[preface]

.[big]*OGC® Engineering Report*

*COPYRIGHT*

Copyright © 2016 Open Geospatial Consortium.
To obtain additional rights of use, visit http://www.opengeospatial.org/


*WARNING*

This document is not an OGC® Standard. This document is an OGC® Public Engineering Report created as a deliverable in an OGC® Interoperability Initiative and is not an official position of the OGC® membership. It is distributed for review and comment. It is subject to change without notice and may not be referred to as an OGC® Standard. Further, any OGC® Engineering Report should not be referenced as required or mandatory technology in procurements. However, the discussions in this document could very well lead to the definition of an OGC® Standard. 

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.[big]*Abstract*

This engineering report describes a protocol for synchronizing data between two
enterprise servers.  While the protocol itself is generic, this engineering
report describes its application to web feature servers.

In the simplest terms, the protocol involves each synchronization peer accessing
the other’s "Sync" resource to get the set of changed objects since the last
time the "Sync" resource was accessed.  In the case of web feature servers, the
objects are features.  The requesting peer then compare that list of changed
features with the identically identified features in its data store and performs
any necessary changes so that the feature states match.

Continuing the work done in Testbed-11, this engineering report describes the
implementation of a Sync operation in a WFS server that:

. Enhances the Sync operation from Testbed-11 to include an abstract query element where each service type can then substitute their specific query syntax for identifying the specific sub-set of changed features to be synchronized.  In the case of the WFS, several query syntaxes may be used including the wfs:Query element and a REST based feature type URI with query parameters.

. Extends the definition of the Sync operation with the addition of a "resultType" parameter to allow a client to obtain a hit count of the number of features that a Sync operation shall return.

. Shall investigate the proper procedure for handling resource references.  Implementing the resolvePath parameter alone is not sufficient to ensure complete data set synchronization.

. Shall investigate concurrency and consistency issues.

NOTE: With regard to the resolvePath parameter; in order to be able to test the
use of this parameter, the feature encoding needs to support the ability to
reference resources (i.e. property values, other features, etc.).  Although
Testbed-11 used GeoJSON, this format is not sufficiently mature in this regard
and so we propose using GML as the feature encoding format for this round of
sync testing.

.[big]*Business Value*

Data synchronization is a fundamental means by which data consistency is 
achieved and maintained over time between a source and target data system.  It
is fundamental to a wide variety of applications including, for example, file
synchronization and synchronization between mobile devices and especially in
a low-connectivity environment such as might exist after a natural disaster.
Anyone who has ever synchronized their iPod/iPhone with iTunes has experienced
data synchronization in action.

.[big]*What does this ER mean for the Working Group and OGC® in general*

With regard to the WFS/FES SWG, this engineering report defines the means
by which peer-to-peer data synchronization can be performed with web
feature services.  This can be synchronization between two web feature
services or synchronization between a web feature service and some other
system (e.g. a web processing service that manages geopackages).

With regard to the GeoSynchronization 1.0 SWG, the current draft GSS standard 
defines a publish-subscribe architecture for synchronization between services
with the GeoSynchronization service acting as the broker.  This engineering
report describes and alternative synchronization architecture that should
be considered by the GSS SWG.

Although the testing platform for synchronization for Testbed-12 is the web
feature service, the protocol described in this engineering report is 
applicable to a number of OGC® standards and so there is a strong likelihood
that some of the elements of the description should end up in the OWS Common
standard.

For the OGC® in general, this engineering report defines a generic
synchronization protocol that can be used to maintain data consistency across
a variety of systems ranging from the very small (e.g. mobile devices with 
geopackages) to the very large (e.g. enterprise level data services such as
web feature services).

.[big]*How does this ER relates to the work of the Working Group*

Although the protocol described in this engineering report is generic, the
web feature service was used as the testing platform in Testbed 12.  As such,
this engineering report describes modification and extensions to the web
feature service standard that need to be considered by the WFS/FES SWG.

As mentioned above, one part of the GSS draft standard describes a 
publish-subscribe architecture for synchronization.  The work described in
this engineering report is directly related to synchronization and is thus
directly related to the work of the GSS SWG.

Although the WFS was used as a testing platform, the synchronization protocol
described in this engineering report is generic and could be applicable to a
variety of OGC® standards.  As such, it is related to one of the primary
purposes of the OWS Common 1.2 SWG which is to collect common elements among
OGC® standards (e.g. BBOX encoding) into a single standard where this
information can be referenced rather than be copied over and over.

.[big]*Keywords*
ogcdocs, testbed-12, synchronization, web feature service, WFS, filter, change set, low connectivity, checkpoint, service identifier, REST, XML, KVP

.[big]*Proposed OGC® Working Group for Review and Approval*

This engineering report shall be submitted to the WFS/FES SWG, the 
GeoSynchronization 1.0 SWG and the OWS Common 1.2 SWG for review and comment.