Currently we're using single redis instance(String datastructure is used, here i am able to fetch data using "get key" and "mget key1 key2"). To scale the redis now created redis cluster here i get this error: ERR CROSSSLOT Keys in request don't hash to the same slot.
If I give hashtag then all data is storing in one slot. All other slots are unused.
I am new to redis, Please suggest a solution how to make use of all the slots. Thanks in advance.
RTFM: https://redislabs.com/blog/redis-clustering-best-practices-with-keys/
Keys are hashed. Using a hash forces the same hash result (aka slot) so that multiple command operations can work.
Related
I'm under the impression that one should hash (i.e. sha3) their Redis key before adding data to it. (It might have even been with regard to memcache.) I don't remember why I have this impression or where it came from but I can't find anything to validate (or refute) it. The reasoning was the hash would help with even distribution across a cluster.
When using Redis (in either/both clustered and non-clustered modes) is it best pracatice to hash the key before calling SET? e.g. set(sha3("username:123"), "owlman123")
No, you shouldn't hash the key. Redis Cluster hashes the key itself for the purpose of choosing the node:
There are 16384 hash slots in Redis Cluster, and to compute what is the hash slot of a given key, we simply take the CRC16 of the key modulo 16384.
You can also use hash tags to control which keys share the same slot.
It might be a good idea if your keys are very long, as recommended in the official documentation:
A few other rules about keys:
Very long keys are not a good idea. For instance a key of 1024 bytes is a bad idea not only memory-wise, but also because the lookup of the key in the dataset may require several costly key-comparisons. Even when the task at hand is to match the existence of a large value, hashing it (for example with SHA1) is a better idea, especially from the perspective of memory and bandwidth.
source: https://redis.io/docs/data-types/tutorial/#keys
I'm have hashes in redis cache like:
Hash Key Value
hashme:1 Hello World
hashme:2 Here Iam
myhash:1 Next One
My goal is to get the Hashes as output in the CLI like:
hashme
myhash
If there's no such option, this is ok too:
hashme:1
hashme:2
myhash:1
I didn't find any relevant command for it in Redis API.
Any suggestions ?
You can use the SCAN command to get all keys from Redis. Then for each key, use the TYPE command to check if it's a hash.
UPDATE:
With Redis 6.0, the SCAN command supports TYPE subcommand, and you can use this subcommand to scan all keys of a specified type:
SCAN 0 TYPE hash
Also never use KEYS command in production environment!!! It's a dangerous command which might block Redis for a long time.
keys *
is work for me. you Can try it.
The idea of redis (and others K/v stores) is for you to build an index. The database won't do it for you. It's a major difference with relational databases, which conributes to better performances.
So when your app creates a hash, put its key into a SET. When your app deletes a hash, remove its key from the SET. And then to get the list of hash IDs, just use SMEMBERS to get the content of the SET.
connection.keys('*') this will bring all the keys irrespective of the data type as everything in redis is stored as key value format
for redis in python, you can use below command to retrieve keys from redis db
def standardize_list(bytelist):
return [x.decode('utf-8') for x in bytelist]
>>> standardize_list(r.keys())
['hat:1236154736', 'hat:56854717', 'hat:1326692461']
here r variable is redis connection object
I am trying to insert multiple key/values at once on Redis (some values are sets, some are hashes) and I get this error: ERR CROSSSLOT Keys in request don't hash to the same slot.
I'm not doing this from redis-cli but from some Go code that needs to write multiple key/values to a redis cluster. I see other places in the code where multiple key values are done this way and I don't understand why mine don't work. What are the hash requirements to not have this error?
Thanks
In a cluster topology, the keyspace is divided into hash slots. Different nodes will hold a subset of hash slots.
Multiple keys operations, transactions, or Lua scripts involving multiple keys are allowed only if all the keys involved are in hash slots belonging to the same node.
Redis Cluster implements all the single key commands available in the
non-distributed version of Redis. Commands performing complex
multi-key operations like Set type unions or intersections are
implemented as well as long as the keys all belong to the same node.
You can force the keys to belong to the same node by using Hash Tags
ERR CROSSSLOT Keys in request don't hash to the same slot
As the error message suggests, only if all of the keys belong to same slot will the operation succeed. Otherwise, you will see this failure message. This error would be seen even though both/all slots belongs to the same node. The check is very strict and, as per the code, all keys should hash to same slot.
I have a Redis Cluster. I am using JedisCluster client to connect to my Redis.
My application is a bit complex and I want to basically control to which partition data from my application goes. For example, my application consists of sub-module A, B, C. Then I want that all data from sub-module A should go to partition 1 for example. Similarly data from sub-module B should go to partition 2 for example and so on.
I am using JedisCluster, but I don't find any API to write to a particular partition on my cluster. I am assuming I will have same partition names on all my Redis nodes and handling which data goes to which node will be automatically handled but to which partition will be handled by me.
I tried going through the JedisCluster lib at
https://github.com/xetorthio/jedis/blob/b03d4231f4412c67063e356a7c3acf9bb7e62534/src/main/java/redis/clients/jedis/JedisCluster.java
but couldn't find anything. Please help?
Thanks in advance for the help.
That's not how Redis Cluster works. With Redis Cluster, each node (partition) has a defined set of keys (slots) that it's handling. Writing a key to a master node which is not served by the master results in rejection of the command.
From the Redis Cluster Spec:
Redis Cluster implements a concept called hash tags that can be used in order to force certain keys to be stored in the same node.
[...]
The key space is split into 16384 slots, effectively setting an upper limit for the cluster size of 16384 master nodes (however the suggested max size of nodes is in the order of ~ 1000 nodes).
Each master node in a cluster handles a subset of the 16384 hash slots.
You need to define at Cluster configuration-level which master node is exclusively serving a particular slot or a set of slots. The configuration results in data locality.
The slot is calculated from the key. The good news is that you can enforce a particular slot for a key by using Key hash tags:
There is an exception for the computation of the hash slot that is used in order to implement hash tags. Hash tags are a way to ensure that multiple keys are allocated in the same hash slot. This is used in order to implement multi-key operations in Redis Cluster.
Example:
{user1000}.following
The content between {…} is used to calculate the slot. Key hash tags allow you to group keys on particular nodes and enforce the same data locality when using arbitrary hash tags.
You can also go a step further by using known hash tags that map to slots (you'd need either precalculate a table or see this Gist). By using known hash tags that map to a specific slot you're able to select the slot and so the master node on which the data is located.
Everything else is handled by your Redis client.
I couldn't find a way to "page" through redis hashes (doc).
I've got ~5million hash entries in 1 redis db. I am trying to iterate through all of them without having to resort to building a list of entry keys.
Can this be achieved?
Since all the redis hash commands require the key element. You need to store your set of keys to page your hash.
See my answer to this question for an example of key iteration by using extra sets.
There is no way to avoid storing extra sets (or lists) and still iterate on a huge number of keys. The KEYS command is not an option.
I had exactly the same requirement of Redis Hash Pagination and yes it is possible to page through redis hash using HSCAN command. Detailed documentation of the same is present at SCAN.
Usage: Hscan <your key/hash name> <cursor-id> count <page-size>.
cursor id which should be passed initially is 0 and it returns a cursor-id and data which is of page-size. The cursor id returned needs to be passed in the subsequent call for fetching subsequent data.