- Open Access
Contextualization: dynamic configuration of virtual machines
© Armstrong et al. 2015
- Received: 31 March 2015
- Accepted: 30 June 2015
- Published: 15 July 2015
New VM instances are created from static templates that contain the basic configuration of the VM to achieve elasticity with regards to capacity. Instance specific settings can be injected into the VM during the deployment phase through means of contextualization. So far this is limited to a single data source and data remains static throughout the lifecycle of the VM.
We present a layered approach to contextualization that supports different classes of contextualization data available from several sources. The settings are made available to the VM through virtual devices. Inside each VM data from different classes are layered on top of each other to create a unified file hierarchy.
Context data can be modified during runtime by updating the contents of the virtual devices, making our approach the first contextualization approach to natively support recontextualization. Recontextualization enables runtime reconfiguration of an executing service and can act as a trigger and key enabler of self-* techniques. This trigger provides a service with a mechanism to adapt or optimize itself in response to a changing environment. The runtime reconfiguration using recontextualization and its potential gains are illustrated in an example with a distributed file system, demonstrating the feasibility of our approach.
- Cloud Computing
- Virtual machine
One of the key characteristics of cloud computing is rapid elasticity ; the ability to quickly provision or release resources assigned to a cloud service in order to respond to current demand. In the Infrastructure as a Service (IaaS) model, cloud services are normally comprised of a set of different components, each defined using a Virtual Machine (VM) template. To achieve elasticity, the capacity of a cloud service can be adapted during runtime by adjusting the number of running VM instances of each template. This makes it possible to scale each part of the service independently.
Each VM instance started from a template needs to be customized with some unique settings, e.g., networking configuration to ensure each instance is assigned a unique IP-address. The settings need to be applied dynamically, normally done as part of the VM boot process. This boot-time customization process is called contextualization [2, 3].
Several different usage scenarios that join isolated cloud infrastructures together into a larger unified infrastructure are being considered [4, 5]. Conceptually, a virtual market of cloud resources not limited by technological boundaries would offer diversity in terms of (but not limited to) pricing, availability, and a choice of different geographical locations to use for hosting. Cloud infrastructures commonly offer supporting services such as network based storage, and today’s major infrastructure providers (IPs) such as Amazon wrap the infrastructure specific functionality into pre-configured VM templates. In a multi-cloud scenario the VM templates need to be generic enough to be deployed across a wide range of infrastructures. Therefore, the contextualization stage is needed to support interactions with any infrastructure-specific services or settings.
VMs can be migrated between physical hosts without being restarted using live-migration. This enables an IP to re-distribute the current VM load across available server resources during runtime. Because the VM is not restarted during the migration, the contextualization procedure is not triggered post-migration. Therefore, any VM customization (such as loading infrastructure specific settings) will remain unchanged.
In our earlier work  we present and define recontextualization, an emerging technology to allow contextual data inside running VMs to be updated during runtime. This work extends on earlier work on recontextualization and contextualization  and presents a novel layered based approach to contextualization. In this approach, multiple independent sets of contextual data are merged into a unified structure realized by a custom file system whose data sets can be dynamically added or removed during runtime. This paper presents the requirements, design, and evaluation of a system that support this multi-layered approach to contextualization, which is the first contextualization solution to natively support recontextualization. It is worth noting that the research presented in this paper uses a scientific methodology based on functional prototyping and system performance analysis. We do not use mathematical modelling. Additionally and for clarity’s sake, the results of this research aims to facilitate and enhance current cloud provider interoperability beyond the state of the art but we do not claim to solve all problems of VM or IaaS interoperability.
The rest of this article is organized as follows; In Section ‘Background and related work’ the background of topics related to the field of contextualization and recontextualization are presented. Section ‘Contextualization’ presents an overview of contextualization including challenges and a summary of our earlier work on the subject. The corresponding information for recontextualization is presented in Section ‘Recontextualization’. Section ‘Context-aware lifecycle management’ shows how the two techniques are used in conjunction to provide adaptable usage throughout the lifecycle of a service. The unified approach is demonstrated and evaluated in Section ‘Functional evaluation’. Finally, conclusions and future work are presented in Section ‘Conclusions and future work’.
Our research on contextualization of cloud applications started with an evaluation into the limitations of the current cloud infrastructures. It quickly became apparent that, although virtualization brings several key benefits and is a critical enabler of cloud computing, it also increases the complexity of managing an application deployed in a cloud.
Modern virtualization technologies enable rapid provisioning of VMs and thus allow cloud services to scale up and down on-demand. This elasticity [26, 27] comes with a new set of challenges for dynamic service configuration. The focus of this research is on horizontal elasticity where scaling is achieved by adding or removing VMs to a service during its operation. The related case of vertical elasticity, i.e. application scaling through VM resizing, is much easier from a contextualization perspective. For horizontal elasticity scenarios, predefined yet flexible contextualization mechanisms enable the manipulation of VM images during the development of a software service. Previously, this required the complex and time consuming configuration of software within base images.
For clarity, our definition of contextualization is as follows:
Contextualization. The autonomous configuration of individual components of an application and supporting software stack during deployment to a specific environment. 
In this definition, an environment is considered to be the specific composition of underlying physical and/or virtual hardware in addition to any value added services provisioned by an IP.
There are a number of challenges that make the contextualization of cloud services a non-trivial affair. In our previous work we argued that contextualization in cloud computing is a highly pervasive key technological requirement of any cloud service, where elastic resource management is critical to the on-demand scalability of a service . The holistic nature of the services deployed on clouds makes it difficult to provide flexible generic and open tools without limiting the heterogeneity of supported services. We identified three inherent challenges to providing elasticity through contextualization where VMs are added and removed during service operation.
SaaS: web-based applications and services hosted on-line, usually reachable from standard interfaces such as a web browser.
PaaS: systems that offer deployment of applications designed for execution on a specific platform or software environment. For example, the opportunity to upload a Java archive and have it executed on remote resources.
IaaS: infrastructures that offer the provisioning of remote resources on which the consumer can execute arbitrary software as if the resources were dedicated hardware servers. Virtualization is usually used as a layer between the hardware and the consumer software systems, which makes it possible for the consumer to design the software stack from the operating system upwards.
In this work, we focus on IaaS contextualization of VMs comprising a cloud service. We touch upon using the suggested approach to access platform software services such as network based storage or database services that could be considered stand-alone PaaS services. In IaaS, the challenge pertains to low-level contextualization of virtual resources, as found in IPs, where virtual devices require context to enable VMs to be bootstrapped to existing virtual infrastructures. This approach has been partially explored by RESERVOIR .
The second challenge to overcome is contextualization across multiple IaaS domains for reasons of interoperability. Many IaaS providers offer platform services that are not interoperable with those of other providers. Also, as contextualization of VMs is performed as part of service development, the service will be customized to a single provider only. To solve this challenge, we need to extend contextualization to support run-time recontextualization.
The third challenge pertains to a set of functional requirements for real world clouds and their impact on contextualization. Notable among these are end-to-end security through contextualization mechanisms that support a Virtual Private Network (VPN) overlay and software license management systems. Both of these have unique contextualization requirements: contextualization must be secure with no VPN keys stored unless in use; and contextualization that is able to accommodate license protected software and licensing tokens.
In our previous work  we discuss the nature of contextualization in the light of the OPTIMIS project . We present details of the architecture and implementation of our tool for the contextualization of platform level services as well as virtual infrastructure. We discuss the implication of contextualization in clouds, the motivation behind our work and suggest a landscape for the evolution of contextualization tools across all classes of clouds within the ecosystem of the future. We contribute to both the image and instance level contextualization of VM’s and illustrate the potential effectiveness of our tool through a simple use case.
In addition, as part of our previous work on contextualization, a proof of concept contextualization tool was developed and its performance tested. This prototype was used to confirm the validity of our contextualisation approach on a cloud testbed. This provided evidence on the potential performance of our approach for contextualization with regards to preparing generic VM images of sizes in the range of 1-5 GB in increments of 1 GB and with varying numbers of concurrent user requests from 10-100, for the purpose of creating ISO CD images containing 1 MB of context data. The results showed adequate scalability and response time over ten iterations of the experiment with minimal variance.
Recontextualization can be used to adapt a running service to any system changes, including making newly migrated VMs operate properly in the (potentially different) system environment of a new host. A definition of recontextualization is as follows:
Recontextualization. The autonomous updating of configuration for individual components of an application and supporting software stack during runtime for the purpose of adapting to a new environment .
In this definition, a new environment is considered to be a change in the underlying physical or virtual hardware, for example when a VM is migrated from one host to another. In addition, changes in infrastructure or platform level services that are in active use by a cloud application would also be considered to be a new environment.
An analogy to recontextualization can be made with printers and printer drivers. Imagine a laptop user moving from one physical location to another (i.e. a new environment) wanting to access a printer at the new location. Instead of manually installing a driver for an associated printer and having to configure the driver before the printing device is usable, the environment itself could detect that the laptop has entered the printer domain, and supply the needed software and settings to the laptop operating system automatically.
Using recontextualization, VM instances can be automatically offered settings and context data to adapt to a new execution environment. Without the support of recontextualization, adapting the instances to their environment may require a time-consuming manual process that limits the flexibility and scalability of the cloud. When migrating VM instances, parts of the virtual infrastructure (e.g., virtual networks, storage areas, databases) may have to be migrated as well. For example, assume that a cloud service is running on the resources of an IP and that it employs a network based storage service offered by that provider. When migrating to another IP, there are three main tasks; migrating the VM itself, relocating the data on the network based storage service to the corresponding service offered by the new IP (to avoid cross-network transfer costs), and ensuring that the VM is able to communicate with the new storage system without manual intervention. Recontextualization can be used to achieve the latter of these three.
The key benefits of our approach to recontextualization are: i) minimal changes to the existing cloud infrastructure, i.e. there is no need to make alterations to the hypervisor and ii) the preservation of security through the selection of a recontextualization mechanism that gathers contextualization data from a secured source. In addition to these benefits, there are a number of challenges that must be overcome before recontextualization can reach widespread adoption. These are due in part to a lack of IP interoperability and the difficulties in creating an approach to recontextualization that can be applied to the wide diversity of applications deployed into cloud computing environments.
A motivational factor behind the need for runtime recontextualization stems from VM migration in clouds [29, 30]. Using migration, a VM can be transferred from one physical host to another without explicitly shutting down and subsequently restarting the VM . The entire state of the VM, including e.g., memory pages, are transferred to the new host and the VM can resume its execution from its state prior to migration. As a consequence of this, no contextualization is triggered again when the VM is resumed, as the level of abstraction provided by virtualization is insufficient for platform services. In this research migration from and to identical hypervisor technology is considered. The topic of hypervisor interoperability for migration is out of scope but is discussed in work by Liu et al. . The idea of reconfiguring computing services is by all means not a new subject area. Mohamed et al.  present work on reconfiguration of Web Services during runtime. In their approach, remote APIs for modifying values in running services are compiled into Java-based Web Services and are made accessible using remote method invocation.
Bursting - The partial or full migration of an application to a third party IP. This may occur when local resources are near exhaustion.
Federation - The migration of an applications workload between a group of IPs. This normally occurs when a single IP’s resources are insufficient for maintaining the high availability of an application through redundancy.
Brokering - The automated migration of an application’s VMs across IPs. This is normally done for the purpose of maintaining an agreed Quality of Service (QoS) in the case of an end-user utilizing a broker to select an IP given a set of selection criteria.
These scenarios have also been used to guide the defining of requirements for any potential recontextualization mechanism. In the bursting scenario, if an IaaS provider is not obligated to divulge third party providers used for outsourcing of computational resources, an application may end up deployed on to a third party’s infrastructure that requires use of their local infrastructure services. A dynamic federation of IaaS providers created during negotiation time that alters during the operation phase requires infrastructure services to be discovered dynamically. The same is applicable in the case of a broker, knowledge of a provider’s local infrastructure services is not available during deployment until after the broker has selected a provider.
The following general requirements have been extracted from the scenarios and are considered as imperative:
A triggering mechanism for recontextualization on VM migration or other events.
A secure process to gather and recreate contextualization data after migration or other events.
A hypervisor agnostic solution that maintains IaaS provider interoperability.
An approach that is non-pervasive and minimizes modifications at the IaaS level.
The lack of knowledge on the attributes of an IaaS provider’s local infrastructure service available during deployment time further motivates this research. An example of such a service that exhibits configuration issues after resource migration is application-level monitoring and is discussed in detail in our previous work .
A recontextualization mechanism
With the requirements in mind, we have considered several approaches for mechanisms that would support recontextualization . An approach using dynamic virtual device mounting is found to be the most promising solution for recontextualization due to inherent interoperability and support in all major operating systems. Dynamic virtual device mounting is based on dynamically mounting virtual media containing newly generated content in a running VM via the reuse of existing hypervisor interfaces and procedures. Interoperability is achieved by reusing existing drivers for removable media such as USB disks or CD-ROM drives. Recontextualization can be detected by the guest OS by reacting to events triggered when new USB or CD-ROM media is available. The ability to manage virtual devices is also offered by the libvirt API , inferring that there is fundamental support for these operations in most major hypervisors.
The suggested approached has been implemented and the performance has been assessed to confirm the feasibility of the approach in a real cloud infrastructure. For all tests, libvirt version 0.9.9 is used to monitor and manage the VMs. QEMU-KVM version 1.0.50 and Xen version 4.0.0 are used as hypervisors, both running on the same hardware using CentOS 5.5 (final) with kernel version 188.8.131.52. The hosts used in these tests are on the same subnet, have shared storage and are comprised of a quad core Intel Xeon X3430 CPU 2.40 GHz, 4 GB DDR3 @ 1333 MHz, 1 GBit NIC and a 250 GB 7200 RPM WD RE3 HDD.
Based on the evaluation we conclude that the recontextualization process adds about an 18 % overhead using either hypervisor compared to normal migrations . For KVM, most of the extra time required for recontextualization is spent outside the bounds of our component. The delay is likely associated with processing events and extra overhead imposed by preparing migration with virtual devices attached. In the case of Xen the device management functionality in libvirt proved unreliable and we therefore had to bypass the libvirt API. We instead had to rely on sub-process calls from the recontextualizer to Xen using the xm utility. This workaround increases the time needed for recontextualization in the Xen case. The internal logic of the recontextualizer is discussed in full detail as part of Section ‘Design and architecture’ and within Procedure ??.
Contextualization versus recontextualization
Although contextualization and recontextualization share many similarities in that they help adapt services to running in the cloud, they do however differ in one fundamental way. Contextualization can only be used during deployment time to configure an application to a specific IP’s environment. Recontextualization, however, enables an application to be migrated freely between dissimilar environments during runtime through the use of an additional level of abstraction. One could consider recontextualization to be an extension of contextualization, and base contextualization of services during deployment (or development) is also needed to enable runtime recontextualization.
As discussed in Section ‘Service lifecycle’, a cloud service is prepared with configuration data built in during the definition phase. Complementing contextualization data, either generated or read from a repository, is injected by the infrastructure during deployment. Parts of the contextualization data may also be recontextualized during runtime. In this work, we strive to create a model and mechanism capable of managing any kind of configuration and contextualization data in a fail-safe manner that appears seamless to the software running inside as part of the service.
Web server configuration data is normally provided by the service developer, so this data would belong to the service class.
WAN network data is by necessity provided by the infrastructure, and would belong to the infrastructure class.
Other classes of data may include, e.g., legislative data that depends on the current legislation in which the service is hosted, or data injected by a broker during an advanced service deployment.
Defining a complete ontology of service classes and their relations is out of scope at this point, but is an important part of future work. The model presented in this paper supports an arbitrary number of service classes, and relies on an internal prioritization of the classes to determine which class should take precedence if the same data is available from multiple sources. The feasibility of the approach is illustrated using a use-case featuring a distributed file system. The use-case employs data from service and infrastructure classes, and infrastructure-class data is configured to have precedence in case of overlapping datasets.
A generic approach with support for several classes of data enables a service to be designed for generic cloud deployment with a set of default settings. These default settings may be complemented and partly replaced with settings provided by, e.g., the infrastructure to which the service is deployed. Through future formalization of the type of data offered by different actors, dynamic services with inherit support for self-* operations, e.g. self-configuration, can be constructed and deployed to the cloud.
For example, in previous work on recontextualization , a monitoring use case was used to illustrate how the infrastructure can update a setting used for internal processes in the VM during runtime. In this work, we illustrate how the procedure can be used at the service level to reconfigure the internal connections of a distributed file system, adapting its operation to a new geographic layout resulting from migration of VMs (see Section ‘Functional evaluation’).
Further use cases related to other aspects of self-* management are part of future work. For self-protection, the ability to change the security level of the software executing inside the VM depending on, e.g., the geographical location of the current IP is an interesting area of research. This could include adjusting the level of encryption of data streams passing in and out of the service component to manage the balance between security and performance. If the hosted service contains data only allowed to be hosted within certain legislations, recontextualization could be used to ensure that the operations of the service component is temporary suspended if the VM is (erroneously) migrated to a disallowed environment.
Design and architecture
As described in Section ‘A recontextualization mechanism’, earlier work explored several approaches to passing data between the hypervisor and the processes running inside the VM. The work in this paper retains the use of virtual devices as a transport mechanism, due to its native support in major hypervisors and operating systems, and does not depend on network access. This is a key feature, since network access is one of the primary things that contextualized settings can be used to enable. As previously mentioned, the core idea of our suggested approach is to dynamically generate customized ISO images which are made available to the running VM instance. As the ISO images can be freely changed (and re-mounted) during run-time, the context data can be updated at any point in the VM lifecycle.
Contextualizer offers API-level integration with the surrounding cloud control system to integrate the contextualization process in the workflow executed prior to deployment. The result of the contextualization call is an ISO image file customized to suit the running hypervisor with the necessary context data. Once completed the file is uploaded to a data repository from where it can be fetched later during the service deployment procedure.
Image service contains the code used for image generation that is shared between contextualization and recontextualization. This includes the coordination of calls to image creation, fabrication, and conversion.
ISO image creator manipulates a set of template ISO images. In addition, this sub-component is responsible for the creation of ISO images and the embedding of context data. The ISO Image Creator has access to a single set of context data processing scripts that are agnostic of a VM’s operating system, embedded in the ISO image and responsible for reading context data from the ISO image at run time. An internal interface provides access to contextualization scripts embedded in the VM image.
Image fabricator manipulates VM images, passed to it by the VM Contextualizer, using underlying system tools. This sub-component installs context data originating from a manifest. A set of prefabricated operating system specific contextualization scripts, from a data store, are embedded for each operating system with foreseen use at the IP level. The main function of these scripts are to set the context of operating system level components such as configuring the network for DHCP access, or connecting to a service-level VPN.
Image converter transforms images from one format to another for the purpose of supporting interoperability between IPs using different hypervisor technology. Depending on the hypervisor and operating system, conversion can require the changes made by the Image Fabricator to be reverted and reapplied. The Image Converter supports Xen, KVM, VMware, and VirtualBox hypervisors and has built-in support for a range of different image formats.
Recontextualizer is a stand alone component that monitors events related to VM domains during runtime. The recontextualizer is responsible for triggering the creation and association of new infrastructure class context data when applicable domains are migrated to the infrastructure. The recontextualizer is integrated with libvirt, and relies on libvirt’s unified approach to domain and device management to support runtime reactive virtual device management across a multitude of different hypervisors.
Figure 3 also includes the order in which components are called. From the figure it can be seen that the Contextualizer component is invoked by the VM Manger during application deployment (step 1) to create ISO images (steps 2, 3), create VM images (steps 4, 5) and/or manipulate VM images (steps 6, 7). After images have been created and/or manipulated, they are stored in a local data repository (step 8) for deployment by the VM Manager. During operation, if an event from the underlying hypervisor indicates that a VM has been stopped, started or migrated (step 9), alterations to the existing ISO images are made (steps 10, 11) and reinserted into the VM’s virtual device (step 12).
The flow of logic through the contextualizer component is as follows: context data is gathered from a range of sources, it is processed into a usable fashion for use within a VM, stored within an ISO image and finally associated with the virtual hardware of a domain for use during booting. The recontextualization mechanism (i.e. the internal logic), outlined in Procedure ??, registers interest with libvirt in a VM’s lifecycle by means of a callback function before beginning its operation. When an event is registered in the callback function it is classified and appropriate action taken. The exact events used to represent a VM stopping or starting as a result of migration may differ depending on the underlying hypervisor. In the example shown we use vmStop and vmStart events to also cover migration, as these are the only events available with the XEN hypervisor and libvirt.
If a stop event is detected the previous device containing context data is detached and the device change is committed via libvirt, which proceeds to propagate the virtual hardware change at the hypervisor level. In the case of a start event being detected, infrastructure class context data is generated then packaged into an ISO image. The recontextualization mechanism interacts with libvirt to create a new virtual device with the contents of the dynamically generated ISO image. A failed VM migration will not affect recontextualization as the corresponding events are only sent after a migration has been successful.
Although not shown in the illustration, the infrastructure can trigger recontextualization in response to any event, not just migration. This enables the infrastructure to update the contents of the virtual device by generating new context data and detaching and re-attaching the virtual device. Using this construct, infrastructure specific settings can be changed during runtime at the convenience of the provider.
Flexible file system
Flexible File System (FFS) is a lean, read-only file system implementation that allows separate directory trees to be mounted into a single location. The resulting file system looks and acts like a single unified directory structure while retaining all file contents and properties in the underlying directory trees.
In essence, the FFS file system solves many of the complications of dealing with file-sets from different sources, as files from one set can overshadow a file from another set, without making permanent changes to either of the files. In the example shown in Fig. 4, the file ’F’ from Layer 1 will not be visible to the user as it has the same name and path as a file from another file-set (Layer 2), which has precedence. If the Layer 2 file-set is removed (as can happen due to recontextualization, see Section ‘Recontextualization using FFS’), file ’F’ from Layer 1 would be visible to the user, and the running applications are able to reconfigure to use the available settings.
FFS is implemented using Filesystem in Userspace (FUSE) , a kernel module that allows custom filesystem implementations to run in userspace without the need for any changes in kernel space. The kernel module is available for most Unix-like system, including OpenSolaris and OS X, and was merged into the main Linux kernel tree in 2011. Work on FUSE compatible technologies for Windows is ongoing .
readdir(path) List all items in a path. In this case, FFS returns the union of listings from all layers.
getattr(path) List attributes for the file or folder indicated by the path. FFS will return the actual file attributes, with the attributes at the highest layer taking precedence if the path exists in more than one layer.
open(path, flags) Similarly to getattr(), the access flags for the file with the highest precedence will apply.
read(path, size, offset) Reads size bytes starting from offset in the file indicated by path, again being resolved to the file located at the layer with the highest precedence if more than one layer exists.
The required logic for managing several classes of data is implemented within these method calls, creating a virtual file structure based on the set of files currently available.
Recontextualization using FFS
The file system is initialized and started during the deployment phase of the cloud service (see Table 1), with the required files being part of the context data class originating from the service manifest. If multi-layered contextualization is not required for a particular service, the associated files can simply be omitted from the set of files specified in the manifest. When other layers of context data (such as infrastructure specific settings) are available they will be made available as a separate layer in the FFS stack. The order of the layers (configurable as part of the initial context files) determine the priority of files in case of overlapping datasets.
Since layers can be added and removed during runtime, recontextualization is natively supported by this approach. In the following section, we show results of testing using this setup to replace infrastructure specific data after migration to a new environment.
As the use of virtualized hosting becomes more widespread, application-level systems need to be updated to better respond to previously unexpected changes in the hosting environment. For example, the degraded performance could in theory be detected by internal self-optimization techniques during runtime. However, from the perspective of the individual VM a performance loss as a result of migration would be hard to distinguish from a loss of performance due to heavy load on the network. This is one factor that makes application-level self-optimization approaches more challenging to design.
By supporting recontextualization, the proposed contextualization framework offers unambiguous means of triggering internal system events as a result of external events such as VM migration or a change in the surrounding hosting infrastructure. In the distributed file system case, recontextualization can be used to trigger a new optimization phase post-migration, either by calling designated functions in the file system client (if supported), or simply by restarting the client process.
It is important to note that the performance results are strongly correlated with the traffic shaping settings, and the numerical results shown in the figures should not be emphasized. The purpose of these tests are to illustrate the functional capabilities of the suggested approach, and demonstrate that contextualization and recontextualization can be used to adapt a running VM to changing execution environments.
As illustrated using these tests, considerable performance gains can be achieved by optimizing the setup of application-level systems such as distributed file systems during runtime. This is achieved by using recontextualization to update the application with new settings containing details of what server node to use for its interactions. This demonstrates how the suggested approach can be used as a corner stone in application-level self-* operations.
In this work we have suggested a contextualization approach that supports several classes of data. The data can originate from different sources, e.g. data related to the service the VM instance is part of or data related to the platform on which the instance is running. Data of different classes are layered on top of each other using a lean virtual file system implementation, making it possible to add or remove data layers during runtime. For example, the platform specific class of data can be replaced by the infrastructure as a response to, e.g., a migration of the VM instance to a new executing environment. This way, the suggested contextualization solution is the first known approach to natively support recontextualization. Recontextualization offer means to reconfigure the VM instance as a result of external events or changes in the execution environment.
Recontextualization can be used as a trigger for self-* operations that cannot misinterpret events. This is in contrast to a VM trying to analyse when an event had occurred via its own internal procedures, for example based on when its external IP address changes. A distributed file system use case demonstrated the feasibility of our approach. In the use case recontextualization is used to reconfigure the VM to recover from performance degradation experienced as a result of VM migration. Although the degradation is partly synthetic and correlated to the network settings used, we demonstrate that performance gains can be achieved by runtime optimization triggered by recontextualization.
The limitations of our research are constrained to the self-configuration property of autonomic computing. Future work includes evaluating further use cases related to other self-* aspects such as self-protection. This includes the ability to adjust the security level and functionality of the service component depending on the current geographical location.
Additionally, we have mostly limited our research to IaaS level applications of contextualization and recontextualization. We touch upon using these technologies also for accessing platform specific functionality, such as network based storage, offered as a service by IPs. There is great potential for employing these techniques within PaaS systems, for example enabling dynamic code replacement where software libraries (JAR files, DLLs, etc.) can be made available by the infrastructure for execution within the service. This would allow for arbitrary recontextualization also at the application level.
Future technical work includes formalizing and standardizing the use of different classes of context data and their semantics. A common approach to contextualization is key to enable wide-spread and compatible support across cloud boundaries. A unified representation and interpretation of contextualization meta-data in manifests and service definitions also needs to be further developed.
The research that led to these results is partially supported by the European Commission’s Seventh Framework Programme (FP7/2001-2013) under grant agreement numbers 257115 (OPTIMIS) and 610874 (ASCETiC). The authors would also like to thank Tomas Forsman for technical assistance and expertise.
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