Intro to

Tim Bond

July 20 2017

for PHP

What is Redis?

  • REmote DIctionary Store
  • Started out as LLOOGG
    • Realtime log analyzer web app
    • "web 2.0 tail -f access.log"
    • Written in PHP and MySQL
    • MySQL couldn't keep up
  • Redis 4.0.0 just came out last week!
  • Sponsored by RedisLabs since June 2015

What is Redis?

  • Fast
  • Key-value store
  • Queue
  • Pub/Sub Messaging
  • Fast
  • Data structure server

Who uses Redis?

  • GitHub
  • Twitter
  • Craigslist
  • StackOverflow
  • Flickr
  • Slack
  • Hulu
  • SnapChat
  • Napster
  • Just to name a few

Redis in the real world

  • Codebase: 70,000 lines
  • Hulu's distributed setup sustains 1m ops/second (2015)
  • A RedisLabs project sustained 2.65 million votes per second on a low spec AWS instance (2015)
  • One Twitter datacenter storing timelines (2015):
    • ~40TB allocated heap
    • ~30 million queries/sec
    • >6,000 instances
  • Network is usually the bottleneck--up to 97% of time for an operation
  • Caching API responses to work around slowness/rate limiting

Redis vs Memcached

  • More than just strings
  • Durable
  • Transactions
  • Scripting
  • No hard limit (unless you want it)
  • Single threaded
  • Not all client libraries serialize

Datatypes: The basics

  • Regardless of datatype, every object has a unique key
  • Convention is to use colons as separators
    • e.g. user:123 or page:456
  • Keys can be up to 512 MB
  • Optional TTL (expiration)

Strings

  • Any sort of text
  • Binary safe
  • Can also be integers
    • Use incr/incrby commands
  • Commands are O(1)

Big O Notation

  • Worst case runtime
  • Useful for describing growth rate rather than runtime
  • n = number of elements
  • O(1) is the holy grail
  • Most of Redis' commands are O(n) or better

Hashes

  • Think PHP's associative arrays
  • Each field pretty much the same as a Redis string
  • Can retreive all fields O(n), one field O(1), or just some O(n)

Hashes vs Strings

Use case: Instagram needed a quick way to get the user ID for a given media ID

  • Strings: Key = media ID, value = user ID
    • Example: media:1155315 = 939
    • 70 MB to store 1 million keys
  • Hashes: Same, but buckets of 1,000
    • Example: mediabucket:1155 = 1155315 => 939
    • 16 MB to store 1 million keys
  • Memcached used 52 MB

Lists

  • Same functionality of a doubly linked list
  • Internal representation varies depending on size
  • Accessing the head or tail is O(1)
  • Accessing by index is O(n)
  • Can set TTL on the list but not the members

List operations

  • LPUSH/RPUSH
  • LPOP/RPOP
  • LRANGE
  • BLPOP/BRPOP
    • Blocking, good for job queue daemons
  • (B)RPOPLPUSH
    • (Blocking) right pop, left push on to another list

Lists: Examples

  • Job queues: Process uploaded files, build reports, etc.
    • Webapp will PUSH to list
    • Background daemon uses blocking POP
  • Recent interactions: user uploaded photo, new article published, etc
    • Webapp will PUSH to list (and might also insert into MySQL)
    • Homepage will run LRANGE 0 9 which will show the first 10 items in the list

Sets

  • A lot like a list, sort of like a hash
  • No field names, unlike hash
  • A value can only exist once
  • No order
  • Can set TTL on the set but not the members
  • Lots of comparison functions
    • Intersection (members that exist in both sets)
    • Difference (members that only exist in one set)

Sorted Sets

  • Just like sets, but each member has a score (float)
    • Two members can have the same score
      • String sort is used to break "ties"
  • Members are sorted when stored
    • Inserts are fast: O(log(set length))
    • Lookups are more or less the same
  • Commands similar to list
  • Can also retrieve by score, range of scores, or sort by values

Sorted sets: Examples

  • Leaderboards
    • Every time a player finishes a game, insert their ID in to a list with their points as the score
    • ZRANGE myset 0 4 will return the top five
    • ZREVRANGE myset 0 4 will return the bottom five
  • Most popular articles
  • Autocomplete
    • Store all members with the same score
    • Retrieve by value (ZRANGEBYLEX)

Pub/Sub

  • One or more clients subscribes to a channel
  • One or more clients publishes to a channel
  • If a message is published to a channel and no one is around to subscribe to it, it does not make a sound.

What Datatype Do I Use?

Answer:
How do you want to look it up?

phpredis

  • Written in C
  • PECL extension

predis

  • Written in PHP
  • Composer project

Neither does serialization!

Serialization

$foo = new stdClass();
$foo->bar = 'baz';
$redis->set('foo_obj', $foo);
var_dump($redis->get('foo_obj'));

string(6) "Object"

Serialization Methods

  • json_encode()
  • serialize()
  • igbinary_serialize()
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