Case

Seeder: Pre-generating tiles

• Multi thread/process seeding
• "Drill-Down" mode to take advantage of caching on source WMS
• Seed particular subset
  • Given Dimension
  • Regenerate old tiles
  • Restricted min/max zoom and/or extent
  • Restrict to geometry (e.g. seed to level 18 all areas where population density is over a given value)
• Pruning mode

Dimensions

• Multiple Dimensions per Tileset
• E.g. Forecast Time, Elevation
• Multiple Types
  • List of Values
  • Intervals
  • Regular Expression
• Use-Case: BoM MetEye

Time

• Each timestamp corresponds to a limited extent
• Dynamic Mosaic given a requested time interval
• c.f. ESA ngEO Presentation at 15:00

What's coming next

• TMS/GDAL source
• Native
• S3, Riak Caches
• Vector Tiles (UTFGrid)

Thank you for your Attention