4. Deployment Architecture

The SpringSource dm Server offers several choices when it comes to deploying applications. Each choice offers certain advantages, and it is important to understand those in order to make the right choice for your application. In this chapter, we take a closer look at the choices offered, compare them, and provide guidelines in choosing the right one based on your specific needs.

The dm Server supports standard self-contained WAR files thus allowing you to use the SpringSource dm Server as an enhanced web server. The dm Server also supports the Shared Libraries WAR format which allows for slimmer WAR files that depend on OSGi bundles instead of including JAR files inside the WAR. The Shared Services WAR format allows developers to further reduce the complexity of standard WARs by deploying services and infrastructure bundles alongside the WAR. A shared services WAR will then consume the services published by those bundles. To complete the picture, the dm Server supports a new OSGi-based Web Module deployment format for web applications that builds on the benefits provided by a shared services WAR and provides additional conveniences for developing and deploying Spring MVC based web applications.

For applications consisting of multiple bundles and web applications, the PAR format is the primary deployment model which takes advantage of OSGi capabilities. We will explore all of these formats and their suitability later in this guide.

4.1 Supported Deployment Formats

The SpringSource dm Server supports applications packaged in the following formats:

When you deploy an application to the dm Server, each deployment artifact (e.g., a single bundle, WAR, or PAR) passes through a deployment pipeline. This deployment pipeline supports the notion of personality-specific deployers which are responsible for processing an application with a certain personality (i.e., application type). The 1.0.2.RELEASE release of the dm Server natively supports personality-specific deployers analogous to each of the aforementioned packaging options. Furthermore, the deployment pipeline can be extended with additional personality deployers, and future releases of the dm Server will provide support for personalities such as Batch, Web Services, etc.

Let's take a closer look now at each of the supported deployment and packaging options to explore which one is best suited for your applications.

4.1.1 Raw OSGi Bundles

At its core, the SpringSource dm Server is an OSGi container. Thus any OSGi-compliant bundle can be deployed directly on the dm Server unmodified. You'll typically deploy an application as a single bundle or a set of stand-alone bundles if you'd like to publish or consume services globally within the container via the OSGi Service Registry.

4.1.2 WAR Deployment Formats

For Web Application Archives (WAR), the SpringSource dm Server provides support for the following three formats.

Each of these formats plays a distinct role in the incremental migration path from a standard Java EE WAR to an OSGi-ified web application. Standard WAR

Standard WAR files are supported directly in the dm Server. At deployment time, the WAR file is transformed into an OSGi bundle and installed into Tomcat. All the standard WAR contracts are honored, and your existing WAR files should just drop in and deploy without change. Support for standard, unmodified WAR files allows you to try out the SpringSource dm Server on your existing web applications and then gradually migrate toward the Shared Libraries WAR , Shared Services WAR , and Web Module formats.

In addition to the standard support for WARs that you would expect from Tomcat, the dm Server also enables the following features:

  1. Spring-driven load-time weaving (see Section 6.8.4, “Load-time weaving with AspectJ in the Spring Framework").

  2. Diagnostic information such as FFDC (first failure data capture)

The main benefit of this application style is familiarity -- everyone knows how to create a WAR file! You can take advantage of the dm Server's added feature set without modifying the application. The application can also be deployed on other Servlet containers or Java EE application servers.

You may choose this application style if the application is fairly simple and small. You may also prefer this style even for large and complex applications as a starting point and migrate to the other styles over time as discussed in Chapter 6, Migrating to OSGi . Shared Libraries WAR

If you have experience with developing and packaging web applications using the standard WAR format, you're certainly familiar with the pains of library bloat. So, unless you're installing shared libraries in a common library folder for your Servlet container, you have to pack all JARs required by your web application in /WEB-INF/lib . Prior to the release of the SpringSource dm Server, such library bloat has essentially been the norm for web applications, but now there is a better solution! The Shared Libraries WAR format reduces your application's deployment footprint and eradicates library bloat by allowing you to declare dependencies on libraries via standard OSGi manifest headers such as Import-Package and Require-Bundle . The dm Server provides additional support for simplifying dependency management via the Import-Library and Import-Bundle manifest headers which are essentially macros that get expanded into OSGi-compliant Import-Package statements.


For detailed information on which libraries are already available, check out the SpringSource Enterprise Bundle Repository . Shared Services WAR

Once you've begun taking advantage of declarative dependency management with a Shared Libraries WAR, you'll likely find yourself wanting to take the next step toward reaping further benefits of an OSGi container: sharing services between your OSGi-compliant bundles and your web applications. By building on the power and simplicity of Spring-DM, the Shared Services WAR format puts the OSGi Service Registry at your finger tips. As a best practice you'll typically publish services from your domain, service, and infrastructure bundles via <osgi:service ... /> and then consume them in your web application's ApplicationContext via <osgi:reference ... />. Doing so promotes programming to interfaces and allows you to completely decouple your web-specific deployment artifacts from your domain model, service layer, etc., and that's certainly a step in the right direction. Of the three supported WAR deployment formats, the Shared Services WAR is by far the most attractive in terms of modularity and reduced overall footprint of your web applications.

4.1.3 Web Modules

Above and beyond WAR-based deployment formats, the SpringSource dm Server introduces a deployment and packaging option for OSGi-compliant web applications, the Web Module format. Web modules have a structure similar to a Shared Services WAR and can therefore take full advantage of all three WAR deployment formats. In addition, web modules benefit from reduced configuration for Spring MVC based applications via new OSGi manifest headers such as Web-DispatcherServletUrlPatterns and Web-FilterMappings. For further details on these and other Web-* manifest headers, please consult Section, “Web Module Manifest Headers”.

If you're building a Spring MVC based web application as a web module, you won't need to worry about configuring a root WebApplicationContext or an ApplicationContext for your DispatcherServlet. Based on metadata in your web module's /META-INF/MANIFEST.MF, the dm Server will auto-generate an appropriately configured web.xml for you on-the-fly, and your application will use the ApplicationContext created for your web module by Spring-DM. Future releases of the dm Server will add additional support to simplify configuration of Spring Web Flow based web applications as well.

4.1.4 PAR

A PAR is a standard JAR which contains all of the modules of your application (e.g., service, domain, and infrastructure bundles as well as a WAR or web module for web applications) in a single deployment unit. This allows you to deploy, refresh, and undeploy your entire application as a single entity. If you are familiar with Java EE, it is worth noting that a PAR can be considered a replacement for an EAR (Enterprise Archive) within the context of an OSGi container. As an added bonus, modules within a PAR can be refreshed independently and on-the-fly, for example via the SpringSource dm Server Tool Suite (see Chapter 8, Tooling).

Many of the benefits of the PAR format are due to the underlying OSGi infrastructure, including:

  • Fundamentally modularized applications: instead of relying on fuzzy boundaries between logical modules in a monolithic application, this style promotes physically separated modules in the form of OSGi bundles. Then each module may be developed separately, promoting parallel development and loose coupling.

  • Robust versioning of various modules: the versioning capability offered by OSGi is much more comprehensive than any alternatives. Each module can specify a version range for each of its dependencies. Bundles are isolated from each other in such a way that multiple versions of a bundle may be used simultaneously in an application.

  • Improved serviceability: each bundle may be deployed or undeployed in a running application. This allows modifying the existing application to fix bugs, improve performance, and even to add new features without having to restart the application.

Furthermore, PARs scope the modules of your application within the dm Server. Scoping provides both a physical and logical application boundary, effectively shielding the internals of your application from other PARs deployed within the dm Server. This means your application doesn't have to worry about clashing with other running applications (e.g., in the OSGi Service Registry). You get support for load-time weaving, classpath scanning, context class loading, etc., and the dm Server does the heavy lifting for you to make all this work seamlessly in an OSGi environment. If you want to take full advantage of all that the SpringSource dm Server and OSGi have to offer, packaging and deploying your applications as a PAR is definitely the recommend choice.

[Tip]OSGi != multiple JARs

Note that while physically separated modules can, in theory, be implemented simply using multiple JARs, complex versioning requirements often make this impractical. For example, consider the situation depicted in the diagram below.

  • Bundle A depends on version 1.0.0 of bundle B and version 2.0.0 of bundle C.

  • Bundle B depends on version 1.0.0 of bundle C.

Suppose that versions 1.0.0 and 2.0.0 of bundle C are neither backward nor forward compatible. Traditional monolithic applications cannot handle such situations: either bundle A or bundle B would need reworking which undermines truly independent development. OSGi's versioning scheme enables this scenario to be implemented in a robust manner. If it is desirable to rework the application to share a single version of C, then this can be planned in and is not forced.