Explain the basic architecture of ElasticSearch.

ElasticSearch follows a distributed architecture with these key components:

Node: A single running instance of ElasticSearch
Cluster: A collection of nodes that work together
Index: A collection of documents with similar characteristics (like a database)
Document: Basic unit of information stored as JSON
Shard: A subset of an index's data, distributed across nodes
Replica: A copy of a shard for redundancy and increased search performance

Cluster
├── Node 1
│   ├── Index A (Primary Shard 0)
│   └── Index B (Replica Shard 1)
├── Node 2
│   ├── Index A (Primary Shard 1)
│   └── Index B (Primary Shard 0)
└── Node 3
    ├── Index A (Replica Shard 0)
    └── Index B (Replica Shard 0)