3.2. Shared State Cache

-3.2. Shared State Cache

- By design, the OpenEmbedded build system builds everything from scratch unless - BitBake can determine that parts don't need to be rebuilt. - Fundamentally, building from scratch is attractive as it means all parts are - built fresh and there is no possibility of stale data causing problems. - When developers hit problems, they typically default back to building from scratch - so they know the state of things from the start. -

- Building an image from scratch is both an advantage and a disadvantage to the process. - As mentioned in the previous paragraph, building from scratch ensures that - everything is current and starts from a known state. - However, building from scratch also takes much longer as it generally means - rebuilding things that don't necessarily need rebuilt. -

- The Yocto Project implements shared state code that supports incremental builds. - The implementation of the shared state code answers the following questions that - were fundamental roadblocks within the OpenEmbedded incremental build support system: -

What pieces of the system have changed and what pieces have not changed?
How are changed pieces of software removed and replaced?
How are pre-built components that don't need to be rebuilt from scratch - used when they are available?

- For the first question, the build system detects changes in the "inputs" to a given task by - creating a checksum (or signature) of the task's inputs. - If the checksum changes, the system assumes the inputs have changed and the task needs to be - rerun. - For the second question, the shared state (sstate) code tracks which tasks add which output - to the build process. - This means the output from a given task can be removed, upgraded or otherwise manipulated. - The third question is partly addressed by the solution for the second question - assuming the build system can fetch the sstate objects from remote locations and - install them if they are deemed to be valid. -

- The rest of this section goes into detail about the overall incremental build - architecture, the checksums (signatures), shared state, and some tips and tricks. -