It's easy to make a cluster non-responsive in Elasticsearch prior to 5.x, by using the attachment mapper. The metadata extraction from a document requires a very high CPU operation and if you are ingesting a lot of documents, your cluster is under-loaded.
To prevent this scenario, Elasticsearch introduces the ingest node. An ingest node can be held under very high pressure without causing problems to the rest of the Elasticsearch cluster.
The attachment processor allows us to use the document extraction capabilities of Tika in an ingest node.
