How do you change a mapping using reindex and aliases?

Question

IT Interview Practice · Accepted Answer

An alias is a stable name that points to an index. When a field needs an incompatible mapping change, build a new index and move that name after checking the copied data.

## Picture the change

```text
Before: application -> products -> products-v1
After:  application -> products -> products-v2
```

The application continues using `products`. First create `products-v2` with the desired mapping and settings. Reindex copies documents; it does not automatically copy the source index's configuration.

## Use a simple migration when a write pause is acceptable

- Pause application writes, including background writers, while keeping reads available if appropriate.
- Reindex v1 into v2, inspect failures, and check representative documents and queries after making the copied data searchable.
- Change the alias, test application reads and writes, then resume writers.

For an alias currently pointing only to v1, this Kibana Dev Tools request performs the switch in one alias update:

```http
POST /_aliases
{
  "actions": [
    {"remove":{"index":"products-v1","alias":"products","must_exist":true}},
    {"add":{"index":"products-v2","alias":"products","is_write_index":true}}
  ]
}
```

Inspect the response before resuming writes. This avoids a separate remove/add gap; it does not copy changes that were missed during migration.

## If writes cannot pause

You need a durable record of changes made during copying and a way to replay updates and deletes without older data overwriting newer data. This is a separate synchronization design. Keep v1 temporarily, but do not assume switching back is lossless after new writes have reached v2.

[Official documentation](https://www.elastic.co/docs/manage-data/data-store/aliases).

Picture the change

Use a simple migration when a write pause is acceptable

If writes cannot pause

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