This change adds `access` format support for notifications to a
Elasticsearch server, and it refactors `namespace` format support.
In the case of `access` format, for each event in Minio, a JSON
document is inserted into Elasticsearch with its timestamp set to the
event's timestamp, and with the ID generated automatically by
elasticsearch. No events are modified or deleted in this mode.
In the case of `namespace` format, for each event in Minio, a JSON
document is keyed together by the bucket and object name is updated in
Elasticsearch. In the case of an object being created or over-written
in Minio, a new document or an existing document is inserted into the
Elasticsearch index. If an object is deleted in Minio, the
corresponding document is deleted from the Elasticsearch index.
Additionally, this change upgrades Elasticsearch support to the 5.x
series. This is a breaking change, and users of previous elasticsearch
versions should upgrade.
Also updates documentation on Elasticsearch notification target usage
and has a link to an elasticsearch upgrade guide.
This is the last patch that finally resolves#3928.
