backup servers/ proxy_next_upstream questions

[Harish Sundararaj]

Hello All,
I have a few questions on backup servers and proxy_next_upstream in general:

For the backup server the doc says: "only uses this server if the
non-backup servers are all down or busy"

1) So what exactly constitutes down or busy? Is it just response codes
- 502,503 and 504  or anything else?
2) Will the same request which received such an error be sent to a
backup server or is it only subsequent requests?

3) Can I say 504s happen when proxy_connect_timeout/proxy_read_timeout
is reached? Is there a way to find out which among these is happening?

4) Also, In proxy_next_upstream :

error — an error has occurred while connecting to the server, sending
a request to it, or reading its response;
timeout — occurred timeout during the connection with the server,
transfer the request or while reading response from the server;

I'm not able to clearly distinguish between both of these. What kind
of scenarios explain these?

5) In a case when proxy_next_upstream is off and when a backup server
is provided, will nginx still try all non-backup servers before
sending it to backup servers?

Thanks
Harish