Date: 2019-10-17
Rejected
The benefits do not outweigh the costs.
Though very handy to have around, the source field does incur storage overhead within the index.
Disable _source for ElasticSearch indexed documents.
Given a dataset composed of small text/plain messages, we notice a 20% space reduction of data stored on ElasticSearch.
However, patch updates can no longer be performed upon flags updates. Upon flag update we need to fully read the mail content, then mime-parse it, potentially html parse it, extract attachment content again and finally index again the full document.
Without _source field, flags update is two times slower, 99 percentile 4 times slower, and this impact negatively other requests.
Note please that _source allows admin flexibility like performing index level changes without downtime, amongst others: