Readonly traffic fallback on primary if all replicas down

[Vonng]

Scenario

If all replica instances are down. Current setup will NOT route those traffic to the only primary. which means readonly service are not available.

It nice to have an additional option to configure that in case of all replicas failure:

• Leave primary be, so at least readwrite service will survival the avalanche case
• Route readonly traffic to primary so the service still intact.

Proposal

Add an option haproxy_weight_fallback which controls this behaviro

• When set to 0, it is same as before, primary will not take any read-only traffic
• When set to a value larger than 0, It will be used as relative weight for primary instance in replica services group.

In normal situation, the primary will take a very little portion of read-only traffic. In case of all replica down. It will become the only available backend in replica service. And takeover all read-only traffics.

[Vonng]

Resolved by d87854b and d87854b