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From Garren Smith <gar...@apache.org>
Subject [DISCUSS] Map Indexes
Date Mon, 15 Apr 2019 14:24:48 GMT
Hi Everyone,

I want to start a discussion around creating map/reduce view indexes. One
way to get views indexes to work with FoundationDB is to break up a view
index into indexes for the map functions and indexes for the reduce
functions. Along those lines, I’m going to break the discussions into two,
this discussion around map functions and indexes and then a another one on
reduce functions and the indexes that go with those.

## Data model
For a map function, we need to store the emitted keys and the emitted
values:

{?DATABASE, ?VIEWS, ?VIEW_SIGNATURE, ?VIEWS, <view_id>, ?MAP, <keys>,
<_id>} -> <emitted_value>

To briefly explain what the above means, it creates a views subspace in a
database subspace, then every view defined on a design doc is grouped via
the design doc’s view signature. The view_id is the name of the view in the
design doc - we can look at ways to make that smaller to save some key
space. The ?MAP groups the key/value into the view’s map index subspace,
then we have the keys that were emitted for the map function and finally
the _id field of the document used to create the keys for this row.

## Emitted Value
The value stored for the row is the emitted value from the map function.
Because we have a limitation on the size of the value field one caveat
around this design is that a user will run into issues if they emit a
document that exceeds 100KB. In CouchDB we don’t recommend users emitting
the doc, but there are some nice speed optimisations you get by emitting
the document as the value. With CouchDB on FDB that performance
optimisation won’t be required and so we will have to actively discourage
users from doing that.

Just to note, a user would experience the same issue if they emit a value
exceeding 100KB.

## Key ordering
There are some changes to how we will manage keys emitted from a map
function. For strings we will need to generate a ICU sort string upfront
instead of using the ICU comparison. To maintain the way CouchDB currently
does view collation [1], we need to prepend a type value to each key so
that we get the correct sort order of null < boolean < numbers < strings <
arrays < objects. CouchDB currently allows duplicate keys to be emitted for
an index, to allow for that a counter value will be added to the end of the
keys.

## Index Key Management
For every document that needs to be processed for an index, we have to run
the document through the javascript process to get the emitted keys and
values. This means that it won’t be possible to update a map/reduce index
in the same transaction that a document is updated. To account for this, we
will need to keep an `id index` similar to the `id tree` we current keep.
This index will hold the document id as the key and the value would be the
keys that were emitted. We would then use this information to know which
fields need to be updated or removed from the index when a document is
changed.  A data model for this would be:

{?DATABASE, ?VIEWS, ?VIEW_SIGNATURE, ?VIEWS, <view_id>, ?ID_INDEX, <_id>,
<view_id>} -> [emitted keys]

## Updating an index
To help in knowing which documents have changed since a view was last
updated, we will need to keep the latest update sequence. This will change
from the really long string we currently have, to using the sequence value
defined in the _changes RFC [2].

Based on all of that, a slightly expanded data model for map functions
inside a database subspace would look like:

* views
    * <signature>
        * update_seq
        * idtree
            * (<_id>, <viewid>) -> [keys]
        * views
            * <viewid>
                * map
                    * (<key>, <_id>) -> <value>

## Size limits
There are some size limits that are worth listing and keeping in mind.

* Emitted keys will not be able to exceed 10 KB
* Values cannot exceed 100 KB
* Following from Alex’s email on how transaction sizes are calculated [3],
there could be rare cases where the number of key-value pairs emitted for a
map function could lead to a transaction either exceeding 10 MB which isn’t
allowed or exceeding 5 MB which impacts the performance of the cluster. We
will have to detect for those situations and split the transaction into
smaller transactions

What do you think of that? Any questions or thoughts on this? Once again a
big acknowledgment to Adam who did the initial investigation and design
ideas around this.

Cheers
Garren

[1]
http://docs.couchdb.org/en/stable/ddocs/views/collation.html#collation-specification
[2] https://github.com/apache/couchdb-documentation/pull/401
[3]
https://lists.apache.org/thread.html/4976e0b7e3df89c5d64f37b5299b04c2ed01088f357be8aceaeedec1@%3Cdev.couchdb.apache.org%3E

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