1. European Space Astronomy Centre (ESAC) Villafranca del Castillo, MADRID (SPAIN) Aurélien Stébé Homogeneous Access to Tabular Data Beijing, China - May 2007 Homogeneous Access to Tabular Data Aur é lien St é b é I ñ aki Ortiz, Kona Andrews, Guy Rixon

2. European Space Astronomy Centre (ESAC) Villafranca del Castillo, MADRID (SPAIN) Aurélien Stébé Homogeneous Access to Tabular Data Beijing, China - May 2007 Introduction  Three input query methods: Simple Access Query - Key Value Pair filters Complete Access Query - ADQL (synchronous) Asynchronous Querying - ADQL and UWS  Output result formats / error response handling common to all methods  New proposals for output format selection, empty results, error messages http://esavo.esac.esa.int/doc/Homogeneous_Access.pdf

3. European Space Astronomy Centre (ESAC) Villafranca del Castillo, MADRID (SPAIN) Aurélien Stébé Homogeneous Access to Tabular Data Beijing, China - May 2007 Simple Access Query  Intended to be easy to implement for server and client  Uses Key-Value Pairs to filter the data to be returned  Allows minimal control over quantity of data to be returned  Presents dataset as flat, single table, hiding inner structure  Invoked via HTTP GET to: http://service.endpoint.saq{?,&}PARAM=value[&…]  PARAMs should not modify output format or alter data  PARAMs may only limit the quantity of data (rows), the type of specific data or the quantity of information (columns)  Define generic types / families of parameters  Reserve a few parameter names (POS, SIZE, BAND, TIME, …)

4. European Space Astronomy Centre (ESAC) Villafranca del Castillo, MADRID (SPAIN) Aurélien Stébé Homogeneous Access to Tabular Data Beijing, China - May 2007 Simple Access Query - PARAMs  Single value type: PARAM_NAME=value PARAM_column = “value” PARAM_column > value (upper limit parameter)  List value type: PARAM_NAME=value1,value2,value3 PARAM_column IN (“value1”, “value2”, “value3”)  Interval value type: PARAM_NAME=valueMIN/valueMAX PARAM_column BETWEEN valueMIN AND valueMAX PARAM_column > valueMIN (open upper limit interval)  Interval PARAM type: PARAM_columnMIN value PARAM_columnMIN valueMIN PARAM_columnMAX > valueMIN (open upper limit interval)

5. European Space Astronomy Centre (ESAC) Villafranca del Castillo, MADRID (SPAIN) Aurélien Stébé Homogeneous Access to Tabular Data Beijing, China - May 2007 Complete Access Query  Intended to give total access and control over the dataset  Uses full query language (defined in other specifications)  Makes dataset’s inner structure available to the client  Users become responsible for data-level considerations  Invoked via HTTP POST to: http://service.endpoint.caq  Sending the message body: queryType=queryString  Three query types: nativeADQL - ADQL against the service’s table/column names uTypeADQL - ADQL against formal data model using uTypes directQuery - pass-through to the DBMS, any query language

6. European Space Astronomy Centre (ESAC) Villafranca del Castillo, MADRID (SPAIN) Aurélien Stébé Homogeneous Access to Tabular Data Beijing, China - May 2007 Asynchronous Querying  Interface identical to the Complete Access Query, plus DEST  Using the UWS for job management and workflow  The DEST parameter for delivery: DEST=LOCAL For local staging of the data at the service DEST=http://my.server/~john/out.vot DEST=ftp://my.server/~john/out.vot DEST=vos://my.server!vospace/john/out.vot For respectively HTTP, FTP or VOSpace delivery

7. European Space Astronomy Centre (ESAC) Villafranca del Castillo, MADRID (SPAIN) Aurélien Stébé Homogeneous Access to Tabular Data Beijing, China - May 2007 Output Result Formats  Various formats possible, default is VOTable-v1.1  Two methods to select the output format: HTTP header “Accept:” with MIME types OUTPUT parameter with fixed values  The OUTPUT method always overrides the “Accept:” one  Response to valid queries must have the “200 OK” status code and “Content-Type:” header with MIME type of the output format  Empty response to valid queries must have the “204 No Content” status code and no message body  Formats defined: VOTable, CSV or TSV, XML, …

8. European Space Astronomy Centre (ESAC) Villafranca del Castillo, MADRID (SPAIN) Aurélien Stébé Homogeneous Access to Tabular Data Beijing, China - May 2007 Output Result Formats  The use of OUTPUT and “Accept:” HTTP header Advantage: consistent across query methods, allows direct human readable webpage interface for a web browser Disadvantage: output format must be a MIME type, need two methods to select format, because “Accept:” cannot always be used Alternative: only use the OUTPUT parameter method NOTE: OUTPUT is not the equivalent to FORMAT from DAL  The use of “204 No Content” for empty responses Advantage: consistent across result formats, processing power spared Disadvantage: current clients may not check for this status code Alternative: return empty VOTable (or equivalent in output format)

9. European Space Astronomy Centre (ESAC) Villafranca del Castillo, MADRID (SPAIN) Aurélien Stébé Homogeneous Access to Tabular Data Beijing, China - May 2007 Metadata Access Format  All the information needed to call the service should come from it  Should come from a unique endpoint to invoke via HTTP GET  For Simple Access Query, we need: input PARAMs, output FIELDs  For Complete Access Query, we need: tables/columns information  Encoding this information in Registry format would ease many things  If filling those requirements, would use VOSI for Metadata access

10. European Space Astronomy Centre (ESAC) Villafranca del Castillo, MADRID (SPAIN) Aurélien Stébé Homogeneous Access to Tabular Data Beijing, China - May 2007 Error Responses  Error output is done using the HTTP error codes  Here are a few examples: 400 Bad Request - malformed input query 404 Not Found - query to unsupported method 500 Internal Server Error - general misc server error 501 Not Implemented - unimplemented optional method 502 Bad Gateway - backend error (DBMS, store, …)  The message body should contain the error text explanation  Advantage: same system regardless of output format  Disadvantage: limited list of codes we don’t control  Alternative: classical VOTable-based error output

11. European Space Astronomy Centre (ESAC) Villafranca del Castillo, MADRID (SPAIN) Aurélien Stébé Homogeneous Access to Tabular Data Beijing, China - May 2007 Conclusion  Need work on Metadata access, reserved PARAMs list, output format, empty responses, error mechanism to be complete  Compliant with second generation DAL services: the Simple Access Query method represents the first step (queryData method)  Questions ?