1. Redis Tips
charsyam@naver.com

2. • OpenSource Contributor
• REDIS
• TWEMPROXY

3. Before Presentation

4. Agenda
•Dangerous Commands
•Memory Policies
•Replication
•RDB

5. Dangerous Commands
•Keys *(pattern)
•FlushAll

6. Dangerous Commands
•Why?
–Single Thread.
–Each command should spend
small time.

7. Keys *• Don’t use keys command in production.
–“Warning: consider KEYS as a command
that should only be used in production
environments with extreme care” is
written in redis.io keys commands
manual.

8. Keys *
di = dictGetSafeIterator(c->db->dict);
allkeys = (pattern[0] == '*' && pattern[1] == '0');
while((de = dictNext(di)) != NULL) {
……
stringmatchlen(pattern,plen,key,sdslen(key),0)
}

9. FlushAll
Cache Item Count Time
Memcache 1,000,000 1~2ms
Redis 1,000,000 1000ms(1 second)

10. FlushAll
• Redis’s FlushAll is slow and paused.
• But memcache’s flush is really fast.
–memcache doesn’t depend on Item count.
–Redis depends on Item count.

11. FlushAll
• What is different from memcache’s flush.
• Memcache just sets time to flush.
• Redis deletes all items.
–It spends 1 second in 1M or 2M items

12. FlushAll-Redis
long long emptyDb() {
for (int j = 0; j < server.dbnum; j++) {
dictEmpty(server.db[j].dict);
dictEmpty(server.db[j].expires);
}
return removed;
}

13. FlushAll-Redis

14. FlushAll-Memcachestatic void process_command(conn *c, char *command) {
if (exptime > 0)
settings.oldest_live = realtime(exptime) - 1;
else /* exptime == 0 */
settings.oldest_live = current_time - 1;
}

15. FlushAll-Memcacheif (settings.oldest_live != 0 &&
settings.oldest_live <= current_time &&
it->time <= settings.oldest_live) {
do_item_unlink(it, hv);
do_item_remove(it);
it = NULL;
}

16. Redis Memory Policy• Volatile-lru
• Allkeys-lru
• Volatile-random
• Allkeys-random
• Volatile-ttl
• noeviction

17. Redis Memory Policy
• In 64bit, Memory Policy doesn’t work,
if you don’t set maxmemory
parameter.
• In 32bit, maxmemory is set 4GB as
default

18. Default Setting in 64
• There is no memory limitation.
• So If you insert bigger data than
memory. It will be slow.
• But But But. Make sure
Vm.overcommit_memory property.

19. Memory Policy
• If you use Memory-Policy and
replication.
– It can cause some consistency problem.
Master SlaveDelete
propagation
Delete items because
of its own memory
policy

20. Replication
•Support Chained Replication
•Common mistake

21. Replication
•Support Chained Replication
Master
Slave Slave
Slave Slave

22. Replication
•Common mistake.
–Don’t forget “slaveof no one”
•Slave’s replicationCron keeps
track of master’s health.

23. Replication
Master Slave
replicationCron
Health check

24. Replication
Master Slave
replicationCron
Health check

25. Replication
Master Slave
replicationCron
When master reruns, Resync with Master

26. Replication
Master Slave
replicationCron
Slave will has no data after resyncing
If master has no data.

27. Initial Replication Step

28. RDB
• Good and Bad.
• It causes all of problems.

29. RDB - GOOD
• Can store data into persistent layer.

30. RDB - BAD
• Bad Performance in Large memory.
• Twice memory problem.
• Denying write problem when storing RDB
fails.
• If you just want Cache. Turn off RDB

31. RDB – Large Memory
• Performance is relevant to Memory
Size.
17.1 GB 34.2 GB
68.4 GB
117 GB

32. RDB - BAD
• Using bgsave in slave.
• Turning off RDB.
• Using save in master or slave.

33. RDB – Twice Memory
• fork() and COW(copy on write) Issue
–In Write Heavy System:

34. RDB – Twice Memory

35. RDB – Twice Memory

36. RDB – Twice Memory

37. Real Case Study
• Background
–Can’t write to Redis Server
–Sentinel doesn’t find Server’s failure.

38. Real Case Study• Reason
– If redis fails to save RDB, Redis basically denies write
operations from client.
– “MISCONF Redis is configured to save RDB snapshots,
but is currently not able to persist on disk.
Commands that may modify the data set are disabled.
Please check Redis logs for details about the error.”

39. Real Case Studyif (server.stop_writes_on_bgsave_err &&
server.saveparamslen > 0
&& server.lastbgsave_status == REDIS_ERR &&
c->cmd->flags & REDIS_CMD_WRITE)
{
flagTransaction(c);
addReply(c, shared.bgsaveerr);
return REDIS_OK;
}

40. Real Case Study
• Solution #1
• Solution #2
config set stop-writes-on-bgsave-error no
Turn off RDB Setting
2.6.12 부터 conf 에서 stop-writes-on-bgsave-error
설정이 가능해짐.

41. SELECT Command
• Don’t use “SELECT” Command.
–You will not be able to use “SELECT” in
Redis Cluster.
• Redis Proxy can make some troubles.

42. Response Time
• Why memcache’s response time is
uniformed?
–Difference of Memory Allocator
Memcache Redis
- Slab Allocator - Just malloc()

43. Redis VS Memcache
• The popular question 
–Redis and Memcache: What is better?
Memcache Redis
- Uniformed Response
Time
- More stable
- Data Structure
- Persistent

44. Sentinel

45. Sentinel
• Sentinel is Failover Solution for Redis.
Master Slave
Sentinel
Sentinel periodically checks Redis Master

46. Sentinel
Master Slave
Sentinel
Send “slaveof no one” to slave node

47. Sentinel
Master Slave
Sentinel
Notify to client about changing master
Client

48. Sentinel
redis 127.0.0.1:2003> psubscribe *
Reading messages... (press Ctrl-C to quit)
1) "pmessage"
2) "*"
3) "+switch-master"
4) "resque 127.0.0.1 1999 127.0.0.1 2002"

49. Sharding or Clustering

50. Two Approches.

51. Client Library
VS
Server Proxy

52. Client Library
NHN Case Study:
http://helloworld.naver.com/helloworld/294797
Craiglist Case Study:
http://blog.zawodny.com/2011/02/26/redis-
sharding-at-craigslist/

53. Client Library
ZooKeeper
or
health Checker
Redis Library

54. Craigslist

55. NHN Application Servers
ZooKeeper
Redis Cluster
Manager
Redis
Shard-1
Redis
Shard-2
Redis
Shard-3

56. NHN

57. Client Library
• Benefits
–No hob(faster than server proxy)
• Liabilities
–Consistency Problem(Timing Issue)

58. Server Proxy
Redis Cloud Services
Twemproxy
https://github.com/twitter/twemproxy

59. Twemproxy
• Simple and fast memcache/redis proxy
Client
Client
Client
Client
Twemproxy
Redis
Redis
Redis
Redis

60. Twemproxy
If you use redis as a store,
Twemproxy might not be
good for your purpose

61. Server Proxy
• Benefits
–Client doesn’t need any other libraries
• Liabilities
–Hob.(less speed and proxy is also SPOF)

62. Q & A

63. Thank you!