Elasticsearch (ES) is a powerful tool offering multiple search, content, and analytics capabilities. You can extend its capacity and relatively quickly horizontally scale the cluster by adding more nodes. When data is indexed in some Elasticsearch index, the index is not typically placed in one node but is spread across different nodes such that each node contains a “shard” of the index data. The shard (called primary shard) is replicated across the cluster into several replicas.

Disk-related issues with Elasticsearch can present themselves through various symptoms. It is important to understand their root causes and know how to deal with them when they arise. As an Elasticsearch cluster administrator, you are likely to encounter some of the following cluster symptoms.

Let's say you log into your amazon or eBay account and start searching for a gift clothing. First you would filter out gender-specific collections, then you might fix a particular color or even a set of colors of your choice, and following that, you can fix a price range. When you apply these filters one by one, you can see the aggregate products displayed each time varies (their total number changing according to the availability of aggregates). This is exactly what aggregations do.