3.3. Shared State Cache

+3.3. Shared State Cache

+ By design, the OpenEmbedded build system builds everything from + scratch unless BitBake can determine that parts do not 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 do not necessarily need + to be 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 do not 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. +

Note

+ The OpenEmbedded build system does not maintain + PR + information as part of the shared state packages. + Consequently, considerations exist that affect maintaining + shared state feeds. + For information on how the OpenEmbedded build system + works with packages and can track incrementing + PR information, see the + "Automatically Incrementing a Binary Package Revision Number" + section in the Yocto Project Development Tasks Manual. +

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