# Reindex and Zero-Downtime Mapping Changes — Elasticsearch

Source: https://www.skillbyai.com/en/elasticsearch/o-reindex

> Change mappings without an outage.

## The alias swap pattern

Because field types cannot change in place, mapping changes follow a pattern. (1) Applications always read and write through an alias, for example `products` pointing at `products_v1`. (2) Create `products_v2` with the new mapping and analyzers. (3) Copy data with the `_reindex` API (for large indices run it with `wait_for_completion=false` and follow the task) or, often better, re-load from the source database. (4) Catch up writes that happened during the copy, for example by dual-writing to both indices during the migration or re-syncing changes since a timestamp. (5) Atomically move the alias to `products_v2` in one `_aliases` call. (6) Keep v1 briefly for rollback, then delete it.

## Reindex and swap the alias

Kibana Dev Tools console syntax; send the same requests with curl or a client library.

```http
PUT /products_v2
{
  "mappings": {
    "properties": {
      "sku":  { "type": "keyword" },
      "name": { "type": "text", "fields": { "raw": { "type": "keyword" } } }
    }
  }
}

POST /_reindex?wait_for_completion=false
{
  "source": { "index": "products_v1" },
  "dest":   { "index": "products_v2" }
}
# -> { "task": "<node-id>:<task-number>" }

GET /_tasks/<node-id>:<task-number>

# atomic switch: both actions apply together
POST /_aliases
{
  "actions": [
    { "remove": { "index": "products_v1", "alias": "products" } },
    { "add":    { "index": "products_v2", "alias": "products", "is_write_index": true } }
  ]
}
```

## Use an alias from day one

Even if you never plan to reindex, creating products_v1 behind a products alias costs nothing and turns a future migration from an outage into a routine change.

**Quiz:** Why is the alias switch done in a single _aliases request?

- [ ] It is faster to type
- [x] The remove and add actions apply atomically, so clients never see no index
- [ ] Aliases can only be changed once
- [ ] It triggers an automatic reindex

*Answer:* The remove and add actions apply atomically, so clients never see no index. Atomic alias updates make the cutover seamless.