Firefox has it's own built-in update system. The update system supports 2 types of updates: complete and incremental. Completes can be applied to any older version, unless there are some incompatible changes in the MAR format. Incremental updates can be applied only to a release they were generated for.
Usually for the beta and release channels we generate incremental updates against 3-4 versions. This way we try to minimize bandwidth consumption for our end users and increase the number of users on the latest version. For Nightly and Developer Edition builds we generate 5 incremental updates using funsize.
Both methods assume that we know ahead of time what versions should be used for incremental updates. For releases and betas we use ADI stats to be as precise as possible. However, these methods are static and don't use real-time data.
The idea to generate incremental updates on demand has been around for ages. Some of the challenges are:
Acquiring real-time (or close to real-time) data for making decisions on incremental update versions
Size of the incremental updates. If the size is very close to the size of the corresponding complete, there is reason to serve incremental updates. One of the reasons is that the that the updater tries to use the incremental update first, and then falls back to the complete in case if something goes wrong. In this case the updater downloads both the incremental and the complete.
Ben and I talked about this today and to recap some of the ideas we had, I'll put them here.
We still want to "pre-seed" most possible incremental updates before we publish any updates
Whenever Balrog serves a complete-only update, it should generate a structured log entry and/or an event to be consumed by some service, which should contain all information required to generate a incremental update.
The "new system" should be able to decide if we want to discard incremental update generation, based on the size. These decisions should be stored, so we don't try to generate incremental update again next time. This information may be stored in Balrog to prevent further events/logs.
Before publishing the incremental update, we should test if they can be applied without issues, similar to the update verify tests we run for releases, but without hitting Balrog. After they pass this test, we can publish them to Balrog and check if Balrog returns expected XML with partial info in it.
Minimize the amount of served completes, if we plan to generate incremental updates. One of the ideas was to modify the client to support responses like "Come in 5 minutes, I may have something for you!"
The only remaining thing is to implement all these changes. :)