Briefly, this error occurs when Elasticsearch detects an index that is not associated with any cluster state. This is known as a “dangled” index. It usually happens due to a failed deletion or a node that rejoins a cluster after a long disconnection. To resolve this issue, you can manually delete the dangled index if it’s not needed. Alternatively, you can use the ‘dangling_index_import’ setting to automatically import the dangled index into the cluster. However, be cautious as this might lead to data duplication if the index was already re-created.
This guide will help you check for common problems that cause the log ” found dangled index [{}] on node [{}]. This index cannot be ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: index, node, dangled.
Overview
In Elasticsearch, an index (plural: indices) contains a schema and can have one or more shards and replicas. An Elasticsearch index is divided into shards and each shard is an instance of a Lucene index.
Indices are used to store the documents in dedicated data structures corresponding to the data type of fields. For example, text fields are stored inside an inverted index whereas numeric and geo fields are stored inside BKD trees.
Examples
Create index
The following example is based on Elasticsearch version 5.x onwards. An index with two shards, each having one replica will be created with the name test_index1
PUT /test_index1?pretty
{
"settings" : {
"number_of_shards" : 2,
"number_of_replicas" : 1
},
"mappings" : {
"properties" : {
"tags" : { "type" : "keyword" },
"updated_at" : { "type" : "date" }
}
}
}List indices
All the index names and their basic information can be retrieved using the following command:
GET _cat/indices?v
Index a document
Let’s add a document in the index with the command below:
PUT test_index1/_doc/1
{
"tags": [
"opster",
"elasticsearch"
],
"date": "01-01-2020"
}Query an index
GET test_index1/_search
{
"query": {
"match_all": {}
}
}Query multiple indices
It is possible to search multiple indices with a single request. If it is a raw HTTP request, index names should be sent in comma-separated format, as shown in the example below, and in the case of a query via a programming language client such as python or Java, index names are to be sent in a list format.
GET test_index1,test_index2/_search
Delete indices
DELETE test_index1
Common problems
- It is good practice to define the settings and mapping of an Index wherever possible because if this is not done, Elasticsearch tries to automatically guess the data type of fields at the time of indexing. This automatic process may have disadvantages, such as mapping conflicts, duplicate data and incorrect data types being set in the index. If the fields are not known in advance, it’s better to use dynamic index templates.
- Elasticsearch supports wildcard patterns in Index names, which sometimes aids with querying multiple indices, but can also be very destructive too. For example, It is possible to delete all the indices in a single command using the following commands:
DELETE /*
To disable this, you can add the following lines in the elasticsearch.yml:
action.destructive_requires_name: true
Overview
To put it simply, a node is a single server that is part of a cluster. Each node is assigned one or more roles, which describe the node’s responsibility and operations. Data nodes store the data, and participate in the cluster’s indexing and search capabilities, while master nodes are responsible for managing the cluster’s activities and storing the cluster state, including the metadata.
While it is possible to run several node instances of Elasticsearch on the same hardware, it’s considered a best practice to limit a server to a single running instance of Elasticsearch.
Nodes connect to each other and form a cluster by using a discovery method.
Roles
Master node
Master nodes are in charge of cluster-wide settings and changes – deleting or creating indices and fields, adding or removing nodes and allocating shards to nodes. Each cluster has a single master node that is elected from the master eligible nodes using a distributed consensus algorithm and is reelected if the current master node fails.
Coordinating (client) node
There is some confusion in the use of coordinating node terminology. Client nodes were removed from Elasticsearch after version 2.4 and became coordinating nodes.
Coordinating nodes are nodes that do not hold any configured role. They don’t hold data and are not part of the master eligible group nor execute ingest pipelines. Coordinating nodes serve incoming search requests and act as the query coordinator running query and fetch phases, sending requests to every node that holds a shard being queried. The coordinating node also distributes bulk indexing operations and route queries to shards based on the node’s responsiveness.
Log Context
Log “found dangled index [{}] on node [{}]. This index cannot be ” classname is LocalAllocateDangledIndices.java.
We extracted the following from Elasticsearch source code for those seeking an in-depth context :
newIndexMetadata = IndexMetadata.builder(newIndexMetadata).settings(
Settings.builder().put(newIndexMetadata.getSettings()).put(
IndexMetadata.SETTING_HISTORY_UUID; UUIDs.randomBase64UUID())).build();
} catch (Exception ex) {
// upgrade failed - adding index as closed
logger.warn(() -> new ParameterizedMessage("found dangled index [{}] on node [{}]. This index cannot be " +
"upgraded to the latest version; adding as closed"; indexMetadata.getIndex(); request.fromNode); ex);
newIndexMetadata = IndexMetadata.builder(indexMetadata).state(IndexMetadata.State.CLOSE)
.version(indexMetadata.getVersion() + 1).build();
}
metadata.put(newIndexMetadata; false);
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