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Diffstat (limited to 'doc/manual/introduction.xml')
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1 files changed, 176 insertions, 176 deletions
diff --git a/doc/manual/introduction.xml b/doc/manual/introduction.xml index 5eea76459..feabeef9c 100644 --- a/doc/manual/introduction.xml +++ b/doc/manual/introduction.xml @@ -15,74 +15,74 @@ <title>Build management</title> <para> - Build management tools are used to perform <emphasis>software - builds</emphasis>, that is, the construction of derived products - (<emphasis>derivates)</emphasis>) such as executable programs from - source code. A commonly used build tool is Make, which is a standard - tool on Unix systems. These tools have to deal with several issues: - <itemizedlist> - - <listitem> - <para> - <emphasis>Efficiency</emphasis>. Since building large systems - can take a substantial amount of time, it is desirable that build - steps that have been performed in the past are not repeated - unnecessarily, i.e., if a new build differs from a previous build - only with respect to certain sources, then only the build steps - that (directly or indirectly) <emphasis>depend</emphasis> on - those sources should be redone. - </para> - </listitem> - - <listitem> - <para> - <emphasis>Correctness</emphasis> is this context means that the - derivates produced by a build are always consistent with the - sources, that is, they are equal to what we would get if we were - to build the derivates from those sources. This requirement is - trivially met when we do a full, unconditional build, but is far - from trivial under the requirement of efficiency, since it is not - easy to determine which derivates are affected by a change to a - source. - </para> - </listitem> - - <listitem> - <para> - <emphasis>Variability</emphasis> is the property that a software - system can be built in a (potentially large) number of variants. - Variation exists both in <emphasis>time</emphasis>---the - evolution of different versions of an artifact---and in - <emphasis>space</emphasis>---the artifact might have - configuration options that lead to variants that differ in the - features they support (for example, a system might be built with - or without debugging information). - </para> - - <para> - Build managers historically have had good support for variation - in time (rebuilding the system in an intelligent way when sources - change is one of the primary reasons to use a build manager), but - not always for variation in space. For example, - <command>make</command> will not automatically ensure that - variant builds are properly isolated from each other (they will - in fact overwrite each other unless special precautions are - taken). - </para> - </listitem> - - <listitem> - <para> - <emphasis>High-level system modelling language</emphasis>. The - language in which one describes what and how derivates are to be - produced should have sufficient abstraction facilities to make it - easy to specify the derivation of even very large systems. Also, - the language should be <emphasis>modular</emphasis> to enable - components from possible different sources to be easily combined. - </para> - </listitem> - - </itemizedlist> + Build management tools are used to perform <emphasis>software + builds</emphasis>, that is, the construction of derived products + (<emphasis>derivates)</emphasis>) such as executable programs from + source code. A commonly used build tool is Make, which is a standard + tool on Unix systems. These tools have to deal with several issues: + <itemizedlist> + + <listitem> + <para> + <emphasis>Efficiency</emphasis>. Since building large systems + can take a substantial amount of time, it is desirable that build + steps that have been performed in the past are not repeated + unnecessarily, i.e., if a new build differs from a previous build + only with respect to certain sources, then only the build steps + that (directly or indirectly) <emphasis>depend</emphasis> on + those sources should be redone. + </para> + </listitem> + + <listitem> + <para> + <emphasis>Correctness</emphasis> is this context means that the + derivates produced by a build are always consistent with the + sources, that is, they are equal to what we would get if we were + to build the derivates from those sources. This requirement is + trivially met when we do a full, unconditional build, but is far + from trivial under the requirement of efficiency, since it is not + easy to determine which derivates are affected by a change to a + source. + </para> + </listitem> + + <listitem> + <para> + <emphasis>Variability</emphasis> is the property that a software + system can be built in a (potentially large) number of variants. + Variation exists both in <emphasis>time</emphasis>---the + evolution of different versions of an artifact---and in + <emphasis>space</emphasis>---the artifact might have + configuration options that lead to variants that differ in the + features they support (for example, a system might be built with + or without debugging information). + </para> + + <para> + Build managers historically have had good support for variation + in time (rebuilding the system in an intelligent way when sources + change is one of the primary reasons to use a build manager), but + not always for variation in space. For example, + <command>make</command> will not automatically ensure that + variant builds are properly isolated from each other (they will + in fact overwrite each other unless special precautions are + taken). + </para> + </listitem> + + <listitem> + <para> + <emphasis>High-level system modelling language</emphasis>. The + language in which one describes what and how derivates are to be + produced should have sufficient abstraction facilities to make it + easy to specify the derivation of even very large systems. Also, + the language should be <emphasis>modular</emphasis> to enable + components from possible different sources to be easily combined. + </para> + </listitem> + + </itemizedlist> </para> </sect2> @@ -91,33 +91,33 @@ <title>Package management</title> <para> - After software has been built, is must also be - <emphasis>deployed</emphasis> in the intended target environment, e.g., - the user's workstation. Examples include the Red Hat package manager - (RPM), Microsoft's MSI, and so on. Here also we have several issues to - contend with: - <itemizedlist> - <listitem> - <para> - The <emphasis>creation</emphasis> of packages from some formal - description of what artifacts should be distributed in the - package. - </para> - </listitem> - <listitem> - <para> - The <emphasis>deployment</emphasis> of packages, that is, the - mechanism by which we get them onto the intended target - environment. This can be as simple as copying a file, but - complexity comes from the wide range of possible installation - media (such as a network install), and the scalability of the - process (if a program must be installed on a thousand systems, we - do not want to visit each system and perform some manual steps to - install the program on that system; that is, the complexity for - the system administrator should be constant, not linear). - </para> - </listitem> - </itemizedlist> + After software has been built, is must also be + <emphasis>deployed</emphasis> in the intended target environment, e.g., + the user's workstation. Examples include the Red Hat package manager + (RPM), Microsoft's MSI, and so on. Here also we have several issues to + contend with: + <itemizedlist> + <listitem> + <para> + The <emphasis>creation</emphasis> of packages from some formal + description of what artifacts should be distributed in the + package. + </para> + </listitem> + <listitem> + <para> + The <emphasis>deployment</emphasis> of packages, that is, the + mechanism by which we get them onto the intended target + environment. This can be as simple as copying a file, but + complexity comes from the wide range of possible installation + media (such as a network install), and the scalability of the + process (if a program must be installed on a thousand systems, we + do not want to visit each system and perform some manual steps to + install the program on that system; that is, the complexity for + the system administrator should be constant, not linear). + </para> + </listitem> + </itemizedlist> </para> </sect2> @@ -136,95 +136,95 @@ <itemizedlist> <listitem> - <para> - <emphasis>Reliable dependencies.</emphasis> Builds of file system - objects depend on other file system object, such as source files, - tools, and so on. We would like to ensure that a build does not - refer to any objects that have not been declared as inputs for that - build. This is important for several reasons. First, if any of the - inputs change, we need to rebuild the things that depend on them to - maintain consistency between sources and derivates. Second, when we - <emphasis>deploy</emphasis> file system objects (that is, copy them - to a different system), we want to be certain that we copy everything - that we need. - </para> - - <para> - Nix ensures this by building and storing file system objects in paths - that are infeasible to predict in advance. For example, the - artifacts of a package <literal>X</literal> might be stored in - <filename>/nix/store/d58a0606ed616820de291d594602665d-X</filename>, - rather than in, say, <filename>/usr/lib</filename>. The path - component <filename>d58a...</filename> is actually a cryptographic - hash of all the inputs (i.e., sources, requisites, and build flags) - used in building <literal>X</literal>, and as such is very fragile: - any change to the inputs will change the hash. Therefore it is not - sensible to <emphasis>hard-code</emphasis> such a path into the build - scripts of a package <literal>Y</literal> that uses - <literal>X</literal> (as does happen with <quote>fixed</quote> paths - such as <filename>/usr/lib</filename>). Rather, the build script of - package <literal>Y</literal> is parameterised with the actual - location of <literal>X</literal>, which is supplied by the Nix - system. - </para> + <para> + <emphasis>Reliable dependencies.</emphasis> Builds of file system + objects depend on other file system object, such as source files, + tools, and so on. We would like to ensure that a build does not + refer to any objects that have not been declared as inputs for that + build. This is important for several reasons. First, if any of the + inputs change, we need to rebuild the things that depend on them to + maintain consistency between sources and derivates. Second, when we + <emphasis>deploy</emphasis> file system objects (that is, copy them + to a different system), we want to be certain that we copy everything + that we need. + </para> + + <para> + Nix ensures this by building and storing file system objects in paths + that are infeasible to predict in advance. For example, the + artifacts of a package <literal>X</literal> might be stored in + <filename>/nix/store/d58a0606ed616820de291d594602665d-X</filename>, + rather than in, say, <filename>/usr/lib</filename>. The path + component <filename>d58a...</filename> is actually a cryptographic + hash of all the inputs (i.e., sources, requisites, and build flags) + used in building <literal>X</literal>, and as such is very fragile: + any change to the inputs will change the hash. Therefore it is not + sensible to <emphasis>hard-code</emphasis> such a path into the build + scripts of a package <literal>Y</literal> that uses + <literal>X</literal> (as does happen with <quote>fixed</quote> paths + such as <filename>/usr/lib</filename>). Rather, the build script of + package <literal>Y</literal> is parameterised with the actual + location of <literal>X</literal>, which is supplied by the Nix + system. + </para> </listitem> <listitem> - <para> - <emphasis>Support for variability.</emphasis> - </para> - - <para> - As stated above, the path name of a file system object contain a - cryptographic hash of all inputs involved in building it. A change to - any of the inputs will cause the hash to change--and by extension, - the path name. These inputs include both sources (variation in time) - and configuration options (variation in space). Therefore variants - of the same package don't clash---they can co-exist peacefully within - the same file system. So thanks to Nix's mechanism for reliably - dealing with dependencies, we obtain management of variants for free - (or, to quote Simon Peyton-Jone, it's not free, but it has already - been paid for). - </para> + <para> + <emphasis>Support for variability.</emphasis> + </para> + + <para> + As stated above, the path name of a file system object contain a + cryptographic hash of all inputs involved in building it. A change to + any of the inputs will cause the hash to change--and by extension, + the path name. These inputs include both sources (variation in time) + and configuration options (variation in space). Therefore variants + of the same package don't clash---they can co-exist peacefully within + the same file system. So thanks to Nix's mechanism for reliably + dealing with dependencies, we obtain management of variants for free + (or, to quote Simon Peyton-Jone, it's not free, but it has already + been paid for). + </para> </listitem> <listitem> - <para> - <emphasis>Transparent source/binary deployment.</emphasis> - </para> + <para> + <emphasis>Transparent source/binary deployment.</emphasis> + </para> </listitem> <listitem> - <para> - <emphasis>Easy configuration duplication.</emphasis> - </para> + <para> + <emphasis>Easy configuration duplication.</emphasis> + </para> </listitem> <listitem> - <para> - <emphasis>Automatic storage management.</emphasis> - </para> + <para> + <emphasis>Automatic storage management.</emphasis> + </para> </listitem> <listitem> - <para> - <emphasis>Atomic upgrades and rollbacks.</emphasis> - </para> + <para> + <emphasis>Atomic upgrades and rollbacks.</emphasis> + </para> </listitem> <listitem> - <para> - <emphasis>Support for many simultaneous configurations.</emphasis> - </para> + <para> + <emphasis>Support for many simultaneous configurations.</emphasis> + </para> </listitem> <listitem> - <para> - <emphasis>Portability.</emphasis> Nix is quite portable. Contrary - to build systems like those in, e.g., Vesta and ClearCase [sic?], it - does not rely on operating system extensions. - </para> + <para> + <emphasis>Portability.</emphasis> Nix is quite portable. Contrary + to build systems like those in, e.g., Vesta and ClearCase [sic?], it + does not rely on operating system extensions. + </para> </listitem> </itemizedlist> @@ -236,20 +236,20 @@ <itemizedlist> <listitem> - <para> - <emphasis>Build management.</emphasis> In principle it is already - possible to do build management using Fix (by writing builders that - perform appropriate build steps), but the Fix language is not yet - powerful enough to make this pleasant. The <ulink - url='http://www.cs.uu.nl/~eelco/maak/'>Maak build manager</ulink> - should be retargeted to produce Nix expressions, or alternatively, - extend Fix with Maak's semantics and concrete syntax (since Fix needs - a concrete syntax anyway). Another interesting idea is to write a - <command>make</command> implementation that uses Nix as a back-end to - support <ulink - url='http://www.research.att.com/~bs/bs_faq.html#legacy'>legacy</ulink> - build files. - </para> + <para> + <emphasis>Build management.</emphasis> In principle it is already + possible to do build management using Fix (by writing builders that + perform appropriate build steps), but the Fix language is not yet + powerful enough to make this pleasant. The <ulink + url='http://www.cs.uu.nl/~eelco/maak/'>Maak build manager</ulink> + should be retargeted to produce Nix expressions, or alternatively, + extend Fix with Maak's semantics and concrete syntax (since Fix needs + a concrete syntax anyway). Another interesting idea is to write a + <command>make</command> implementation that uses Nix as a back-end to + support <ulink + url='http://www.research.att.com/~bs/bs_faq.html#legacy'>legacy</ulink> + build files. + </para> </listitem> </itemizedlist> |