Update configuration for actively used index without data loss - elasticsearch

Question

Sometimes, I need to update mappings, settings, or bind default pipelines to the actively used index.
For the time being, I am using a method with data loss as follows:
update the index template with proper mapping (or binding the default pipeline by index.default_pipeline);
create a_new_index (matching the template index_patterns);
reindex the index_to_fix to a_new_index to migrate the data already indexed;
use alias to redirect the coming indexing request to a_new_index (the alias will have the same name as index_to_fix to ensure the indexing is undisturbed) and delete the index_to_fix;
But between step 3 and step 4, there is a time gap, during which the newly indexed data are lost in the original index_to_fix.
Is there a way, to update configurations for actively used index without any data loss?

Answer 1

Thanks for the help of #LeBigCat, after some discussions. I think this problem could be solved in three steps.
Use Alias for CRUD
First thing first, try not to use index directly, use alias if possible; since you can't use an alias with the same name as the existed indices, directly you can't replace the index even if it's broken (badly designed). The easiest way is to use a template and include the index name directly in the alias.
PUT _template/test
{
...
"aliases" : {
"{index}-alias" : {}
}
}
Redirect the Indexing
Since the index_to_fix is being actively used, after updating the template and create a new index a_new_fix, we can use alias to redirect the indexing to a_new_fix.
POST /_aliases
{
"actions" : [
{ "add": { "index": "a_new_index", "alias": "index_to_fix-alias" } },
{ "remove": { "index": "index_to_fix", "alias": "index_to_fix-alias" } }
]
}
Migrating the Data
Simply use _reindex to migrate all the data from index_to_fix to a_new_index.
POST _reindex
{
"source": {
"index": "index_to_fix"
},
"dest": {
"index": "index_to_fix-alias"
}
}