ML Wiki

Redis

Redis is an in-memory database, good to be used as

• a cache
• a message queue

Installation

Ubuntu

Installing Redis on ubuntu

• sudo apt-get install redis-tools
• default port is 6379
• Use redis-cli to enter redis commands
• if you need redis instance be outside, do nano /etc/redis/redis.conf and replace bind 127.0.0.1 to bind 0.0.0.0
• restart the server service redis-server restart
• to check if you can connect to redis from another server, do redis-cli -h 192.168.x.x ping, should get PONG in return

Python

Python library:

• sudo pip install redis or
• sudo easy_install install redis or

Building Blocks

Databases

• Each redis server can have several databases
• they are indexed by integers: 0, 1, 2, ...
• 0 is the default db
• to choose the 1st db, use select 1
• redis keeps all the data in memory, but by default, redis also snapshots DB to disk after each $X$ changes
• there's also append-only mode

Key/Value Pairs

String key/values

• key can be anything, e.g. users:ololo, : doesn't have any specific meaning, but can be used to separate "table name" and "key name"
• command set users:ololo '{"name":"ololo", ...}' - the content can be any string, not necessarily JSON
• to get the value, use get users:ololo
• you can query only by key, never by value

String functions:

• strlen key returns the lenth of the value associated with key key
• getrange key 0 10 returns the substring from 0 to 10 of the value stored by key
• append key value2 appends 'value2' to the current value stored by key

Counters

• You can store counters in redis
• incr key increment the integer value stored in key, or create a new key with value 1
• decr key decrement the value
• incrby and decrby are used to increase/decrease the values by some number

Bitmaps

• you can store byte arrays in redis and set/get their specific bits
• commands: setbit and getbit
• this can be used for implementing Sketching Algorithms, e.g. Bloom Filters

Hashes

• hashes in redis are associative arrays
• to store a hash, not a string, use hset and hget
• hset hashname field 30 is approximately equal to hashname[field] = 30
• if hash hashname doesn't exist, it will be created. If exists, but field doesn't, field will be created
• hgetall hashname returns all fields of the hash hashname
• hdel hashname field removes field from hashname

Lists

• lists keep an array of values associated with a specific key
• lpush listname new_value adds the new value to the front of listname
• ltrim listname 0 49 removes first 50 values from the list
• lrange listname 0 9 returns first 10 values

Sets

Unordered sets

• sadd set1 val1 val2 val3 adds values "val1" "val2" "val3" to set setkey
• smember set1 val1
• sinter set1 set2 intersection of two sets set1 and set2
• sinterstore set1 set2 new_set store instersection of two sets in a new set new_set

Ordered sets

• the same commands, but they start with "z":
• zadd, zmember, ...

Usage Patterns

• in redis it's fine to issue several requests ("round trips" to db) - because it's so fast

Avoiding Duplication

• suppose users can log in using their ids or emails - so you have two keys for the same user
• how can you avoid storing the same user twice?
• first, keep the user data at user:id, e.g. users:9001
• use hash to lookup the id for email: hget user:emails ololo@mlwiki.org

Expiration

• you can set time to live of a key
• expire key 30 where 30 is the time to live in seconds
• expireat key unix_timestamp
• ttl key to see how much time the key has left to live

Publish/Subscribe

• it's easy to implement the Publish-Subscribe Model in redis - which makes it a lightweight message queue
• blpop and brpop - return and remove first (last) element of a list - or blocks until something is avialable
• can use it to implement a Queue
• also can use subscribe q_id and publish q_it message

A P/S model implemented in Scala: https://gist.github.com/debasishg/7056696

Lua Scripting

• finally, it's possible to write stored procedures in Lua

Sources

• Little Redis Book: https://github.com/karlseguin/the-little-redis-book