Introduction to CouchDB - LA Hacker News

Michael Parker
February 24, 2013

What is CouchDB?
● One of those hipster NoSQL databases
● Document-oriented, not relational
○ No joins, prefer denormalization

● RESTful API for all operations
● Views add structure via secondary indexes
● Other perks:
○
○
○
○

Multiversion concurrency control (lock-free)
MapReduce framework
Great web admin interface
Multi-master replication

Document-oriented
● Like a key-value store, flat namespace
○ In CouchDB, key is called a document identifier, or
doc id

● Documents stored in JSON format
○ "Schemaless," but the schema resides in your app
○ JSON also format for HTTP body

● Similar: MongoDB (no relation), Redis,
Cassandra

JSON Document in
CouchDB
{
"_id": "emp_001",
"_rev": "1-4c6114c65e295552ab1019e2b046b10e",
"name": {
"first": "Dante",
"last": "Hicks",
},
"phone": "310-555-1212",
"interests": ["philosophy", "Star Wars"]
}

RESTful URLs
● All resources in database identified by URL
● Basic URL structure:
○ /db_id: A database
○ /db_id/doc_id: A document in a database

● URL components and JSON fields starting
with an underscore are special, e.g.:
○ /_config: CouchDB configuration parameters
○ /db_id/_all_docs: A cursor across all
documents
○ /db_id/_design/design_doc: A design
document for a database

RESTful Methods
● HTTP methods for CRUD actions
○ GET: Read data, typically a document
○ HEAD: Like GET without a body, typically used to
check if a document exists
○ PUT: Creates new databases, documents, and other
resources
○ POST: Updates these resources
○ DELETE: Deletes these resources

RESTful Status Codes
● HTTP status codes for server responses
○ 200 OK: Request completed successfully (e.g.
retrieving, updating, deleting documents)
○ 201 Created: Resource created (used with PUT)
○ 202 Accepted: Request completed and operation
pending (e.g. for background operations)
○ 401 Unauthorized: Bad username or password
○ 404 Not Found: Resource missing
○ 409 Conflict: MVCC failure, or concurrent
modification to a document
○ 500 Internal Server Error: Everybody panic

Creating a Database
● Request (abbr.):
PUT /new_db/ HTTP/1.1
● Response (abbr.):
HTTP/1.1 201 Created
{"ok": true}

Creating a Database
GET /_all_dbs HTTP/1.1
HTTP/1.1 200 OK
["new_db"]

Creating a Document
PUT /my_db/emp_001 HTTP/1.1
{
"name": {
"first": "Dante", "last": "Hicks",
},
"phone": "310-555-1212",
}

Creating a Document
HTTP/1.1 201 Created
{
"ok": true,
"id": "emp_001",
"rev": "1-4c6114c65e295552ab1019e2b046b10e"
}

Retrieving a Document
GET /my_db/emp_001 HTTP/1.1

HTTP/1.1 200 OK
{
"_id": "emp_001",
"_rev": "1-4c6114c65e295552ab1019e2b046b10e",
"name": {
"first": "Dante", "last": "Hicks",
},
"phone": "310-555-1212",
}

GET /my_db/missing_emp_007 HTTP/1.1

HTTP/1.1 404 Object Not Found
{
"error": "not_found",
"reason": "missing"
}

Views
● Secondary indexes for querying by other
than _id
● Written in JavaScript, executed with Mozilla
SpiderMonkey engine
● Defined in the design document

Views
● Define view emps_by_interest:
function(emp_doc) {
for (var i = 0; i < emp_doc.interests.length; ++i) {
var interest = emp_doc.interests[i];
emit(interest.toLowerCase(), null);
}
}

● Request (abbr):
GET
/my_db/_design/my_dd/_view/emps_by_interest?key=philosophy
HTTP/1.1

Views
● Response (abbr):
HTTP/1.1 200 OK
{
"total_rows": 1, "offset": 0,
"rows": [
{"id": "emp_001", "key": "philosophy", "value": null}
]
}

● Append &include_docs=true in request to
return documents with results

Multiversion Concurrency
Control (MVCC)
● Every document has a _rev attribute
○ Only required field other than _id

● Ensures that client is updating latest data
○ No accidental clobbering

● Lock-free concurrency control
● If multiple clients attempt to write
concurrently, exactly one succeeds every
time
○ Always making "forward progress"

Multiversion Concurrency
Control (MVCC)
CLIENT 1

CLIENT 2
GET id=doc_id

GET id=doc_id

_rev=X, v=1

_rev=X, v=1
_rev=X, v=2

_rev=Y, v=2
HTTP 200, _rev=Y

_rev=X, v=2
HTTP 409
GET id=doc_id
_rev=Y, v=2
_rev=Y, v=3

_rev=Z, v=3
t

HTTP 200, _rev=Z

Benchmarking
https://github.com/mgp/iron-cushion

● Setup:
○
○
○
○
○

Server: Intel Core 2 2.83GHz quad-core, 4GB RAM
Client: 1.83 GHz Intel Core Duo MacBook
100Mbit LAN, 100 concurrent connections
first, bulk insert 2,000,000 documents
second, intersperse 20,000 create and read
operations, 30,000 update and delete operations

● Caveat: no indexes

Benchmarking
bulkInsertRate: 10,003.030 docs/sec
createProcessingRate: 949.141
readProcessingRate: 9,015.862
updateProcessingRate: 980.172
deleteProcessingRate: 980.154

docs/sec
docs/sec
docs/sec
docs/sec

Thanks!
http://couchdb.apache.org/
michael.g.parker@gmail.com
https://github.com/mgp
http://mgp.github.com/couchdb-la-hn.pdf

Introduction to CouchDB - LA Hacker News

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