I am new to redis, I do not know the meaning of "keyspace" and "key space" in redis terminology which I encountered in redis official website. Can someone help me to clear that? Thanks.
These terms refer to the internal dictionary that Redis manages, in which all keys are stored. The keyspace of a Redis database is managed by a single server in the case of a single instance deployment, and is divided to exclusive slot ranges managed by different nodes when using cluster mode.
In a key-value database, all keys can be in one node or divided in multiple nodes. Suppose I am storing telephone dictionary as key-value store with name as key and phone number as a value. If I store names A-L on one node and M-Z on another node, I divide my database into two key spaces. When I run query to search number of Smith, I need to search only second key space or node. This divides the query on multiple nodes and divide the work giving faster result. This could be shared-nothing model of working.
Related
i have two plan for my redis key , one for separate several db ,one for each project ,another is put the keys on a whole big db, use the key name prefix to distinct them like " db1:xxx ,db2:xxx ",which one is better ? why?
Please reference to this post: What's the Point of Multiple Redis Databases?
In my opinion, it's not good to use multiple databases in a single redis instance, as Salvatore(Author of Redis) said here: https://groups.google.com/d/msg/redis-db/vS5wX8X4Cjg/8ounBXitG4sJ
But if just for development, both of your choice are ok.
I'm new to redis, have doubt on below commands
In redis-cli i have entered info keyspace, for this i got the following output
# Keyspace
db0:keys=2,expires=0,avg_ttl=0
What does the above lines represent. N how to get the value from key.
You might want to look at https://redis.io/commands/INFO
To be specific the keyspace is, as mention in the official doc:
Database related statistics
The keyspace section provides statistics on the main dictionary of each database. The statistics are the number of keys, and the number of keys with an expiration.
For each database, the following line is added:
dbXXX: keys=XXX,expires=XXX
So in your case you have 2 keys present in redis
In the aerospike documentation, it is mentioned that aerospike has 4096 logical partitions and each key is hashed and eventually mapped to any of the partitions between 1 to 4096, which determines in which node the data for that key should be stored.
However if we have two keys "A" and "AB" and we want to store them in the same node, is there a way?
In Redis it can be achieved by making the keys as "A" and "{A}B" that will make sure that the key "{A}B" will go to a node where "A" is hashed and stored.
In Apache Ignite, same can be done using "AffinityKey".
Does a similar idea exist in Aerospike?
Thanks
Aerospike was designed as a distributed database. Redis was designed to run on a single node, and lacks concepts such as data distribution, clustering, replication, failover, at least natively. I'm aware that you can use various application-side shenanigans to make it into an ad-hoc cluster.
Don't worry about the implementation details of Aerospike's data distribution. Those happen automatically between the client and cluster, and don't require you to do anything on the application side. Instead, think about your access patterns.
First, your Aerospike cluster will make sure the data is evenly distributed. Because work is directly proportional to data, you should make sure the nodes are homogeneous. You can then expect multi-node operations to wrap up in roughly the same amount of time on each node.
You can create a secondary index on the fields that you'll be querying often to enhance the speed of the query. Release 3.12 adds predicate filtering, allowing you to create more complex query predicates on top of the initial secondary index based filter (also see the Java client's PredExp class).
If you don't want to use secondary indexes (there are several valid reasons), you can create your own lookup using external records. In a set called country-school you can have a record for each country (keys such as 'india', 'luxembourg') with the value being a list containing the IDs of the schools in that country. You can get the list with a single get (or a batch-get if it's several records, such as india-1, india-2, ... , india-9999), then use the results to compose a batch-get operation for the schools. Batch reads return results in the ordered you asked so you can get a large batch, check whether the last element is null, and if not get another batch.
('ns1', 'country-school', 'us-california') => [ 1, 2, 3, 5, 8, 11, .. ]
Similarly, you can create permutations such as country-state-city, (example, us-california-oakland) with smaller lists. This costs some extra space, but gives you faster (key-value based) retrieval without spending memory on secondary indexes.
('ns1', 'country-school', 'us-california-oakland') => [ 1, 5, 42, .. ]
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.