CODEWITHSUNDEEP
mongodb
Zane Claes
Zane Claes

Reputation: 14954

What is the advantage to explicitly connecting to a Mongo Replica Set?

Obviously, I know why to use a replica set in general.

But, I'm confused about the difference between connecting directly to the PRIMARY mongo instance and connecting to the replica set. Specifically, if I am connecting to Mongo from my node.js app using Mongoose, is there a compelling reason to use connectSet() instead of connect()? I would assume that the failover benefits would still be present with connect(), but perhaps this is where I am wrong...

The reason I ask is that, in mongoose, the connectSet() method seems to be less documented and well-used. Yet, I cannot imagine a scenario where you would NOT want to connect to the set, since it is recommended to always run Mongo on a 3x+ replica set...

Upvotes: 4

Views: 2865

Answers (1)

itsbruce
itsbruce

Reputation: 4843

If you connect only to the primary then you get failover (that is, if the primary fails, there will be a brief pause until a new master is elected). Replication within the replica set also makes backups easier. A downside is that all writes and reads go to the single primary (a MongoDB replica set only has one primary at a time), so it can be a bottleneck.

Allowing connections to slaves, on the other hand, allows you to scale for reads (not for writes - those still have to go the primary). Your throughput is no longer limited by the spec of the machine running the primary node but can be spread around the slaves. However, you now have a new problem of stale reads; that is, there is a chance that you will read stale data from a slave.

Now think hard about how your application behaves. Is it read-heavy? How much does it need to scale? Can it cope with stale data in some circumstances?

Incidentally, the point of a minimum 3 members in the replica set is to offer resiliency and safe replication, not to provide multiple nodes to connect to. If you have 3 nodes and you lose one, you still have enough nodes to elect a new primary and have replication to a backup node.

Upvotes: 2

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