# Motivation

Following aneoconsulting/ArmoniK.Infra#168, this
PR aims to enable the deployment of a sharded MongoDB as ArmoniK's
database.

A more complete motivation is provided in AEP 004
(aneoconsulting/ArmoniK.Community#48). This PR
is meant to treat a previsible bottleneck on ArmoniK's database by
enabling a sharded architecture to it.

For now, when a sharded database is specified, the following collections
will be sharded :
- Result
- TaskData
- SessionData

# Description

This PR adds : 
- A method `ShardCollectionAsync` to the `IMongoDataModelMapping`
interface.
- 2 new `MongoOptions` : a string `AuthSource` and a boolean `Sharding`.
- A new class `ShardingExt` containing an extension method
`shardCollection` for the `IClientSessionHandle` interface.

When the `MongoOption` `Sharding` is true, the `MongoCollectionProvider`
calls `ShardCollectionAsync`. Then the implementation depends on whether
the collection is wanted to be sharded :
- If the collection has to be sharded, the `shardCollection` extension
method will be called.
- If the collection isn't wanted to be sharded, the method directly
returns a complete `Task`.

The new `MongoOption` `AuthSource` is required because the `MongoClient`
has to authenticate as an administrator to be able to shard a
collection.

# Testing

It has been tested manually. Since these developmentsts are very coupled
with ArmoniK's database, it is difficult to write unit tests. Anyhow, it
is still possible to have automatized integration tests that would
verify in a first time if a deployment of ArmoniK using these
deployments succeeds, and in a second time if sharding is indeed enabled
on the right collections. Workflows testing a deployment of ArmoniK with
a sharded MongoDB are currently being studied.

# Impact

In the end, it is expected to enhance ArmoniK's strong scalability.

# Additional Information

To better understand how MongoDB sharding works see [MongoDB's
documentation](https://www.mongodb.com/docs/manual/sharding/).

To understand why this new MongoOption is required, see : MongoDB's
documentation on the [shardCollection database
command](https://www.mongodb.com/docs/manual/reference/command/shardCollection/#mongodb-dbcommand-dbcmd.shardCollection)
and on [connection string's authSource
option](https://www.mongodb.com/docs/manual/reference/connection-string-options/#mongodb-urioption-urioption.authSource)

# Checklist

- [X] My code adheres to the coding and style guidelines of the project.
- [X] I have performed a self-review of my code.
- [X] I have commented my code, particularly in hard-to-understand
areas.
- [ ] I have made corresponding changes to the documentation.
- [X] I have thoroughly tested my modifications and added tests when
necessary.
- [X] Tests pass locally and in the CI.
- [ ] I have assessed the performance impact of my modifications.