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| <h1 class="page">Dependency Manager - Background</h1> |
| <div class="sect1"> |
| <h2 id="_background"><a class="anchor" href="#_background"></a>Background</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>In an OSGi framework, services are deployed using bundles and these bundles feature two types of dependencies:</p> |
| </div> |
| <div class="olist arabic"> |
| <ol class="arabic"> |
| <li> |
| <p>Package dependencies. |
| A bundle can export a package which others import. |
| These dependencies, although dynamic, are relatively easy to handle and the whole resolution process is handled by the OSGi framework for you.</p> |
| </li> |
| <li> |
| <p>Service dependencies. |
| Services, implemented by components inside of bundles, can have their own life cycle within that of their containing bundle and therefore can be registered and unregistered at any time. |
| Other components often depend on these services and need to deal with changes in their availability.</p> |
| </li> |
| </ol> |
| </div> |
| <div class="paragraph"> |
| <p>When you look at dependency management, there are two aspects you need to take into account:</p> |
| </div> |
| <div class="paragraph"> |
| <p>The first is managing software configurations. |
| This means you need to manage the dependencies from a configuration standpoint. |
| What you are managing are bundles, since those are the units of deployment. |
| What you need to manage are the package and service dependencies between bundles. |
| Package dependencies are always visible by examining the bundle manifest and when a bundle is installed the framework will try to resolve such dependencies before that bundle can even be started. |
| Service dependencies are only optionally described in the manifest by a list of services a bundle might export as well as a list it might use (import). |
| The words 'optionally' and 'might' already indicate that these aren’t things we can depend on. |
| Even worse, these keywords have by now been deprecated. |
| Besides that, the framework doesn’t have to perform any checks on these attributes.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The second is managing service dependencies at runtime. |
| As mentioned before, a service oriented architecture is dynamic by design, so your implementation should be able to deal with this. |
| Bundles can start in any order and any service can go away or be replaced by a different implementation at any point in time. |
| OSGi itself offers basic assistance for tracking services. |
| You can track them yourself by registering as a service listener. |
| A slightly more advanced way is to create a service tracker, which you can subsequently query, or have it notify you on changes. |
| All of these are too low-level to be good building blocks for developers.</p> |
| </div> |
| <div class="paragraph"> |
| <p>In real implementations, you are probably going to track multiple services. |
| Using service trackers in such a scenario has the tendency to result in dependency logic that is entangled in the implementation instead of being expressed in a declarative way. |
| Using a declarative way to specify dependencies has clear advantages when it comes to monitoring and managing them, a task that becomes more and more important in modern, federated, service oriented environments.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The Dependency Manager provides you with the right building blocks to declaratively specify dependencies using a straightforward Java API that is easy to maintain and refactor.</p> |
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| </div> |
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| <div class="sect1"> |
| <h2 id="_design_goals"><a class="anchor" href="#_design_goals"></a>Design Goals</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>The goals that drove the design of the dependency manager are:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>Provide a clean separation between a component implementation and the "glue" that binds it to the OSGi framework. |
| The component implementation should not have to contain any OSGi specific code. |
| In other words, it should be a POJO (Plain Old Java Object).</p> |
| </li> |
| <li> |
| <p>Minimize the amount of code that needs to be written. |
| The specification and management of dependencies should be automated as much as possible, whilst still providing enough flexibility to customize the system.</p> |
| </li> |
| <li> |
| <p>Be extensible. |
| Even though the core bundle provides a lot of different dependencies already, you should be able to add your own types of dependencies easily.</p> |
| </li> |
| <li> |
| <p>Easy to monitor and debug. |
| Being a dynamic system, it’s important to be able to see what the state of all components and dependencies are at any point in time.</p> |
| </li> |
| <li> |
| <p>Supporting powerful high level design patterns. |
| When building real-world applications, more complex patterns need to be used, such as aspects and adapters. |
| Support for these needs to be built into the core.</p> |
| </li> |
| </ul> |
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