Noticed this due to high indexing failures reported by elasticsearch today, looks like the log is malformed and can't be parsed as valid json. Not sure if this is aqs-specific or related to some library.
[2020-09-15T12:52:43,315][WARN ][logstash.outputs.elasticsearch] Could not index event to Elasticsearch. {:status=>400, :action=>["index", {:_id=>nil, :_index=>"logstash-2020.09.15", :routing=>nil, :_type=>"_doc"}, #<LogStash::Event:0x2e39b975>], :response=>{"index"=>{"_index"=>"logstash-2020.09.15", "_type"=>"_doc", "_id"=>"5SbTkXQBGIXcFGALEnhm", "status"=>400, "error"=>{"type"=>"mapper_parsing_exception", "reason"=>"failed to parse field [res.body] of type [text] in document with id '5SbTkXQBGIXcFGALEnhm'. Preview of field's value: '{stack=Error: Operation timed out - received only 1 responses.
at ResponseError.DriverError (/srv/deployment/analytics/aqs/deploy-cache/revs/95d6432bab3f85ff30a70cc776ee5dd06f478340/node_modules/cassandra-driver/lib/errors.js:14:19)
at new ResponseError (/srv/deployment/analytics/aqs/deploy-cache/revs/95d6432bab3f85ff30a70cc776ee5dd06f478340/node_modules/cassandra-driver/lib/errors.js:51:24)
at FrameReader.readError (/srv/deployment/analytics/aqs/deploy-cache/revs/95d6432bab3f85ff30a70cc776ee5dd06f478340/node_modules/cassandra-driver/lib/readers.js:317:13)
at Parser.parseBody (/srv/deployment/analytics/aqs/deploy-cache/revs/95d6432bab3f85ff30a70cc776ee5dd06f478340/node_modules/cassandra-driver/lib/streams.js:194:66)
at Parser._transform (/srv/deployment/analytics/aqs/deploy-cache/revs/95d6432bab3f85ff30a70cc776ee5dd06f478340/node_modules/cassandra-driver/lib/streams.js:137:10)
at Parser.Transform._read (_stream_transform.js:190:10)
at Parser.Transform._write (_stream_transform.js:178:12)
at doWrite (_stream_writable.js:410:12)
at writeOrBuffer (_stream_writable.js:394:5)
at Parser.Writable.write (_stream_writable.js:294:11)
at Protocol.ondata (_stream_readable.js:667:20)
at Protocol.emit (events.js:189:13)
at addChunk (_stream_readable.js:284:12)
at readableAddChunk (_stream_readable.js:265:11)
at Protocol.Readable.push (_stream_readable.js:220:10)
at Protocol.Transform.push (_stream_transform.js:151:32), err={consistencies=6, stack=Error: Operation timed out - received only 1 responses.
at ResponseError.DriverError (/srv/deployment/analytics/aqs/deploy-cache/revs/95d6432bab3f85ff30a70cc776ee5dd06f478340/node_modules/cassandra-driver/lib/errors.js:14:19)
at new ResponseError (/srv/deployment/analytics/aqs/deploy-cache/revs/95d6432bab3f85ff30a70cc776ee5dd06f478340/node_modules/cassandra-driver/lib/errors.js:51:24)
at FrameReader.readError (/srv/deployment/analytics/aqs/deploy-cache/revs/95d6432bab3f85ff30a70cc776ee5dd06f478340/node_modules/cassandra-driver/lib/readers.js:317:13)
at Parser.parseBody (/srv/deployment/analytics/aqs/deploy-cache/revs/95d6432bab3f85ff30a70cc776ee5dd06f478340/node_modules/cassandra-driver/lib/streams.js:194:66)
at Parser._transform (/srv/deployment/analytics/aqs/deploy-cache/revs/95d6432bab3f85ff30a70cc776ee5dd06f478340/node_modules/cassandra-driver/lib/streams.js:137:10)
at Parser.Transform._read (_stream_transform.js:190:10)
at Parser.Transform._write (_stream_transform.js:178:12)
at doWrite (_stream_writable.js:410:12)
at writeOrBuffer (_stream_writable.js:394:5)
at Parser.Writable.write (_stream_writable.js:294:11)
at Protocol.ondata (_stream_readable.js:667:20)
at Protocol.emit (events.js:189:13)
at addChunk (_stream_readable.js:284:12)
at readableAddChunk (_stream_readable.js:265:11)
at Protocol.Readable.push (_stream_readable.js:220:10)
at Protocol.Transform.push (_stream_transform.js:151:32), coordinator=10.64.16.78:9042, code=4608, isDataPresent=1, query=select \"project\",\"article\",\"granularity\",\"timestamp\",\"aa\",\"ab\",\"as\",\"au\",\"da\",\"db\",\"ds\",\"du\",\"maa\",\"mab\",\"mas\",\"mau\",\"mwa\",\"mwb\",\"mws\",\"mwu\",\"_domain\",\"_del\",\"_tid\" from \"local_group_default_T_pageviews_per_article_flat\".\"data\" where \"project\" = ? AND \"article\" = ? AND \"granularity\" = ? AND \"timestamp\" >= ? AND \"timestamp\" <= ? AND \"_domain\" = ?, name=ResponseError, received=1, message=Operation timed out - received only 1 responses., blockFor=2, info=Represents an error message from the server}, title=Error in Cassandra table storage backend, type=query_error, req={headers={}, body={\"table\":\"pageviews.per.article.flat\",\"attributes\":{\"project\":\"es.wikipedia\",\"article\":\"Primera_Guerra_Mundial\",\"granularity\":\"daily\",\"timestamp\":{\"between\":[\"2020090700\",\"2020091100\"]},\"_domain\":\"ana, uri=/analytics.wikimedia.org/sys/table/pageviews.per.article.flat/}}'", "caused_by"=>{"type"=>"illegal_state_exception", "reason"=>"Can't get text on a START_OBJECT at 1:792"}}}}}