Last week’s announcement that HTTP PATCH has been adopted as an official verb via RFC 5789 has generated a lot of excitement (and questions). As a summary, the intention of each verb is:

• POST to create a new resource when the client cannot predict the identity on the origin server (think a new order)
• PUT to override the definition of a specified resource with what is passed in from the client
• PATCH to override a portion of a specified resource in a predictable and effectively transactional way (if the entire patch cannot be performed, the server should not do any part of it)

The goal is to convey the intent of the patch more clearly than is possible with the more generic POSTing of modifications. Specific patch diff formats will emerge for modifying plain text, HTML, XML, etc.

The Semantic Web community has been interested in this capability for quite some time, particularly with respect to modification of RDF graphs. SPARQL/Update is a proposed (currently just a W3C Member Submission) language for remotely updating RDF Graphs. It does not directly specify how the requests will be transferred, but the updates themselves look something like this (from the spec):

Add a title and creator relationship to a book: 

PREFIX dc: <http://purl.org/dc/elements/1.1/>
INSERT DATA
{ 
   <http://example/book3> dc:title "A new book" ;
                 dc:creator  "A.N.Other" .
}

Remove any records associated with older books:

PREFIX dc:  <http://purl.org/dc/elements/1.1/>
PREFIX xsd: <http://www.w3.org/2001/XMLSchema#>
DELETE
{ ?book ?p ?v }
WHERE
  { ?book dc:date ?date .
    FILTER ( ?date < "2000-01-01T00:00:00"^^xsd:dateTime )
    ?book ?p ?v
  }

The SPARQL 1.1 Protocol and Uniform HTTP Protocol for Managing RDF Graphs (currently W3C Working Drafts) will map these updates into HTTP verbs to create or modify named graphs. The graphs are named either through their own URI or a graph query parameter.

As an example, an HTTP GET would act as a query into a named graph. An HTTP PUT of an enclosed body of RDF would translate to the equivalent SPARQL/Update (from the spec):

DROP SILENT GRAPH <graph_uri>
CREATE SILENT GRAPH <graph_uri>
INSERT DATA [ INTO <graph_uri> ] 
{ .. RDF payload .. }

As a PUT represents an idempotent overwrite action, it must first drop the existing graph should it exist, create a new one and then dump the specified contents into the new graph.

An HTTP POST should be used to additively extend the RDF graph with the specified body and a DELETE would remove the graph. There is no support yet for a PATCH-level view of RDF graphs.

Barring standardization on this PATCH front, developers have had to create their own implementations.

The Talis platform supports ChangeSets which allow the identification of collections of modifications to apply to an existing store. Here we see the ordered removal of an old title and the addition of a new one:

<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" 
    xmlns:cs="http://purl.org/vocab/changeset/schema#">
  <cs:ChangeSet rdf:about="http://example.com/changesets#change">
    <cs:subjectOfChange rdf:resource="http://example.com/res#thing"/>
    <cs:createdDate>2006-01-01T00:00:00Z</cs:createdDate>
    <cs:creatorName>Anne Onymous</cs:creatorName>
    <cs:changeReason>Change of title</cs:changeReason>
    <cs:removal>
      <rdf:Statement>
        <rdf:subject rdf:resource="http://example.com/res#thing"/>
        <rdf:predicate rdf:resource="http://purl.org/dc/elements/1.1/title"/>
        <rdf:object>Original Title</rdf:object>
      </rdf:Statement>
    </cs:removal>
    <cs:addition>
      <rdf:Statement>
        <rdf:subject rdf:resource="http://example.com/res#thing"/>
        <rdf:predicate rdf:resource="http://purl.org/dc/elements/1.1/title"/>
        <rdf:object>New Title</rdf:object>
      </rdf:Statement>
    </cs:addition>
  </cs:ChangeSet>
</rdf:RDF>

@prefix guo: <http://webr3.org/owl/guo#> .
@prefix dcterms: <http://purl.org/dc/terms/> .
_:u1 a guo:UpdateInstruction ;
     guo:target_graph <http://webr3.org> ;
     guo:target_subject <http://webr3.org/r/Special_Document> ;
     guo:delete _:d1 ;
     guo:insert _:i2 .
_:d1 dcterms:title "Draft Special Document"@en .
_:i2 dcterms:title "Special Document"@en ;
     dcterms:published "2010-03-18T15:26:13Z" .

DELETE DATA FROM <http://webr3.org> {
  <http://webr3.org/r/Special_Document> <http://purl.org/dc/terms/title> "Draft Special Document"@en .
}
INSERT DATA INTO <http://webr3.org> {
  <http://webr3.org/r/Special_Document> <http://purl.org/dc/terms/title> "Draft Special Document"@en ;
  <http://purl.org/dc/terms/published> "2010-03-18T15:26:13Z" .
}

@prefix guo: <http://webr3.org/owl/guo#> .
@prefix dcterms: <http://purl.org/dc/terms/> .
_:u1 a guo:UpdateInstruction ;
      guo:target_subject <http://example.com/resource/London> ;
      guo:insert <http://dbpedia.org/resource/London>,
                 <http://mpii.de/yago/resource/London>,
                 <http://statistics.data.gov.uk/id/eer?name=London> .

@prefix guo: <http://webr3.org/owl/guo#> .
@prefix dcterms: <http://purl.org/dc/terms/> .
_:u1 a guo:UpdateInstruction ;
      guo:merge <http://dbpedia.org/data/Linked_Data>,
                <http://dbpedia.org/data/Semantic_Web> .

<rdf:RDF xmlns="http://webr3.org/owl/guo#"
    xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
    <UpdateInstruction>
        <insert rdf:resource="http://dbpedia.org/sparql?query=construct+{+%3Fs+%3Fp+%3Fo+}+where+{..."/>
        <target_subject rdf:resource="http://webr3.org/r/linked_to_haxe"/>
    </UpdateInstruction>
</rdf:RDF>