Merge pull request #6429 from IntersectMBO/mkarg/bench-master

New tracing: Support environment variable for logging hostname

Author: mgmeier

nix/workbench/service/nodes.nix

@@ -290,6 +290,8 @@ let
         ''
         #!${pkgs.stdenv.shell}
 
+        export TRACE_DISPATCHER_LOGGING_HOSTNAME=${name}
+
         ${service.script}
         ''
       ;

trace-dispatcher/CHANGELOG.md

@@ -1,9 +1,10 @@
 # Revision history for trace-dispatcher
 
-## 2.11.1 -- Jan 2026
+## 2.11.1 -- Feb 2026
 
 * Add strict `contramap'` (infix alias `>!$!<`) to the API, capturing a common pattern to avoid unintentional space leaks when composing tracers
 * Increase `PrometheusSimple` robustness by restarting the backend upon crash, adding start/stop traces and more eagerly reaping of dangling sockets
+* Setting the `TRACE_DISPATCHER_LOGGING_HOSTNAME` environment variable will override the system hostname in trace messages.
 * Increased strictness when storing traced `DataPoints`
 * Drastically reduced fallback value for forwarding queue capacity to minimize impact of forwarding service interruption on heap size and retention
 * Removed `TraceConfig.tcPeerFrequency` and hence `TraceOptionPeerFrequency` from config representation

trace-dispatcher/src/Cardano/Logging/Formatter.hs

@@ -39,14 +39,20 @@ import           Data.Text.Lazy.Builder as TB
 import           Data.Text.Lazy.Encoding (decodeUtf8)
 import           Data.Time (defaultTimeLocale, formatTime, getCurrentTime)
 import           Network.HostName
+import           System.Environment (lookupEnv)
 import           System.IO.Unsafe (unsafePerformIO)
 
 
--- If the hostname in the logs should be anything different from the system reported hostname,
--- a new field would need to be added to PreFormatted to carry a new hostname argument to preFormatted.
+-- | If the @TRACE_DISPATCHER_LOGGING_HOSTNAME@ environment variable is set,
+--   it overrides the system hostname in the trace message. This is useful when
+--   multiple instances of a service or application on the same host.
 hostname :: Text
 {-# NOINLINE hostname #-}
-hostname = unsafePerformIO $ T.pack <$> getHostName
+hostname = unsafePerformIO $
+  lookupEnv "TRACE_DISPATCHER_LOGGING_HOSTNAME" >>= maybe hostNameOnly (pure . T.pack)
+  where
+    -- disregard FQDNs
+    hostNameOnly = T.pack . takeWhile (/= '.') <$> getHostName
 
 
 -- | Format this trace as metrics