use zsync to update offline routing data

[Martin Kolman]

• use zsync to update routing data from the server
• like this, only changes would be transferred, making frequent routing data updates feasible
  • packs for large countries can take up multiple GBs

• zsync: ​http://zsync.moria.org.uk/

• how would an update look like:
  • before starting the update, mark the map package as dirt
    • .#dirty file ?
  • modRana downloads a file-list for the package, that contains also a link to a zsync control file for every file
  • iterate over the zsync files and update every local file
  • at the end, remove all files that are present in the local folder but not on the file-list (exclude the .#dirty file)
  • remove the .#dirty file

• drawbacks:
  • needs to store two copies of the routing data
    • one archive for direct download
    • one in uncompressed form + zsync control file for updates only
      • needs zsync files for all files
      • on the other hand as zsync won't overwrite the file until the download is finished, working on multiple smaller files helps