Briefly, this error occurs when a node in an Elasticsearch cluster that was previously acting as the master is no longer in that role. This could be due to network issues, resource constraints, or a new master being elected. To resolve this, you can check the cluster health and logs to identify any issues. If it’s a network problem, ensure all nodes are properly connected. If it’s a resource issue, consider adding more resources or nodes. If a new master was elected due to the old one being unresponsive, investigate the cause of the unresponsiveness.
This guide will help you check for common problems that cause the log ” no longer master ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: master, routing, allocation, cluster.
Overview
In Elasticsearch, routing refers to document routing. When you index a document, Elasticsearch will determine which shard the document should be routed to for indexing.
The shard is selected based on the following formula:
shard = hash(_routing) % number_of_primary_shards
Where the default value of _routing is _id.
It is important to know which shard the document is routed to, because Elasticsearch will need to determine where to find that document later on for document retrieval requests.
Examples
In twitter index with 2 primary shards, the document with _id equal to “440” gets routed to the shard number:
shard = hash( 440 ) % 2
PUT twitter/_doc/440
{
...
}Notes and good things to know
- In order to improve search speed, you can create custom routing. For example, you can enable custom routing that will ensure that only a single shard will be queried (the shard that contains your data).
- To create custom routing in Elasticsearch, you will need to configure and define that not all routing will be completed by default settings. ( v <= 5.0)
PUT my_index/customer/_mapping
{
"order":{
"_routing":{
"required":true
}
}
}- This will ensure that every document in the “customer” type must specify a custom routing. For Elasticsearch version 6 or above you will need to update the same mapping as:
PUT my_index/_mapping
{
"order":{
"_routing":{
"required":true
}
}
}Log Context
Log “no longer master” class name is PendingListenersQueue.java. We extracted the following from Elasticsearch source code for those seeking an in-depth context :
if (listeners.isEmpty() == false) {
threadPool.generic().execute(() -> {
if (isMaster) {
ActionListener.onResponse(listeners; null);
} else {
ActionListener.onFailure(listeners; new NotMasterException("no longer master"));
}
});
}
}
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