Wikidata Query Service/User Manual/MWAPI

MediaWiki API Query Service
MediaWiki API Service allows to call out to MediaWiki API from SPARQL, and receive the results from inside the SPARQL query.

Service
The query is initiated by SERVICE with URL.

Input
Query inputs are specified as service parameters, in a clause such as this: This defines the parameter " " in the query.

There are two special parameters,  and , which are mandatory and identify the endpoint host (wiki) and the specific API used. The API endpoints are pre-defined in configuration; see below.

On the Wikidata Query Service, all Wikimedia sites are allowed (all Wikipedia language editions, Wikimedia Commons, Wikidata itself, etc.).

Example: The rest of the parameters use predicates starting with  and the name is the name of the parameter in the query, e.g.  . The value is the value that will be used in the query; it can be a constant or a variable. If the variable is not bound when the query is run, the parameter is skipped or, if it is required (as defined by the configuration), an error occurs.

It is permissible to add input parameters not specified in the configuration; they will be passed to the service query. Please refer to the API documentation for the parameters each service has. The configuration also defines some default parameters. If you override them in the service parameters, you may want to merge the default value with your desired value to ensure the output looks as expected by MWAPI (e. g. use  instead of just  ).

Pagination
By default, WDQS will try to fetch all the results of the query, with continuations. You can set the page size by using service-specific limit parameters, but the service will use continuation to fetch the following pages. This can be controlled by the  parameter, which will limit total number of rows fetched in one API request: This will fetch only 50 rows, regardless of pagination settings. Note that SPARQL  clause will cause the same effect, but will be applied only after the API call has finished - all the data has been fetched and then the extra data has been discarded. It is more efficient to use the internal limit.

A special value of "once" disables the continuation mechanism: This makes the service to stop after the first API call and not use continuations. The number of results depends on service parameters.

There is a hard-coded limit of 10000 rows which always applies.

Two additional limit parameters exist to fine-tune some requests (see the original Gerrit patch):
 * to limit the number of internally requested continuations (by default 1000 continuations, hard-coded limit of 1000);
 * to limit the number of empty consecutive continuations (when N empty consecutive continuations is reached, the global request to MediaWiki API is ended) (by default 25 empty continuations, hard-coded limit of 1000).

Output
MediaWiki API services are asked to return the results in XML, from which they are extracted to output parameters using XPath. There are two ways to specify output parameters: by referring to the configured parameter and by specifying XPath directly.

The configured output parameter could be used like this: This results in the variable  being bound to the XPath result defined by the " " output variable. Note that the configured output parameters expect certain input parameters to be present, and if you override the configured defaults the output parameters may break: for example, the configuration specifies a default parameter of, and if you override it with  , then the   output parameter will no longer be able to get the   from the response. (You would fix this by looking up the default  in the configuration and then changing your query to use  .)

The direct XPath expression is used like this: In this case, the object of the triple is an XPath string and not  URI.

In both cases, XPath is evaluated relative to item path, specified in service configuration. There's no difference in XPath evaluation for both forms.

The predicates that can be used are,   and. The first form results in a string taken literally; the second interprets the string as Item ID and constructs a full item URI; and the third interprets the string as an URI and constructs an URI value.

Predicate  allows to define variable that would be set to the number of the result in the original API output. Note that due to the multithreaded nature of the SPARQL engine the results are not necessarily returned in the order in which they were received by the API, which may be useful for queries like search where result order is important. Using  will allow to recover the original order. The object of the triple is currently ignored.

Supported services
Currently the following services are supported:

Required parameters are in bold. Other input parameters may also be supported – please refer to the service documentation (linked in Service column) for supported API parameters and their meaning.

The full list of services can be seen in the production config file.

Configuration
The service is configured by a JSON file listing particular API configurations and a list of endpoints. Please see the example configuration.

On the top level, there is a list of endpoint hostnames under " " and API configs under " ", with the keys being service names for use with.

Only endpoints listed in  are allowed. The list can also contain prefixes - i.e.  allows any hostname that ends with.

Individual service configuration
The service configuration has two required elements -  for input parameters and   for output parameters. All the rest are currently ignored by the code, but  and   are used to store description and link to documentation about particular API. It is safe to assume that config entries starting with @ will be always ignored by the code.

Input
Input parameters are keyed by parameter name, and the value can be either constant - in which case the parameter is fixed, always present and its value can not be changed, or object, which contains description of variable parameter.

Variable parameter is bound by the query template or variable. The configuration specifies its type, which is currently ignored but may be enforced later. The configuration can also specify the, which will be used in case the parameter is not specified or not bound. If the default is the parameter will be omitted in case it is not specified or not bound. If no default is specified, the parameter is treated as required and omitting it will result in an error.

Output
Output is defined by , which specifies the XPath for extracting specific result elements for the query, and set of output variables under. Each key in  is variable name (to be used with  ) and the value is XPath to this particular value, calculated relative to the items path. Note that it is possible to also specify absolute paths for specific variables, but in that case you will get copies of that value for each result set.

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