Expected array for " + field + " - Common Causes and Quick Fixes

Opster Team

July-20, Version: 1.7-8.0

Before you begin reading this guide, we recommend you try running the Elasticsearch Error Check-Up which can resolve issues that cause many errors.

This guide will help you check for common problems that cause the log ” expected array for ” + field + ” ” to appear. It’s important to understand the issues related to the log, so to get started, read the general overview on common issues and tips related to the Elasticsearch concepts: repositories.

Advanced users might want to skip right to the common problems section in each concept or try running the Check-Up which analyses ES to pinpoint the cause of many errors and provides suitable actionable recommendations how to resolve them (free tool that requires no installation).

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 + “]”classname 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]");
  }