NoSQL Databases - CouchDB

By Tom Sausner

Agenda

• Introduction • Review of NoSQL storage options

CAP Theorem Review categories of storage options

• CouchDB Overview Interacting with data Examples

• Technologies applying Couch DB

What does it mean?

• Not Only SQL or NO! SQL• A more general definition… a datastore

that does not follow the relational model including using SQL to interact with the data.

• Why? One size does not fit all Relational Model has scaling issues Freedom from the tyranny of the DBA?

CAP Theorem

• Eric Brewer of U.C. Berkeley, Seth Gilbert and Nancy Lynch, of MIT

• Relates to distributed systems• Consistency, Availability, Partition

Tolerance… pick 2• A distributed system is built of

“nodes” (computers), which can (attempt to) send messages to each other over a network….

Consistency

• “is equivalent to requiring requests of the distributed shared memory to act as if they were executing on a single node, responding to operations one at a time.” Not the same as “ACID”

• Linearizability ~ operations behave as if there were no concurrency.

• Does not mention transactions

Available

• “every request received by a non-failing node in the system must result in a response.”

• says nothing about the content of the response. It could be anything; it need not be “successful” or “correct”.

Partition Tolerant

• any guarantee of consistency or availability is still guaranteed even if there is a partition.

• if a system is not partition-tolerant, that means that if the network can lose messages or any nodes can fail, then any guarantee of atomicity or consistency is voided.

Implications of CAP

• How to best scale your application? The world falls broadly into two ideological camps: the database crowd and the non-database crowd.

• The database crowd, unsurprisingly, like database technology and will tend to address scale by talking of things like optimistic locking and sharding

• The non-database crowd will tend to address scale by managing data outside of the database environment (avoiding the relational world) for as long as possible.

Types of NoSQL datastores

• Key - value stores• Column stores• Document stores• Oject stores

Key Value stores

• Memcache ( just merged with CouchDB)

• Redis• Riak

Column Stores

• Big Table ( Google )• Dynamo• Cassandra• Hadoop/HBase

Document Stores

• Couch DB• Mongo

Graph, Object Stores

• Neo4J• db4o

Couch DB - relax ( taken from website)

• An Apache project create by….Damien Katz…• A document database server, accessible via a

RESTful JSON API.• Ad-hoc and schema-free with a flat address

space.• Distributed, featuring robust, incremental

replication with bi-directional conflict detection and management.

• Recently merged with Membase

More on CouchDB

• The CouchDB file layout and commitment system features all Atomic Consistent Isolated Durable (ACID) properties.

• Document updates (add, edit, delete) are serialized, except for binary blobs which are written concurrently.

• CouchDB read operations use a Multi-Version Concurrency Control (MVCC) model where each client sees a consistent snapshot of the database from the beginning to the end of the read operation.

• Eventually Consistent

Couch DB Access via CURL

• curl http://127.0.0.1:5984/

• curl -X GET http://127.0.0.1:5984/_all_dbs

• curl -X PUT http://127.0.0.1:5984/baseball

// error.... already exist• curl -X PUT http://127.0.0.1:5984/baseball

• curl -X DELETE http://127.0.0.1:5984/baseball

Adding Doc’s via CURL

• curl -X PUT http://127.0.0.1:5984/albums• curl -X PUT http://127.0.0.1:5984/albums/1000 -d

'{"title":"Abbey Road","artist":"The Beatles"} '• Uuids curl -X GET http://127.0.0.1:5984/_uuids• curl -X GET http://127.0.0.1:5984/albums/1000• _rev - If you want to update or delete a document,

CouchDB expects you to include the _rev field of the revision you wish to change

• curl -X PUT http://127.0.0.1:5984/albums/1000 -d '{"_rev":"1-42c7396a84eaf1728cdbf08415a09a41","title":"Abbey Road", "artist":"The Beatles","year":"1969"}'

Futon… Couch DB Maintenence

• http://127.0.0.1:5984/_utils/index.html

• Albums database review Add another document

• Tools• Database, Document, View Creation• Secuity, Compact & Cleanup• Create and Delete

Demo Setup

• Examples implemented in Groovy• Use HttpBuilder to interact with the

database• Groovy RESTClient• Use google GSON to move objects

between JSON and Java/Groovy • Use Federal Contribution database for

our dataset.• Eclipse

Data Loading Review

• Limited input to NY candidates, and only year 2010

• contributions.fec.2010.csv• Groovy bean for input data• Readfile.groovy• contribDB.put(path:"fed_contrib_test/$

{contrib.transactionId}", contentType: JSON, requestContentType: JSON, body:json )

Couch DB Design Documents

• CouchDB is designed to work best when there is a one-to-one correspondence between applications and design documents.

• _design/”design_doc_name”• Design Documents are applications

Ie. A CouchDB can be an application.

Design Documents contents

• Update Handler updates: {"hello" : function(doc, req) {…}

• Views ( more on this later)• Validation• Shows• Lists• Filters• libs

Updates

• If you have multiple design documents, each with a validate_doc_update function, all of those functions are called upon each incoming write request

• If any of the validate functions fail then the document is not added to the database

Validation

• Validation functions are a powerful tool to ensure that only documents you expect end up in your databases.

• validate_doc_update section of the view document

• function(newDoc, oldDoc, userCtx) {} throw({forbidden : message}); throw({unauthorized : message});

Ok, how can I see my data?

• CouchDB design documents can contain a “views” section

• Views contain Map/Reduce functions• Map/Reduce functions are

implemented in javascript However there are different Query Servers

available using different languages

Views

• Filtering the documents in your database to find those relevant to a particular process.

• Building efficient indexes to find documents by any value or structure that resides in them

• Extracting data from your documents and presenting it in a specific order.

• Use these indexes to represent relationships among documents.

Map/Reduce dialog

• Bob: So, how do I query the database?• IT guy: It’s not a database. It’s a key-

value store.• Bob: OK, it’s not a database. How do I

query it?• IT guy: You write a distributed map-

reduce function in Erlang.• Bob: Did you just tell me to go screw

myself?• IT guy: I believe I did, Bob.

Map/Reduce in CouchDB

• Map functions have a single parameter a document, and emit a list of key/value pairs of JSON values CouchDB allows arbitrary JSON structures to be

used as keys• Map is called for every document in the

database Efficiency?

• emit() function can be called multiple times in the map function

• View results are stored in B-Trees

Reduce/Rereduce

• The reduce function is optional• used to produce aggregate results for that view• Reduce functions must accept, as input, results

emitted by its corresponding map function as well as results returned by the reduce function itself(rereduce).

• On rereduce the key = null• On a large database objects to be reduced will

be sent to your reduce function in batches. These batches will be broken up on B-tree boundaries, which may occur in arbitrary places.

More on Map/Reduce

• Linked Documents - If you emit an object value which has {'_id': XXX} then include_docs=true will fetch the document with id XXX rather than the document which was processed to emit the key/value pair.

• Complex Keys emit([lastName, firstName, zipcode], doc)

• Grouping• Grouping Levels

Restrictions on Map/Reduce

• Map functions must be referentially transparent. Given the same doc will always issue the same key/value pairs Allows for incremental update

• reduce functions must be able reduce on its own output This requirement of reduce functions allows

CouchDB to store off intermediated reductions directly into inner nodes of btree indexes, and the view index updates and retrievals will have logarithmic cost

List Donors

• Map:function(doc) { if(doc.recipientName){ emit(doc.recipientName, doc); } else if(doc.recipientType){

emit(doc.recipientType, doc) }}No reduce function

List of Query Parameters

• key• startkey, endkey• startkey_docid , endkey_docid• limit, skip, stale, decending• group, grouplevel• reduce• include_docs, inclusive_end

List all NY candidates

• Want a list of all of the unique candidates in the database

• Map: emit(doc.recipientType, null);

• Reduce: return true

• Must set group = true

Total Candidate Donations

• List the total campaign contributions for each candidate

• Map: emit(doc.recipientType, doc.amount)

• Reduce: function(keys, values) {

var sum = 0;for(var idx in keys) { sum = sum + parseFloat(values[idx]); }return sum;

• Must set group=true

Donation Totals by Zip

• Complex Keys• In the map function:

emit([doc.recipientType, doc.contributorZipCode], doc.amount);

• Reduce: function(keys, values) {

var sum = 0;for(var idx in keys) { sum = sum + parseFloat(values[idx]); }return sum;}

Referencing other documents

Conflict Management

• Multi-Version Concurrency Control (MVCC)• CouchDB does not attempt to merge the

conflicting revisions this is an application• If there is a conflict in revisions between

nodes App is ultimately responsible for resolving the

conflict All revisions are saved One revision is selected as the most recent _conflict property set

Database Replication

• “CouchDB has built-in conflict detection and management and the replication process is incremental and fast, copying only documents and individual fields changed since the previous replication.”

• replication is a unidirectional process.• Databases in CouchDB have a sequence

number that gets incremented every time the database is changed.

Replication Continued

• "continuous”: true… automatically replicate over any new docs as

they come into the source to the target…there’s a complex algorithm determining the ideal moment to replicate for maximum performance.

• Create albums_backup using futon replicator• curl -X PUT

http://127.0.0.1:5984/albums/1010 -d '{"title":"Let It Be","artist":"The Beatles"} '

Replication & Conflict

• Replicate albums db via Futon• curl -X PUT

http://127.0.0.1:5984/albums/1050 -d '{"title":”RJUG Roundup","artist":"Rob", ”year":”2010"} ’

• Replicate again• curl -X PUT

http://127.0.0.1:5984/albums_backup/1050 -d '{"title":”RJUG Roundup","artist":"Rob", ”year":”2011"} ’

• Replicate, review

Notifications

• Polling , long polling _changes

• If executing not from a browser can request continuous changes

• Filters can be applied to changes Ex only notify when level = error

• filterName:function(doc, req) Req contains query parameters Also contains userCtx

Security

• ships with OAuth, cookie auth handler, default - standard http

• Authorizations Reader - read/write document Database Admin - compact, add/edit

views Server Admin - create and remove

databases

CouchDB Applied

• CouchOne Hosting Services CouchDB on Android

• CouchApp HTML5 applications

• jCouchDB Java layer for CouchDB access

• CouchDB Lounge Clustering support

Links

• http://couchdb.apache.org/• http://wiki

.apache.org/couchdb/FrontPage• http://guide.couchdb.org/editions/1/

en/index.html

Questions?

• Thanks!