"InterfaceError: connection already closed" when using multiprocessing.Pool on black box function that queries PostgreSQL database - postgresql

Question

I've been given a Python (2.7) function that takes 3 strings as arguments, and returns a list of dictionaries. Due to the nature of the project, I can't alter the function, which is quite complex, calling several other non-standard Python modules and querying a PostgreSQL database using psychopg2. I think that it's the Postgres functionality that's causing me problems.
I want to use the multiprocessing module to speed up calling the function hundreds of times. I've written a "helper" function so that I can use multiprocessing.Pool (which takes only 1 argument) with my function:
from function_script import function
def function_helper(args):
return function(*args)
And my main code looks like this:
from helper_script import function_helper
from multiprocessing import Pool
argument_a = ['a0', 'a1', ..., 'a99']
argument_b = ['b0', 'b1', ..., 'b99']
argument_c = ['c0', 'c1', ..., 'c99']
input = zip(argument_a, argument_b, argument_c)
p = Pool(4)
results = p.map(function_helper, input)
print results
What I'm expecting is a list of lists of dictionaries, however I get the following errors:
Traceback (most recent call last):
File "/local/python/2.7/lib/python2.7/site-packages/variantValidator/variantValidator.py", line 898, in validator
vr.validate(input_parses)
File "/local/python/2.7/lib/python2.7/site-packages/hgvs/validator.py", line 33, in validate
return self._ivr.validate(var, strict) and self._evr.validate(var, strict)
File "/local/python/2.7/lib/python2.7/site-packages/hgvs/validator.py", line 69, in validate
(res, msg) = self._ref_is_valid(var)
File "/local/python/2.7/lib/python2.7/site-packages/hgvs/validator.py", line 89, in _ref_is_valid
var_x = self.vm.c_to_n(var) if var.type == "c" else var
File "/local/python/2.7/lib/python2.7/site-packages/hgvs/variantmapper.py", line 223, in c_to_n
tm = self._fetch_TranscriptMapper(tx_ac=var_c.ac, alt_ac=var_c.ac, alt_aln_method="transcript")
File "/local/python/2.7/lib/python2.7/site-packages/hgvs/decorators/lru_cache.py", line 176, in wrapper
result = user_function(*args, **kwds)
File "/local/python/2.7/lib/python2.7/site-packages/hgvs/variantmapper.py", line 372, in _fetch_TranscriptMapper
self.hdp, tx_ac=tx_ac, alt_ac=alt_ac, alt_aln_method=alt_aln_method)
File "/local/python/2.7/lib/python2.7/site-packages/hgvs/transcriptmapper.py", line 69, in __init__
self.tx_identity_info = hdp.get_tx_identity_info(self.tx_ac)
File "/local/python/2.7/lib/python2.7/site-packages/hgvs/decorators/lru_cache.py", line 176, in wrapper
result = user_function(*args, **kwds)
File "/local/python/2.7/lib/python2.7/site-packages/hgvs/dataproviders/uta.py", line 353, in get_tx_identity_info
rows = self._fetchall(self._queries['tx_identity_info'], [tx_ac])
File "/local/python/2.7/lib/python2.7/site-packages/hgvs/dataproviders/uta.py", line 216, in _fetchall
with self._get_cursor() as cur:
File "/local/python/2.7/lib/python2.7/contextlib.py", line 17, in __enter__
return self.gen.next()
File "/local/python/2.7/lib/python2.7/site-packages/hgvs/dataproviders/uta.py", line 529, in _get_cursor
cur.execute("set search_path = " + self.url.schema + ";")
File "/local/python/2.7/lib/python2.7/site-packages/psycopg2/extras.py", line 144, in execute
return super(DictCursor, self).execute(query, vars)
DatabaseError: SSL error: decryption failed or bad record mac
And:
Traceback (most recent call last):
File "/local/python/2.7/lib/python2.7/site-packages/variantValidator/variantValidator.py", line 898, in validator
vr.validate(input_parses)
File "/local/python/2.7/lib/python2.7/site-packages/hgvs/validator.py", line 33, in validate
return self._ivr.validate(var, strict) and self._evr.validate(var, strict)
File "/local/python/2.7/lib/python2.7/site-packages/hgvs/validator.py", line 69, in validate
(res, msg) = self._ref_is_valid(var)
File "/local/python/2.7/lib/python2.7/site-packages/hgvs/validator.py", line 89, in _ref_is_valid
var_x = self.vm.c_to_n(var) if var.type == "c" else var
File "/local/python/2.7/lib/python2.7/site-packages/hgvs/variantmapper.py", line 223, in c_to_n
tm = self._fetch_TranscriptMapper(tx_ac=var_c.ac, alt_ac=var_c.ac, alt_aln_method="transcript")
File "/local/python/2.7/lib/python2.7/site-packages/hgvs/decorators/lru_cache.py", line 176, in wrapper
result = user_function(*args, **kwds)
File "/local/python/2.7/lib/python2.7/site-packages/hgvs/variantmapper.py", line 372, in _fetch_TranscriptMapper
self.hdp, tx_ac=tx_ac, alt_ac=alt_ac, alt_aln_method=alt_aln_method)
File "/local/python/2.7/lib/python2.7/site-packages/hgvs/transcriptmapper.py", line 69, in __init__
self.tx_identity_info = hdp.get_tx_identity_info(self.tx_ac)
File "/local/python/2.7/lib/python2.7/site-packages/hgvs/decorators/lru_cache.py", line 176, in wrapper
result = user_function(*args, **kwds)
File "/local/python/2.7/lib/python2.7/site-packages/hgvs/dataproviders/uta.py", line 353, in get_tx_identity_info
rows = self._fetchall(self._queries['tx_identity_info'], [tx_ac])
File "/local/python/2.7/lib/python2.7/site-packages/hgvs/dataproviders/uta.py", line 216, in _fetchall
with self._get_cursor() as cur:
File "/local/python/2.7/lib/python2.7/contextlib.py", line 17, in __enter__
return self.gen.next()
File "/local/python/2.7/lib/python2.7/site-packages/hgvs/dataproviders/uta.py", line 526, in _get_cursor
conn.autocommit = True
InterfaceError: connection already closed
Does anybody know what might cause the Pool function to behave like this, when it seems so simple to use in other examples that I've tried? If this isn't enough information to go on, can anyone advise me on a way of getting to the bottom of the problem (this is the first time I've worked with someone else's code)? Alternatively, are there any other ways that I could use the multiprocessing module to call the function hundreds of times?
Thanks

Answer 1

I think what may be happening is that your connection object is used across all workers and when 1 worker has completed all its tasks it closes the connection and meanwhile the other workers are still working and the connection is closed so when one of those workers tries to use the db it is already closed.