Introduction to Redis

"Little Server of Awesome"

2011 Dvir Volk

Software Architect, Do@
dvir@doat.com http://doat.com

What is redis
● Memcache-ish in-memory key/value store
● But it's also persistent!
● And it also has very cool value types:
○ lists
○ sets
○ sorted sets
○ hash tables
○ append-able buffers
● Open source; very helpful and friendly community.
Development is very active and responsive to requests.
● Sponsored by VMWare
● Used in the real world: github, craigslist, engineyard, ...
● Used heavily in do@ as a front-end database, search, geo
resolving

Key Features and Cool Stuff
● All data is in memory (almost)
● All data is eventually persistent (But can be immediately)
● Handles huge workloads easily
● Mostly O(1) behavior
● Ideal for write-heavy workloads
● Support for atomic operations
● Supports for transactions
● Has pub/sub functionality
● Tons of client libraries for all major languages
● Single threaded, uses aync. IO
● Internal scripting with LUA coming soon

A little benchmark

This is on my laptop (core i7 @2.2Ghz)

● SET: 187265.92 requests per second
● GET: 185185.17 requests per second
● INCR: 190114.06 requests per second

● LPUSH: 190114.06 requests per second
● LPOP: 187090.73 requests per second

●
● SADD: 186567.16 requests per second
● SPOP: 185873.61 requests per second

Scaling it up

● Master-slave replication out of the box

● Slaves can be made masters on the fly

● Currently does not support "real" clustered mode....

● ... But Redis-Cluster to be released soon

● You can manually shard it client side

● Single threaded - run num_cores/2 instances on the same
machine

Persistence
● All data is synchronized to disk - eventually or immediately
● Pick your risk level Vs. performance
● Data is either dumped in a forked process, or written as a
append-only change-log (AOF)
● Append-only mode supports transactional disk writes so you
can lose no data (cost: 99% speed loss :) )
● AOF files get huge, but redis can minimize them on the fly.
● You can save the state explicitly, background or blocking

● Default configuration:
○ Save after 900 sec (15 min) if at least 1 key changed
○ Save after 300 sec (5 min) if at least 10 keys changed
○ Save after 60 sec if at least 10000 keys changed

Virtual Memory

● If your database is too big - redis can handle swapping on
its own.

● Keys remain in memory and least used values are swapped
to disk.

● Swapping IO happens in separate threads

● But if you need this - don't use redis, or get a bigger
machine ;)

Show me the features!

Now let's see the key featurs:

● Get/Set/Incr - strings/numbers
● Lists
● Sets
● Sorted Sets
● Hash Tables
● PubSub
● SORT
● Transactions

We'll use redis-cli for the examples.
Some of the output has been modified for readability.

The basics...
Get/Sets - nothing fancy. Keys are strings, anything goes - just quote spaces.
redis> SET foo "bar"
OK
redis> GET foo
"bar"

You can atomically increment numbers
redis> SET bar 337
OK
redis> INCRBY bar 1000
(integer) 1337

Getting multiple values at once
redis> MGET foo bar
1. "bar"
2. "1337"

Keys are lazily expired
redis> EXPIRE foo 1
(integer) 1
redis> GET foo
(nil)
Be careful with EXPIRE - re-setting a value without re-expiring it will remove the
expiration!

Atomic Operations
GETSET puts a different value inside a key, retriving the old one
redis> SET foo bar
OK
redis> GETSET foo baz
"bar"
redis> GET foo
"baz"

SETNX sets a value only if it does not exist
redis> SETNX foo bar
*OK*
redis> SETNX foo baz
*FAILS*

SETNX + Timestamp => Named Locks! w00t!
redis> SETNX myLock <current_time>
OK
redis> SETNX myLock <new_time>
*FAILS*

Note that If the locking client crashes that might cause some problems, but it can be solved
easily.

List operations
● Lists are your ordinary linked lists.
● You can push and pop at both sides, extract range, resize,
etc.
● Random access and ranges at O(N)! :-(
redis> LPUSH foo bar
(integer) 1

redis> LPUSH foo baz
(integer) 2

redis> LRANGE foo 0 2
1. "baz"
2. "bar"

redis> LPOP foo
"baz"

● BLPOP: Blocking POP - wait until a list has elements and pop them. Useful for realtime stuff.
redis> BLPOP baz 10 [seconds]
..... We wait!

Set operations
● Sets are... well, sets of unique values w/ push, pop, etc.
● Sets can be intersected/diffed /union'ed server side.
● Can be useful as keys when building complex schemata.
redis> SADD foo bar
(integer) 1
redis> SADD foo baz
(integer) 1
redis> SMEMBERS foo
["baz", "bar"]

redis> SADD foo2 baz // << another set
(integer) 1
redis> SADD foo2 raz
(integer) 1

redis> SINTER foo foo2 // << only one common element
1. "baz"
redis> SUNION foo foo2 // << UNION
["raz", "bar", "baz"]

Sorted Sets
● Same as sets, but with score per element
● Ranked ranges, aggregation of scores on INTERSECT
● Can be used as ordered keys in complex schemata
● Think timestamps, inverted index, geohashing, ip ranges
redis> ZADD foo 1337 hax0r redis> ZRANGE foo 0 10
(integer) 1 1. "luser"
redis> ZADD foo 100 n00b 2. "hax0r"
(integer) 1 3. "n00b"
redis> ZADD foo 500 luser
(integer) 1 redis> ZREVRANGE foo 0 10
1. "n00b"
redis> ZSCORE foo n00b 2. "hax0r"
"100" 3. "luser"

redis> ZINCRBY foo 2000 n00b
"2100"

redis> ZRANK foo n00b
(integer) 2

Hashes
● Hash tables as values
● Think of an object store with atomic access to object
members

redis> HSET foo bar 1 redis> HINCRBY foo bar 1
(integer) 1 (integer) 2
redis> HSET foo baz 2
(integer) 1 redis> HGET foo bar
redis> HSET foo foo foo "2"
(integer) 1
redis> HKEYS foo
redis> HGETALL foo 1. "bar"
{ 2. "baz"
"bar": "1", 3. "foo"
"baz": "2",
"foo": "foo"
}

PubSub - Publish/Subscribe
● Clients can subscribe to channels or patterns and receive
notifications when messages are sent to channels.
● Subscribing is O(1), posting messages is O(n)
● Think chats, Comet applications: real-time analytics, twitter
redis> subscribe feed:joe feed:moe feed:
boe

//now we wait
.... redis> publish feed:joe "all your base are
<<<<<---------- belong to me"
1. "message" (integer) 1 //received by 1
2. "feed:joe"
3. "all your base are belong to me"

SORT FTW!
● Key redis awesomeness
● Sort SETs or LISTS using external values, and join values
in one go:

SORT key
SORT key BY pattern (e.g. sort userIds BY user:*->age)
SORT key BY pattern GET othervalue

SORT userIds BY user:*->age GET user:*->name

● ASC|DESC, LIMIT available, results can be stored, sorting
can be numeric or alphabetic

● Keep in mind that it's blocking and redis is single threaded.
Maybe put a slave aside if you have big SORTs

Transactions
● MULTI, ...., EXEC: Easy because of the single thread.
● All commands are executed after EXEC, block and return
values for the commands as a list.
● Example:
redis> MULTI
OK
redis> SET "foo" "bar"
QUEUED
redis> INCRBY "num" 1
QUEUED
redis> EXEC
1) OK
2) (integer) 1

● Transactions can be discarded with DISCARD.

● WATCH allows you to lock keys while you are queuing your
transaction, and avoid race conditions.

Gotchas, Lessons Learned
● Memory fragmentation can be a problem with some usage
patterns. Alternative allocators (jemalloc, tcmalloc) ease
that.

● Horrible bug with Ubuntu 10.x servers and amazon EC2
machines [resulted in long, long nights at the office...]

● 64 bit instances consume much much more RAM.

● Master/Slave sync far from perfect.

● DO NOT USE THE KEYS COMMAND!!!

● vm.overcommit_memory = 1

● Use MONITOR to see what's going on

Example: *Very* Simple Social Feed
#let's add a couple of followers
>>> client.rpush('user:1:followers', 2)
>>> numFollowers = client.rpush('user:1:followers', 3)
>>> msgId = client.incr('messages:id') #ATOMIC OPERATION

#add a message
>>> client.hmset('messages:%s' % msgId, {'text': 'hello world', 'user': 1})

#distribute to followers
>>> followers = client.lrange('user:1:followers', 0, numFollowers)

>>> pipe = client.pipeline()
>>> for f in followers:
pipe.rpush('user:%s:feed' % f, msgId)
>>> pipe.execute()

>>> msgId = client.incr('messages:id') #increment id
#....repeat...repeat..repeat..repeat..
#now get user 2's feed
>>> client.sort(name = 'user:2:feed', get='messages:*->text')
['hello world', 'foo bar']

Other use case ideas
● Geo Resolving with geohashing
● Implemented and opened by yours truly https://github.com/doat/geodis

● Real time analytics
● use ZSET, SORT, INCR of values

● API Key and rate management
● Very fast key lookup, rate control counters using INCR

● Real time game data
● ZSETs for high scores, HASHES for online users, etc

● Database Shard Index
● map key => database id. Count size with SETS

● Comet - no polling ajax
● use BLPOP or pub/sub

● Queue Server
● resque - a large portion of redis' user base

Melt - My little evil master-plan
● We wanted freakin' fast access to data on the front-end.

● but our ability to cache personalized and query bound data
is limited.

● Redis to the rescue!

● But we still want the data to be in an RDBMs.

● So we made a framework to "melt the borders" between
them...

Introducing melt
● ALL front end data is in RAM, denormalized and optimized for
speed. Front end talks only to Redis.

● We use Redis' set features as keys and scoring vectors.

● All back end data is on mysql, with a manageable normalized
schema. The admin talks only to MySQL.

● A sync queue in the middle keeps both ends up to date.

● A straightforward ORM is used to manage and sync the data.

● Automates indexing in Redis, generates models from MySQL.

● Use the same model on both ends, or create conversions.

● Central Id generator.

Melt - an example:
#syncing objects:
with MySqlStore:
users = Users.get({Users.id: Int(1,2,3,4)})
with RedisStore:
for user in users:
Users.save(user)

#pushing a new feed item from front to back:
with RedisStore:
#create an object - any object!
feedItem = FeedItem(userId, title, time.time())
#use the model to save it
Feed.save(feedItem)
#now just tell the queue to put it on the other side
SyncQueue.pushItem(action = 'update', model = FeedItem,
source = 'redis', dest = 'mysql',
id = feedItem.id)

Coming soon to a github near you! :)

More resources

Redis' website:
http://redis.io

Excellent and more detailed presentation by Simon Willison:
http://simonwillison.net/static/2010/redis-tutorial/

Much more complex twitter clone:
http://code.google.com/p/redis/wiki/TwitterAlikeExample

Full command reference:
http://code.google.com/p/redis/wiki/CommandReference

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