In this discussion, I will rely on this page for
reference:
http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/search-request-scroll.html
At my level, I cannot really make a recommendation but I can share some
questions going through my head, which if you fill in the blanks, might
help you come to a reasonable decision ...
1. Flat out notice by the docs: "Scrolling is not intended for real time
user requests, but rather for processing large amounts of data" ... but for
a user scrolling an infinite window with 250 results per pseudo-page ...
can we really consider that real time? If not then I guess its reasonable
to look at scoll for infinite paging.
2. If you have 100 users accessing your application then that means you
are serving the infinite-scroll screen via 100 scroll-queries ... what is a
reasonable timeout period for your scroll queries?
1. Should they stay alive for 10m each ... is that how long a user
session is for you on average?
2. Quoting the docs before my next thought: "The scroll parameter
(passed to the search request and to every scroll request) tells
Elasticsearch how long it should keep the search context alive. Its value
does not need to be long enough to process all data — it just needs to be
long enough to process the previous batch of results. Each scroll request
(with the scroll parameter) sets a new expiry time."
3. What happens when a scroll query timeout expires? I think your app
will need to be smart enough to issue a new search_then_scroll chain but:
1. the results will shift a bit if new data has been indexed
2. your app will probably need to keep track of the fact that user
was on pseudo-page 5 of their infinite scroll (looking at results #
1000-1250) and then get the new page #6 by searching-1st and then
scrolling
pages # 2,3,4,5 until it gets to page #6 ... so the app needs to be
very
smart ... depending on your comfort level with writing code, you may
or may
not consider this to be a big deal
3. also there is no way to scroll backwards, which means any pages
you've retrieved, you'll need to cache them all on client side, does
your
app have sufficient memory?
4. will you choose to replace the cache when something like (3.2)
happens? I would btu again the shifting results may or may not feel
odd to
the user.
4. Will you be happy with a scroll query only performance or will you
want something more like the SCAN search_type also? In which case there
is
no sorting at all so when a timeout happens, the rows of data will seem
to
have moved around a LOT to a user with good memory who was scrolling the
page. So it would be best to just make users start from the beginning in
such a scenario
5. I also imagine you will shorten the timeout based on how many
users are hammering your application so when you try and strike a balance
between the average user engagement time and the scroll query timeout,
you'll have a lowerbound determined (somewhat) by the fact that on
average
users stick around on your page and scroll around for no more than 25
seconds on average. I'm making these metrics up but you get the idea. So
you wouldn't want to set a timeout smaller than 25 seconds then.
3. The good news is if you do all this, then people will love you for
sharing your infinite client code :)
4. Some practical advice: I have an angular+ionic app for selling
clothes,shoes etc. where I have NOT setup infinite scroll (default is like
1000 products), and I explain my decision to my team like so: I want my
users to *search* and not scroll. This is just an opinion so if you took
the time to write a smart client, and if it was open-source, I would
totally consider using it :)
Cheers!
- Pulkit
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