S6: Synchronized S5

S5 + AJAX = S6

Carsten Bormann

cabo@tzi.org

Presenter: Press “L” before starting

Student: Press “F” before starting

The classroom problem

• Presenter has one copy of slides
• Uses aging, dim projector to show them
• Students need to squint in the bright sun
• They all have laptops and love looking at the slides there
  • So you hand out PDFs in advance
• Problem: How to move their slides in sync?

Enter S5

• S5 is the solution for web-based presentation
• Why not use S5 for the classroom presentation
• Small matter of programming:
• Write AJAX code to sync multiple copies
• Write server code to provide the synchronization
• → S6
  • Synchronized Simple Standards-based Slide Show Software

How to use S6

• Model: One slideset user (presenter) leads, all others (students) follow
  • There is no explicit protection against multiple leaders, though
• Select roles by typing “L” to lead, “F” to follow
• Whenever leader changes slides, the followers follow
  • Students can veer off (e.g., review last slide), but will be synchronized on next slide change
  • If students really want to browse independently, they can “G”et out of the way, too

How does it work (1)

• In addition to the web server for the slides, there is a synchronization server
  • Using the Apache Rewrite and Proxy modules to map it on the same server — necessary for XMLHttpRequest security model
• Sync server has two requests:
• http://server/lead?url=…&value=…
  • Used by the leader to set the current slide number for the given URL to the given value
  • Returns immediately (giving estimate of number of followers)
• Leader sends lead request after each slide change

How does it work (2)

• http://server/follow?url=…&value=…
  • Used by the follower to wait for a new slide number for the given URL
  • The given value identifies what the follower believes to be the current slide number
  • Returns immediately if the actual current slide number is different
  • Otherwise, waits and returns only when the current slide number becomes different
• Follower sends follow request immediately after becoming a follower, and each time the previous request returns
  • Of course, whenever that request returns, the follower switches to the given new slide first

Implementation

• Sync Server
  • Frontend: 3 lines of Apache configuration
  • Backend: 75 line Ruby program (based on WEBrick)
    • What can I say :-)
• Client: Changes in slides.js
  • Added selected parts of Prototype library (~180 lines), see prototype.conio.net
  • New code: ~75 lines
    • Mostly rather boring
  • Just works in Safari
  • One Firefox fix needed in Prototype library
  • Not yet tested too much in IE (basically works)

To do (1)

• Show Stoppers
  • Doesn’t synchronize subitems (subgo()) yet — SMOP
• Should be fixed
  • The lead/follow indication is ugly (CSS not adapted)
  • Keys don’t always register (some more debugging required)
  • Path for syncserver (on slide server host) hardwired right now

To do (2)

• Hard to fix due to the model used
  • The count that allows the leader to gauge how many followers there are will be off occasionally
    • Can refresh by typing “L” repeatedly
  • New follower starts at old position of previous leader until new leader comes online
    • Could mitigate by aging out syncserver state
    • Can’t really solve