Redis

When you encounter a Redis instance and you quickly want to learn about the setup you just need a few simple commands to peak into the setup. Of course it doesn’t hurt to look at the official full command documentation, but below is a listing just for sysadmins.

Accessing Redis

CLI

First thing to know is that you can use “telnet” (usually on Redis default port 6379)

telnet localhost 6379

or the Redis CLI client

redis-cli

to connect to Redis. The advantage of redis-cli is that you have a help interface and command line history.

CLI Queries

Here is a short list of some basic data extraction commands:

CLI Scripting

For scripting just pass commands to “redis-cli”. For example:

$ redis-cli INFO | grep connected
connected_clients:2
connected_slaves:0
$

Server Statistics

The statistics command is “INFO” and will give you an output as following.

$ redis-cli INFO
redis_version:2.2.12
redis_git_sha1:00000000
redis_git_dirty:0
arch_bits:64
multiplexing_api:epoll
process_id:8353
uptime_in_seconds:2592232
uptime_in_days:30
lru_clock:809325
used_cpu_sys:199.20
used_cpu_user:309.26
used_cpu_sys_children:12.04
used_cpu_user_children:1.47
connected_clients:2         # <---- connection count
connected_slaves:0
client_longest_output_list:0
client_biggest_input_buf:0
blocked_clients:0
used_memory:6596112
used_memory_human:6.29M         # <---- memory usage
used_memory_rss:17571840
mem_fragmentation_ratio:2.66
use_tcmalloc:0
loading:0
aof_enabled:0
changes_since_last_save:0
bgsave_in_progress:0
last_save_time:1371241671
bgrewriteaof_in_progress:0
total_connections_received:118
total_commands_processed:1091
expired_keys:441
evicted_keys:0
keyspace_hits:6
keyspace_misses:1070
hash_max_zipmap_entries:512
hash_max_zipmap_value:64
pubsub_channels:0
pubsub_patterns:0
vm_enabled:0
role:master             # <---- master/slave in replication setup
db0:keys=91,expires=88

Changing Runtime Configuration

The command

CONFIG GET *

gives you a list of all active configuration variables you can change. The output might look like this:

redis 127.0.0.1:6379> CONFIG GET *
 1) "dir"
 2) "/var/lib/redis"
 3) "dbfilename"
 4) "dump.rdb"
 5) "requirepass"
 6) (nil)
 7) "masterauth"
 8) (nil)
 9) "maxmemory"
10) "0"
11) "maxmemory-policy"
12) "volatile-lru"
13) "maxmemory-samples"
14) "3"
15) "timeout"
16) "300"
17) "appendonly"
18) "no"
19) "no-appendfsync-on-rewrite"
20) "no"
21) "appendfsync"
22) "everysec"              # <---- how often fsync() is called
23) "save"
24) "900 1 300 10 60 10000"     # <---- how often Redis dumps in background
25) "slave-serve-stale-data"
26) "yes"
27) "hash-max-zipmap-entries"
28) "512"
29) "hash-max-zipmap-value"
30) "64"
31) "list-max-ziplist-entries"
32) "512"
33) "list-max-ziplist-value"
34) "64"
35) "set-max-intset-entries"
36) "512"
37) "slowlog-log-slower-than"
38) "10000"
39) "slowlog-max-len"
40) "64"

Note that keys and values are alternating and you can change each key by issuing a “CONFIG SET” command like:

CONFIG SET timeout 900

Such a change will be effective instantly. When changing values consider also updating the redis configuration file.

Databases

Multiple Databases

Redis has a concept of separated namespaces called “databases”. You can select the database number you want to use with “SELECT”. By default the database with index 0 is used. So issuing

redis 127.0.0.1:6379> SELECT 1
OK
redis 127.0.0.1:6379[1]>

switches to the second database. Note how the prompt changed and now has a “[1]” to indicate the database selection. To find out how many databases there are you might want to run redis-cli from the shell:

$ redis-cli INFO | grep ^db
db0:keys=91,expires=88
db1:keys=1,expires=0

Dropping Databases

To drop the currently selected database run

FLUSHDB

to drop all databases at once run

FLUSHALL

Replication

Checking for Replication

To see if the instance is a replication slave or master issue

redis 127.0.0.1:6379> INFO
[...]
role:master

and watch for the “role” line which shows either “master” or “slave”. Starting with version 2.8 the “INFO” command also gives you per slave replication status looking like this

slave0:ip=127.0.0.1,port=6380,state=online,offset=281,lag=0

Setting up Replication

If you quickly need to set up replication just issue

SLAVEOF <IP> <port>

on a machine that you want to become slave of the given IP. It will immediately get values from the master. Note that this instance will still be writable. If you want it to be read-only change the redis config file (only available in most recent version, e.g. not on Debian). To revert the slave setting run

SLAVEOF NO ONE

Performance Testing

Benchmark

Install the Redis tools and run the provided benchmarking tool

redis-benchmark -h <host> [-p <port>]

If you are migrating from/to memcached protocol check out how to run the same benchmark for any key value store with memcached protocol.

Debugging Latency

First measure system latency on your Redis server with

redis-cli --intrinsic-latency 100

and then sample from your Redis clients with

redis-cli --latency -h <host> -p <port>

If you have problems with high latency check if transparent huge pages are disabled. Disable it with

echo never > /sys/kernel/mm/transparent_hugepage/enabled

Dump Database Backup

As Redis allows RDB database dumps in background, you can issue a dump at any time. Just run:

BGSAVE

When running this command Redis will fork and the new process will dump into the “dbfilename” configured in the Redis configuration without the original process being blocked. Of course the fork itself might cause an interruption. Use “LASTSAVE” to check when the dump file was last updated. For a simple backup solution just backup the dump file. If you need a synchronous save run “SAVE” instead of “BGSAVE”.

Listing Connections

Starting with version 2.4 you can list connections with

CLIENT LIST

and you can terminate connections with

CLIENT KILL <IP>:<port>

Monitoring Traffic

The propably most useful command compared to memcached where you need to trace network traffic is the “MONITOR” command which will dump incoming commands in real time.

redis 127.0.0.1:6379> MONITOR
OK
1371241093.375324 "monitor"
1371241109.735725 "keys" "*"
1371241152.344504 "set" "testkey" "1"
1371241165.169184 "get" "testkey"

additionally use “SLOWLOG” to track the slowest queries in an interval. For example

SLOWLOG RESET
# wait for some time
SLOWLOG GET 25

and get the 25 slowest command during this time.

Sharding with proxies

There are two major proxy solutions

• Twemproxy (aka nutcracker, by Twitter)
• Codis