By deploying their applications to PaaS, today's software organisations-and small and medium enterprises (SMEs) in particular-can achieve key competitive advantages due to their business agility, how quickly they market the products they offer, and their ability to reach their potential users. PaaS environments have the potential to let organisations increase their abilities to scale up and down-by adding or removing resources to a node in use-, or in and out-by adding or removing nodes to a system-; cut their costs; and shorten the time they spend to market their software solutions. PaaS offerings hide low-level details from the developer, such as the underlying infrastructure including load balancers, virtual machines, servers, data storage and access, and networks. The PaaS developing environment frees developers from software versioning or resource provision and lets them use extra built-in services and capabilities and write code faster. PaaS vendors provide development tools, libraries, and application programming interfaces (APIs) to let developers access their storage resources and deploy their applications to their runtime environment and execute them. Due to the faster PaaS application testing and deployment; development teams easily try different configurations of their applications to test them, assess their applications' performance, and identify potential compatibility issues [1]. From a business perspective, as enterprises outsource computation to PaaS vendors, they can focus on their core business interests and do it better [2].
Problem statement. As a result of the potential advantages offered by PaaS, the demand of PaaS environments has been increasing for the last years and the number of PaaS providers accordingly. A myriad of vendors supply many PaaS offerings at various levels of abstraction with different capabilities, runtime environments, services, and supported platforms. The huge amount of PaaS vendors together with the various PaaS offerings with many configurable parts makes for a lot of options for organisations to chose from. So many options that organisations find difficulties in selecting the right PaaS offering for their software system requirements and the goals they want to achieve by using PaaS environments. These goals range from achieving easier extensibility by using the services offered in PaaS environments to easing the deployment, including improving the application's security and portability. Enterprises can choose from PaaS offerings with various security protocols, QoS guarantees, trust issues, tools form application migration and porting, followed standards, and cost models. SMEs' PaaS migration criteria might lead them to want to select the most reliable, performant, well-reputed, or cost-efficient offering [3]. Likewise, they might simply be interested in using the one which meets their technical requirements without much adaptation or developing effort. It is not obvious how to compare PaaS offerings according to these criteria or dimensions [4]. Additionally, it is not evident how to identify the potential of a PaaS offering over another one, especially on the long term. There is a lack of standards and metrics to identify differences and compare PaaS offerings [5].
The six dimensions identified in [4] to compare PaaS offerings are not independent. As a result some dimensions or sub-dimensions oppose others in some cases. Organisations need assistance to incorporate all this knowledge into the decision-making process involved in migrating applications to PaaS. Additionally, enterprises may face a well-known problem associated with the PaaS model, the vendor lock-in problem when they want to switch from one PaaS vendor to another one. The inability of some PaaS vendors to comply to standards hinders the application and data portability. But even if portability is supported in theory, the high complexity of the process if vendors do not facilitate it and the switching costs still discourage organisations from porting their applications [6]. Organisations want to be able to switch easily and transparently between PaaS providers whenever they need. They might want to move to a different provider due to a Service Level Agreement breach (SLA), a cost increase, or simply because the vendor went out of business. At the same time, organisations do not want to set their data and applications at risk in case they have to port their ap-plications to a different PaaS environment. Typical risky scenarios include data loss or inability to run the application on a different platform.
Related work. The migration of systems and applications to cloud-enabled environments attracts a lot of attention from the research community [7] [3]. The REMICS FP7 project [8] reuses and migrates legacy applications to interoperable cloud services by using concepts from the Model-Driven Architecture and Architecture-Driven Modernization worlds. They leverage cloud computing to provide cost-efficient and reliable services. However, unlike InCLOUDer, they focus only on the technical dimension of the migration but not on the organisational, policy-and business-related aspects as well as recommended by the literature [9] [10]. authors of [11] presented a process framework to migrate legacy software to cloud environments based on their experiences in migrating a legacy open source system. They propose seven distinct activities for cloud migration including the cloud selection but they do not cope with application's constraints related to data privacy or other organisation's criteria.
Goal. This paper describes the process followed by InCLOUDer [12] to migrate applications to PaaS environments and the meta models that support the process. For the first time, this paper formalises and explains the meta models used by InCLOUDer and makes contribution on showing how they work when taking into account the explained six PaaS dimensions. More specifically, In-CLOUDer uses meta models for application profiling, organisations PaaS criteria description, and PaaS offerings selection. Prior to migration, this paper addresses PaaS selection in respect of six PaaS identified dimensions [4]-the application, SLA, deployment, security, portability and usability dimensions-with emphasis in achieving portability to increase the level of service and minimise vendor lock-in. InCLOUDer assist the PaaS selection and supports the decision on how to adapt the application to the selected PaaS configuration.
The InCLOUDer decision support system assists the migration of applications to PaaS environments. Fig. 1 shows a high-level description of InCLOUDer and its three input models marked with red arrows. The organisation migrating the application to the PaaS specifies both the organisation's criteria to PaaS migration-which weights each of the six PaaS migration dimensions in relation to the importance of each one of them for the organisation; and the application profile capturing the application's requirements, constraints, and architecture according to the meta model in Fig. 3. The third input model lets PaaS vendors describe their PaaS offerings-or modellers if they do not-according to the meta model in Fig. 2.
Fig. 1 presents three of the mechanisms used in InCLOUDer, namely the InCLOUDer application components re-scatterer, the InCLOUDer alternatives generator, and the InCLOUDer alternatives selector. The In-CLOUDer application components re-scattering and the InCLOUDer alternatives generator take into account the application profile and the organ- The meta model shown in Fig. 3 lets organisations specify their application's requirements, constraints, and architecture. Organisations can model their application's data and application components, specify in which nodes these components run, and how these components are connected. Additionally, organisations can model the application's requirements and constraints.
Surajbali and Juan-Verdejo described the six PaaS migration dimensions with more detail in [4]. Once the organisation specifies the organisation's PaaS migration criteria and the two input models based on the meta models shown in Fig. 2 and Fig. 3. InCLOUDer generates the alternatives to migrate to PaaS consisting in a model describing-according to the meta model presented in