Global Positioning System Reference
In-Depth Information
response message can dynamically vary based on the client requests (e.g.,
in the WFS GetFeature operation).
The last problem discussed here is how the SOAP protocol can be used
in conjunction with traditional binary data, such as the images returned
by a WMS. Binary data, usually called “opaque data” (Powell 2004) often
constitute a problem for (Web) services based solutions. First of all, the
serialization (i.e., the translation of an object into an XML stream) of such
data into XML documents is not always an easy or feasible solution. For
example, documents with digital signatures could lose their integrity
(Powell 2004). To deal with this problem two predominant techniques can
be followed (Bosworth et al. 2003), namely either embedding, in some way,
the opaque data in an XML element or referencing it as an external entity.
The former is the currently used solution because the latter is inapplicable
in a W3C compliant service environment, being based on XML features
prohibited by the SOAP standard (such as Document Types Declarations).
In XML, the support to binary data is usually achieved by using the Base64
or the hexadecimal text encoding. The result for both is a sequence of octets
(Bosworth et al. 2003). However, although these two encoding solutions are
very simple to implement, a well-known problem concerns the size increase
of the binary data as well as the overhead caused by the processing time
needed to perform the encoding and decoding operations. To overcome
these performance issues other proposals have been suggested, such
as SOAP with Attachment (SwA) (Barton et al. 2000). SwA relies on the
fundamental concept of MIME multipart messages, which simply means
that a message is split into two or more parts and hence can include multiple
attachments. The MIME standard specifi es how these parts should be
combined to form a single message. In SwA, the traditional SOAP message
constitutes the root part of the MIME multipart message and the SOAP Body
element contains explicit references to other parts of the MIME multipart
message, which may contain arbitrary data (Schäffer 2008). A problem with
this approach is that it does not work with other fundamental components
of the W3C service stack, such as the WS-Security protocol (Powell 2004;
Nadalin et al. 2006). Therefore the solution currently used (recommended
also by the previously mentioned OGC working group) is represented by
the Message Transmission Optimization Mechanism (MTOM). MTOM is a
W3C recommendation that provides an effi cient mechanism for exchanging
large binary data by using SOAP messages and is based on another W3C
recommendation, namely the XML binary Optimized Packaging (XOP)
that, among other advantages, does not require the time consuming task
of the Base64 encoding (Schäffer 2008). Opaque data in MTOM are treated
in a manner similar to what happens in SwA but, in this case, the SOAP
message consists of the whole MIME multipart message (Schäffer 2008).
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