How To Solve Issues Related to Log – Ignoring dangled index on node due to an existing alias with the same name

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Updated: Jan-20

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Troubleshooting background

To troubleshoot Elasticsearch log “Ignoring dangled index on node due to an existing alias with the same name” it’s important to understand common problems related to Elasticsearch concepts: alias, dangled, index, indices, node. See detailed explanations below complete with common problems, examples and useful tips.

Alias in Elasticsearch

In Elasticsearch, an alias is a secondary name to refer to one or more indices. Aliases can be created and removed dynamically using _aliases REST endpoint.

What it is used for

Aliases are used for multiple purposes such as to search across more than one index with a single name, perform the reindexing process with zero downtime and query data based on predefined filters.

Examples

Creating an alias on a single index:

POST /_aliases?pretty
{
  "actions": [
    {
      "add": {
        "index": "index_1",
        "alias": "alias_1"
      }
    }
  ]
}

Creating the same alias on more than one index:

POST /_aliases?pretty
{
  "actions": [
    {
      "add": {
        "index": "index_1",
        "alias": "alias_1"
      }
    },
    {
      "add": {
        "index": "index_2",
        "alias": "alias_1"
      }
    }
  ]
}

Creating a filter-based alias:

POST /_aliases?pretty
{
  "actions": [
    {
      "add": {
        "index": "index_1",
        "alias": "alias_2",
        "filter": {
          "term": {
            "organization": "opster"
          }
        }
      }
    }
  ]
}

Listing out all of the available aliases in an Elasticsearch cluster:

GET _cat/aliases

Removing an alias:

POST /_aliases?pretty
{
  "actions": [
    {
      "remove": {
        "index": "index_2",
        "alias": "alias_1"
      }
    }
  ]
}
Notes
  • An Alias cannot be used for the indexing process if it points to more than one index. If attempted, Elasticsearch will throw an exception.
  • Deleting an alias does not delete the actual index.
Common Problems
  • Since a single alias can be attached to multiple indices, it is important to keep in mind that querying an alias will send the query against all the associated indices and this can impact the search performance and high resource utilization.

Index in Elasticsearch

What it is

In Elasticsearch, an index (indices in plural) can be thought of as a table inside a database that has 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 below command:

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


To help troubleshoot related issues we have gathered selected Q&A from the community and issues from Github , please review the following for further information :

1 Github Issue Number 28441  

2Logstash Syslog Configuration In 7  


Log Context

Log ”Ignoring dangled index [{}] on node [{}] due to an existing alias with the same name” classname is LocalAllocateDangledIndices.java
We have extracted the following from Elasticsearch source code to get an in-depth context :

                         }
                        if (currentState.metaData().hasIndex(indexMetaData.getIndex().getName())) {
                            continue;
                        }
                        if (currentState.metaData().hasAlias(indexMetaData.getIndex().getName())) {
                            logger.warn("ignoring dangled index [{}] on node [{}] due to an existing alias with the same name";
                                    indexMetaData.getIndex(); request.fromNode);
                            continue;
                        }
                        importNeeded = true;







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