Friday, August 28, 2009

Windows SharePoint Services - Architectural Perspective

Windows® SharePoint® Services (WSS) provides team-oriented collaboration Web sites, a platform for building Web-based applications that use the WSS collaboration features, and a framework for deploying and managing these sites and applications.

Functional Architecture
WSS is designed to support a broad range of deployments. At a high level, four different types of systems are involved:
1. The End-User Client (EUC) is a computer on which an individual user is requesting specific file operations. These requests are communicated using HTTP-based protocols, including HTTP, DAV and Microsoft FrontPage Server Extensions.
2. The Web Front-End (WFE) is a computer that receives requests from an EUC and provides WSS capabilities by manipulating information stored in a database. These database requests are expressed in the T-SQL language and communicated using the TDS protocol.
3. The Back-End Database (BEDS) is a computer that runs Microsoft® SQL™ Server and responds to requests from the WFE server.
4. An Active Directory server responds to authentication requests from the EUC, WFE, and BEDS. These components could use an alternate authentication mechanism besides Active Directory.
These systems are shown in the following diagram.

Figure 1: Intersystem protocol relationships

WFE and BEDS are considered part of the WSS deployment; the EUC is normally a user's desktop or laptop computer that connects to WSS services, and Active Directory is part of the generally available infrastructure. Both WFE and BEDS can run on a single computer, so the simplest WSS deployment could be just a single server. Alternatively, multiple instances of WFE and BED computers can be installed for greater throughput and redundancy.

Storage Architecture
WSS provides a flexible model for storage which allows for a robust variety of data management and organization techniques. The following diagram presents a high-level view of the containers in this hierarchy. These containers provide important organizational and management tools for content stored in WSS, and also form the core securable objects.

Figure 2: SharePoint storage object hierarchy

Advanced Storage Concepts
In addition to basic file system concepts, WSS provides a variety of specialized file system objects and operations as described below -

For some complex HTML documents, WSS provides the capability to store the document separated into its component sibling documents. This group of documents is treated like a single document for most file operations, but is stored as a set of related documents within the file store location.

Sites and Lists tend to share a relatively small set of common system files across many instances within the BEDS. To avoid having to store these files repeatedly for every Site or List in the BEDS, WSS allows files to be ghosted, meaning that the content of the file is not actually stored in the BEDS. Instead, a reference to a location on the WFE where the source of the file can be found is stored in the file metadata.

WSS can be configured on a per-List or per-Document Library basis to store multiple versions of Documents. If versioning is configured for a storage location, each new version of a Document is stored with an incrementing version number that can be either in the form 'Major Version Number' (#) or 'Major Version Number.Minor Version Number' (#.#).
If Major Version Number.Minor Version Number version numbering is used, individual Documents start their numbering at 0.1, and the version can be promoted to a Major Version using the Publishing feature. Prior versions of a document can be retrieved by users with the appropriate access rights.

WSS can be configured on a per-List or per-Document Library basis to allow publishing features with Documents. If publishing features are configured for a storage location, each version of a Document can be configured as a draft, and therefore, not be available to be viewed by users without the appropriate access rights for viewing unpublished versions.

Large File Access
When dealing with very large files, obtaining the complete file contents in a single buffer as part of one operation can be a burden on system resources. Instead, the BEDS provides functionality to return files larger than a specified size to the WFE in a series of smaller chunks that can be processed more smoothly.

BLOB Storage Outside the Content Database
WSS provides the ability to allow file metadata to be stored by WSS in a Content Database, while allowing the actual file contents to be stored in an external file system. The term Binary Large Object (BLOB) refers to the SQL Server concept of unstructured, binary data streams that are commonly associated with files.
WSS does not natively provide a BLOB store. There are two APIs that allow a BLOB storage provider to be built and registered with WSS:
External BLOB Storage: For more information about External BLOB Storage, see [MSDN-WSSEBS].
Remote BLOB Storage: For more information about Remote BLOB Storage, see [MSDN-SQLRBS]. Windows SharePoint Services 4.0 supports Remote BLOB Storage providers.

Content Databases
A Content Database stores and manages end-user content. Each Web Application will have one or more Content Databases. The WSS Content Database stores the data and documents that end-users have associated with their SharePoint Site. The contents of this database are fully normalized in order to efficiently perform the kinds of operations required for the high-scale, highly-configurable system previously described. The role of a WSS WFE is to fetch the appropriate data from these multiple locations within SQL using the appropriate set of queries and stored procedures and to correctly interpret and map the results into a correct response to the EUC.

Configuration Database
The Configuration Database describes the topology of the farm and global settings. Each farm will have only one Configuration Database. The Configuration Database is essentially the definition of the WSS deployment, for either a single instance of WSS or for a farm. In the farm context, the Configuration Database contains information representing the global settings that are required to provide consistent operation across all servers within the farm, and to map requests to particular Content Databases.

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