Then one needs to express the incompatibility in the versioning. Usually there is some ''major'' version number that, once increased means that the new version is no longer compatible with the previous one. This shall be used rarely, but each [[module system]] needs to provide some way of expressing incompatible versions.

Then one needs to express the incompatibility in the versioning. Usually there is some ''major'' version number that, once increased means that the new version is no longer compatible with the previous one. This shall be used rarely, but each [[module system]] needs to provide some way of expressing incompatible versions.

Common way to implement this is to associate with each version an implicit upper bound. So when one writes <math>some.library >= 1.4.3</math> one actually means any give me some library at least in version 1.4.3 but not 2.0 or higher. From here there is just a little step towards [[RangeDependencies]].

Common way to implement this is to associate with each version an implicit [[upper bound]]. So when one writes <math>some.library >= 1.4.3</math> one actually means any give me some library at least in version 1.4.3 but not 2.0 or higher. From here there is just a little step towards [[RangeDependencies]].

=== [[RangeDependencies]] ===

=== [[RangeDependencies]] ===

== Dependencies ==

== Dependencies ==

Not many [[API]]s can live alone, without support from other parts of the system. Every [[C]] library needs [[wikipedia::Libc|libc]], every [[Java]] code needs some version of [[JDK]]. As each library, also [[API]]s have own environment, which defines what needs to be available around to allow the [[API]] to function properly. Each user of such shared library needs to recreate proper environment, that means to satisfy its dependencies, before its [[API]] can be used. This implies, that the ''dependencies of a library'' form an important [[API]] of the shared library itself.

Not many [[API]]s can live alone, without support from other parts of the system. Every [[C]] application or library needs [[wikipedia::Libc|libc]], every [[Java]] code needs some version of [[JDK]]. As each library, also [[API]]s have own environment, which defines what needs to be available around to allow the [[API]] to function properly. Each user of such shared library needs to recreate proper environment, that means to satisfy its dependencies, before its [[API]] can be used. This implies, that the ''dependencies of a library'' form an important [[API]] of the shared library itself.

These dependencies may not even be visible in external signatures (more about that in [[TypesOfDependencies]])! They may only be needed during the runtime, internally, still changing them constitutes an [[API]] change. Imagine, that users of your API are using your library in some version and it works fine with just plain [[JDK]]. Suddenly, in newer release, you decide to change the library internals and depend on some other library, for example [[wikipedia::Jakarta_Commons|Jakarta Commons]]. That immediately means every user, who migrates to new version of your library, needs to include a version of [[wikipedia::Jakarta_Commons|Jakarta Commons]] in own application as well. This may or may not be a problem, however this is quite an externally visible change.

These dependencies may not even be visible in external signatures (more about that in [[TypesOfDependencies]])! They may only be needed during the runtime, internally, still changing them constitutes an [[API]] change. Imagine, that users of your API are using your library in some version and it works fine with just plain [[JDK]]. Suddenly, in newer release, you decide to change the library internals and depend on some other library, for example [[wikipedia::Jakarta_Commons|Jakarta Commons]]. That immediately means every user, who migrates to new version of your library, needs to include a version of [[wikipedia::Jakarta_Commons|Jakarta Commons]] in own application as well. This may or may not be a problem, however this is quite an externally visible change.

Then one needs to express the incompatibility in the versioning. Usually there is some ''major'' version number that, once increased means that the new version is no longer compatible with the previous one. This shall be used rarely, but each [[module system]] needs to provide some way of expressing incompatible versions.

Then one needs to express the incompatibility in the versioning. Usually there is some ''major'' version number that, once increased means that the new version is no longer compatible with the previous one. This shall be used rarely, but each [[module system]] needs to provide some way of expressing incompatible versions.

Thus this kind of dependency is useful only for modules which can be used separately, but are built and deployed together.

Thus this kind of dependency is useful only for modules which can be used separately, but are built and deployed together.

=== Greater Than ===

=== Greater Or Equal ===

For modules deployed individually one needs to allow certain freedom. The ''equality'' dependency is too restricting. Usually people use ''equal or greater than'' dependency. It is usually expected that if program runs on [[Java]] 1.3, it will also run (relatively) fine on [[Java]] 1.5.

For modules deployed individually one needs to allow certain freedom. The ''equality'' dependency is too restricting. Often people use ''equal or greater than'' dependency. This follows the usually accepted expectation that if program runs on [[Java]] 1.3, it will also run (relatively) fine on [[Java]] 1.5.

This of course requires the vendor of the library to follow rules of [[BackwardCompatibility]] - e.g. really design and especially [[evolution|evolve]] the [[API]] well. However most people designing commonly used libraries understand this and thus we can mostly rely on ''>='' dependencies.

This of course requires the vendor of the library to follow rules of [[BackwardCompatibility]] - e.g. really design and especially [[evolution|evolve]] the [[API]] well. However most people designing commonly used libraries understand this and thus we can mostly rely on ''>='' dependencies.

[[Category:APITypes]]

[[Category:APITypes]]

=== Equality ===

=== Equality ===

One can depend on exact certain version of other library. This is however very restricting. If there is a part of application that needs [[Java]] 1.3 and another that needs [[Java]] 1.5, will you run each part of the application in different [[Hotspot|VM]]?

One can depend on exact certain version of other library. This is however very restricting. If there is a part of application that needs [[Java]] 1.3 and another that needs [[Java]] 1.5, will you run each part of the application in different [[HotSpot|VM]]?

Thus this kind of dependency is useful only for modules which can be used separately, but are built and deployed together.

Thus this kind of dependency is useful only for modules which can be used separately, but are built and deployed together.

=== Equality ===

=== Equality ===

One can depend on exact certain version of other library. This is however very restricting. If there is a part of application that needs [[Java]] 1.3 and another that needs [[Java]] 1.5, will you run each part of the application in different [[VM]]?

One can depend on exact certain version of other library. This is however very restricting. If there is a part of application that needs [[Java]] 1.3 and another that needs [[Java]] 1.5, will you run each part of the application in different [[Hotspot|VM]]?

Thus this kind of dependency is useful only for modules which can be used separately, but are built and deployed together.

Thus this kind of dependency is useful only for modules which can be used separately, but are built and deployed together.

=== Specifying [[Dependencies]] ===

=== Specifying [[Dependencies]] ===

[[Dependencies]] are usually specified as set of pairs. First element in each pair contains unique identification of needed [[API]] and also some specification of the [[API]] versions.

[[Dependencies]] are usually specified as set of pairs. First element in each pair contains unique identification of needed [[API]] and also some specification of the [[API]] versions. Depending on the actual [[module system]], the dependencies may be equality, greater than or even specified as a [[RangeDependencies|range]].

 

 

 

 

 

 

 

This of course requires the vendor of the library to follow rules of [[BackwardCompatibility]] - e.g. really design and especially [[evolution|evolve]] the [[API]] well. However most people designing commonly used libraries understand this and thus we can mostly rely on ''>='' dependencies.

[[Category:APITypes]]

[[Category:APITypes]]

== Dependencies ==

== Dependencies ==

Not many [[API]]s can live alone, without support from other parts of the system. As each library, also [[API]]s have own environment, which defines what needs to be available around to allow the [[API]] to function properly. Each user of the shared library needs to recreate proper environment, that means to satisfy library dependencies, before the [[API]] can be used. This implies, that the ''dependencies of a library'' form an important [[API]] of the shared library itself.

Not many [[API]]s can live alone, without support from other parts of the system. Every [[C]] library needs [[wikipedia::Libc|libc]], every [[Java]] code needs some version of [[JDK]]. As each library, also [[API]]s have own environment, which defines what needs to be available around to allow the [[API]] to function properly. Each user of such shared library needs to recreate proper environment, that means to satisfy its dependencies, before its [[API]] can be used. This implies, that the ''dependencies of a library'' form an important [[API]] of the shared library itself.

These dependencies may not even be visible in external signatures (more about that in [[TypesOfDependencies]])! They may only be needed during the runtime, internally, still changing them constitutes an [[API]] change. Imagine, that users of your API are using your library in some version and it works fine with just plain [[JDK]]. Suddenly, in newer release, you decide to change the library internals and depend on some other library, for example [[wikipedia::Jakarta_Commons|Jakarta Commons]]. That immediately means every user, who migrates to new version of your library, needs to include a version of [[wikipedia::Jakarta_Commons|Jakarta Commons]] in own application as well. This may or may not be a problem, however this is quite an externally visible change.

These dependencies may not even be visible in external signatures (more about that in [[TypesOfDependencies]])! They may only be needed during the runtime, internally, still changing them constitutes an [[API]] change. Imagine, that users of your API are using your library in some version and it works fine with just plain [[JDK]]. Suddenly, in newer release, you decide to change the library internals and depend on some other library, for example [[wikipedia::Jakarta_Commons|Jakarta Commons]]. That immediately means every user, who migrates to new version of your library, needs to include a version of [[wikipedia::Jakarta_Commons|Jakarta Commons]] in own application as well. This may or may not be a problem, however this is quite an externally visible change.

As the [[Determining What Makes a Good API|Chapter 3]] defines [[API]]s as everything that is externally visible, it makes sense to include ''shared library dependencies'' into the family of various types of [[API]]s. In spite the fact, that it is very hidden kind of [[API]], it is in fact one of the highest level kind and the [[API]] that we deal with the most during our day to day work.

As the [[Determining What Makes a Good API|Chapter 3]] defines [[API]]s as everything that is externally visible, it makes sense to include ''shared library dependencies'' into the family of various types of [[API]]s. In spite the fact, that it is very hidden kind of [[API]], it is in fact one of the highest level kind and the [[API]] that we deal with the most during our day to day work.

--[[User:JaroslavTulach|JaroslavTulach]] 10:47, 16 June 2008 (UTC)

=== Specifying [[Dependencies]] ===

 

[[Category:APITypes]]

[[Category:APITypes]]

== Dependencies ==

== Dependencies ==

Not many APIs can live alone, without support from other parts of the system. As each library, also APIs have own environment, which defines what needs to be available around to allow the API to function properly. Each user of the shared library needs to recreate proper environment, that means to satisfy library dependencies, before the API can be used. This implies, that the ''dependencies of a library'' form an important API of the shared library itself.

Not many [[API]]s can live alone, without support from other parts of the system. As each library, also [[API]]s have own environment, which defines what needs to be available around to allow the [[API]] to function properly. Each user of the shared library needs to recreate proper environment, that means to satisfy library dependencies, before the [[API]] can be used. This implies, that the ''dependencies of a library'' form an important [[API]] of the shared library itself.

These dependencies may not even be visible in external signatures (more about that in [[TypesOfDependencies]])! They may only be needed during the runtime, internally, still changing them constitutes an API change. Imagine, that users of your API are using your library in some version and it works fine with just plain JDK. Suddenly, in newer release, you decide to change the library internals and depend on some other library, for example [[wikipedia::Jakarta_Commons|Jakarta Commons]]. That immediately means every user, who migrates to new version of your library, needs to include a version of [[wikipedia::Jakarta_Commons|Jakarta Commons]] in own application as well. This may or may not be a problem, however this is quite an externally visible change.

These dependencies may not even be visible in external signatures (more about that in [[TypesOfDependencies]])! They may only be needed during the runtime, internally, still changing them constitutes an [[API]] change. Imagine, that users of your API are using your library in some version and it works fine with just plain [[JDK]]. Suddenly, in newer release, you decide to change the library internals and depend on some other library, for example [[wikipedia::Jakarta_Commons|Jakarta Commons]]. That immediately means every user, who migrates to new version of your library, needs to include a version of [[wikipedia::Jakarta_Commons|Jakarta Commons]] in own application as well. This may or may not be a problem, however this is quite an externally visible change.

As the [[Determining What Makes a Good API|Chapter 3]] defines APIs as everything that is externally visible, it makes sense to include ''shared library dependencies'' into the family of various types of APIs. In spite the fact, that it is very hidden kind of API, it is in fact one of the highest level kind and the API that we deal with the most during our day to day work.

As the [[Determining What Makes a Good API|Chapter 3]] defines [[API]]s as everything that is externally visible, it makes sense to include ''shared library dependencies'' into the family of various types of [[API]]s. In spite the fact, that it is very hidden kind of [[API]], it is in fact one of the highest level kind and the [[API]] that we deal with the most during our day to day work.

--[[User:JaroslavTulach|JaroslavTulach]] 10:47, 16 June 2008 (UTC)

--[[User:JaroslavTulach|JaroslavTulach]] 10:47, 16 June 2008 (UTC)

[[Category:APITypes]]

[[Category:APITypes]]

Not many APIs can live alone, without support from other parts of the system. As each library, also APIs have own environment, which defines what needs to be available around to allow the API to function properly. Each user of the shared library needs to recreate proper environment, that means to satisfy library dependencies, before the API can be used. This implies, that the ''dependencies of a library'' form an important API of the shared library itself.

Not many APIs can live alone, without support from other parts of the system. As each library, also APIs have own environment, which defines what needs to be available around to allow the API to function properly. Each user of the shared library needs to recreate proper environment, that means to satisfy library dependencies, before the API can be used. This implies, that the ''dependencies of a library'' form an important API of the shared library itself.

These dependencies may not even be visible in external signatures! They may only be needed during the runtime, internally, still changing them constitutes an API change. Imagine, that users of your API are using your library in some version and it works fine with just plain JDK. Suddenly, in newer release, you decide to change the library internals and depend on some other library, for example [[wikipedia::Jakarta_Commons|Jakarta Commons]]. That immediately means every user, who migrates to new version of your library, needs to include a version of [[wikipedia::Jakarta_Commons|Jakarta Commons]] in own application as well. This may or may not be a problem, however this is quite an externally visible change.

These dependencies may not even be visible in external signatures (more about that in [[TypesOfDependencies]])! They may only be needed during the runtime, internally, still changing them constitutes an API change. Imagine, that users of your API are using your library in some version and it works fine with just plain JDK. Suddenly, in newer release, you decide to change the library internals and depend on some other library, for example [[wikipedia::Jakarta_Commons|Jakarta Commons]]. That immediately means every user, who migrates to new version of your library, needs to include a version of [[wikipedia::Jakarta_Commons|Jakarta Commons]] in own application as well. This may or may not be a problem, however this is quite an externally visible change.

As the [[Determining What Makes a Good API|Chapter 3]] defines APIs as everything that is externally visible, it makes sense to include ''shared library dependencies'' into the family of various types of APIs. In spite the fact, that it is very hidden kind of API, it is in fact one of the highest level kind and the API that we deal with the most during our day to day work.

As the [[Determining What Makes a Good API|Chapter 3]] defines APIs as everything that is externally visible, it makes sense to include ''shared library dependencies'' into the family of various types of APIs. In spite the fact, that it is very hidden kind of API, it is in fact one of the highest level kind and the API that we deal with the most during our day to day work.