The document discusses MongoDB and how it works. It includes information on BSON, the wire protocol for messages like insert and query, the anatomy of an insert message, how MongoDB uses memory mapped storage, commands like drop, the query optimizer using indexes, issues with geohashing, replication using replica sets and oplogs, and auto-sharding using config and mongos servers.

1. @mongodb @mdirolf http://www.mongodb.org/

2. How it Works
http://www.flickr.com/photos/tmh9/677919415/

3. BSON
{“hello”: “bson”}
x15 x00 x00 x00 x02 h e
l l o x00 x05 x00 x00
x00 b s o n x00 x00
http://bsonspec.org

4. Messages
• TCP/IP Wire Protocol
• Separate messages for
insert, update, query, get_more, delete, etc.
http://www.mongodb.org/display/DOCS/Mongo+Wire+Protocol

5. Anatomy of an insert
use foo;
db.test.insert({“hello”: “bson”});
message length request id response id op code (insert)
x67x00x00x0 xXXxXXxXX x00x00x00x xd2x07x00x00
0 xXX 00
reserved collection name document(s)
x00x00x00x00 f o o . t e s t x00 BSON({“hello”: “bson”})
http://www.mongodb.org/display/DOCS/Mongo+Wire+Protocol

6. Memory mapped
storage engine

7. Commands
db.foo.drop();
=
db.foo.runCommand({drop: "foo"});
=
db.$cmd.findOne({drop: "foo"});
=
db.$cmd.find({drop: "foo"}).limit(-1);

8. Query optimizer
find({x: 10, y: “foo”})
scan
terminate
index on x
index on y remember

9. Geohashing
(20, 10)
(0001 0100, 0000 1010)
0000 0010 0110 0100
(21, 9) 0000 0010 0110 0011
Problem: bit-flips (127 vs 128)

10. Replication
Replica sets
Master - slave
master slave
master
master slave
slave slave slave
master master
slave
slave master

11. Replication
• oplog - capped collection
• idempotent ($inc -> $set)

12. Auto-sharding
Shards
mongod mongod mongod
...
Config mongod mongod mongod
Servers
mongod
mongod
mongod
mongos mongos ...
client