Excursus on Deployment and Service Models

Different Service Models: Who is responsible for what?

Service Models characterize the specific division of work or labor within the IT stack. Figure 6 shows six IT stacks each referring to a specific Service Model (top of figure). The division of labor is indicated by different color of the elements in the IT stack (center of figure). The figure does not cover cloud computing services only, since the term Service Model is used for IT service provision in general.

Figure 6: Service Models: division of work within the IT stack (source: [8])

The figure gives a general overview about some existing Service Models in regard to whom (user organization or IT service provider) is responsible for what part of the IT stack. The user organization is responsible for the bright/blue-green areas (shown on the left-hand side of the figure). The IT service provider is responsible for the dark/red areas (shown on the right-hand side of the figure). Further, this figure shows which Service Model is typically based on dedicated systems and shared systems.

The three models on the left are services which the IT service provider provides for IT components/systems that are owned by the user organization (often called on-premise services). Refer to Figure 6. The three models on the right refer to cloud computing:

  • Infrastructure-as-a-Service (IaaS),
  • Platform-as-a-Service (PaaS), and
  • Software-as-a-Service (SaaS).

In their plain form these Service Models simply describe what party (CSC or CSP) is supplying and therefore taking responsibility for the IT components running on computer systems which are provided by the Cloud Service Provider (CSP). In case of bare metal virtualization, the “IT components running on computer systems” constitute the Virtual Machines (VM).

This structure is not very granular. Also, management activities (updates, monitoring etc.) are not assigned at all. That’s why we extend the meaning of Service Model [8]. In the wider sense, the Service Model shall also determine the division of work and assignment of responsibility during implementation and operations including the IT service management activities as stipulated in ITIL® and ISO/IEC 20000 [9].

Performing e.g. maintenance activities requires access to components in the IT infrastructure. Consequently, different service models require different access rights, connectivity etc. This shall also be considered by the Service Model (provided that different assignments are possible). Note that these aspects are not shown in Figure 6.

An important difference between the different cloud Service Models is the source of the IT components constituting the Virtual Machines (VM) and/or which are used by the VMs. The extended Service Model also requires defining which party is responsible for and doing what activity in the overall lifecycle from provisioning and configuration all the way to the operations phase (covering management activities including incident management etc.) till decommissioning and/or transfer or deletion of data. Also refer to Figure 3. Note that, depending on this division of labor, the cloud computing infrastructure may need to be architected differently. Even performing monitoring tasks and also executing management activities require having access which in turn may require a specific form of connectivity.