[DOCS-14330] Document that client affinity is required if a proxy/load balancer between the application and the mongos Created: 02/Apr/21  Updated: 30/Oct/23  Resolved: 08/Feb/23

Status: Closed
Project: Documentation
Component/s: manual, Server
Affects Version/s: None
Fix Version/s: Server_Docs_20231030

Type: Task Priority: Major - P3
Reporter: Linda Qin Assignee: Ian Fogelman
Resolution: Fixed Votes: 0
Labels: None
Remaining Estimate: Not Specified
Time Spent: Not Specified
Original Estimate: Not Specified

Issue Links:
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Participants:
Days since reply: 1 year, 2 days ago
Epic Link: DOCSP-11701

 Description   

Description

From the documentation "Sharded Cluster Components" up to version 3.2, we have mentioned that client affinity is required if load balancer is running between the application and the mongos:

> You may also deploy a group of mongos instances and use a proxy/load balancer between the application and the mongos. In these deployments, you must configure the load balancer for client affinity so that every connection from a single client reaches the same mongos.

However, this was no longer mentioned since the [v3.4 documentation](https://docs.mongodb.com/v3.4/core/sharded-cluster-components/#production-configuration). Since this is still a requirement, please add this to the documentation.

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 Comments   
Comment by Jeffrey Yemin [ 06/Feb/23 ]

Hi linda.qin@mongodb.com, the situation is a bit complicated. The load balancer supported added in DRIVERS-720 required both a driver that supports load balancer mode and a sharded clusters configured to support that mode. The server support was added in MongoDB 5.0 and the major drivers delivered the feature in the same time frame. See the language tickets linked to DRIVERS-720 for details.

In Atlas the only clusters which are currently configured for load balancer support are serverless ones. I don't know what has to be done to configure an on-premise sharded cluster for load balancer support, or if that has been documented or advertised.

Comment by Spencer Brown [ 02/Apr/21 ]

Can we add a reference to round-robin DNS also? As it cannot support client affinity, it should not be used between the application and the mongos deployment.

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