shreemaan-abhishek opened a new pull request, #13568:
URL: https://github.com/apache/apisix/pull/13568
### Description
Setting `log_format` on a logger plugin (via `plugin_metadata` or plugin
`conf`) **replaces** the default rich log entry with a flat custom format. The
custom format only substitutes scalar `$variables`, so it silently drops every
default field that has no scalar-variable equivalent: the full request/response
header maps, the parsed querystring, the composed request URL, the APISIX
version, and the computed `apisix_latency`.
Customers often set `log_format` just to add or rename one field, and
unknowingly lose all of those.
This PR adds **`log_format_extra`**: an additive log format that keeps the
default rich log entry intact and only overlays the user-defined fields on top.
It is ignored when `log_format` is set (which keeps the replace semantics).
It also exposes a new core variable **`upstream_unresolved_host`** — the
upstream host *before* DNS resolution (the configured domain or IP of the
picked node). The default log only carries `upstream` (`$upstream_addr`, the
resolved `ip:port`), so this variable lets users log the original hostname via
`log_format_extra`.
### Example
```json
"plugin_metadata": {
"log_format_extra": {
"upstream_host": "$upstream_unresolved_host"
}
}
```
The log keeps every default field (`request`, `response`, `server`,
`route_id`, ...) and gains `upstream_host`.
### Changes
- `apisix/utils/log-util.lua`: `get_log_entry` builds the default log then
overlays `log_format_extra` (http subsystem only, since the default log is
http-only).
- `apisix/core/ctx.lua`: register the `upstream_unresolved_host` variable.
- `apisix/balancer.lua`: stash the pre-DNS host on `ctx` when picking a
server (single-node and balancer paths).
- `apisix/plugins/loki-logger.lua`, `apisix/plugins/file-logger.lua`: accept
`log_format_extra` in conf and metadata schemas.
- Docs: `loki-logger.md`, `apisix-variable.md`.
- Tests: enrich scenario in `t/plugin/file-logger.t` (self-contained) and
`t/plugin/loki-logger.t`.
### Checklist
- [x] Tests added
- [x] Docs updated
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