TopolearnMappings and analyzers can't change in place, so every long-lived index eventually needs a reindex without downtime.
An alias is a secondary name pointing at one or more indices. Applications should read and write through an alias (products) rather than the concrete index (products-v3), because then you can switch the underlying index without touching code.
The zero-downtime workflow when a mapping must change:
- create
products-v4with the new mappings and settings, - copy data with
POST _reindexfromproducts-v3toproducts-v4(it runs as a task; useslicesfor parallelism andwait_for_completion=falsefor long jobs), - catch up writes that happened during the copy (dual-write from the app, or reindex documents changed since the start),
- swap the alias in one atomic
POST _aliasesrequest with aremoveand anaddaction, - delete the old index when you're confident.
Aliases can also carry a filter (a per-tenant view) and routing. When an alias points at several indices, one can be marked is_write_index to receive writes; this is how rollover works for time-based indices.
_reindex can also transform documents with a script or ingest pipeline, and pull from a remote cluster for migrations. Remember it reads _source, so indices with _source disabled can't be reindexed.
Index templates (PUT _index_template/...) apply settings and mappings automatically to new indices matching a pattern, keeping versions consistent.
Example
PUT /products-v4
{ "mappings": { "properties": {
"name": { "type": "text", "analyzer": "english" },
"price": { "type": "scaled_float", "scaling_factor": 100 } } } }
POST /_reindex?wait_for_completion=false&slices=auto
{ "source": { "index": "products-v3" }, "dest": { "index": "products-v4" } }
# { "task": "oTUltX4IQMOUUVeiohTt8A:12345" }
GET /_tasks/oTUltX4IQMOUUVeiohTt8A:12345
POST /_aliases
{ "actions": [
{ "remove": { "index": "products-v3", "alias": "products" } },
{ "add": { "index": "products-v4", "alias": "products" } } ] }
# both actions apply atomically: clients never see a gap
GET /_alias/productsRemember
- Applications use aliases, never concrete index names
- Change mappings by creating a new index and _reindex
- Swap aliases with one atomic _aliases request
- Index templates apply mappings to new indices automatically
Watch out for
- Hard-coding index names in the application, which makes every reindex a deployment.