I need to build 2 nodes of PXC between 2 data centers. In these 2 data centers, there is a public node (more important) and a private node (offline sometimes due to network issue). If there are only 2 nodes and when a node becoming offline, whole cluster will become unreachable.
In order to prevent split-brain, it should be better to build odd nodes. I want to build a extra node as a moderator to prevent this issue. But I could not build a full function node.
Is there any method to build a PXC node without data? Or any package could let a node represent only for quorum?
Thanks!
OK, I found the keyword is "Galera Arbitrator".
Galera Arbitrator could become an additional node in certain cluster operations, such as quorum calculations. It is a separate daemon from Galera Cluster, called garbd.
Ref: http://galeracluster.com/documentation-webpages/arbitrator.html
Related
I am using Cassandra(3.5) with 20 nodes with data center-1 with 10 nodes and data center-2 with 10 nodes and has huge data. All hosts belong to say legacy hosts. Now we have newer generation hosts say generation-2.
I have tried adding new nodes and decommissioning old node. But this will be tie consuming.
Q1: How can I migrate all hosts from legacy hosts to generation-2 host? What is the best approach for that?
Q2: What will be rollback strategy?
Q3: Finally, How can I validate data once I migrate to generation-2 hosts?
If you just replacing the nodes with newer hardware, keeping the same number of nodes, then it's simple (operations should be done on every node):
prepare the new installation on every node, with configuration identical to existing nodes, but with different IP addresses but don't start the nodes
(optional) disable autocompaction with nodetool disableautocompaction - this could help to execute step 5 faster
copy data from old node to new node using rsync (this could take long time)
execute nodetool drain & stop old node
use rsync to synchronize changes happened since initial copying (it should be relatively fast)
make sure that the old node won't start again (for example, remove Cassandra package) - otherwise it could be a chaos
start the new node
This works because Cassandra node is identified by the UUID that is stored in the local table, so changing of IP doesn't affect the operations.
P.S. In future, if you'll need to replace node (not as described, but completely died), use the procedure of node replacement - in this case, you won't stream data twice, as happened when you do decomissioning and then re-adding node.
We are in the process of adding new nodes to our existing cassandra 3.x cluster. Basically following the steps outlined in this article (https://docs.datastax.com/en/cassandra/2.1/cassandra/operations/ops_add_node_to_cluster_t.html).
Currently, out 3.x cluster does incremental repairs. What I'm not 100 percent sure about is if we need to do anything after we do the node cleanup. Specifically, are our new added nodes setup to do incremental repairs after following the procedure listed above?
Thanks
Marshall
Adding a node involves rebalancing the token distribution among nodes in the cluster and bootstrapping the new node. Repair is a regular maintenance process and it's not necessarily needed in the adding node process.
For the token redistribution part, simplified example is if your have 20 tokens and 4 nodes initially. If random enough, each node would be the primary node for 5 tokens. When add a node and change the configuration, 20 tokens would be distributed to 5 nodes and each node would be the primary node for 4 tokens. When you run bootstrap to add the new node, the new node knows what tokens belongs to it and it will stream missing tokens from other existing nodes. After bootstrapping is done, nodetool cleanup on the existing nodes would remove tokens which don't belong to them anymore. To sum up, bootstrapping new node would redistribute tokens and stream objects to new nodes based on the distribution. you don't need repair in this process to stream data. cleanup would remove objects which the ownership is changed.
However, repair is a regular process to guarantee data consistency and incremental or full options are out of scope in terms of talking about adding node.
Good reference to read under the hood on what happens when you change the topology of a cluster
I added a new node to Cassandra cluster by making it seed node and than started rebuilding it (nodetool rebuild ) command. Although node joined the cluster quickly, the rebuild process that started streaming from all nodes in selected caused the whole dc nodes to slow down. The impact on application is severe. I'll have to stop rebuild process in order to keep normal operation ON!.
Here, I'm seeking advice if you guys can share ways/tricks so to minimize the impact of (node rebuild ) operation on rest of dc nodes and application.
I'll much appreciate your suggestions - thanks for reading my message and your help in advance.
While adding a new node you shouldn't make it a seed node. The seed node is used to bootstrap other nodes and join them in cluster. Making the new node as a seed node will not allow to join the new node in the cluster. Follow the steps provided in the Cassandra docs provided in the link below.
https://docs.datastax.com/en/archived/cassandra/2.0/cassandra/operations/ops_add_node_to_cluster_t.html
This is the best way to add a new node in the cluster.
Note: Make sure the new node is not listed in the -seeds list. Do not make all nodes seed nodes. Please read Internode communications (gossip).
As I understand, you added a node as a seed node just so it will not bootstrap and join the cluster instantly. While is approach is right for it joins the cluster quickly, the downside is, it will not bootstrap and hence will not copy all the data from other nodes that it is responsible for. When you run a rebuild on that node, data is blindly copied (without doing any validation) from other nodes, which can choke up existing nodes throughput and your network pipeline. This approach is very safe and is recommended when using adding a new DC but not when adding nodes to an existing DC.
When you are adding a node, the simplest way is to add a node using the procedure described here. https://docs.datastax.com/en/archived/cassandra/2.0/cassandra/operations/ops_add_node_to_cluster_t.html
When the node bootstrap, it will copy data needed from other nodes but will not start taking client connections until it fully bootstraps and validates the data. So, add one node at a time and let it bootstrap so all the necessary data is copied and validated. After you are done adding the number of nodes you desire, run a cleanup on all the previously joined nodes to cleanup all the keys for which the old nodes are not responsible.
I am new to Cassandra and I want to install it. So far I've read a small article on it.
But there one thing that I do not understand and it is the meaning of 'node'.
Can anyone tell me what a 'node' is, what it is for, and how many nodes we can have in one cluster ?
A node is the storage layer within a server.
Newer versions of Cassandra use virtual nodes, or vnodes. There are 256 vnodes per server by default.
A vnode is essentially the storage layer.
machine: a physical server, EC2 instance, etc.
server: an installation of Cassandra. Each machine has one installation of Cassandra. The Cassandra server runs core processes such as the snitch, the partitioner, etc.
vnode: The storage layer in a Cassandra server. There are 256 vnodes per server by default.
Helpful tip:
Where you will get confused is that Cassandra terminology (in older blog posts, YouTube videos, and so on) had been used inconsistently. In older versions of Cassandra, each machine had one Cassandra server installed, and each server contained one node. Due to the 1-to-1-to-1 relationship between machine-server-node in old versions of Cassandra people previously used the terms machine, server and node interchangeably.
Cassandra is a distributed database management system designed to handle large amounts of data across many commodity servers. Like all other distributed database systems, it provides high availability with no single point of failure.
You may got some ideas from the description of above paragraph. Generally, when we talk Cassandra, we mean a Cassandra cluster, not a single PC. A node in a cluster is just a fully functional machine that is connected with other nodes in the cluster through high internal network. All nodes work together to make sure that even if one of them failed due to unexpected error, they as a whole cluster can provide service.
All nodes in a Cassandra cluster are same. There is no concept of Master node or slave nodes. There are multiple reason to design like this, and you can Google it for more details if you want.
Theoretically, you can have as many nodes as you want in a Cassandra cluster. For example, Apple used 75,000 nodes served Cassandra summit in 2014.
Of course you can try Cassandra with one machine. It still work while just one node in this cluster.
What is meant by a node in cassandra?
Cassandra Node is a place where data is stored.
Data center is a collection of related nodes.
A cluster is a component which contains one or more data centers.
In other words collection of multiple Cassandra nodes which communicates with each other to perform set of operation.
In Cassandra, each node is independent and at the same time interconnected to other nodes.
All the nodes in a cluster play the same role.
Every node in a cluster can accept read and write requests, regardless of where the data is actually located in the cluster.
In the case of failure of one node, Read/Write requests can be served from other nodes in the network.
If you're looking to understand Cassandra terminology, then the following post is a good reference:
http://exponential.io/blog/2015/01/08/cassandra-terminology/
Is it possible to add a new node to an existing cluster in cassandra 1.2 without running nodetool cleanup on each individual node once data has been added?
It probably isn't but I need to ask because I'm trying to create an application where each user's machine is a server allowing for endless scaling.
Any advice would be appreciated.
Yes, it is possible. But you should be aware of the side-effects of not doing so.
nodetool cleanup purges keys that are no longer allocated to that node. According to the Apache docs, these keys count against the allocated data for that node, which can cause the auto bootstrap process for the next node to not properly balance the ring. So depending on how you are bringing new user machines into the ring, this may or may not be a problem.
Also keep in mind that nodetool cleanup only needs to be run on nodes that lost keyspace to the new node - i.e. adjacent nodes, not all nodes, in the cluster.