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The . OVERVIEW The WFS specification defines interfaces for describing data manipulation operations of geographic features. Data manipulation operations include the ability to:
The basic Web Feature Service allows querying and retrieval of features. A transactional Web Feature Service allows creation, deletion, and updating of features. A WFS describes discovery, query, or data transformation operations. The request is generated on the client and is posted to a web feature server using HTTP. The web feature server then executes the request. The WFS specification uses HTTP as the distributed computing platform, although this is not a hard requirement. There are two encodings defined for WFS operations: Communication Models The WFS Web Feature Services or Web Feature Server specification supports two communication models:
::A messaging system in which clients address messages to a specific node in a content hierarchy, called a topic. Publishers and subscribers are generally anonymous and can dynamically publish or subscribe to the content hierarchy. The system takes care of distributing the messages arriving from a node's multiple publishers to its multiple subscribers. Messages are generally not persistent and will only be received by subscribers who are listening at the time the message is sent. A special case known as a “durable subscription” allows subscribers to receive messages sent while the subscribers are not active. (Source: http://java.sun.com/j2ee/1.4/docs/glossary.html) The WNS is one of the implementation specifications for the Pub/Sub model. Regardless of the model, URL format is used and specified in the WFS specification. At this time there are no open-standard implementations of WNSs. Vendors plan to release implementations once the standard has been ratified. Data The Geography Markup Language (GML) passes data back and forth between a Web Feature Server and a client. GML normally communicates geospatial data but also supports other types of data. GML GML expresses feature data in and out of a feature server. The GML standard is at V2.0 and is defined by the WFS Web Feature Services or Web Feature Server 1.0 specification. This standard covers the following new topics:
V3.0 of the specification, which is currently being released, includes Topology, which enables the expression of facts such as "Road A ends at Road B." PUBLIC INTERFACES Static Interfaces The static interface model for the OGC Web Service model appears in the figure below. The Transaction and LockFeature operations are also optional. When writing a WFS , you must implement the following operations:
Dynamic interfaces The dynamic request/reply interface model for the OGC Web Service model appears in the figure below. Dynamic interface updates The client gets updates by one of two mechanisms:
WFS dynamic interface web notification model This model uses the OGC Web Notification Service to send update notifications to registered clients. The Notification interface appears in the figure below. WFS dynamic interface web notification polling model The polling model interface appears in the figure below. EXTERNAL LINKS |
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