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2 reasons to use ElasticSearch aliases

  • Oct 06, 2018

ElasticSearch has a very handy feature to use different names to point to indices, aliases.
Why should I care?
Aliases can simplify and fasten deployment if used properly.
Below are 2 examples.

Mapping changes

Elasticsearch has come a long way from v0.9.x to v7.x.
I still remember that prior to v2.x, default mapping of string fields was analysed.
That was a terrible idea as usually a string field just contains a single word and if someone
wants some text-search features, she/he declares a custom mapping.
But no, it was analysed, and you were thinking that everything is fine, until the first prefix query.

How aliases can help alleviate this pain? Mapping changes require a full reindex.
By using an alias instead of index names you can point to a newly created index with corrected/improved mappings so that your applications can index documents without issues while you reIndex existing documents from the old index to the new one.

Logs generally

Most of Elasticsearch users use it as a log aggregation system.
That means that there are indices named application_logs.2018-10-06,20181006.data etc.
By having an alias pointing to the newest index, it’s not mandatory to update properties of all software pieces that need to connect to that latest index.
For example, an application that previously used these settings:

elasticsearch.search.index=application_logs.2018-10-06

will need to be updated every day
while by having this:

elasticsearch.search.index=application_logs

where application_logs is an alias to the latest index, application will always look at the latest index.

Notes

  • An alias can’t have the same name as index. Duh of course.
    But usually indices are not designed in the beginning to have good aliases names.
  • Aliases can’t be used to restore an index, index name must be declared.
  • Below is a simple add/remove alias flow taken from official documentation:
    1. Add alias to an existing index 
      POST /_aliases
{
  "actions" : [
   { "add" : { "index" : "test1", "alias" : "alias1" } }
  ]
}
    2. Remove and add alias to another index 
      POST /_aliases
{
  "actions" : [
   { "remove" : { "index" : "test1", "alias" : "alias1" } },
   { "add" : { "index" : "test2", "alias" : "alias1" } }
  ]
}
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