net/http: Client round-robin across persistent connections

[CAFxX]

When connecting directly to a service spread across multiple hosts with DNS "load-balancing" it would be ideal if http.Client could round-robin requests over persistent connections to the multiple hosts listed in the A record.

Right now, if you want to ensure that load is balanced the only practical option is to disable HTTP keepalives.

One way this could be done is to have idle connection pools for each different A record, and do a DNS resolution for every request to decide which pool to use for that request.

This round-robin mode should likely be opt-in (or opt-out). When enabled, given a DNS record like the following:

example.local   60   IN   A   10.0.0.1
example.local   60   IN   A   10.0.0.2

http.Client would send roughly half the requests to .1 and half to .2, even when using persistent connections, and even if the client only ever sends requests serially.

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Until this is fixed, if someone else also needs this you can take a look at https://github.com/CAFxX/balancer.

[agnivade]

@bradfitz @fraenkel

[bradfitz]

Agreed. I've been thinking about things like this as part of #23707 and making the connection pool interface more explicit/configurable. Currently it's a pretty hidden detail.

[frittentheke]

@bradfitz could you kindly give an update if you are working on load-balancing / round-robing requests at all? #23707 seems dead.

@CAFxX I know you did implement https://github.com/CAFxX/balancer, but do you have any plans to get (something like) this into upstream net/http then? Would be great to have.