What are we running from? • Relational databases are the defacto standard for storing data in a web application. • A lot of times, that data isn’t really relational at all. • RDBMS’s have lots of rules that can impact performance.
Atomicity • If any part of the update fails, it all fails. • Databases have to be able to lock tables and rows for operations, which can block or delay other incoming requests.
Consistency • After a transaction, all copies of the data must be consistent with each other (my interpretation). • Replication across lots of shards is expensive especially if there’s locking involved.
Isolation • Data involved in a transaction must be inaccessible to other operations. • Remember the thing about locked rows and tables? • It’s a bummer.
I come not to bury MySQL... • Relational databases are great for a lot of uses. • If you have data that’s actually relational and you need transactions, joins and have a limited number of data types, then an RDBMS will work for you.
Key-Value Benefits • Simple • High performance (usually) because there are no transactions or relations so it’s a simple bucket and lookup. • Extremely flexible • Commonly used as caches in front of slower resources (like MySQL - bazinga!)
Popular Players • memcached - in memory only, extremely efficient hashing algorithm allows you to scale easily to hundreds of nodes. • Redis - persistent, slightly more complex than memcached (has support for arrays) but still highly performant. • Riak - The Rails Machine guys love it. Jesse?
My Uses • memcached: Read-through cache for Rails with cache-money. • redis: persistent cache for results from our algorithm, partitioned by version and instance.
Wide Column • Family of databases modeled on either Google’s BigTable or Amazon’s Dynamo. • Pick two out of three from the CAP theorem in order to get horizontal scalability. • Data stored by column instead of by row.
CAP? • Consistency: All clients always have the same view of the data. • Availability: Each client can always read and write. • Partition Tolerance: The system works well despite physical network partitions
Use cases • Making sense out of large amounts of data where you know your query scenario ahead of time. • Large = 100s of millions of records. • Data-mining log files and other sources of similar data.
Graph Databases • Store nodes, edges and properties • Think of them as Things, Connections and Properties • Good for storing properties and relationships. • Honestly, I don’t fully understand them... anyone?
Document Stores • Short on relationships, tall on rich data types. • Big on eventual consistency and flexible schemas. • Hybrid of traditional RDBMS and Key-Value stores.
Use Cases • Content Management Systems • Applications with rapid partial updates • Anything you don’t need joins or transactions for that you would normally use a RDBMS for.
MongoDB • Support for rich data types: arrays, hashes, embedded documents, etc • Support for adding and removing things from arrays and embedded documents (addToSet, for example). • Map/Reduce support and strong indexes • Regular expression support in queries
Design Considerations • Embedded Documents - Use only if it the embedded document will always be selected with the parent. • Indexes - MongoDB punishes you much earlier for missing indexes than MySQL. • Document size - Currently, documents are limited to 4MB, which should be large enough, but if it’s not...
Real-World MongoDB • We use MongoDB heavily at MIS. • Statistics application and reporting • Top-secret new application • Web crawler and indexer • CMS
And to get a “thing’s” tags? SELECT `tags`.* FROM `tags` INNER JOIN `taggings` ON `tags`.id = `taggings`.tag_id WHERE ((`taggings`.taggable_id = 237) AND (`taggings`.taggable_type = 'Song'))
Yuck! That’s a lot of pain for something so simple. And I didn’t even show you finding things with tag “x”. Or how to set and unset tags on a “thing”. Ouch.
Let’s Make This Easy... def add_tag(tag) tag = Post.clean_tag(tag) self.tags << tag self.add_to_set(:tags => tag) unless self.new_record? end def remove_tag(tag) tag = Post.clean_tag(tag) self.tags.delete(tag) self.pull(:tags => tag) unless self.new_record? end def self.clean_tag(str) str.strip.downcase.gsub(" ","-").gsub(/[^a-z0-9-]/,"") end def self.clean_tags(str) out = [] arr = str.split(",") arr.each do |t| out << self.clean_tag(t) end out end
Why I Love MongoDB • Document model fits how I build web apps. • For most apps, I don’t need transactions. • Eventual consistency is actually OK. • Partial updates and arrays make things that are a pain in SQL-land absolutely painless. • It’s just smart enough without getting in the way.
What’s NoSQL, really? • The right tool for the job. • We’ve got lots of options for storing application data. • The key is picking the one that solves our real problem. • And if an RDBMS is the right tool, that’s OK too.
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