Application Level Undo & Recovery: Applied to the Pencil Application on Linux

Pencil is an open source, multi-platform QT-based drawing and animation solution. Software bugs in the program can cause the application to exhibit anomalous behaviour or degrade in performance. This can cause the program to hang or crash which can ultimately lead to the loss of user data. We introduce undo and recovery functionality into Pencil that will help the user recover their work by replaying logged operations onto periodically recorded snapshots of the canvas. Evaluation of the operation logging solution concluded acceptable real-time costs in the range of 20-30 ms for performing per mouse movement in an operation, 7-10 kB storage requirement per draw event per second and 4-6 ms of recovery per draw action on application startup following a crash. Similar snapshot collection overheads of 2-3 kB storage and 8-10 ms processing for each draw event were already present in the system. Although some of these numbers may seem prohibitively high, we feel the positive aspect of accurately and completely recovering user work presents a much greater benefit for the use of this recovery feature.