Concurrency Control

To support a writing collaboration of multiple users, Saros uses the Jupiter algorithm for concurrency control. The Concurrency Control keeps all shared data of the participants of a Saros session consistent.



The functionality of the Jupiter package is based on the Jupiter algorithm taken from the Open Source project ACE (A Collaborative Editor).
The Jupiter class implements the client-side core of the Jupiter control algorithm.
It generates JupiterActivities for local operations and transforms received JupiterActivities into operations that need to be executed locally. Operations are changes made on a document.
An operation contains what has changed and the position where it was changed.
These changes made by multiple participants of a Saros session need to be executed in the correct order and position by all participants so that they all have the same consistent copy of this changed workspace.
Both participants of a Saros session C and S start with the text "abc". C performs an insert operation insert(0,"def") and S performs an delete operation delete(1,"b"). The resulting text is "defac". Since the local operation is performed immediately at each participants side, the received operation of the other participant is transformed to ensure the consistency. Therefore for participant C the delete operation is transformed into delete(4,"b"). For participant S the transformation doesn't change the insert operation.


The subpackage management is responsible for managing the state of the shared workspace in a Saros session.
Every user with write-access sends their JupiterActivities to the ConcurrentDocumentServer located on the host side where they are transformed and sent to everybody else.
The transformed JupiterActivities are received and transformed to TextEditActivities in the ConcurrentDocumentClient for execution on the client side.
The host of the session is always responsible for managing the state of all projects (or files), even when a client adds projects or files to the Saros session.


The consistency Watchdog is responsible for monitoring the states of all participant's workspaces and to recover from inconsistent states between the participants of a Saros session.
For all opened files anywhere in the shared session (host and clients) checksums are generated periodically (every 10 seconds) on the host side. The basis for these checksums are the current contents of the editors, not the content of the files as they are written on the harddrive.
These checksums are sent to all clients and the clients check their locally generated checksums against them.
If an inconsistency is detected the inconsistency state is set and the user of the session gets informed (the yellow triangle on the Saros View).
When the user wants to recover from that inconsistency a ChecksumError is sent to the host and waited for the reply.
The user receives the file changes from the host.
The host can never be in an inconsistent state.