MapGuide RFC # - JSON format responses

This page contains a change request (RFC) for the MapGuide Open Source project. More MapGuide RFCs can be found on the RFCs page.

Status

RFC Template Version	(1.0)
Submission Date	August 8, 2007
Last Modified	Jason Chen Timestamp
Author	Jason Chen
RFC Status	proposed (draft, proposed, frozen for vote, adopted, retracted, or rejected)
Implementation Status	under development
Proposed Milestone	1.3
Assigned PSC guide(s)	(when determined)
Voting History	August 8, 2007
+1	Tom, Chris, Bruce, Trevor, Jason
+0
-0
-1

Overview

JSON (​http://www.json.org) will be used to provide a lightweight data-interchange format between the web tier and any clients for requests that can currently return XML responses.

Motivation

The Fusion framework uses JSON in order to keep client side code simple (it much easier to work with than XML) and JSON is a more compact format so communication over the wire is more efficient.

Currently Fusion uses a web tier PHP script to convert the XML responses to JSON, but this is inefficient so we will move the generation of JSON into the server. This should further speed up the generation of the JSON, and where required we can eliminate the intermediate XML form.

Proposed Solution

The solution involves adding an extra "format" parameter on selected API invocations from the Web-tier to the Server. The "format" can be either "text/xml" or "application/json". If no parameter is specified at the Web-tier, the "format" will default to "text/xml". Due to performance considerations, two additional APIs (i.e. DescribeSchemaAsJson and GetFDOCacheInfoAsJson) have been added to handle the special cases.

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An additional OEM component has been added to the source -- jsoncpp. Jsoncpp is a C++ implementation of the JSON specification (​http://sourceforge.net/projects/jsoncpp/).

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Two additional classes have been added to assist the JSON implementation in MapGuide:

1) The MgJsonDoc Class

• MgJsonDoc is a wrapper over the jsoncpp implementation. MgJsonDoc provides many easy-to-use interfaces, such as Begin* and End*, for creating JSON documents from an XML perspective. For example, to create a JSON document for the corresponding XML content, <Node>Text</Node>, one would call BeginObject("Node"), SetValue("Text"), and EndObject() subsequently.

2) The MgXmlJsonConvert Class

• MgXmlJsonConvert converts an XML document into a JSON formatted document.

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The following Web-tier to Server APIs are affected:

Coordinate System

# EnumerateCategories (1)
# EnumerateCoordinateSystems (1)

Resource

# EnumerateResourceReferences (3)
# EnumerateResources (3)
# GetResourceContent (3)
# GetResourceHeader (3)
# EnumerateResourceData (3)
# GetResourceData (3)
# EnumerateUnmanagedData (3)

Feature

# GetFeatureProviders (3)
# GetCapabilities (3)
# GetConnectPropertyValues (1)
# DescribeSchema (4)
# SelectFeatures (2)
# SelectAggregates (1)
# GetSpatialContexts (1)
# GetLongTransactions (1)
# EnumerateDataStores (3)
# GetSchemaMapping (3)
# GetSchemas (1)
# GetClasses (1)
# GetClassDefinition (2)
# GetIdentityProperties (1)
# ExecuteSqlQuery (1)
# GetFdoCacheInfo (4)

Rendering

# GetVisibleMapExtent (1)

where the appended labeling indicates the different implementation schemes:

(1) Web-tier changes -- every ToXml implementation is matched with a ToJson implementation that composes a JSON document in place.

(2) Web-tier changes -- utilizes the MgXmlJsonConvert class to convert XML documents to JSON documents.

(3) Server changes -- an extra parameter "CREFSTRING format" has been added to the respective API call; the MgXmlJsonConvert class is used if format is equaled to the JSON MIME type.

(4) Server changes -- an extra API has been added matching the respective API call (e.g. DescribeSchemaAsXml has DescribeSchemaAsJson as the JSON version, and GetFDOCacheInfo has GetFDOCacheInfoAsJson as its JSON specific implementation); this is because the original APIs return "STRING" and could not be efficiently adapted by the conversion process within the MgXmlJsonConvert class implementation; and for the affected APIs, the MgByteReader* is returned instead.

Implications

The major changes involve differing API signatures (i.e. some functions take an extra parameter "format"). Any API invocation made to the Server will be affected. However, API calls made to the Web-tier will behave the same if the format parameter is not specified (i.e. it defaults to the "text/xml" format). Documentation should be updated to indicate that the above APIs now take an extra parameter, "format".

Test Plan

The following tests should be performed:

• Resource Service unit tests must all pass.
• Feature Service unit tests must all pass.
• API test pages should be checked against the affected APIs.

Funding/Resources

Supplied by Autodesk.