I am using nginx as proxy server. I got following error once:
upstream timed out (110: Connection timed out) while connecting to upstream,

In what situtaion I will get this error? Exaple:
1. Upstream didn't accept connection within x time.
Is there any other case when I can get this error or is it only one case when I will get this error?
Example:
1. Saving or reading to tmp file buffer was slow.
2. Nginx was running out of connection?