Best approch of Elastic Search time based feeds module? - performance

Question

I am new with elastic search and looking for the best solution with which i can create a feed module which have time based feeds along with there group and comment.
I learned little and come up with following.
PUT /group
{
"mappings": {
"groupDetail": {},
"content": {
"_parent": {
"type": "groupDetail"
}
},
"comment": {
"_parent": {
"type": "content"
}
}
}
}
so that will be placed separately as per index.
but than after i found one post where i found that parent child is costly operation for search than nested objects.
something like following is two group(feed) having details with content and comments as nested element.
{
"_index": "group",
"_type": "groupDetail",
"_id": 6829,
"_score": 1,
"_source": {
"groupid": 6829,
"name": "Jignesh Public",
"insdate": "2016-10-01T04:09:33.916Z",
"upddate": "2017-04-19T05:19:40.281Z",
"isVerified": true,
"tags": [
"spotrs",
"surat"
],
"content": [
{
"contentid": 1,
"type": "1",
"byUser": 5858,
"insdate": "2016-10-01 11:20",
"info": [
{
"t": 1,
"v": "lorem ipsum long text 1"
},
{
"t": 2,
"v": "http://www.imageurl.com/1"
}
],
"comments": [
{
"byuser": 5859,
"comment": "Comment 1",
"upddate": "2016-10-01T04:09:33.916Z"
},
{
"byuser": 5860,
"comment": "Comment 2",
"upddate": "2016-10-01T04:09:33.916Z"
}
]
},
{
"contentid": 2,
"type": "2",
"byUser": 5859,
"insdate": "2016-10-01 11:20",
"info": [
{
"t": 4,
"v": "http://www.videoURL.com/1"
}
],
"comments": [
{
"byuser": 5859,
"comment": "Comment 1",
"upddate": "2016-10-01T04:09:33.916Z"
},
{
"byuser": 5860,
"comment": "Comment 2",
"upddate": "2016-10-01T04:09:33.916Z"
}
]
}
]
}
}
{
"_index": "group",
"_type": "groupDetail",
"_id": 6849,
"_score": 1,
"_source": {
"groupid": 6849,
"name": "Xyz Group Public",
"insdate": "2016-10-01T04:09:33.916Z",
"upddate": "2017-04-19T05:19:40.281Z",
"isVerified": false,
"tags": [
"spotrs",
"food"
],
"content": [
{
"contentid": 3,
"type": "1",
"byUser": 5858,
"insdate": "2016-10-01 11:20",
"info": [
{
"t": 1,
"v": "lorem ipsum long text 3"
},
{
"t": 2,
"v": "http://www.imageurl.com/1"
}
],
"comments": [
{
"byuser": 5859,
"comment": "Comment 1",
"upddate": "2016-10-01T04:09:33.916Z"
},
{
"byuser": 5860,
"comment": "Comment 2",
"upddate": "2016-10-01T04:09:33.916Z"
}
]
},
{
"contentid": 4,
"type": "2",
"byUser": 5859,
"insdate": "2016-10-01 11:20",
"info": [
{
"t": 4,
"v": "http://www.videoURL.com/1"
}
],
"comments": [
{
"byuser": 5859,
"comment": "Comment 1",
"upddate": "2016-10-01T04:09:33.916Z"
},
{
"byuser": 5860,
"comment": "Comment 2",
"upddate": "2016-10-01T04:09:33.916Z"
}
]
}
]
}
}
now if i try to think with nested object than i confused if user add comment very frequently than reindexing factor will effect?
So main think i want to ask is which is the best approach with which i can add comment frequently and my content searching result is also faster.

Answer 1

Performance
Parent/child stores relevant data in same shards, as separately doc, which avoid the network;
Parent/child needs a joining process when retrieving data;
Nested object store the inner and outer object together, as a single doc;
So, we can infer:
Update nested object will re-index whole index, which can very expensive if your document is large;
Update parent or child alone will not affect the other one;
Searching nested object is a little fast, which save the process of joining;
Suggestions
As far as I understand your problem, you should use parent/child.
When your group's comments become more and more, adding a new comment will still re-index whole content, which can be very time-consuming;
On the other hand, search a comment with parent/child just need one more look up after finding the child, which is relative acceptable.
Furthermore, you should also take the rate of searching a comment comparing to adding a comment into account:
If you need searching a lot but a little new comments, maybe you can choose nested object;
Otherwise, choose parent/child;
By the way, you may combine both of them:
When this feed is active, use parent/child to store them;
When it is closed, i.e., no more comments can be added, move them to a new index with nested object;

Answer 2

If you do not specify more detailed info other than very frequently it is going to be hard to come up with a recommendation. Also you have not mentioned how your data looks like. A comment in a blog post might be happening rare, even in heated discussions. A comment/reply in a forum post (that will result in a huge document) might be sth very different. I'd personally start with nested and see how it goes, but I also do not know all the requirements, so this might be a very wrong answer.