Shantanu's Blog

Corporate Consultant

January 05, 2012

 

Hashes and sets in redis

Hashes


Redis's implementation of hashes makes for a perfect solution to store the object data application typically use.

Let's also assume we want to store a number of fields about our users, such as a full name, email address, phone and number of visits to our application. We will use Redis's hash management commands like HSET, HGET and HINCRBY to store this information.

redis> hset users:jdoe name "John Doe"
redis> hset users: jdoe email "jdoe@test.com"
redis> hset users: jdoe phone "+1234567890"
redis> hincrby users:jdoe visits 1

With our has built and in place, we can fetch single fields with HGET or the full hash by using the HGETALL command, for e.g.

redis> hget users:jdoe email
"jdoe@test.com"
redis> hgetall users:jdoe
1) "name"
2) "John Doe"
3) "email"
4) "jdoe@test.com"
5) "phone"
6) "+1234567890"
7) "visits"
8) "1"

There are other commands like HKEYS, which return the keys stored in a particular has, and HVALS, which returns only the values. Depending on how you want to retrieve your data, you may find it useful to use HGETALL or one of these to retrieve data from Redis into your application.

redis> hkeys users:jdoe
1) "name"
2) "email"
3) "phone"
4) "visits"

redis> hvals users:jdoe
1) "John Doe"
2) "jdoe@test.com"
3) "+1234567890"
4) "1"

sets


Sets are a natural fit for circles, because sets represent collections of data, and have native functionality to do interesting things like intersections and unions.

redis> sadd circle:jdoe:family users:anna
redis> sadd circle:jdoe:family users:richard
redis> sadd circle:jdoe:family users:mike

redis> sadd circle:jdoe:soccer users:mike
redis> sadd circle:jdoe:soccer users:adam
redis> sadd circle:jdoe:soccer users:toby
redis> sadd circle:jdoe:soccer users:apollo

redis> smembers circle:jdoe:family
1) users:anna
2) users:richard
3) users:mike
redis> hgetall users:mike
(...)


redis> sinter circle:jdoe:family circle:jdoe:soccer
1) "users:mike"

redis> sunion circle:jdoe:family circle:jdoe:soccer

1) users:anna
2) users:richard
3) users:mike
4) users:adam
5) users:toby
6) users:apollo

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January 04, 2012

 

redis tips

BGSAVE can cause redis to hang upto 20 minutes and lose data. The info command will let us know the peak memory used.
used_memory_peak_human:5.46G

This shows peak usage of 5.46G. When you get close to the edge of your RAM limits, redis can start to misbehave on BGSAVE.

Something else to check -- find your .rdb file and make sure it's timestamp is getting updated (ls -l) after the bgsave completes. While the bgsave
processes, you'll see a temp rdb file next to it growing. It should grow by 10's to 100's megs a second.

_____

When redis is started, it is highly likely that the following warning will be displayed. This needs to be corrected before starting to use redis.

[root@ip-10-134-170-210 mynew]# src/redis-server
[3240] 02 Mar 14:53:10 # Warning: no config file specified, using the default config. In order to specify a config file use 'redis-server /path/to/redis.conf'
[3240] 02 Mar 14:53:10 * Server started, Redis version 2.4.8
[3240] 02 Mar 14:53:10 # WARNING overcommit_memory is set to 0! Background save may fail under low memory condition. To fix this issue add 'vm.overcommit_memory = 1' to /etc/sysctl.conf and then reboot or run the command 'sysctl vm.overcommit_memory=1' for this to take effect.
[3240] 02 Mar 14:53:10 - Accepted 127.0.0.1:51868

The following 2 commands will do the needful.

echo 'vm.overcommit_memory = 1' >> /etc/sysctl.conf
sysctl vm.overcommit_memory=1

_____

You can write your own pooling layer in PHP:

$connection = $pool->getConnection(...);
...
$pool->releaseConnection($connection);

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