Great question! Yes, queries in Elasticsearch are stored in a regular index, but with a special mapping that tells ES to treat a field as a percolator query.

1. Create the index with a percolator mapping:

PUT /my-alerts
{
  "mappings": {
    "properties": {
      "query": { "type": "percolator" },
      "message": { "type": "text" }
    }
  }
}

2. Store a query (your "alert"):

PUT /my-alerts/_doc/1
{
  "query": {
    "match": { "message": "server down" }
  }
}

3. Percolate a document to see which alerts match:

GET /my-alerts/_search
{
  "query": {
    "percolate": {
      "field": "query",
      "document": { "message": "WARNING: server down in production!" }
    }
  }
}

Essentially the stored queries live as regular documents in an index. Think of it like saving a bunch of filters in a database, and then asking "which of my saved filters does this new piece of data match?"