[Chinese](zh-cn/debug-function.md)
## `5xx` response status code
Similar `5xx` status codes such as 500, 502, 503, etc., are the status codes in response to a server error. When a request has a `5xx` status code; it may come from `APISIX` or `Upstream`. How to identify the source of these response status codes is a very meaningful thing. It can quickly help us determine the problem.
## How to identify the source of the `5xx` response status code
In the response header of the request, through the response header of `X-APISIX-Upstream-Status`, we can effectively identify the source of the `5xx` status code. When the `5xx` status code comes from `Upstream`, the response header `X-APISIX-Upstream-Status` can be seen in the response header, and the value of this response header is the response status code. When the `5xx` status code is derived from `APISIX`, there is no response header information of `X-APISIX-Upstream-Status` in the response header. That is, only when the status code of `5xx` is derived from Upstream will the `X-APISIX-Upstream-Status` response header appear.
## Example
>Example 1: `502` response status code comes from `Upstream` (IP address is not available)
```shell
$ curl http://127.0.0.1:9080/apisix/admin/routes/1 -H 'X-API-KEY: edd1c9f034335f136f87ad84b625c8f1' -X PUT -d '
{
"methods": ["GET"],
"upstream": {
"nodes": {
"127.0.0.1:1": 1
},
"type": "roundrobin"
},
"uri": "/hello"
}'
```
Test:
```shell
$ curl http://127.0.0.1:9080/hello -v
......
< HTTP/1.1 502 Bad Gateway
< Date: Wed, 25 Nov 2020 14:40:22 GMT
< Content-Type: text/html; charset=utf-8
< Content-Length: 154
< Connection: keep-alive
< Server: APISIX/2.0
< X-APISIX-Upstream-Status: 502
<
502 Bad Gateway
502 Bad Gateway
openresty
```
It has a response header of `X-APISIX-Upstream-Status: 502`.
>Example 2: `502` response status code comes from `APISIX`
```shell
$ curl http://127.0.0.1:9080/apisix/admin/routes/1 -H 'X-API-KEY: edd1c9f034335f136f87ad84b625c8f1' -X PUT -d '
{
"plugins": {
"fault-injection": {
"abort": {
"http_status": 500,
"body": "Fault Injection!\n"
}
}
},
"uri": "/hello"
}'
```
Test：
```shell
$ curl http://127.0.0.1:9080/hello -v
......
< HTTP/1.1 500 Internal Server Error
< Date: Wed, 25 Nov 2020 14:50:20 GMT
< Content-Type: text/plain; charset=utf-8
< Transfer-Encoding: chunked
< Connection: keep-alive
< Server: APISIX/2.0
<
Fault Injection!
```
There is no response header for `X-APISIX-Upstream-Status`.
>Example 3: `Upstream` has multiple nodes, and all nodes are unavailable
```shell
$ curl http://127.0.0.1:9080/apisix/admin/upstreams/1 -H 'X-API-KEY: edd1c9f034335f136f87ad84b625c8f1' -X PUT -d '
{
"nodes": {
"127.0.0.3:1": 1,
"127.0.0.2:1": 1,
"127.0.0.1:1": 1
},
"retries": 2,
"type": "roundrobin"
}'
```
```shell
$ curl http://127.0.0.1:9080/apisix/admin/routes/1 -H 'X-API-KEY: edd1c9f034335f136f87ad84b625c8f1' -X PUT -d '
{
"uri": "/hello",
"upstream_id": "1"
}'
```
Test：
```shell
$ curl http://127.0.0.1:9080/hello -v
< HTTP/1.1 502 Bad Gateway
< Date: Wed, 25 Nov 2020 15:07:34 GMT
< Content-Type: text/html; charset=utf-8
< Content-Length: 154
< Connection: keep-alive
< Server: APISIX/2.0
< X-APISIX-Upstream-Status: 502, 502, 502
<
502 Bad Gateway
502 Bad Gateway
openresty
```
It has a response header of `X-APISIX-Upstream-Status: 502, 502, 502`.