Asynchronously migrate Searchkick created Elasticsearch indices between clusters (e.g. for a major version upgrade), for 0-downtime

reindex.rb

# Connection to new v8 cluster
v8_es = Elasticsearch::Client.new(host: NEW_V8_ES_URL)

# v7 cluster details
host = Searchkick.client.transport.connections[0].host

# For every model we create an indentical (but empty) index and matching alias in the v8 cluster,
# then we create an async task in the new v8 cluster to reindex the contents of the index
# remotely from the v7 cluster.
Searchkick.models.each do |model|
  index = model.search_index
  index.all_indices.each do |name|
    v8_es.indices.create(index: name, body: index.index_options) rescue nil
    v8_es.indices.update_aliases(body: { actions: { add: { index: name, alias: index.name } } }) rescue nil
    puts v8_es.reindex(body: {
      source: {
        index: index.name,
        remote: {
          host: "https://#{host[:host]}:#{host[:port]}",
          username: host[:user],
          password: host[:password]
        }
      },
      dest: { index: name, version_type: 'external' },
      conflicts: 'proceed'
    }, wait_for_completion: false)
  end
end

After switching over to the new ES cluster, you should run a Searchkick reindex job on each model:

# See the Searchkick readme for more information about configuring asynchronous reindex jobs,
# we use Sidekiq for ours on a custom queue for YMMV

Searchkick.models.each do |model|
  model.reindex(mode: async)
end

You shouldn't need to promote indices with as Searchkick.promote_index(index_name) after this as SK only creates new indices when the schema that created them differs, but you might want to check the indices created in the new cluster to be sure.