When using multiple databases on a single redis instance and its memory is full, when I'm trying to insert new data it samples a number of keys and it applies an algorithm to them to determine which ones should be evicted.
But, if I'm using db0 and db1 and I'm trying to insert a new record into db1, will redis sample keys from the same database or does it sample them globally?
When it does eviction, Redis chooses eviction candidate from all databases.
In your case, it might evict keys from db0 or db1.
Related
We have two redis instances that are overprovisioned on AWS (oldhost1, oldhost2) and I'm wondering if it's possible to combine that data into a single newhost1
Is it possible to replicate
oldhost1/db0 -> newhost1/db0
oldhost2/db1 -> newhost1/db1
I tried setting up newhost1 to replcate from oldhost1, which sucked in oldhost1's data - but then when I repointed replication at oldhost2, it clobbered the data already replicated.
I'm absolutely a newbie using redis.
I need to:
list all databases
list all data structures
I've connected to redis 4.0.11 server using redis-cli.
Redis is a key value storage not a database, You can't query or structure the Redis like you do in a database. You can only receive the relevant value from the key that you are passing.
Usually instead of database a key value storage like redis is used to to high performance key value storage and retrieve, if performance of a database is not enough.
I am new to crate.io and I am not very familiar with the term of "sherd" and I am trying to understand why when I am running my local db it creates 4 different shards?
I need to reduce this to one single shard because it causes problems when I try to export the data from crate into json files (it creates 4 different shards!)
Most users run crate on multiple servers. To distribute the records of a table between multiple servers it needs to be splitted. One piece of that table is called shards.
To make sure that the database still has records CrateDB by defaults create on replica of each shard. A copy of the data that is located on a different server.
While the system doesn't have full copies of the shards the cluster state is yellow / underreplicated.
CrateDB running on a single node will never be able to create a redundant copy (because it is only one server).
To change the amount of replicas you can use the command ALTER TABLE my_table SET(number_of_replicas=...)
There are 0 to 15 databases in redis.
I want to create my own database using redis-cli.
Is there any command for it?
Redis database is not an equivalent of database names in DBMS like mysql.
It is a way to create isolation and namespacing for the keys, and only provides index based naming, not custom names like my_database.
By default, redis has 0-15 indexes for databases, you can change that number
databases NUMBER in redis.conf.
And then you use SELECT command to select the database you want to work on.
You don't create a database in Redis with a command - the number of databases is defined in the configuration file with the databases directive (the default value is 16). To switch between the databases, call SELECT.
Use select, for example:
select 1
select 2
...
I found this relevant when I encountered the same question:
Redis different selectable databases are a form of namespacing: all the databases are anyway persisted together in the same RDB / AOF file. However different databases can have keys having the same name, and there are commands available like FLUSHDB, SWAPDB or RANDOMKEY that work on specific databases.
In practical terms, Redis databases should mainly used in order to, if
needed, separate different keys belonging to the same application, and
not in order to use a single Redis instance for multiple unrelated
applications.
The bolding is my addition.
Read more here: https://redis.io/commands/select
For the question of how to select a "database", it is the same answer given here:
$ select 1
And also some useful stuff about persistence, if RDB/AOF were mentioned: https://redis.io/topics/persistence
I have postgres 9.3 db and I want to use redis to cache calls the the DB (basically like memcached). I followed these docs, which means I have basically configured redis to work as an LRU cache. But am unsure what to do next. How do I tell redis to track calls to the DB and cache their output? How can I tell it's working?
In pseudo code:
see if redis has the record by 'record_type:record_id'
if so return the result
if not then query postgres for the record_id in the record_type table
store the result in redis by 'record_type:record_id'
return the result
This might have to be a custom adapter for the query engine that you are using.