[Tarantool-patches] [PATCH vshard 5/6] router: introduce automatic master discovery

[Vladislav Shpilevoy]

>>> And we even don't have exposed replicaset_wait_connected/replicaset_wait_master
>>>
>>> to call it before master will be known. As result several seconds we will get failed requests.
>> I didn't expose them yet because I am very reluctant towards exposing
>> anything until there is an explicit request from a user. Don't want to
>> overload the API.
>>
>>> Also I saw TODO in test that "RO requests should be able to go to replicas" and I expect questions
>>>
>>> about RTO 0 on read requests from our customers.
>> What do you mean as "RTO 0 on read requests"?
>>
> We faced it when developed CRUD module. To perform requests we need to know space schema.
> 
> But all remote schema changes are visible only after request that triggers schema refetch.
> 
> And we had working cluster but the first portion of requests failed because of outdated schema.
> 
> 
> Will we have something similar when we doesn't discover master yet but have replicas and our
> 
> read requests will fail? If it's so please file at least an issue to fix it.

Yes, I understand now. Right, you might have failures, I am afraid. Depending on
your timeout. If there is no master, your request will try to wait for a master's
appearance during the given timeout. It won't fail immediately.

But sometimes it does not even need to wait - it can simply go to a working replica
right away if there is one. I mentioned it in XXX comments in the patch. I created
a ticket: https://github.com/tarantool/vshard/issues/288.