Configure backward compatibility on Group.Rebalance.GroupStrategies

[wathenjiang]

In d66826a commit, new configuration options Group.Rebalance.GroupStrategies have been introduced.

But it is not backward compatibility.

If I do the below starategy set in my older code, but use the latest sarama package, will encounter Consumer.Group.Rebalance.GroupStrategies and Consumer.Group.Rebalance.Strategy cannot be set at the same time error. Please see https://github.com/Shopify/sarama/blob/v1.37.0/config.go#L808

// config setting in my older setting
kconfig.Consumer.Group.Rebalance.Strategy = sarama.BalanceStrategyRoundRobin

That is because sarama always set a default value on Group.Rebalance.GroupStrategies. Please see https://github.com/Shopify/sarama/blob/v1.37.0/config.go#L542

The code is very confusing, util I read d66826a commit. Does it a violated Object-oriented design pattern? The Config should not exposed, so set Consumer.Group.Rebalance.Strategy and Consumer.Group.Rebalance.GroupStrategies can be atomic.
Just setting Consumer.Group.Rebalance.Strategy Deprecated is not enough.

[wathenjiang]

By default sarama will not set https://github.com/Shopify/sarama/issues/2351, but set c.Consumer.Group.Rebalance.GroupStrategies.
If c.Consumer.Group.Rebalance.Strategy != nil is true, must set by user, user should have a higher priority than default setting. Or we might just delete the c.Consumer.Group.Rebalance.Strategy config.

See #2352.

[napallday]

My bad. I think we can just delete this validation

case c.Consumer.Group.Rebalance.Strategy != nil && len(c.Consumer.Group.Rebalance.GroupStrategies) != 0:
	return ConfigurationError("Consumer.Group.Rebalance.GroupStrategies and Consumer.Group.Rebalance.Strategy cannot be set at the same time")

to make it compatible.

Because c.Consumer.Group.Rebalance.Strategy takes precedence of c.Consumer.Group.Rebalance.GroupStrategies.
wdyt?

[wathenjiang]

My bad. I think we can just delete this validation

case c.Consumer.Group.Rebalance.Strategy != nil && len(c.Consumer.Group.Rebalance.GroupStrategies) != 0:
	return ConfigurationError("Consumer.Group.Rebalance.GroupStrategies and Consumer.Group.Rebalance.Strategy cannot be set at the same time")

to make it compatible.

Because c.Consumer.Group.Rebalance.Strategy takes precedence of c.Consumer.Group.Rebalance.GroupStrategies. wdyt?

I does not know the internal mechanism of saram well, If delete it has no side effect, we could delete the above case segment.
May u or matainer do code review on #2352?

[dnwe]

Fixed by #2352 — thanks for raising this