Elasticsearch alias not pointing to new indexes created by the rollover strategy - elasticsearch

Question

Elasticsearch: 7.15.2
We have an alias created in this way:
POST _aliases
{
"actions": [
{
"add": {
"index": "my-index-*",
"alias": "my-alias",
"is_write_index": false
}
}
]
}
And if I get the alias info I can see
GET _alias/my-alias
{
"my-index-2021.11.30-000001" : {
"aliases" : {
"my-alias" : { }
}
}
}
However there is another index which has been created automatically by the rollover policy: my-index-2021.11.30-000002 but this index is not pointed by the alias created before my-alias
If I create a new alias from scratch with the same index pattern I can see both:
POST _aliases
{
"actions": [
{
"add": {
"index": "my-index-*",
"alias": "my-alias-2",
"is_write_index": false
}
}
]
}
GET _alias/my-alias-2
{
"my-index-2021.11.30-000001" : {
"aliases" : {
"my-alias-2" : { }
}
},
"my-index-2021.11.30-000002" : {
"aliases" : {
"my-alias-2" : { }
}
}
}
Is there something that I am missing? I was expecting to see also the *-000002 index pointed by the alias my-alias without any manual operation.
The rollover policy is just creating a new index if the index size is grater then X GBs
or maybe do I have to modify the index template in order to add the "read" alias automatically? I have already the alias specified in the template but that is the write alias for the rollover policy (which I cannot use for search because our custom elasticsearch configuration)
{
"index": {
"lifecycle": {
"rollover_alias": "my-write-alias"
}
}
}

Answer 1

When you create an alias using POST _aliases it will just create the alias on the matching indexes that currently exist, but if a new index is created later and matches the criteria, the alias will not be added to that index.
What you need to do is to:
create an index template containing your alias definition
assign your rollover policy to your template index setting (i.e. the index.lifecycle.name and index.lifecycle.rollover_alias settings)
Basically, like this:
PUT _index_template/my_index_template
{
"index_patterns": ["my-index-*"],
"template": {
"alias": {
"my-alias": {}
},
"mappings": {
...
},
"settings": {
"index.lifecycle.name": "my_policy",
"index.lifecycle.rollover_alias": "my_write_alias"
}
}
}
After this is set up, every time the lifecycle policy creates a new index my-index-2021.MM.dd-000xyz, that new index will be pointed to by my-alias. Also worth noting that my_write_alias will always point to the latest index of the sequence, i.e. the write index.