In web infrastructure, an upstream is the backend a proxy, load balancer or CDN forwards requests to, such as an application server, a pool of servers or an origin. Traffic flows from the client through the proxy "up" to the upstream.
Why it matters
Most gateway errors are statements about the upstream. A 502 means the proxy got an invalid response or none; a 504 means the upstream did not answer in time. Naming the layers clearly speeds up debugging: ask which component generated the error, then look at the one behind it. The word also appears in dependency graphs, where "upstream" means a service you depend on.
Example
An nginx upstream group with two servers and passive failure detection:
upstream app {
server 10.0.0.11:8080 max_fails=3 fail_timeout=30s;
server 10.0.0.12:8080;
}
server {
location / { proxy_pass http://app; }
}If 10.0.0.11 fails three times in 30 seconds, nginx stops sending it traffic for 30 seconds. The error log line upstream timed out tells you the problem is behind nginx, not in it.
Common mistakes
- Debugging the proxy config when the upstream is down.
- Mixing up "upstream" in proxies with "upstream" in open-source projects, where it means the original source repository.
- Ignoring that the upstream may be a third-party API you do not control.
Related
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