MongoDB: tips, trick and hacks

Tips, Trick and Hacks
Get’n it done!

Basic Expectations
No transactions
Fast-N-Loose by default (no safe/w/GLE)
Indexing order matters; query optimizer helps
One write thread
One JS thread (M/R, $where, group)
Many query threads (non-js)
Memory Mapped Data Files
BSON everywhere

Indexes
Index on A, B, C works for [A], [A,B], [A,B,C]
Query Optimizer figures out order
Hint when you know
Missing values are indexed as “null” value
Just like real “null” values
Unique indexes include missing/nulls
Sort works on last field

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

Connections
Order 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

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 }

getLastErrorParams
Params
w: number of replicas to write to
wtimeout: time to wait for acknowledgements
fsync: 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 }

Shell Command Line
--eval
it/cursor
Printing values; be careful
Pass in a script

Enable Profiling
setProfilingLevel(lvl, <ms>)
0: none
1: time-based
2: all
Reading from profile collection
>db.system.profile.find()

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

mongod.conf
Specify a logpath or it goes to /dev/null
Quiet = true/anything
Don’t do Quiet = false
Flag options used no matter what values
V[v*] = true for verbose logging

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

Bulk Data Loading
Initial load
Create the index after load
Start with new DB if possible
Updates
Be careful of excessive indexes
Sometimes better to drop and recreate indexes

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

Backups
Only way to get a consistent state
Don’t use mongoexport (lacks type fidelity)
Use mongodump (snap-shotted query)
Or fsync+copy
1.) fsync + lock
2.) backup dbpath files
3.) release lock

mongostat
Watch
% idx miss
faults/sec
flushes/sec

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

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MongoDB: tips, trick and hacks

by Scott Hernandez

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