Discovered which is also master but with an older cluster state telling to rejoin the cluster – How to solve this OpenSearch error

Overview

Search or indexing requests will usually be load-balanced across the OpenSearch data nodes, and the node that receives the request will relay requests to other nodes as necessary and coordinate the response back to the user.

Notes and good things to know

The key elements to clustering are:

Cluster State – Refers to information about which indices are in the cluster, their data mappings and other information that must be shared between all the nodes to ensure that all operations across the cluster are coherent.

Master Node – Each cluster must elect a single master node responsible for coordinating the cluster and ensuring that each node contains an up-to-date copy of the cluster state.

Cluster Formation – OpenSearch requires a set of configurations to determine how the cluster is formed, which nodes can join the cluster, and how the nodes collectively elect a master node responsible for controlling the cluster state. These configurations are usually held in the opensearch.yml config file, environment variables on the node, or within the cluster state.

Node Roles – In small clusters it is common for all nodes to fill all roles; all nodes can store data, become master nodes or process ingestion pipelines. However as the cluster grows, it is common to allocate specific roles to specific nodes in order to simplify configuration and to make operation more efficient. In particular, it is common to define a limited number of dedicated master nodes.

Replication – Data may be replicated across a number of data nodes. This means that if one node goes down, data is not lost. It also means that a search request can be dealt with by more than one node.

Common problems

Many OpenSearch problems are caused by operations which place an excessive burden on the cluster because they require an excessive amount of information to be held and transmitted between the nodes as part of the cluster state. For example:

• Shards too small
• Too many fields (field explosion)

Problems may also be caused by inadequate configurations causing situations where the OpenSearch cluster is unable to safely elect a Master node. These type problems include: 

• Master node not discovered
• Split brain problem

Backups

Because OpenSearch is a clustered technology, it is not sufficient to have backups of each node’s data directory. This is because the backups will have been made at different times and so there may not be complete coherency between them. As such, the only way to backup an OpenSearch cluster is through the use of snapshots, which contain the full picture of an index at any one time.

Cluster resilience

When designing an OpenSearch cluster, it is important to think about cluster resilience. In particular – what happens when a single node goes down? And for larger clusters where several nodes may share common services such as a network or power supply – what happens if that network or power supply goes down? This is where it is useful to ensure that the master eligible nodes are spread across availability zones, and to use shard allocation awareness to ensure that shards are spread across different racks or availability zones in your data center.

Overview

How it works

Upon startup, each node looks for other nodes, firstly by contacting the IP addresses of eligible master nodes held in the previous cluster state.  If they are not available, it will look for nodes based upon the seed host provider mechanisms available.

Seed host providers may be defined in 3 ways: list based, file based or plugin based. All of these methods provide a list of IP addresses or hostnames which the node should contact in order to obtain a list of master eligible nodes. The node will contact all of these addresses in turn, until either an active master is found, or failing that, until sufficient nodes can be found to elect a new master node.

Examples

The simplest form is to define a list of seed host providers in opensearch.yml:

discovery.seed_hosts:
   - 192.168.1.10:9300
   - 192.168.1.11 
   - seeds.mydomain.com

An alternative way is to refer to a file using the following setting:

discovery.seed_providers: file

The file MUST be placed in the following filepath: $ES_PATH_CONF/unicast_hosts.txt

10.10.10.5
10.10.10.6:9305
10.10.10.5:10005
# an IPv6 address
[2001:0db8:85a3:0000:0000:8a2e:0370:7334]:9301

Note that the use of a port is optional. If not used, then the default port range of 9300-9400 will be used.

If you use AWS or GCS then you can install and use a plugin to obtain a list of seed hosts from an API.  A plugin also exists for Azure but is deprecated since version 5.

AWS plugin

A typical configuration could be as follows:

discovery.seed_providers: ec2
discovery.ec2.tag.role: master
discovery.ec2.tag.environment: dev
discovery.ec2.endpoint: ec2.us-east-1.amazonaws.com
cloud.node.auto_attributes: true
cluster.routing.allocation.awareness.attributes: aws_availability_zone

The above configuration would look for all nodes with a tag called “environment” set to “dev” and a tag called “role” set to “master”, in the AWS zone us-east-1. The last two lines set up cluster routing allocation awareness based upon aws availability zones. (Not necessary, but nice to have).

GCE plugin

A typical configuration could be as follows:

discovery.seed_providers: gce
cloud.gce.project_id: <your-google-project-id>
cloud.gce.zone: <your-zone>
discovery.gce.tags: <my-tag-name>

The above configuration would look for all virtual machines inside your project, zone and with a tag set to the tag name you provide.

Notes and good things to know

Cluster formation depends on correct setup of the network.host settings in opensearch.yml.  Make sure that the nodes can reach each other across the network using their IP addresses / hostname, and are not getting blocked due to firewall settings on the ports required.