This document provides tips and explanations for various MongoDB concepts and operations. It discusses basic expectations of MongoDB including indexing order, write and query threads, and memory mapped data files. It also summarizes indexes, shell functions, connections, getLastError commands, profiling, log configuration, limiting and sorting queries, bulk data loading, collection statistics, backups, and the mongostat monitoring tool.

1. Tips, Trick and HacksGet’n it done!

2. Basic ExpectationsNo transactionsFast-N-Loose by default (no safe/w/GLE)Indexing order matters; query optimizer helpsOne write threadOne JS thread (M/R, $where, group)Many query threads (non-js)Memory Mapped Data FilesBSON everywhere

3. IndexesIndex on A, B, C works for [A], [A,B], [A,B,C]Query Optimizer figures out orderHint when you knowMissing values are indexed as “null” valueJust like real “null” valuesUnique indexes include missing/nullsSort works on last field

4. Shell: FunctionsLeave off the () to see the function:> db.coll.findfunction (query, fields, limit, skip) { return new DBQuery(this._mongo, this._db, this, this._fullName, this._massageObject(query), fields, limit, skip); }

5. ConnectionsOrder of ops is only preserved in the same connection (socket/port)Connection pools are good, but could be a problem (getLastError/ordered ops)Server only executes one (concurrent) operation per connection

6. getLastError()More like getLastOpStatus()Returns useful data on update/findAndModify/insert/remove ops> db.t.update({x:1}, {$inc:{y:1}}, true, true)> db.getLastErrorObj(){ "err" : null, "updatedExisting" : false, "upserted" : ObjectId("4c49f54cab620000000071b7"), "n" : 1, "ok" : 1 }

7. getLastErrorParamsParamsw: number of replicas to write towtimeout: time to wait for acknowledgementsfsync: flush to disk{getlasterror : 1, w : 40, wtimeout: 3000}{ "err" : null, "n" : 0, "wtimeout" : true, "waited" : 3006, "errmsg" : "timed out waiting for slaves", "ok" : 0 }

8. Shell Command Line--evalit/cursorPrinting values; be carefulPass in a script

9. Enable ProfilingsetProfilingLevel(lvl, <ms>)0: none1: time-based2: allReading from profile collection>db.system.profile.find()

10. _id: ObjectId Generated on ClientMost drivers create “_id” field (if not set)No way to get “_id” after insert (from server)Nothing you can’t do yourselfclass Foo {ObjectId id = new ObjectId() ….}

11. mongod.confSpecify a logpath or it goes to /dev/nullQuiet = true/anythingDon’t do Quiet = falseFlag options used no matter what valuesV[v*] = true for verbose logging

12. Limit/Sort/PaginationSort fields should be the last in the indexLimit helps with in-memory sorts (diff alg.)Skip/Offset still walks through indexPagination is best using last “_id”/sort valueSet batchsize = pagesize (under 4MB)

13. Bulk Data LoadingInitial loadCreate the index after loadStart with new DB if possibleUpdatesBe careful of excessive indexesSometimes better to drop and recreate indexes

14. Printing Collection Statsdb.getCollectionNames().forEach(function(x){ print(“Collection: “ + x);printjson(db[x].stats());})

15. BackupsOnly way to get a consistent stateDon’t use mongoexport (lacks type fidelity)Use mongodump (snap-shotted query)Or fsync+copy1.) fsync + lock2.) backup dbpath files3.) release lock

16. mongostatWatch % idx missfaults/secflushes/sec

17. QuestionsYou don’t have to go home, but you can’t…ScottHernandez@gmail.com