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10 7 CAST platform deployment model The way in which solutions and customisations are deployed on the platform differs, depending on the artefacts/code that come with their associated apps, and how the latter are deployed to the runtime infrastructure. Within CAST, we distinguish between three types of apps with respect to their method of deployment: purely declarative apps (type 1), apps that involve code execution at the client-side (type 2), and apps that involve code execution at both client-side and server-side (type 3). The increasing numbering of app types denotes increasing capabilities but also increased deployment complexity. This concept applies not only to apps, but to app extensions as well. 7.1 Type 1: Purely declarative apps A major objective for the CAST platform is to promote declarative configurations over native code. If an app is created using declarative specifications only, it is classified as type 1. Such apps can be deployed during normal execution of the CAST platform, without the need to stop and resume any subsystems or affect any active users. During deployment to the platform’s runtime infrastructure, a type 1 app comprises the following (not necessarily all) types of XML-based artefacts: • Database schema descriptions, for extending already existing data object types or defining new ones. • Declarative specifications of data validators employed to enforce constraints on data object types. • Definitions of custom UI elements (XAML-based), page flows, and mappings from page types to forms. • Extensions to business logic through definitions of data-centric rules, or declarative specifications of invocations to external Web services. • Definitions of translation/localisation resources. • Specifications of app extension points. In addition, each app comes with a manifest that describes additional important properties, such as dependencies on other apps, version information, provider information, licensing information, pricing information, etc. 7.2 Type 2: Client-side code apps Type 2 apps offer all of the possibilities that are provided by apps of type 1, and in addition, include the possibility to use native code which is deployed and executed at the client’s machine. In the CAST platform, this is C# code that is intended for runtime deployment to a Silverlight runtime instance within the client’s browser (making use of Silverlight’s ability for dynamic loading of libraries from a Web server). This allows the deployment and execution of customised command handlers
11 for implementing new custom client logic, customised Model/ViewModel implementations and new custom controls. Since the presentation layer of the CAST platform executes on the user's machine within a Silverlight plug-in, security risks are lower compared to code that runs within the CAST platform’s application servers. In case a type 2 app crashes, the only user that is affected is the one currently using the app. However, to prevent potential problems with bugs causing crashes to client machines and creating a negative impact with regard to the platform’s reputation, type 2 apps need to undergo a review and certification process by the platform provider for quality assurance. Due to the presence of native code to be reviewed, this requires substantially more effort compared to conducting review and certification for purely declarative apps (type 1). 7.3 Type 3: Client-side and server-side code apps Type 3 apps enable customisation and extensibility to the fullest extent possible with the CAST platform. In addition to declarative customisations and native code to be executed at the client’s side, type 3 apps include the possibility to write native code to be executed by the CAST platform’s application servers. To that end, the CAST platform runtime defines dedicated extension points to enhance the existing functionality that is built-in to the platform. This further allows developers to implement customised business logic for their apps that is impossible to express using the platform’s facility for data-centric rules, and providing totally new business operations that are registered within the CAST platform application servers and can be called from clients upon request. Since the server-side code of type 3 apps runs within the same execution context as the core CAST platform, the latter’s stability can be seriously threatened. Thus, apps of type 3 can only be deployed to the CAST platform application servers by the platform provider. Prior to this deployment, type 3 apps need to be reviewed and certified, similarly to apps of type 2. The deployment of a type 3 app is a lengthy process. On the one hand, this is due to the review, testing and certification processes. On the other hand, a deployment of new native server code requires a shutdown of the application servers. Shutting down and restarting the platform can only happen when it is necessary to perform important maintenance activities, or to deploy new platform releases. 8 Conclusions The model of Platform as a Service (PaaS), whereby a computing platform and a software development stack are being offered as a combined service to prospective application/solution developers, represents an important trend within cloud computing today. The PaaS model carries a great number of benefits for developers and PaaS providers alike. However, the design of a mature platform to support such a model to its fullest extent possible is a rather complex and challenging undertaking in which the majority of today’s PaaS providers face several difficulties.
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