Smalltalk - Image-based Persistence

Image-based Persistence

Most popular programming systems separate static program code (in the form of class definitions, functions or procedures) from dynamic, or run time, program state (such as objects or other forms of program data). They load program code when a program starts, and any prior program state must be recreated explicitly from configuration files or other data sources. Any settings the program (and programmer) does not explicitly save must be set up again for each restart. A traditional program also loses much useful document information each time a program saves a file, quits, and reloads. This loses details such as undo history or cursor position. Image based systems don't force losing all that just because a computer is turned off, or an OS updates.

Many Smalltalk systems, however, do not differentiate between program data (objects) and code (classes). In fact, classes are objects themselves. Therefore most Smalltalk systems store the entire program state (including both Class and non-Class objects) in an image file. The image can then be loaded by the Smalltalk virtual machine to restore a Smalltalk-like system to a prior state. This was inspired by FLEX, a language created by Alan Kay and described in his M.Sc. thesis.

Other languages that model application code as a form of data, such as Lisp, often use image-based persistence as well.

Smalltalk images are similar to (restartable) core dumps and can provide the same functionality as core dumps, such as delayed or remote debugging with full access to the program state at the time of error.