Why Wordnik went non-relational

NoSQL Now 2011Why Wordnik went Non-Relational
Tony Tam
@fehguy

What this Talk is About
5 Key reasons why Wordnik migrated into a Non-Relational database
Process for selection, migration
Optimizations and tips from living survivors of the battle field

Why Should You Care?
MongoDB user for almost 2 years
Lessons learned, analysis, benefits from process
We migrated from MySQL to MongoDB with no downtime
We have interesting/challenging data needs, likely relevant to you

More on Wordnik
World’s fastest updating English dictionary
Based on input of text up to 8k words/second
Word Graph as basis to our analysis
Synchronous & asynchronous processing
10’s of Billions of documents in NR storage
20M daily REST API calls, billions served
Powered by Swagger OSS API framework
swagger.wordnik.com
Powered API

Architectural History
2008: Wordnik was born as a LAMP AWS EC2 stack
2009: Introduced public REST API, powered wordnik.com, partner APIs
2009: drank NoSQL cool-aid
2010: Scala
2011: Micro SOA

Non-relational by Necessity
Moved to NR because of “4S”
Speed
Stability
Scaling
Simplicity
But…
MySQL can go a LONG way
Takes right team, right reasons (+ patience)
NR offerings simply too compelling to focus on scaling MySQL

Wordnik’s 5 Whys for NoSQL

Why #1: Speed bumps with MySQL
Inserting data fast (50k recs/second) caused MySQL mayhem
Maintaining indexes largely to blame
Operations for consistency unnecessary but "cannot be turned off”
Devised twisted schemes to avoid client blocking
Aka the “master/slave tango”

Why #2: Retrieval Complexity
Objects typically mapped to tables
Object Hierarchy always => inner + outer joins
Lots of static data, so why join?
“Noun”is not getting renamed in my code’s lifetime!
Logic like this is probably in application logic
Since storage is cheap
I’ll choose speed

One definition = 10+ joins
50 requests per second!

Embed objects in rows “sort of works”
Filtering gets really nasty
Native XML in MySQL?
If a full table-scan is OK…
OK, then cache it!
Layers of caching introduced layers of complexity
Stale data/corruption
Object versionitis
Cache stampedes

Why #3: Object Modeling
Object models being compromised for sake of persistence
This is backwards!
Extra abstraction for the wrong reason
OK, then performance suffers
In-application joins across objects
“Who ran the fetch all query against production?!” –any sysadmin
“My zillionth ORM layer that only I understand” (and can maintain)

Why #4: Scaling
Needed "cloud friendly storage"
Easy up, easy down!
Startup: Sync your data, and announce to clients when ready for business
Shutdown: Announce your departure and leave
Adding MySQL instances was a dance
Snapshot + bin files
mysql> change master to MASTER_HOST='db1', MASTER_USER='xxx', MASTER_PASSWORD='xxx', MASTER_LOG_FILE='master-relay.000431', MASTER_LOG_POS=1035435402;

Why #4: Scaling
What about those VMs?
So convenient! But… they kind of suck
Can the database succeed on a VM?
VM Performance:
Memory, CPU or I/O—Pick only one
Can your database really reduce CPU or disk I/O with lots of RAM?

Why #5: Big Picture
BI tools use relational constraints for discovery
Is this the right reason for them?
Can we work around this?
Let’s have a BI tool revolution, too!
True service architecture makes relational constraints impractical/impossible
Distributed sharding makes relational constraints impractical/impossible

Why #5: Big Picture
Is your app smarter than your database?
The logic line is probably blurry!
What does count(*)really mean when you add 5k records/sec?
Maybe eventual consistency is not so bad…
2PC? Do some reading and decide!
http://eaipatterns.com/docs/IEEE_Software_Design_2PC.pdf

Ok, I’m in!
I thought deciding was easy!?
Many quickly maturing products
Divergent features tackle different needs
Wordnik spent 8 weeks researching and testing NoSQL solutions
This is a long time! (for a startup)
Wrote ODM classes and migrated our data
Surprise! There were surprises
Be prepared to compromise

Choice Made, Now What?
We went with MongoDB ***
Fastest to implement
Most reliable
Best community
Why?
Why #1: Fast loading/retrieval
Why #2: Fast ODM (50 tps => 1000 tps!)
Why #3: Document Models === Object models
Why #4: MMF => Kernel-managed memory + RS
Why #5: It’s 2011, is there no progress?

More on Why MongoDB
Testing, testing, testing
Used our migration tools to load test
Read from MySQL, write to MongoDB
We loaded 5+ billion documents, many times over
In the end, one server could…
Insert 100k records/sec sustained
Read 250k records/sec sustained
Support concurrent loading/reading

Migration & Testing
Iterated ODM mapping multiple times
Some issues
Type Safety
cur.next.get("iWasAnIntOnce").asInstanceOf[Long]
Dates as Strings
obj.put("a_date", "2011-12-31") !=
obj.put("a_date", new Date("2011-12-31"))
Storage Size
obj.put("very_long_field_name", true) >>
obj.put("vsfn", true)

Migration & Testing
Expect data model iterations
Wordnik migrated table to Mongo collection "as-is”
Easier to migrate, test
_id field used same MySQL PK
Auto Increment?
Used MySQL to “check-out” sequences
One row per mongo collection
Run out of sequences => get more
Need exclusive locks here!

Migration & Testing
Sequence generator in-process
SequenceGenerator.checkout("doc_metadata,100")
Sequence generator as web service
Centralized UID management

Migration & Testing
Expect data access pattern iterations
So much more flexibility!
Reach into objects
> db.dictionary_entry.find({"hdr.sr":"cmu"})
Access to a whole object tree at query time
Overwrite a whole object at once… when desired
Not always! This clobbers the whole record
> db.foo.save({_id:18727353,foo:"bar"})
Update a single field:
> db.foo.update({_id:18727353},{$set:{foo:"bar"}})

Flip the Switch
Migrate production with zero downtime
We temporarily halted loading data
Added a switch to flip between MySQL/MongoDB
Instrument, monitor, flip it, analyze, flip back
Profiling your code is key
What is slow?
Build this in your app from day 1

Flip the Switch

Flip the Switch
Storage selected at runtime
valh = shouldUseMongoDb match {
case true => new MongoDbSentenceDAO
case _ => new MySQLDbSentenceDAO
}
h.find(...)
Hot-swappable storage via configuration
It worked!

Then What?
Watch our deployment, many iterations to mapping layer
Settled on in-house, type-safe mapper
https://github.com/fehguy/mongodb-benchmark-tools
Some gotchas (of course)
Locking issues on long-running updates (more in a minute)
We want more of this!
Migrated shared files to Mongo GridFS
Easy-IT

Performance + Optimization
Loading data is fast!
Fixed collection padding, similarly-sized records
Tail of collection is always in memory
Append faster than MySQL in every case tested
But... random access started getting slow
Indexes in RAM? Yes
Data in RAM? No, > 2TB per server
Limited by disk I/O /seek performance
EC2 + EBS for storage?

Moved to physical data center
DAS & 72GB RAM => great uncached performance
Good move? Depends on use case
If “access anything anytime”, not many options
You want to support this?

Inserts are fast, how about updates?
Well… update => find object, update it, save
Lock acquired at “find”, released after “save”
If hitting disk, lock time could be large
Easy answer, pre-fetch on update
Oh, and NEVER do “update all records” against a large collection

Indexes
Can't always keep index in ram. MMF "does it's thing"
Right-balanced b-tree keeps necessary index hot
Indexes hit disk => mute your pager
17
15
27

More Mongo, Please!
We modeled our word graph in mongo
50M Nodes
80M Edges
80mS edge fetch

More Mongo, Please!
Analytics rolled-up from aggregation jobs
Send to Hadoop, load to mongo for fast access

What’s next
Liberate our models
stop worrying about how to store them (for the most part)
New features almost always NR
Some MySQL left
Less on each release

Questions?
See more about Wordnik APIs
http://developer.wordnik.com
Migrating from MySQL to MongoDB
http://www.slideshare.net/fehguy/migrating-from-mysql-to-mongodb-at-wordnik
Maintaining your MongoDB Installation
http://www.slideshare.net/fehguy/mongo-sv-tony-tam
Swagger API Framework
http://swagger.wordnik.com
Mapping Benchmark
https://github.com/fehguy/mongodb-benchmark-tools
Wordnik OSS Tools
https://github.com/wordnik/wordnik-oss

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Why Wordnik went non-relational

by Tony Tam

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