- 6Wunderkinder is a Berlin-based startup with 18 employees that created the task management app Wunderlist which has over 600k registered users - They initially used a LAMP stack for Wunderlist but added Redis for caching and storing statistics since user growth impacted performance - Redis allows them to store daily increment counts and sets of active user IDs to track metrics like daily syncs and active users - 6Wunderkinder also uses Redis extensively in their new productivity platform Wunderkit for caching, statistics, logging, and asynchronous job queues

1. Redis @ 6Wunderkinder
- more than just a cache -
Sebastian Kreutzberger, CTO

2. About 6Wunderkinder
• Startup based in Berlin, 18 employees
• VC-backed
• „Wunderlist“ - free, multi-platform task app
- >600k registered users, >1,6 million downloads
- Android, iPhone, iPad, Web, Mac & Windows
- App of the Week in 69 Apple App Stores
• „Wunderkit“ - productivity platform, currently
in development, „next big thing“

3. Redis @ Wunderlist
• Wunderlist backend based on LAMP
• No cache was used in the beginning
• After incredible growth in userbase the
marketing & BI needed statistics
• Solution: Redis!
• Why? Persistent, fast & super-simple

4. Statistics with Redis
• Store 2 types of statistical data:
- simple daily increments to count
syncs, hits & registrations, etc.
- daily Sets of user-ids who are active on
that day, important for marketing to see
how many people are using Wunderlist
for how many days

5. Example
<?php
$cache = new Redis();
/* .... connection stuff .... */
// increment daily sync counter
$key = "sync: " . date("Ymd"); // day-based key
$cache->incr($key);
// store current user in today’s active-users Set
$key = "active: " . date("Ymd");
$cache->sAdd($key, $user_id);
// get all user-ids who were active yesterday
$yesterday = strtotime("-1 days 00:00");
$key
= "active: " . date("Ymd", $time);
$users = $cache->sMembers($key);
?>

6. More details ...
• PHPRedis as module for PHP is very fast!
• Wrote cache abstraction layer for possible
replacement of Redis with Memcached
• Currently just do caching on user objects
• Tasks & lists change too often for caching
• Put a complete object into cache, not just a
database row

7. Redis @ Wunderkit
• Wunderkit uses MongoDB + async jobs
• Redis is fundamental part of architecture
• Even more important than database
• Heavy usage of Redis Lists (a.k.a. queues)

8. Usage at Wunderkit
• Caching of nearly all objects
• Statistics
• Logging
• Job Queue

9. Logging with Redis
• Wrote own PHP Error Handler
• Extended it for own logging
• Store PHP errors & log in Redis List
• Keys are based upon day & log level
• Very simple retrieval
• High-performance & scalable

10. Job Queue I
• Queue is a simple Redis List
• Wrote a helper class for easy queue input
• Every app server can „outsource“
processing by putting a job onto the queue
• Each job contains the following data:
a URL to a worker server + parameters
=> very simple, light-weight & scalable!

11. Job Queue II
• Dozens of parallel PHP scripts running as
daemon(!) listen for new jobs on the queue
• brPop($key, $time) listens for new jobs
• No concurrencies, Redis is single-threaded
• Job URL is optimistically called, no waiting
for response, „fire & forget“
=> many jobs per second!

12. Example Daemon
<?php
/* ... do crazy PHP daemon stuff here ... */
$cache = new Redis();
/* .... connection stuff .... */
while (1) // infinite loop
{
$ret = $cache->brPop("queue", 30); // listen for 30 seconds
if ($ret != false) // something new on the queue
{
fireAsyncRequest($ret["url"], $ret["params"]);
}
}
?>

13. Todos
• Replacing PHP daemon with Erlang
• Adding of message queue incl. callbacks
• Diving into Redis Sharding
• Putting even more stuff into Redis. Why?
Because we love Redis!

14. Thanks!
• Contact:
Twitter: @skreutzb
E-Mail: sebastiank@6wunderkinder.com
Web: www.6wunderkinder.com
• P.S.: We are hiring!