optionally support caching & compression

[derhuerst]

What applies to gtfs-realtime-validator (client) also applies to this project (server). From https://github.com/CUTR-at-USF/gtfs-realtime-validator/issues/124:

We should be sure to look at the following, which will optimize the fetching of feeds (both GTFS and GTFS-realtime):

• If-Modified-Since header

  • https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.25

• Last-Modified header - allows us to see if the data has changed since our last request -

  • http://stackoverflow.com/a/15544499/937715
  • https://www.mnot.net/cache_docs/
  • https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Last-Modified

• accept-content: gzip - allows us to download compressed content

  • http://www.rgagnon.com/javadetails/java-HttpUrlConnection-with-GZIP-encoding.html

I run a medium-sized GTFS-Realtime feed (usually 100-300 TripUpdates & 100-300 VehiclePositions). I just checked, gzipping (with the default level) brought the feed's size down from 246kb to 50kb.

[derhuerst]

In case someone is interested in a JavaScript/Node.js implementation: I have implemented the headers mentioned above as part of another project, using serve-buffer.