DAP4, SOAP and THREDDS
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Transcript of DAP4, SOAP and THREDDS
DAP4, SOAP and THREDDS
James Gallagher
OPeNDAP
Presented by Steve Hankin
Overview
• DAP2 and DAP4: Caveat, DAP4 is still in development.
• DAP4 SOAP interfaces
• SOAP and binary data
• SOAP and REST
DAP 2 versus DAP 4
• Metadata in two responses: DAS & DDS
• Encoded using ‘unique’ notation
• Data returned in non-standard document mixing text and binary
• REST interface only
• Metadata in one response: DDX
• Encoded using XML• Data returned in a
Multipart-MIME document; text/xml and binary
• Both SOAP and REST interfaces
Other Responses
• Most DAP servers support additional responses such as: ASCII, HTML interface for data requests, version information, directory browsing
• These ‘other responses’ will be documented more formally.
• The ad hoc directory response will be replaced by THREDDS in OPeNDAP’s servers.
THREDDS
• The DAP2 servers from OPeNDAP (and most other sources) supported a very limited ‘directory browsing feature:’– Tied to the file system which held data files– Unable to support data in databases– Dependent on a web daemon to provide the HTML
directory listing; varied from server to server
• THREDDS improves this by:– Supports provider-defined organizations for data– No longer dependent on files and file systems– Uniform across different servers
About the SOAP Interface• Requests:
– GetVersion: DAP protocol version– GetDDX: Metadata– GetData: Data– GetTHREDDSCatalog: Machine readable catalog
• Responses:– Version, DDX, DataDDX, ErrorX, THREDDS– Each response contains the matching request
• Pooling: Each request includes a Request ID and several requests can be sent to the server in one SOAP envelope.
• DAP4 uses ‘SOAP with Attachments’ (SWA)
SOAP and Binary Data• There are two general ways to send binary data and
XML:– “Out-of-band transmissions” (embed a URL)– SWA
• Out-of-band is simple, but not standard• SWA: briefly, this uses a multipart MIME document
where the XML is in the ‘main’ part and zero or more binary parts are in attachments.
• SWA is as efficient as Out-of-band because:– once the SOAP envelope is closed, the response can be
sent, which provides a way to stream the binary data.– Non-xml information is never seen by the XML parser.
SOAP versus REST
• SOAP:– Standard– Integrated discovery mechanism– Good for machine-to-machine interactions
• REST/GET:– Entire request encoded in URL - Simple– Integrates well with web browsers– Works well with hypertext/human interfaces
• Strategy: Support both– design systems so that’s easy