Windows Communication Foundation hosting methods for distributed industrial applications.

Gastermann, Bernd ; Stopper, Markus

1. INTRODUCTION

Nowadays, heterogeneous software solutions are applied in many automated businesses. Although these solutions cover their destined application area, they often also harbour the risk of being incompatible with software systems from other operational levels. While progress continues and new demands need to be satisfied, new services for staff and customers have to be provided continuously, which in turn steadily increases the complexity of the whole system. Because of that, companies often take advantage of building a service-oriented architecture (SOA). Most distributed systems base upon the design principles of SOA. A service-oriented architecture consists of several independent--and therefore loosely-coupled --services. Due to standards which apply to all manufacturers, these services are completely interoperable. Usually, a service is a self-contained business function and its functionality is exposed via a structured messaging scheme. From a technology point of view SOA is nothing new but its advantages are definitely standardisation, reusability of single components or services and easy exchange--and extensibility due to their independence (Klein, 2007).

2. WINDOWS COMMUNICATION FOUNDATION

In order to implement a service-oriented architecture many approaches and technologies can be used. Windows Communication Foundation (WCF) is one of them. WCF is a software development kit which is explicitly devoted to communication between software systems. Hence, it can be used to develop services as well as corresponding applications for use in a service-oriented architecture. WCF is part of Microsoft's .NET Framework 3.0 and higher. It allows consistent communication and is well suited for various scenarios in which applications need to exchange data with other services. WCF unifies some of the best features of several other communication technologies and models like Web Services, Remoting or Microsoft Message Queue (MSMQ). All these technologies are encapsulated in order to form a common programming model and provide unitary access. Even though it offers a broad spectrum of communication mechanisms, it can easily be extended to include custom functionality (McMurtry et al., 2007). One advantage of WCF is its support for open industry standards (WS-*) and it is therefore perfectly suited for communication between heterogeneous systems. These standards allow client applications to consume services running on different platforms. At the core of this interoperability lies XML. Message exchange works in particular on basis of SOAP and WSDL. Due to message-based communication WCF is well suited for use in a service-oriented architecture. In consideration of all these aspects, three design goals which took an important part during the development of WCF can be identified (Peiris & Mulder, 2007):

* Unification of established technologies

* Interoperability between platforms

* Service-oriented development

3. WCF BASE ARCHITECTURE ASPECTS

Several key components have to be considered if WCF is used for communication with services: Endpoints including their ABCs to describe a service as well as a hosting environment in which the service ultimately runs. An endpoint acts as a service-access-point from the outside. Each endpoint needs to be described by the ABC concept (Klein, 2007). ABC stands for Address, Binding and Contract. The path to an endpoint is specified by an address. Transport and communication scheme to that endpoint is characterised by the binding. It defines among others, how messages between client and service are exchanged, which transport protocol should be used and which security mechanisms have to be applied. Through contracts all operations offered on a specific endpoint are disclosed to the client. Beyond that contracts also define the internal message structure and layout. Through contracts platform independence is maintained. Definition of endpoints and ABCs has to be done during development. However, in order to run a service it has to be hosted in an appropriate environment (Lowy, 2008). This paper deals with the question which service host is best for use in an industrial application environment.

4. WCF HOSTING TECHNIQUES

In order to access a WCF service it needs to be hosted in an appropriate environment. Such an environment consists of a process in which the service itself runs. The only requirement for the host is to support .NET application domains. For this reason the following four common hosting methods are to be considered: Managed .NET applications, Windows Services, Internet Information Services (IIS) and Windows Activation Services (WAS). Hosting in any custom application type is possible as long as it supports application domains. Each of these types can be classified to one of two categories: Self-Hosted and Hosted.

A type of host the developer has to write on its own is considered a self-hosted environment. As it has to be written completely by the developer himself these hosts do not contain any manageability features by default. However, this approach offers full control over the hosted service and its life cycle.

Hosted environments are--contrary to self-hosted environments--prefabricated hosts that do not need to be developed and already contain several management features. As these pre-existing hosts handle the service in their own way, the developer only has limited influence on the service's life cycle while hosted in this type of host (Peiris & Mulder, 2007).

Each of these hosting environments has certain advantages and disadvantages. Thus, they are not suitable for every application area. In a service-oriented architecture the right host is essential for service robustness. When choosing a hosting environment, it is important to identify the type of service and its availability demands. As the service implementation in WCF is platform agnostic it can easily be ported from one host to another. The following topics have to be considered when selecting a host (Microsoft, 2009):

* Availability

* Reliability

* Manageability

* Versioning

* Deployment

* State

Over the next chapters each of WCF's standard hosting environments is introduced.

4.1 Managed Applications

Managed .NET applications are categorised as self-hosted environment as they have to be written by the developer. That also includes graphical applications like Windows Forms and WPF applications but also console applications. This type of host is easy to develop with only a few lines of code, effortless to install and poses almost no demands to the system it is running on--except for the .NET Framework itself. Unlike other hosting methods this type is ideal for testing, debugging and demonstrating. It can also be used for user interaction and service control if necessary. As this is a completely self-made host it does not contain any features for high availability, easy manageability, robustness, recoverability, versioning and deployment scenarios as IIS does for example. Therefore, it is the developer's responsibility to write the code for starting and stopping the service. This implies that the service needs to be activated manually. The developer thus gains full control over the service's life cycle. Managed applications usually run under limited user accounts and have to be started and stopped explicitly. A service hosted this way is only available when the host runs. However, managed applications support all kinds of transport protocols available for WCF (Peiris & Mulder, 2007).

4.2 Windows Services

A Windows Service is maintained by the operating systems Service Control Manager (SCM). Although the developer has to write the Windows Service which in turn hosts the WCF service, this Windows Service itself is hosted and maintained by the SCM. This allows the developer to have full control over the life time of the WCF service as it has to be explicitly opened and closed from code. Windows Services are easy to develop but do not provide any graphical user interface. This type of host is ideal for long-running services as they are continuously monitored by the SCM of Windows. It offers limited support for features like auto-start on system boot and recovery in case of an error. This makes the service available as soon as the computer starts, regardless of whether a user is logged in or not. Furthermore, it allows each service to have its own account and security permission. Windows Services are easy to maintain by administrators as they often do already have knowledge about how to configure them. Nevertheless, installation software is necessary to install a service on the system (Peiris & Mulder, 2007).

4.3 Internet Information Services Hosting

WCF services can also be hosted using Microsoft's Internet Information Services (IIS) version 5.1 or higher. As IIS is primarily used as web server it only supports HTTP as transport protocol for WCF services. However, IIS 7.0 already includes Windows Activation Services (WAS) which enables support for all WCF protocols. IIS is categorised as a hosted environment and therefore controls each WCF service's instantiation. This can only be influenced by a custom Factory written by the developer. The huge advantage of IIS lies in its many management features like process health monitoring, idle shutdown, process recycling, resource throttling and logging. It offers the same functionality for WCF services as for ASP .NET applications. Since IIS is a complete, stable environment no additional development effort is necessary. Everything is done solely by configuration. Compared to managed applications and Windows Services IIS uses automatic, message-based activation. It is important to know that many of its features as well as its activation mechanism can also cause unexpected behaviour compared to other hosting methods (Peiris & Mulder, 2007).

4.4 Windows Activation Services Hosting

Windows Activation Services (WAS) features all advantages of IIS. Although it is part of IIS 7.0 it can be installed, configured and operated independently. WAS also uses message-based activation for services but contrary to IIS, WAS supports all available transport protocols of WCF, including HTTP, TCP, MSMQ and Pipe. For that purpose it installs listener adapters for each protocol. WAS is only available on Windows Vista and Windows Server 2008 or higher (Microsoft, 2009).

5. CONCLUSION

Hosting distributed applications in a company is subject to certain requirements. These applications have to be hosted primarily in a stable, highly available, maintainable and resource-effective environment. Usually it is not necessary for a service to include a graphical user interface. Under these circumstances managed applications are definitely not well suited because they offer neither management nor high availability features by default. Given that, Windows Services and IIS are the better choice. Nevertheless, this paper's recommendation for hosting distributed applications in an industrial environment is WAS. From all discussed hosting methods WAS is the one which delivers most benefits. It satisfies all previously mentioned hosting requirements for distributed applications. Most companies already use web servers and have web server administrators at their disposal. Regarding these aspects WAS is a good choice that could be implemented even without staff-training as its configuration is similar to IIS.

6. REFERENCES

Klein, S. (2007). Professional WCF Programming, Wiley Publ. Inc., ISBN: 978-0-470-08984-2, Indianapolis, USA

Lowy, J. (2008). Programming WCF Services, 2nd Edition, O'Reilley Media Inc., ISBN: 978-0-596-52130-1, Sebastopol, USA

McMurtry, C., Mercuri, M., Watling, N. & Winkler, M. (2007). Windows Communication Foundation Unleashed, Sams Publishing, ISBN: 978-0-672-32948-7, Indianapolis, USA

Microsoft (2009). Hosting in Windows Process Activation Service, Available from: http://msdn.microsoft.com/enus/library/ms734677.aspx, Accessed: 2009-10-20

Peiris, C. & Mulder, D. (2007). Pro WCF--Practical Microsoft SOA Implementation, Apress Inc., ISBN: 978-1-59059-7026, Berkley, USA