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FABs in Production

While FABs have plenty of uses for prototyping & feedback (e.g. https://linc.sh), they're fundamentally designed as a production artefact. This document talks a little about some of the ideas/decisions that have influenced their shape.

Content-addressable compilation

A FAB can be built anywhere and produce an identical result—that is, for a given set of inputs, the fab.zip file that results is byte-for-byte identical no matter when, or on which machine, the compilation takes place. This includes using deterministic-zip to strip modification timestamps out of files as they're zipped up, so fab build will produce consistent results.

This means that if, after compiling a FAB, it has the same MD5/SHA hash as the currently-deployed FAB, no new deployment is required, as the contents are guaranteed to be the same. No more triggering re-deploys when all you've changed is the README.md or the end-to-end tests!

Production settings baked-in

Following on from this, anything that changes production must change the FAB. We achieve this by compiling in the settings in the settings: { production: {} } part of the config into the FAB itself. Changes to other environments do not change the compiled FAB, but can be used at runtime to override the baked-in values. See settings for more information.

Overrides without recompiling

Any pre-production testing can be performed on a production-ready FAB by injecting overridden environment settings. This means what you test is what you release, rather than testing a staging bundle then throwing it away and releasing a production one. See environments for more information.