These slides cover data modeling, indexing and DBA tools. Talk given at Meteor London MeetUp

1. featuring special guest Sean Bean

2. Let’s talk about production
MongoDB
Database Happiness
Developer Fun

3. But it doesn’t need to be
this way.

4. Let’s use House Stark as an
example

5. It starts with the data model
_id name home p_id
0 Ned Stark Winterfell 0
1 Arya Stark Winterfell 1
2 Bran Stark Winterfell 2
p_id proficiencies
0
keeping a cool
head
1
pulling off the short
hair look
2 riding piggyback

6. Arrays in MongoDB
{
_id: ObjectId(“000”),
name: “Ned Stark”,
relatives: [
ObjectId(“001”),
ObjectId(“002”)
]
}
{
_id: ObjectId(“000”),
name: “Ned Stark”,
relatives: [
{
_id: ObjectId(“001”),
name: “Arya Stark”,
…
},
{
_id: ObjectId(“002”),
name: “Bran Stark”,
…
}
]
}
vs.

7. What are “total normalization”
trade-offs?
{
_id: ObjectId(“000”),
name: “Ned Stark”,
relatives: [
ObjectId(“001”),
ObjectId(“002”)
]
}

8. Here is Ned Stark.
His relatives are:
ObjectId(“001”) ObjectId(“002”)

9. And “partial denormalization”?
{
_id: ObjectId(“000”),
name: “Ned Stark”,
relatives: [
{
_id: ObjectId(“001”),
name: “Arya Stark”,
img: “../arya.png”
}
{
_id: ObjectId(“002”),
name: “Bran Stark”,
img: “../bran.png”
}
]
}

10. Here is Ned Stark.
His relatives are:
Arya Stark Bran Stark

11. Rules on Arrays
High reads, low updates/writes
Array should be bound in size
Don’t need to frequently access
embedded items individually

12. #1 cause of slow databases:
(lack of) indexes

13. cursor.explain() tells us if we’re
indexed
{
"cursor" : "<Cursor Type and Index>",
"n" : <num>,
"nscannedObjects" : <num>,
"nscanned" : <num>,
"nscannedObjectsAllPlans" : <num>,
"nscannedAllPlans" : <num>,
"scanAndOrder" : <boolean>,
"indexOnly" : <boolean>,
"millis" : <num>,
"indexBounds" : { <index bounds> },
"allPlans" : [
{ "cursor" : "<Cursor Type and Index>",
"n" : <num>,
"nscannedObjects" : <num>,
"nscanned" : <num>,
"indexBounds" : { <index bounds> }
},
...
]
}

14. Remember our indexing
essentials
scanAndOrder
n vs nScanned
NO BasicCursor!!!!!!!!!!!!! NO!
Tier 3
Tier 2
Tier 1

16. How to save your database
• mongod logs
• db.currentOp() & db.killOp()

17. There’s lots of good info in the
logs
• 2014-10-09T02:26:58.994-0700 [conn367784] query gotDB.people query: {
$query: { isAlive: true }, $orderby: { age: 1 } } planSummary: IXSCAN { isAlive:
1 }, cursorid:1736436984952 ntoreturn:10 ntoskip:0 nscanned:7677
nscannedObjects:7677 scanAndOrder:1 keyUpdates:0 numYields:0
locks(micros) r:115557 nreturned:10 reslen:34036 115ms
• 2014-10-09T21:26:58.994-0700 [conn127156] command gotDB.$cmd
command: mapReduce { $msg: "query not recording (too large)" }
planSummary: COLLSCAN keyUpdates:0 numYields:3843 locks(micros)
W:2840 r:85085260 w:26899 reslen:187 76657ms

18. Use currentOp() to triage slow
operations
db.currentOp().inprog.forEach(
function(op) {
if(op.secs_running > 5) printjson(op);
}
)

19. Terminate slowness
{
"locks": {
"^myDB": "R"
},
"ns": "myDB.bar",
"op": "query",
"opid": 1344808,
"query": {},
"secs_running": 53,
"waitingForLock": false
}
db.killOp(1344808)

20. Thanks for listening!
chris@mongolab.com
@chrisckchang
@mongolab