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| 1 | <html> | ||
| 2 | <head> | ||
| 3 | <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> | ||
| 4 | <title>3.3. Shared State Cache</title> | ||
| 5 | <link rel="stylesheet" type="text/css" href="../book.css"> | ||
| 6 | <meta name="generator" content="DocBook XSL Stylesheets V1.76.1"> | ||
| 7 | <link rel="home" href="index.html" title="Getting Started With Yocto Project"> | ||
| 8 | <link rel="up" href="overview-concepts.html" title="Chapter 3. Yocto Project Concepts"> | ||
| 9 | <link rel="prev" href="cross-development-toolchain-generation.html" title="3.2. Cross-Development Toolchain Generation"> | ||
| 10 | <link rel="next" href="overall-architecture.html" title="3.3.1. Overall Architecture"> | ||
| 11 | </head> | ||
| 12 | <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="section" title="3.3. Shared State Cache"> | ||
| 13 | <div class="titlepage"><div><div><h2 class="title" style="clear: both"> | ||
| 14 | <a name="shared-state-cache"></a>3.3. Shared State Cache</h2></div></div></div> | ||
| 15 | <p> | ||
| 16 | By design, the OpenEmbedded build system builds everything from | ||
| 17 | scratch unless BitBake can determine that parts do not need to be | ||
| 18 | rebuilt. | ||
| 19 | Fundamentally, building from scratch is attractive as it means all | ||
| 20 | parts are built fresh and there is no possibility of stale data | ||
| 21 | causing problems. | ||
| 22 | When developers hit problems, they typically default back to | ||
| 23 | building from scratch so they know the state of things from the | ||
| 24 | start. | ||
| 25 | </p> | ||
| 26 | <p> | ||
| 27 | Building an image from scratch is both an advantage and a | ||
| 28 | disadvantage to the process. | ||
| 29 | As mentioned in the previous paragraph, building from scratch | ||
| 30 | ensures that everything is current and starts from a known state. | ||
| 31 | However, building from scratch also takes much longer as it | ||
| 32 | generally means rebuilding things that do not necessarily need | ||
| 33 | to be rebuilt. | ||
| 34 | </p> | ||
| 35 | <p> | ||
| 36 | The Yocto Project implements shared state code that supports | ||
| 37 | incremental builds. | ||
| 38 | The implementation of the shared state code answers the following | ||
| 39 | questions that were fundamental roadblocks within the OpenEmbedded | ||
| 40 | incremental build support system: | ||
| 41 | </p> | ||
| 42 | <div class="itemizedlist"><ul class="itemizedlist" type="disc"> | ||
| 43 | <li class="listitem"><p> | ||
| 44 | What pieces of the system have changed and what pieces have | ||
| 45 | not changed? | ||
| 46 | </p></li> | ||
| 47 | <li class="listitem"><p> | ||
| 48 | How are changed pieces of software removed and replaced? | ||
| 49 | </p></li> | ||
| 50 | <li class="listitem"><p> | ||
| 51 | How are pre-built components that do not need to be rebuilt | ||
| 52 | from scratch used when they are available? | ||
| 53 | </p></li> | ||
| 54 | </ul></div> | ||
| 55 | <p> | ||
| 56 | </p> | ||
| 57 | <p> | ||
| 58 | For the first question, the build system detects changes in the | ||
| 59 | "inputs" to a given task by creating a checksum (or signature) of | ||
| 60 | the task's inputs. | ||
| 61 | If the checksum changes, the system assumes the inputs have changed | ||
| 62 | and the task needs to be rerun. | ||
| 63 | For the second question, the shared state (sstate) code tracks | ||
| 64 | which tasks add which output to the build process. | ||
| 65 | This means the output from a given task can be removed, upgraded | ||
| 66 | or otherwise manipulated. | ||
| 67 | The third question is partly addressed by the solution for the | ||
| 68 | second question assuming the build system can fetch the sstate | ||
| 69 | objects from remote locations and install them if they are deemed | ||
| 70 | to be valid. | ||
| 71 | </p> | ||
| 72 | <div class="note" title="Note" style="margin-left: 0.5in; margin-right: 0.5in;"> | ||
| 73 | <h3 class="title">Note</h3> | ||
| 74 | The OpenEmbedded build system does not maintain | ||
| 75 | <a class="link" href="../ref-manual/var-PR.html" target="_self"><code class="filename">PR</code></a> | ||
| 76 | information as part of the shared state packages. | ||
| 77 | Consequently, considerations exist that affect maintaining | ||
| 78 | shared state feeds. | ||
| 79 | For information on how the OpenEmbedded build system | ||
| 80 | works with packages and can track incrementing | ||
| 81 | <code class="filename">PR</code> information, see the | ||
| 82 | "<a class="link" href="../dev-manual/automatically-incrementing-a-binary-package-revision-number.html" target="_self">Automatically Incrementing a Binary Package Revision Number</a>" | ||
| 83 | section in the Yocto Project Development Tasks Manual. | ||
| 84 | </div> | ||
| 85 | <p> | ||
| 86 | </p> | ||
| 87 | <p> | ||
| 88 | The rest of this section goes into detail about the overall | ||
| 89 | incremental build architecture, the checksums (signatures), shared | ||
| 90 | state, and some tips and tricks. | ||
| 91 | </p> | ||
| 92 | </div></body> | ||
| 93 | </html> | ||