Web Services Composition and Geographic Information - Geographical Information Systems: Trends and Technologies - page 117

Web Feature Services
While a WMS gives users the possibility to retrieve maps from multiple
sources, a Web Feature Service (WFS) provides interfaces to access and
manipulate the previously mentioned geographic features. The guidelines
for implementing an OGC compliant WFS can be found in (Vretanos
2005). According to the standard, besides the mandatory GetCapabilities
operation, a WFS has also to support operations allowing to Insert, Update,
Delete or Discovery geographic features expressed in GML. To accomplish
these tasks, fi ve operations are defi ned:
￿ DescribeFeatureType: an operation used to describe the structure of
any feature that a WFS can service. The only mandatory output is a
GML application Schema (presently GML version 3). Such a Schema
describes the features encoding (either for input or output operations)
expected by the WFS.
￿ GetFeature: a function used to request and retrieve feature instances.
A client has to be able to specify the desired properties for each
feature it wants to fetch. WFS may respond to a GetFeature request
in two ways. It can return either a complete document or simply a
counter corresponding to the number of features that the GetFeature
request would return. The optional resultType attribute in the request
message is used to specify how a WFS should respond to a GetFeature
request.
￿ GetGmlObject: a client specifi es the identifi er (ID) of a GML object and
the WFS returns it.
￿ Transaction: a transaction request is made up of operations (create,
update, delete) that may alter the state of one or more features.
￿ LockFeature: when supported, a client can ask WFS to lock one or
more instances of a feature type during a transaction. This operation
is crucial to ensure consistency in scenarios where a client modifi es
a feature and sends it back to the server (using the above-mentioned
Transaction operation).
Depending on the supported functions, WFS can belong to two major
categories: Standard (or read only) and Transactional. A Standard WFS
has to support the DescribeFeatureType and the GetFeature operations,
a Transactional WFS has to implement the Transaction operation and
optionally the GetGmlObject and the LockFeature operations. The encoding
of the requests can be done by using either KVP values or XML, while the
state of geographic features should be encoded using GML.