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Briefly, this error occurs when Elasticsearch expects an array for a specific field, but the provided input is not an array. This usually happens when the data type defined in the mapping is not consistent with the actual data being indexed. To resolve this issue, you can either change the data type in the mapping to match the actual data or modify the data being indexed to match the expected array format. Another solution is to use scripts or ingest pipelines to transform the data into the expected format before indexing.
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This guide will help you check for common problems that cause the log ” expected array for [” + field + “] ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: repositories.
Overview
An Elasticsearch snapshot provides a backup mechanism that takes the current state and data in the cluster and saves it to a repository (read snapshot for more information). The backup process requires a repository to be created first. The repository needs to be registered using the _snapshot endpoint, and multiple repositories can be created per cluster. The following repository types are supported:
Repository types
| Repository type | Configuration type |
|---|---|
| Shared file system | Type: “fs” |
| S3 | Type : “s3” |
| HDFS | Type :“hdfs” |
| Azure | Type: “azure” |
| Google Cloud Storage | Type : “gcs” |
Examples
To register an “fs” repository:
PUT _snapshot/my_repo_01
{
"type": "fs",
"settings": {
"location": "/mnt/my_repo_dir"
}
}Notes and good things to know
- S3, HDFS, Azure and Google Cloud require a relevant plugin to be installed before it can be used for a snapshot.
- The setting, path.repo: /mnt/my_repo_dir needs to be added to elasticsearch.yml on all the nodes if you are planning to use the repo type of file system. Otherwise, it will fail.
- When using remote repositories, the network bandwidth and repository storage throughput should be high enough to complete the snapshot operations normally, otherwise you will end up with partial snapshots.
Log Context
Log “expected array for [” + field + “]” class name is RepositoryData.java. We extracted the following from Elasticsearch source code for those seeking an in-depth context :
if (metaGenerations.isEmpty() == false) {
indexMetaLookup.put(snapshotId; metaGenerations);
}
}
} else {
throw new ElasticsearchParseException("expected array for [" + field + "]");
}
} else if (INDICES.equals(field)) {
if (parser.nextToken() != XContentParser.Token.START_OBJECT) {
throw new ElasticsearchParseException("start object expected [indices]");
}
