Symfony Cache has been around for a few releases. But what is happening behind the scenes? Talk focuses on how is it working, down to detail level on Redis for things like datatypes, Redis Cluster sharding logic, how it differs from Memcached and more. Hopefully you’ll learn how you can make sure to get optimal performance, what opportunities exists, and which pitfalls to try to avoid. https://amsterdam2019.symfony.com/speakers NOTE: This talk is recorded and available on SymfonyCasts, in the future it will also be uploaded to youtube.

1. HEAD FIRST INTO SYMFONY
CACHE, REDIS & REDIS CLUSTER
André Rømcke (@andrerom)
VP Technical Services & Support @ eZ Systems (@ezsystems)
November 22nd 2019 - Amsterdam - SymfonyCon

2. So what is this talk about?
How Symfony 4.3/4.4 ended up
with new Adapters:
FilesystemTagAware &
RedisTagAware
Agenda:
1. Where eZ fits in
2. Some THEORY:
- Symfony Cache
- Cache tagging
- Redis & Redis Cluster
- Memcached vs Redis
5. New Adapters
6. Demo time
7. BONUS & Edge cases

3. Who?
๏André Rømcke | @andrerom
๏Economics, Consulting, Engineering, Lead, VP, now doing Services & Support
๏Tried to contribute to Symfony, FOS, Composer, PHP-FIG, Docker Compose
๏eZ Systems AS | ez.no
๏75+ people across 7+ countries. Partners & community in many many more
๏eZ Platform | ezplatform.com
๏Open Source CMS, feature rich, very extendable, flexible Full & Headless
๏On Symfony since 2012
๏Commercial with additional features: eZ Platform Enterprise & eZ Commerce

4. Symfony Cache in eZ Platform
๏Moved over from Stash in 2017
๏Heavily relies on Cache tagging feature
๏Contributed improvements on performance issues with Redis/Redis Cluster/…
๏… and for 4.3 /4.4: optimized TagAware adapters for Redis and FileSystem

5. Quick recap:
Symfony Cache

6. Recap: Symfony Cache Component
๏PSR-6 compliant Cache component
๏Aims to be fast: Among others supports multi get calls to Redis and Memcached
๏Is progressively being used in several places in Symfony Framework, e.g.:
๏PropertyInfo
๏Serializer
๏Validator
๏(…)
๏.. maybe HTTP Cache at some point

7. Recap: Symfony Cache Adapters
๏Adapters:
๏APCu
๏Array
๏Chain
๏Doctrine
๏FileSystem
๏Opcache based: PhpFile + PhpArray
๏Proxy (PSR-6) + Psr16
๏Redis
๏Memcached
๏And “TagAware”..

8. Cache labeling:
Tags?

9. Tags?
๏Data/Entities often has secondary indexes, e.g:
๏entity type id
๏placement id
๏variant type, …
๏Operations in your application sometimes affect those => Affecting bulk of entities
๏By tagging cache you can directly invalidate:
๏E.g. key: ez-content-66 tags: content-66, type-2, location-44 (…)

10. TagAware: Secondary index for invalidation
/**
* Interface for invalidating cached items using tags.
*
* @author Nicolas Grekas <p@tchwork.com>
*/
interface TagAwareAdapterInterface extends AdapterInterface
{
/**
* Invalidates cached items using tags.
*
* @param string[] $tags An array of tags to invalidate
*
* @return bool True on success
*
* @throws InvalidArgumentException When $tags is not valid
*/
public function invalidateTags(array $tags);
}

11. TagAwareAdapter
๏Wraps your normal adapter, stores item Tags in separate key
๏Does a separate lookup for expiry timestamp for tags => “2-RTT”

12. Topics around:
Redis & Redis Cluster

13. Redis: Datatypes
๏ Strings
๏ Lists
๏ Sets
๏ SortedSets
๏ Hashes
๏ Bitmaps
๏ HyperLogLogs
๏ Streams (As of Redis 5.0)

14. Redis: Commands
๏There are 227 commands and counting
๏There are generic key, cluster, connection, Pub/Sub, Scripting, Transaction commands
๏And Datatypes allows for specific operations/commands:
๏E.g. “String”: GET, SET, APPEND, BITPOS, DECR, …
๏"SET”:
๏SADD, SREM, SPOP
๏SDIFF, SINTER, SUNION, SMOVE
๏SMEMBERS, SSCAN
๏…

15. Redis: Eviction
maxmemory-policy:
๏noeviction
๏volatile-random
๏volatile-ttl
๏volatile-lru
๏volatile-lfu (As of Redis 4.0)
๏allkeys-random
๏allkeys-lru
๏allkeys-lfu (As of Redis 4.0)

16. Redis Cluster: processe
๏Allows you to scale up Redis by running several instances
๏Multi process on same server and/or across servers
๏Coordinates cache across “cache slots", deals with replication, …
๏Unlike Memcached, several operations has limitations on Cluster

17. Redis Cluster: limitations
๏Does not support “pipline”: capability to perform several operations in one call
๏PHPRedis mainly supports multi operations on MGET and MSET with cluster
๏Examples of affected operations:
๏RENAME won’t work if the new key ends up in another “Cache Slot”
๏EVAL (Lua Script) likewise can only be given keys that maps to same node
ERR CROSSSLOT Keys in request don't hash to the same slot

18. FYI on strength and weaknesses:
Memcached vs Redis

19. Memcached vs Redis: Overview
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๏Donec congue erat nibh, ac luctus erat accumsan tempor.
๏Mauris bibendum ac eros eu tempor. Duis libero libero, luctus quis posuere quis, porta vel
turpis.
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lorem ultrices facilisis ac sit amet velit.
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volutpat dapibus.
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condimentum ut, semper eget nulla.
๏Vivamus ultricies massa lectus, id varius orci sodales quis.
Memcached Redis
Multi operations (get, set, ..) V V
Datatypes String
String, List, Set, Hash, Sorted Set, …
Streams
Control over eviction X V
Persistance X V
Pipeline / Lua X V
Some limitations on Redis Cluster
Multiserver V V
Using e.g. Redis Cluster OR Redis Sentinel
Multithreaded V X
But multi process with Redis Cluster *
* Redis 6 is adding partly threading with background thread handling slow operations

20. New Adapters in Symfony 4.3/4.4:
FilesystemTagAware & RedisTagAware

21. Tags storage
๏Moves tags to be a “relation” to cache key, instead of expiry time
๏Avoids the lookup tags on getItem(s), instead does it on invalidation
➡ 1-RTT for lookups
๏FilesystemTagAwareAdapter: Uses a file for tag “relations”
๏RedisTagAwareAdapter: Uses “Set” for tag “relation” => w/o expiry

22. Why the efforts on 1-RTT lookups?
๏AWS ElasticCache Redis instances has latency of around 0.2-0.5ms
๏E.g. Simple page with 20 articles shown:
๏~40 lookups x latency = 5-20 ms
๏E.g. News site landing page with 1000 articles listed:
๏~2000 lookups x latency = 0.4 - 1 seconds

23. In Symfony Cache:
๏Pipeline used instead of MGET => Allows parallel lookups with Redis Cluster
๏Remove need for cache versioning lookups on Redis cluster
๏TagAwareAdapter micro ttl cache for tag lookups
Lookup optimizations done over the last year

24. On Application side (eZ Platform):
๏Changs take better advantage of Multi Get
๏Introduce optimized RedisTagAwareAdapter
๏Introduced Application specific in-memory cache
Lookup optimizations done over the last year

25. End result example:
๏Had 17.000 lookups in Symfony Cache on our Admin dashboard
๏Due to Symfony Cache logic this was ~40-60k lookups on Redis Cluster
๏30 seconds in worst case, just waiting for Redis Cluster
๏After all fixes it went down to 63 lookups in Symfony Cache
Lookup optimizations done over the last year

26. Demo time:
Lets adapt symfony/demo to make it cached
Demo code: https://github.com/andrerom/sfcon-amsterdam-2019-redis-cache-demo

27. Bonus and Edge cases:
Some things to be aware of

28. Bonus: Edge cases in Caching
๏Race conditions
๏When caching entities: Transactions
๏Async / Stale cache

29. Bonus: Adapter recommendations
๏RedisTagAwareAdapter
Requirement: maxmemory-policy: volatile-ttl / volatile-lru / volatile-lfu (Redis 4.0+)
Pro: 1-RTT
Con: Consumes more memory, risk of running out of evictable memory
๏RedisAdapter + TagAwareAdapter:
Pro: Uses less memory then RedisTagAwareAdapter + all can be freed
Con: 2-RTT
๏MemcachedAdapter + TagAwareAdapter:
Pro: Uses less memory due to simpler data structure + all can be freed
Cable off handling more traffic
Con: 2-RTT

30. Bonus: Simulating RedisTagAware in bash
๏Save:
redis-cli set mykey data
redis-cli sadd type-article:tags mykey anotherkey
๏Read:
redis-cli mget mykey anotherkey
๏Invalidation:
tmp=$(cat /dev/urandom | tr -dc 'a-zA-Z0-9' | fold -w 32 | head -n 1)
redis-cli rename type-article:tags {type-article:tags}-$tmp
members=`redis-cli smembers {type-article:tags}-$tmp`
redis-cli del {type-article:tags}-$tmp $members
TIP:
To run these commands:
docker run --name myredis -p 6379:6379 -d redis
docker exec -ti myredis bash
After “exit”, you can clean up using:
docker rm -f myredis

31. Fin..
Questions?
Other talks: http://www.slideshare.net/andreromcke
Twitter: @andrerom
eZ Platform: https://ezplatform.com/
Redis: https://redis.io/
Memcached: https://memcached.org
Demo code: https://github.com/andrerom/sfcon-amsterdam-2019-redis-cache-demo