在Linux中从python执行sparql查询到virtuoso服务器？

Question

I am having problem for running the following program ( sparql_test.py ). I am running it from Linux machine. I am installing Virtuoso server in the same Linux machine. In the Linux server, I don't have sudo permission nor browser access. But, I can execute SPARQL query from isql prompt ( SQL> ) successfully.

Program: sparql_test.py

from SPARQLWrapper import SPARQLWrapper, JSON
sparql = SPARQLWrapper("http://localhost:8890/sparql")
sparql.setQuery("select ?s where { ?s a <http://ehrofip.com/data/Admissions>.} limit  10")
sparql.setReturnFormat(JSON)
result = sparql.query().convert()
for res in result["results"]["bindings"]:
    print(res)

---

I got the following error:

[suresh@deodar complex2vec]$ python sparql_test.py
Traceback (most recent call last):
File "sparql1.py", line 14, in "<module>"
  result = sparql.query().convert()
File "/home/suresh/.local/lib/python2.7/site-packages/SPARQLWrapper/Wrapper.py", line 687, in query
  return QueryResult(self._query())
File "/home/suresh/.local/lib/python2.7/site-packages/SPARQLWrapper/Wrapper.py", line 667, in _query
  raise e
urllib2.HTTPError: HTTP Error 502: Bad Gateway

However, the above program run smoothly in my own laptop. What might be the problem? Is this issue of connection?

Thank you

Best,

Suresh

Answer 1

I do not believe this error is raised by Virtuoso. I believe it is raised by SPARQLWrapper.

It looks like there's something between the outside world (which includes the Linux machine itself) and the Virtuoso listener on port 8890 . The "Bad Gateway" suggests there may be two things -- a reverse proxy, and a firewall.

Port 8890 (set as [HttpServer]:Listen in the INI file) must be open to communications, direct or proxied, for SPARQL access to work.

iSQL talks to port 1111 (set as [Parameters]:Listen in the INI file), which apparently doesn't have a similar block/proxy.