I am working with couchbase. I see several couchbase servers running with one as master and rest as replica server for a particular read/write request. Does this mean the complete data of the database is copied on all the server? Let's say there are 10 server, does that mean there will be 10 copies of the database on 10 different servers? Is this not inefficient use of storage space?
During failover, there will only update in the vBucket Map, no transfer of data to failover server to other server as rest of the server already contain complete data of the database. Is my understanding correct?
I read the document available on couchbase website but not able to completely understand the answer to above questions.
Can anyone help me to get the answer to above questions.
Thanks in advance
Trond Norbye has an excellent explanation of vBuckets and replication on his blog.
To address your questions directly:
The way that Couchbase distributes data throughout the cluster is via the concept of vBuckets. These can be thought of as 'shards' or 'partitions' of your data. The default amount of vBuckets in a cluster is 1024, so your data will be split into 1024 parts and these are shared equally to every node in the cluster. Therefore, in your example of a cluster with 10 nodes each node will be responsible for just over 100 vBuckets of data. The replication system also uses vBuckets and distributes the same vBuckets but to different nodes in the cluster. So the active and replica vBuckets will always be on separate nodes. If the node with the active vBucket failsover, the replica node will seamlessly begin serving traffic for that vBucket.
In the above blog post, Trond Norbye has posted a handy table to visualise this:
+------------+---------+---------+---------+
| vbucket id | active | replica | replica2|
+------------+---------+---------+---------+
| 0 | node A | node B | node D |
| 1 | node B | node C | node A |
| 2 | node C | node D | node B |
| 3 | node D | node A | node C |
+------------+---------+---------+---------+
So if you specify a single replica for your data, your data will be stored twice in Couchbase, 2 replicas will store three copies of the data in the cluster. So no wasted storage space. :)
You are correct about the failover situation, as there are already replica vBuckets ready to take over the traffic there is no need for data to be transferred between nodes. However, you will now have one node in the cluster serving traffic for more vBuckets than it was originally responsible for, so the cluster will be imbalanced. To resolve this you should either bring the failed node back up or complete a rebalance.
In addition to the the architecture overview documentation there are also some good introductory videos on the Couchbase YouTube channel, this one in particular provides a good overview on the basics of Couchbase. The architecture white paper is also good.
Related
I am having 2 Couchbase node, having 3 ephemeral buckets. The buckets are non replicated.
Lets name the nodes as A and B. Now I want to keep node B and remove node A.
Our client services is having the IP of node B, so I want to remove node A.
Can I remove node A directly from the Couchbase console and perform rebalancing. Am I going to lose data.
Any help will be appreciated.
I just tried this locally:
I created an ephemeral bucket with 0 replicas on a 2-node cluster.
I put 6 total documents in the bucket.
I removed one node.
I rebalanced the cluster.
After the rebalance was complete, I still had 6 documents in the ephemeral bucket.
So it appears that you will NOT lose data. HOWEVER, I would highly recommend taking advantage of the distributed nature of Couchbase and turn on replication in order to get high availability (in case something goes wrong with one of the nodes that you didn't plan for).
I have a problem with my cluster couchbase, I have 4 (four) servers in my cluster, I have 3 (three) replicated number, that is the original data and + 3 copies, my created documents are 100% available for consultation while the cluster contains the 4 nodes, what caught my attention and generated very strange is the fact that if one of the machines is unavailable the data of the document are not available or are located when I perform a search.
To improve understanding, I have this cluster with 4 servers to ensure resilience and high availability of data, but it is to lose one of the machines and the data is no longer found when I execute query in one of the 3 replicas.
My 4 nodes of this server are with data, index and query services enabled.
Has anyone gone through this before?
this my cluster services
in this image, my big question is if this option would help "View index replicas"
I intend to run a Point of Sale software in a galera cluster (percona xtradb). Each POS terminal would be its own cluster and then there will be an Amazon EC2 in addition to help avoid split-brain scenarios.
Is the above setup an ideal cluster setup? My POS terminals could range from 1 to N nodes within a local network and I will always only have 1 EC2 instance outside the network.
Thanks,
Yes. To provide automatic failover, 3 nodes is required. If you have 3 nodes in the same building, etc, then you are not safe against floods, earthquakes, tornadoes, data center failure, etc. "Within the local network" -- see what Amazon means by that, then read between the lines; it may or may not protect you from various possible disasters.
Do not plan on having "too many" nodes in the cluster -- all writes go to all other nodes; this can add up to a lot of network traffic. (I have not heard of more than something like a dozen nodes. But I don't know what the practical limit is.)
You could have multiple clusters and have data replicated off-cluster to some central server for reporting, etc. That replication would be ordinary MySQL replication, not the Galera type.
I'm building a very small NDB cluster with only 3 machines. This means that machine 1 will serve as both MGM Server, MySQL Server, and NDB data node. The database is only 7 GB so I plan to replicate each node at least once. Now, since a query might end up using data that is cached in the NDB node on machine one, even if it isn't node the primary source for that data, access would be much faster (for obvious reasons).
Does the NDB cluster work like that? Every example I see has at least 5 machines. The manual doesn't seem to mention how to handle node differences like this one.
There are a couple of questions here :
Availability / NoOfReplicas
MySQL Cluster can give high availability when data is replicated across 2 or more data node processes. This requires that the NoOfReplicas configuration parameter is set to 2 or greater. With NoOfReplicas=1, each row is stored in only one data node, and a data node failure would mean that some data is unavailable and therefore the database as a whole is unavailable.
Number of machines / hosts
For HA configurations with NoOfReplicas=2, there should be at least 3 separate hosts. 1 is needed for each of the data node processes, which has a copy of all of the data. A third is needed to act as an 'arbitrator' when communication between the 2 data node processes fails. This ensures that only one of the data nodes continues to accept write transactions, and avoids data divergence (split brain). With only two hosts, the cluster will only be resilient to the failure of one of the hosts, if the other host fails instead, the whole cluster will fail. The arbitration role is very lightweight, so this third machine can be used for almost any other task as well.
Data locality
In a 2 node configuration with NoOfReplicas=2, each data node process stores all of the data. However, this does not mean that only one data node process is used to read/write data. Both processes are involved with writes (as they must maintain copies), and generally, either process could be involved in a read.
Some work to improve read locality in a 2-node configuration is under consideration, but nothing is concrete.
This means that when MySQLD (or another NdbApi client) is colocated with one of the two data nodes, there will still be quite a lot of communication with the other data node.
I'm reading everywhere that the minimum for a mysql 7 cluster is 3 physical machines, but the cluster exists out of 4 nodes
1 mysql node
2 data nodes
1 management node
So this means at least 1 machine must be hosting 2 types of nodes but I cannot find anywhere which machine shares which nodes.
I've read that sharing MySQL and data nodes is not recommended so then it must be the management node and MySQL node which are sharing a machine?
Could anyone please advice me on this..
Just a small edit: I'm currently setting this up because we now have 1 normal MySQL server and we're pretty much hitting its limit. I'm mainly trying to setup the cluster for performance gain (2 data/MySQL nodes should be faster then 1 right?), expanding it with more server to gain redundancy is next on the list.
You can co-locate the management nodes with the SQL nodes to reduce your footprint to 3 x physical hosts
I would recommend taking a look at the Evaluation Guide (note, opens a pdf) which can talk you through these factors, as well as providing some tips / best practices when moving from a single MySQL node to a fully distributed storage engine such as MySQL Cluster:
http://dev.mysql.com/downloads/MySQL_Cluster_72_DMR_EvaluationGuide.pdf