Feature Production and Modification

44 Copyright © 2006 Open Geospatial Consortium – All rights reserved Intergrated Client WFS getCapabilities CapabilitiesDocument describeFeatureType typeName XMLSchema getFeatures Query GML END GML START Figure 14 Direct Feature Access It is important to be aware of the schema as it plays its established role in validation of an XML document. Care should be taken to ensure that the DescribeFeatureType operation is not called twice for both validation of the XML document and description of the presented Features. Direct access as a raw XML document is often associated with SVG based rendering systems, in which only XML transformations are applied to produce visualization. Care should be taken to avoid this technique when extremely large schema documents are being used as the original XML document stream may timeout. During the course of OWS-3, schema files 12MB took 30 seconds to parse.

11.2 Feature Production and Modification

The ability to interact with and produce spatial information is an important role of an integrated client. The Open Web Services allow for this use through the use of the Web Feature Server Transaction operation. Conformant Web Feature Servers are allowed to forgo implementation of the Transaction operation, and are still referred to as valid WFS servers. The abbreviation WFS-T is used to indicate fully transaction web feature servers. Copyright © 2006 Open Geospatial Consortium – All rights reserved 45 Since this is the only opportunity for an Integrated Client to produce content, WFS-T support is viewed as essential to success. A Managingediting features contained in a WFS-T A Transactional Web Feature Server WFS-T allows users to retrieve and modify feature data. After acquiring data the user can construct a transaction request consisting of a series of delete, update, and additions to be performed by the web feature server. Two models of “success” are supported by WFS 1.0, partial success in which the modifications that could not be performed are indicated. Intergrated Client WFS getCapabilities CapabilitiesDocument describeFeatureType typeName XMLSchema getFeatures Query GML Transaction TransactionResponse Figure 15 Transaction Support Although the use of a WFS-T was not covered during our demo scenario, this document reflects the experience of those implementing support outside of the bounds of the OWS- 3 experiment. B Web Feature Server Locking Support In addition to direct access through the transaction operation the WFS specification outlines a simple locking system. These Locking facilities represent an interesting compromise going beyond short term locking, while falling short of a complete strong- transaction support system. 46 Copyright © 2006 Open Geospatial Consortium – All rights reserved Intergrated Client WFS getCapabilities CapabilitiesDocument describeFeatureType typeName XMLSchema getFeatures Query GML Transaction LockID, ... TransactionResponse LockRequest Query LockResponse Figure 16 Locking The locking model is based the use of tokens, which are obtained for a requested duration. These lock tokens are consumed during the course of a transaction. Depending on the method of use the Lock may be relinquished on just the features modified by the Transaction request. While not providing facilities such as checkpoints, or conflict resolution this does represent a workable middle ground.

11.3 Issues and Tradeoffs with Web Feature Service Communication