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From Ilya Khlopotov <iil...@apache.org>
Subject [DISCUSS] : things we need to solve/decide : changes feed
Date Mon, 04 Feb 2019 19:18:40 GMT
One of the features of CouchDB, which doesn't map cleanly into FoudationDB is changes feed.
The essence of the feature is: 
- Subscriber of the feed wants to receive notifications when database is updated. 
- The notification includes update_seq for the database and list of changes which happen at
that time. 
- The change itself includes docid and rev. 
Hi, 

There are multiple ways to easily solve this problem. Designing a scalable way to do it is
way harder.  

There are at least two parts to this problem:
- how to structure secondary indexes so we can provide what we need in notification event
- how to notify CouchDB about new updates

For the second part of the problem we could setup a watcher on one of the keys we have to
update on every transaction. For example the key which tracks the database_size or key which
tracks the number of documents or we can add our own key. The problem is at some point we
would hit a capacity limit for atomic updates of a single key (FoundationDB doesn't redistribute
the load among servers on per key basis). In such case we would have to distribute the counter
among multiple keys to allow FoundationDB to split the hot range. Therefore, we would have
to setup multiple watches. FoundationDB has a limit on the number of watches the client can
setup (100000 watches). So we need to keep in mind this number when designing the feature.


The single key update rate problem is very theoretical and we might ignore it for the PoC
version. Then we can measure the impact and change design accordingly. The reason I decided
to bring it up is to see maybe someone has a simple solution to avoid the bottleneck. 

best regards,
iilyak

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