[
https://issues.apache.org/jira/browse/BEAM-7866?focusedWorklogId=288221&page=com.atlassian.jira.plugin.system.issuetabpanels:worklog-tabpanel#worklog-288221
]
ASF GitHub Bot logged work on BEAM-7866:
----------------------------------------
Author: ASF GitHub Bot
Created on: 02/Aug/19 20:12
Start Date: 02/Aug/19 20:12
Worklog Time Spent: 10m
Work Description: jkff commented on pull request #9233: [BEAM-7866] Fix
python ReadFromMongoDB potential data loss issue
URL: https://github.com/apache/beam/pull/9233#discussion_r310280167
##########
File path: sdks/python/apache_beam/io/mongodbio.py
##########
@@ -194,18 +225,72 @@ def display_data(self):
res['mongo_client_spec'] = self.spec
return res
- def _get_avg_document_size(self):
+ def _get_split_keys(self, desired_chunk_size, start_pos, end_pos):
+ # if desired chunk size smaller than 1mb, use mongodb default split size of
+ # 1mb
+ if desired_chunk_size < 1:
+ desired_chunk_size = 1
+ if start_pos >= end_pos:
+ # single document not splittable
+ return []
with MongoClient(self.uri, **self.spec) as client:
- size = client[self.db].command('collstats', self.coll).get('avgObjSize')
- if size is None or size <= 0:
- raise ValueError(
- 'Collection %s not found or average doc size is '
- 'incorrect', self.coll)
- return size
-
- def _get_document_count(self):
+ name_space = '%s.%s' % (self.db, self.coll)
+ return (client[self.db].command(
+ 'splitVector',
+ name_space,
+ keyPattern={'_id': 1},
+ min={'_id': start_pos},
+ max={'_id': end_pos},
+ maxChunkSize=desired_chunk_size)['splitKeys'])
+
+ def _get_last_document_id(self):
with MongoClient(self.uri, **self.spec) as client:
- return max(client[self.db][self.coll].count_documents(self.filter), 0)
+ cursor = client[self.db][self.coll].find(filter={}, projection=[]).sort([
+ ('_id', DESCENDING)
+ ]).limit(1)
+ try:
+ return cursor[0]['_id']
+ except IndexError:
+ raise ValueError('Empty Mongodb collection')
+
+
+class _ObjectIdRangeTracker(OrderedPositionRangeTracker):
+ """RangeTracker for tracking mongodb _id of bson ObjectId type."""
+
+ def _id_to_int(self, id):
+ # id object is bytes type with size of 12
+ ints = struct.unpack('>III', id.binary)
+ return 2**64 * ints[0] + 2**32 * ints[1] + ints[2]
+
+ def _int_to_id(self, numbers):
Review comment:
This kind of bignum arithmetic is extremely hard to get right, please make
sure the unit tests cover enough corner cases. I would recommend to add some
randomized property testing.
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Issue Time Tracking
-------------------
Worklog Id: (was: 288221)
Time Spent: 1h 40m (was: 1.5h)
> Python MongoDB IO performance and correctness issues
> ----------------------------------------------------
>
> Key: BEAM-7866
> URL: https://issues.apache.org/jira/browse/BEAM-7866
> Project: Beam
> Issue Type: Bug
> Components: sdk-py-core
> Reporter: Eugene Kirpichov
> Assignee: Yichi Zhang
> Priority: Blocker
> Fix For: 2.15.0
>
> Time Spent: 1h 40m
> Remaining Estimate: 0h
>
> https://github.com/apache/beam/blob/master/sdks/python/apache_beam/io/mongodbio.py
> splits the query result by computing number of results in constructor, and
> then in each reader re-executing the whole query and getting an index
> sub-range of those results.
> This is broken in several critical ways:
> - The order of query results returned by find() is not necessarily
> deterministic, so the idea of index ranges on it is meaningless: each shard
> may basically get random, possibly overlapping subsets of the total results
> - Even if you add order by `_id`, the database may be changing concurrently
> to reading and splitting. E.g. if the database contained documents with ids
> 10 20 30 40 50, and this was split into shards 0..2 and 3..5 (under the
> assumption that these shards would contain respectively 10 20 30, and 40 50),
> and then suppose shard 10 20 30 is read and then document 25 is inserted -
> then the 3..5 shard will read 30 40 50, i.e. document 30 is duplicated and
> document 25 is lost.
> - Every shard re-executes the query and skips the first start_offset items,
> which in total is quadratic complexity
> - The query is first executed in the constructor in order to count results,
> which 1) means the constructor can be super slow and 2) it won't work at all
> if the database is unavailable at the time the pipeline is constructed (e.g.
> if this is a template).
> Unfortunately, none of these issues are caught by SourceTestUtils: this class
> has extensive coverage with it, and the tests pass. This is because the tests
> return the same results in the same order. I don't know how to catch this
> automatically, and I don't know how to catch the performance issue
> automatically, but these would all be important follow-up items after the
> actual fix.
> CC: [~chamikara] as reviewer.
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