In a redis cluster what is the default behaviour for the read operations that happen? Does a client read from the master? I know that a client writes/updates/deletes to the master but what about read operations? If the default behaviour is to read from the master node how can I configure it to read from the slave nodes instead?
It all depends on Redis client library. For Jedis/Lettuce, all operations(CRUD) will be send to corresponding master node, and slaves are only used for failover.
If you wan to implement READONLY slave, you need some customization on Redis client.
The default behavior of replica nodes in cluster-mode enabled clusters
is to redirect all client read/write requests to an authoritative
master node of the shard that belongs to the key's hash slot. The
replica node serves the read request only if that shard belongs to the
hash slot and a readonly command was initiated by the client. This
means that the replica node processes the request only if readonly is
issued by the client before the request. Otherwise, the request is
redirected to the primary node of the shard that the hash slot belongs
to.
https://aws.amazon.com/premiumsupport/knowledge-center/elasticache-redis-client-readonly/
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I have a redis cluster created with master slave mode. I want to create a redisson client to access the cluster but I want to specify separate endpoints for reads and writes. Writes should go to master and reads should happen from the slaves. There is a config readMode that can I set to SLAVE to read only from slave nodes but how do I restrict writes to master only?
In Redis, writes happen only in master nodes. So you don't need separate config to handle that.
There is a config readMode that can I set to SLAVE to read only from slave nodes but how do I restrict writes to master only?
Writes are executed in master nodes.
All commands are executed in the master nodes, by default: while Redis Enterprise allows for a multi-master active-active clusters, Redis (Open Source) only allows one master node and zero or more replicas per slot range. In all cases, all nodes can receive both read and write commands but, by default, replicas reply with a -MOVED redirection error along with the endpoint of the master which is believed to handle a given target key. Clients may use that information to contact the master which will actually execute the command.
With that being said, replicas can be configured to reply to read-only commands - provided they handle the slot range of the given target key. In that context, most cluster-aware Redis clients allow to read from replicas with the goal of distributing the load - with the risk of reading stale data: Redisson manages that through the readMode setting and automatically deals with the aforementioned connection configuration.
I've started exploring Redis Cluster and it's C client(hiredis). I've been unable to find much info about the client's interaction with the Redis cluster. I've got some queries in this regard:
Does the client make a connection with all the nodes of the cluster(master and slaves) in the beginning?
Is there any coordinator node which proxies the client's request to the correct node?
If not, does the client periodically get the info about the hash-slot holdings of each node in the cluster(in order to send its request to the correct node)?
Which client-cluster connection specific parameters are configurable?
Does the client make a connection with all the nodes?
Yes, the client maintains a connection with all the masters at least.
Is there a coordinator node which proxies the client's request to the correct node?
No, there isn't. By design, redis cluster does not have a proxy.
(Aside: There is some talk of developing a proxy solution for redis - but I don't expect it to be released any time soon.)
Does the client periodically get info about hash slot bindings?
When a client starts up, it builds up a cache of hash-slot mappings. Then, at runtime, if a slot is migrated to another master, redis cluster will return a specific error that will tell the client the new owner for that slot. The client is then expected to cache the new owner, and retry the request against the new node.
As a result of this design, clients usually have a very good cache of every slot and it's owner, and there is very little overhead.
which client connection parameters are configurable?
The most important parameter is the list of server nodes to connect to the cluster. You don't have to specify all the nodes - the client can auto-discover all the masters. As long as even one node is active, the client will discover all the other nodes.
Apart from that, you have connection timeout parameters, parameters to control TLS.
Why Redis client use multiple address in cluster-mode for create connection? is this to switch between addresses when one of them has failed?
Thanks.
Redis uses multiple address to setUp application with all the master and slave node available in redis cluster. Redis never switch address it is just redis-cluster responsibility to promote the slave node to master if any one of them failed. After that subsequent request can be served directly from that redis node.
More details here : https://redis.io/topics/cluster-tutorial
I want to configure slave to enable writes (slave-read-only no). The use case is to enable ephemeral cache.
However, this paragraph in the documentation made me concerned:
Normally slave nodes will redirect clients to the authoritative master for the hash slot involved in a given command, however clients can use slaves in order to scale reads using the READONLY command.
– http://redis.io/commands/readonly
Does setting slave-read-only no will make slave confirm every hash lookup with the master?
Please take note that slave-read-only config refers to replication and READONLY refers to the redis-cluster command.
If you are not using redis-cluster, you can safely ignore the READONLY command documentation. Refer to https://raw.githubusercontent.com/antirez/redis/2.8/redis.conf instead. Writes should not replicate nor require lookups to the master. My wireshark dumps on redis with slave-read-only no shows no indication of any communication with master as a consequence of writes to the slave itself.
If you are using redis-cluster on the other hand, and referring to the READWRITE behavior: Cluster nodes' communication with each other for hash slot updates and other cluster specific messages are optimized to use minimal bandwidth and the least processing time. Communicating hash slot updates most likely do not happen for every write on the slave.
I am using redis(Redis 3.1) as session store for tomcat(Tomcat 7). To ensure high availability, there is a sentinel setup and two instances(master and slave) of redis server. The slave is configured as read-only. After running few tests and verifying the statistics, it's observerd there are no read requests sent to the slave. All the read requests are processed by the master alone.
Could you please let me know how I can make the slave serve the read requests?
You could use Redis based Tomcat Session Manager provided by Redisson. It allows to manage which type of node use for read operation (master, slave or both master and slave). Perfectly works in Sentinel/Cluster modes.