NoSQL Data Stores - Roma Tre Universitytorlone/bd2/noSQL-2.pdfNoSQL Systems > Memcached • What...

NoSQL Data Stores

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NoSQL Systems > Timeline

2003 Memcached2006 Google BigTable2007 Amazon Dynamo

2007 HBase2008 Cassandra, CouchDB2009 P.Voldemort, Redis, Riak, MongoDB

30/05/2011 Sistemi NoSQL 2

NoSQL Systems > Memcached

• What ismemcached– Caching system intended to alleviate database load.– In‐memory key‐value store for small chunks of data.

• Extremely successful– Facebook, Yahoo, Wikipedia, Ebay, Digg, ….


Memcached > How does it work


Super simple!

v = memcachedClient.get(key);if(v == NULL) {

v = db.query( SOME SLOW QUERY );memcachedClient.set(key, v);

}

Key‐value cache1. Keys are hashed2. Hash table span across an

arbitrary number of servers

NoSQL Systems > Google BigTable


NoSQL Systems > Google BigTable


• BigTable is a distributed storagesystem for managing structureddata that is designed to scale to a very large size.

• Petabytes of data across thousands of commodity servers.

• Built on top of Google File System

Google BigTable > Data Model


…

Row Id ColumnFamily1 ColumnFamily2 … ColumnFamilyN

rowid1 qualifier1 = “abc”qualifier2 = “def”qualifier3 = “123”…

qualifier1 = “xyz”qualifier5 = “fgh”

… …

rowid2…

• Column Families are (the only things) defined in the schema• Qualifiers are added dynamically.

• Simple queries• Get a row by key• Get a range of rows by (start key, end key)

Google BigTable > Data Model > Example


• Student – Course– 1 student > many courses– 1 course > many students

Studentsid PKnameemailbirthdate

Courseid PKtitledescriptionteacher_id

Student2Coursestudent_idcourse_id

Google BigTable > Data Model > Example


De‐normalized data

Single key‐space

Google BigTable > Infrastructure

• Partition model: sharding on the row key :– Data is divided into tablets– Each tablet is defined by the range of row keys it isresponsible for (start key – end key)

– Each tablet is served by one tablet server at a time– Each tablet server may serve (has the lock for) manytablets.

• Distributed locking service called Chubby– Manages tablet servers lifecycle



• Three‐level hierarchy to store tablet location– Analogous to a B+ Tree


Master ServerTablet Servers



Client Master Server

Tablet Server

Tablet Server

Tablet Server

Tablet Serverrequest

request

response

• Strong consistency– Only one tablet server is responsible for a given piece of data.– Replication is handled on the GFS layer

• Trade‐off with availability– If a tablet server fails, its portion of data is temporarily unavailable until a new

server is assigned

NoSQL Systems > Amazon Dynamo

“An extra tenth of second in response times will cost us1% in sales” ‐ Amazon

• Dynamo: Highly available key‐value store

• Challenge: reliability at massive scale– Tens of millions of customers.– Tens of thousands of servers.


Amazon Dynamo > Data Model

• Binary objects (i.e. blobs) identified by uniquekeys

• Query model: – Simple read and write operations to data retrievedby primary key

– No operations span multiple data items


Amazon Dynamo > Infrastructure

• Partitioning similar to P2P (Chord, Pastry, etc.)– Keys are hashed.– The range of the hash function is treated as a circular space (ring).

– Each node is responsible for a region of the ring.– Distributed Hash Table (DHT)


AA

N=1N=1

N=2N=2

N=2N=2

N=3N=3



“AE107FB…”

• Each node is responsiblefor the region between itand its N predecessors.

• N is tuned on per‐nodebasis


• Replication– Each data item is replicated at many hosts

• Eventual consistency– Updates are propagated to replicas asynchronously– The system eventually reaches a consistent state

• Tradeoff between consistency and availability– Number of replicas is crucial


Case Study > Facebook Messages


Case Study > Facebook Messages

• Real‐time, reliable messaging system that combines chat, messagesand emails.

• 135+ billion messages per month

• Two main usage patterns– A short set of temporal data that tends to be volatile– An ever growing set of data that rarely gets accessed

• Candidate systems: – MySQL– Apache Cassandra– Apache HBase


Facebook Messages > MySQL

• Attractive choice:+ Facebook core infrastructure is MySQL‐based

• It is indeed a giant LAMP application+ Facebook team has extensive knowledge in running and managing MySQL

• But…– MySQL clustering is hard to mantain (and scale)– MySQL performances suffer with large indexes and data sets


Facebook Messages > Apache HBase

• BigTable’s open‐source clone– Extensible record store– Strong consistency

• Availability trade‐off

• Part of the Hadoop ecosystem– Built on top of HDFS– Integrates with Hive, ZooKeeper, etc.


Facebook Messages > Apache Cassandra

• Marriage between BigTable and Dynamo– Data model: Extensible record store (BigTable)– Infrastructure: Distributed Hash Table (Dynamo)

• Eventual consistency• High availability

• Developed by Facebook itself– To serve the (old) inbox system


Facebook Messages > Evaluation results

• MySQL soon discarded• Hbase vs Cassandra


Data model Consistency model Availability

HBase Extensible record store

Strong consistency ‐ Replicas managed by HDFS

‐ Region servers are singlepoints of failure

Cassandra Extensible record store

Eventual consistency ‐ No single point of failure

Facebook Messages > Evaluation results

• MySQL soon discarded• Hbase vs Cassandra


Data model Consistency model Availability

HBase Extensible record store

Strong consistency ‐ Replicas managed by HDFS

‐ Region servers are singlepoints of failure

Cassandra Extensible record store

Eventual consistency ‐ No single point of failure

• Hbase won– Strong consistency is a better match for real‐time systems

NoSQL Systems > Overview

• We have seen:– Extensible record stores

• BigTable, HBase, Cassandra

– Key‐value stores• Dynamo

• There’s more to it!– Document stores


NoSQL Systems > Document stores

• Systems that store collections of documents

• What is a document?– Generally, an object with a number of fields, whosevalues can be scalars, lists, or nested documents aswell

• e.g.: XML, JSON


Case Study > Guardian.co.uk


Guardian.co.uk > 2005‐09

Modern Java application– Strong model in Java– Oracle RDBMS– Database abstractedwith ORM


Problems: increasing complexity– Complex Hibernate binding (10.000+ lines of XML config)– Lots of optimisations– Complex caching strategy– Load becoming an issue– …



• Introduce yet more caching

Memcached

• Decouple applications from db by building APIs– Power APIs using scalable technologies (Apache Solr)– JSON results

DB Load



Three models now:– RDBMS Tables– Java objects– JSON API

JSON model is very simple:– Multiple domain objects expressed in a single doc– Can be designed in a forwardly extensible way

headache



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What if the JSON API was the primary model?• CouchDB• MongoDB

What if the JSON API was the primary model?• CouchDB• MongoDB

NoSQL Systems > MongoDB vs CouchDB


CouchDB MongoDB

Data Model Collections of JSON docs Collections of BSON docs

Queries Low‐level query language Rich, declarative query language

Consistency Model Eventual Consistency Strong Consistency (tunable though)

Replication Master‐Master Master‐Slave

Scalability Through replication Sharding

NoSQL Systems > MongoDB vs CouchDB


CouchDB MongoDB

Data Model Collections of JSON docs Collections of BSON docs

Queries Low‐level query language Rich, declarative query language

Consistency Model Eventual Consistency Strong Consistency (tunable though)

Replication Master‐Master Master‐Slave

Scalability Through replication Sharding

• MongoDB was chosen:• Can easily express complex queries• Good if you come from RDBMS• No need for extreme scalability (where CouchDB shines)

NoSQL Systems > Links and References

• Rick Cattel – Scalable SQL and NoSQL Datastores• R.Cattel, M.Stonebraker – Ten Rules for Scalable Performance in “Simple

Operation” Datastores• M.Stonebraker – SQL vs NoSQL Databases

• A.Popescu – MyNoSQL Blog

• Chang et al. – Google BigTable• DeCandia et al – Amazon Dynamo

• We have encountered:– Cassandra – cassandra.apache.org– Hbase ‐ hbase.apache.org– CouchDB ‐ couchdb.apache.org– MongoDB ‐ http://www.mongodb.org– Memcached ‐memcached.org


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