Martin Sevior
School of Physics
University of Melbourne
Freelance abiword Hacker

Real time collaboration between any number of sessions running on different computers around the world.

Appliations: schools, universities, businesses, journalists, OLPC.

Can operate without a server and without locking.

Nobody ever stopped from typing. Any errors and we correct later. Collisions and we correct later.

Abiword model

Document Model: PieceTable is a double linked list containing text and formatting (Text), embedded objects (Object), and paragraph descriptors (Strux).

Views: for looking at document, greating something interesting, e.g. screen view or printout.

ChangeRecords (CR): emitted by each PieceTable as it is created destroyed or changed.

AlbCollab: serialises list of changes (ChangeRecords) and squirts across to remote site.

Core principles

Documents must always be identical

Users should not be surprised, eg. undo should undo last change local user made.

Designed to work without a server. Direct connections without locking.

Transports: XMPP (Jabber), TCP connections, Telepathy (OLPC).

Issues

Internet lag

• Can loose synchronisation because of finite propagation time.
• InsertText, at 12, ‘e’ - positions given by absolute character numbers.
• Record the CR number Position, and send with CR number sent to remote. Each end adjusts offsets as required by difference in CR number.
• Pairwise connection: master node to keep track of changes users make.
• If master dies, another master takes its place.
• Local CR number may be different to remote CR number.

Undo/Redo

• Pressing undo should only undo your changes.
• AbiWord records all the CRs applied to the model.
• Undo shouldn’t undo changes made remotely.
• Remote person may be changing other page entirely.
• Need to tag remote CR and record their adjustments.
• If remote user changes your text, you can’t undo it.

Complex operations

• conglomeration of smaller operations
• shouldn’t be interrupted
• Glue operations between GLOBs together on reception and do them all together. If network failure, nothing gets processed. Atomic transactions.

Collisions

• One user types “X” another user types “Y” at same point.
• Collision. Fight to the death.
• Master document tells the remote to undo until we’re back in synch. Remote acknowledges.
• Try and fix it and let user work out what is going on.
• First change received wins.

Embeddable AbiWord

GtkWidget

• python and C# bindings
• AbiCollab built in

basis of OLPC Write, which has radically different UI.

Conclusion

https://abicollab.net/

The code works and is being widely deployed.

Questions:

Somebody drops offline while editing document?

• Need to manually merge changes.
• abicollab.net will always work.

Only need to be wrong once in handling changes, and it breaks badly. Getting all the changes right is difficult.

Abiword and ODF?

• Very well.
• Silly mistakes in ODF?
• Image positions relative to page number. Problem for websites, requires two passes.