Consistency Control in Distributed Collaboration Tools CISMM: Computer Integrated Systems for Microscopy and Manipulation Project Leads: Diane Sonnenwald* and Mary Whitton Investigators: Tom Hudson +, Kevin Jeffay (UNC-CS), Russ Taylor * Now at University of Borås, Sweden + Now at the University of North Carolina at Wilmington November 2003 The Problem: Provide application consistency AND interactivity The Solution: Use optimistic techniques wherever possible Implementing Optimistic Concurrency Control in the Collaborative nM Funding for this project was provided by NIH National Center for Research Resources The nanoManipulator Collaboratory enables non- collocated users to work together to perform real-time experiments and/or analyze previously collected data from the Atomic Force Microscope (AFM). To ensure user acceptance, it had to have the same functionality as the single user system and maintain the same high level of interactivity. In this system the application is replicated at each site and the network carries control signals, video, and application data between sites. Concurrency Technique Suitable Usage Scenarios Explicit LocksNot losing work is critical, even at the cost of latency and a changed workflow Implicit LocksNot interrupting natural workflow is critical, even at the cost of latency or lost work Coarse-Grained Locks Objects to be acted upon are interdependent, either algorithmically or in users’ planning. Fine-Grained Locks Objects to be acted upon can be separated into independent subsets OptimismAs for fine-grained, implicit locks; ideally, conflicting user actions can be automatically reconciled; actions have little/no risk of damage (to devices or data) and are easy to undo Techniques for consistency control which use locks require one or more network round-trips before parameters change at any collaborator’s site. Locks insure application state consistency and prevent damage to data and/or devices. Optimistic techniques allow parameter changes to occur immediately at the local site, maintaining interactivity. When the change is propagated to remote sites, they either implement or ignore it depending on intervening events that happened at those remote sites. Optimistic techniques maintain sufficient consistency for manipulations of individual visualization parameters; we use explicit locks to preserve users’ intent across sequences of manipulations of the microscope and prevent damaging the device. To identify the appropriate concurrency technique for each shared item of data in our system, we used parallel and hierarchical applications of the Model- View-Controller paradigm. The figure above shows two Model-View-Controller triples capture the different semantics of the microscope state and visualization parameters: microscope state must be pessimistically shared, while visualization parameters can be shared optimistically. The 3D rendering draws from both Models to provide an additional View. Data with different consistency requirements was grouped into different models, while data that dealt with coherent units of program function was grouped into submodels within the model. We could vary the coupling and concurrency control of each model and submodel as necessary, giving us a very flexible infrastructure for experimenting with alternate implementation approaches and user interfaces for the Collaborative nanoManipulator. Viewing Parameter Controls (submodel) Replay Controls (submodel) Elapsed time in replay Replay rate Color Controls (submodel) Source plane for color values Colormap Contour Map Controls (submodel) Lighting Controls (submodel) Visualization Model Rendered 3D Graphics View Microscope Controller Microscope Views Visualization Controller Visualization Model (Optimistic Concurrency Control) Visualization Views Microscope Model (Floor Control; Coarse-Grained Locking) PUBLIC NETWORK LAN PC-1: Shared nM application - AFM control - visualization Phantom PC-2: Net Meeting: - video conf. - shared apps. Phantom PC AFMAFM User at Location AUser at Location B PC-1: Shared nM application - AFM control - visualization PC-2: Net Meeting: - video conf. - shared apps.