MongoDB (Or: How I learned to stop worrying, and love the meshuggah DB)

A high level overview of MongoDB given by Jeff Hickson on May 9th, 2014, including its sharding capabilities common gotchas.

1. MongoDB Or: How I learned to stop worrying, and love the meshuggah DB.
2. How I know mongo so well. I have spent about 30+ hours like this, talking to mongo people around the world. At all times of day.
3. How mongo works. Main Components: • mongod ◦ config server • mongos • tools • driver (language)
4. Components - Non-Sharded Mongod • Main database program. • Stores data, serves queries. Driver • Programming language component to allow you to query mongo from $language.
5. Components - Non-Sharded Tools! • Tailing the log. Very useful. • mongostat - Like top for mongo. • mongotop - a not as useful top for mongo. • mongodump/restore - backup and load. • mongo - a mongo shell. • mongoimport/export - backup to other formats.
6. Components - Sharded Mongos • Acts as a pass through, serves queries to shards, assembles results. ◦ Sorts, merges results, etc. • Can be a performance bottleneck! ◦ Not single-threaded, it just sucks.
7. Components - Sharded Mongod - Config Server • Normal mongod running with a special start option. • Holds metadata about what information is on what shard. ◦ Super-critical information. Cannot actively re-gen.
8. The Ws of shards. What? • It breaks your data into chunks and distributes over a number of servers. Why? • To scale write performance. Read performance can scale with replicas.
9. The Ws of shards. When? • Before you have write throughput issues. How? • Very Carefully, and with some downtime. ◦ 2-3 restarts to enable it. ◦ Time to migrate data to other shards (days-weeks) ◦ Create a sharded cluster at the start.
10. Turning on sharding. First, you need config servers. • You can only have one or three. Never have only one. Second, every server should have at least one replica. • No, config servers cannot have replicas (this is dumb).
11. Turning on sharding. Third, prepare your shard keys. • Prepare these EXTREMELY carefully. ◦ These determine if the cluster will hotspot, how much it needs to migrate data. ◦ Changing it is extremely painful, hours of downtime. Fourth, add shards into the sharding config, and enable sharding on collections with your keys.
12. Sharding - top-down view
13. Databases, collections, documents. Difference? • Mongo does not enforce schema. You can throw anything into anywhere. ◦ No Pancakes. Mongo MySQL Database Database Collection Table Document Row
14. Indexes • Always use them. • Mongo is picky over using them. • Tack on .explain() to any query to see what mongo wants to do (query plan). • Desperately try to avoid table scans. • Mongo is really slow without indexes.
15. Who loves mongo now?
16. Questions? Do you have any?
17. The end?