Using Stackdriver Logging on App Engine Standard and Cloud Run is complicated. It doesn't have to be that way.

package main

import (
  "fmt"
  "log"
  "net/http"
  "os"

  "github.com/mtraver/gaelog"
)

// wrappedHandler must be wrapped using gaelog.Wrap or gaelog.WrapWithID so that the
// request context can be used with the package-level logging functions.
type wrappedHandler struct{}

func (h wrappedHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
  if r.URL.Path != "/" {
    http.NotFound(w, r)
    return
  }

  ctx := r.Context()

  gaelog.Debugf(ctx, "Debug")
  gaelog.Infof(ctx, "Info")
  gaelog.Noticef(ctx, "Notice")
  gaelog.Warningf(ctx, "Warning")
  gaelog.Errorf(ctx, "Error")
  gaelog.Criticalf(ctx, "Critical")
  gaelog.Alertf(ctx, "Alert")
  gaelog.Emergencyf(ctx, "Emergency")

  message := struct {
    Places []string
  }{
    []string{"Kings Canyon", "Sequoia", "Yosemite", "Death Valley"},
  }

  gaelog.Info(ctx, message)

  fmt.Fprintf(w, "Hello!")
}

// manualHandler creates and closes a logger manually. This usage does not require
// gaelog.Wrap or gaelog.WrapWithID.
func manualHandler(w http.ResponseWriter, r *http.Request) {
  lg, err := gaelog.New(r)
  if err != nil {
    // The returned logger is valid despite the error. It falls back to logging
    // via the standard library's "log" package.
    lg.Errorf("Failed to make logger: %v", err)
  }
  defer lg.Close()

  lg.Warningf("Some important info right here, that's for sure")

  fmt.Fprintf(w, "Hello!")
}

func main() {
  // Wrap the handler.
  http.Handle("/", gaelog.Wrap(wrappedHandler{}))

  http.HandleFunc("/manual", manualHandler)

  port := os.Getenv("PORT")
  if port == "" {
    port = "8080"
  }
  log.Fatal(http.ListenAndServe(fmt.Sprintf(":%s", port), nil))
}

At its core, gaelog is setting up the MonitoredResource and trace ID of the underlying Stackdriver Logging logger. The trace ID is retrieved from the X-Cloud-Trace-Context header and the app info is retrieved from environment variables.

On App Engine the MonitoredResource is set as follows to match that set by App Engine itself on request logs. The type, "gae_app", is the literal value that App Engine uses (and gaelog mimics). The other strings below are placeholders for your app's info.

resource: {
  labels: {
    module_id: "my-service-id"
    project_id: "my-project-id"
    version_id: "my-version-id"
  }
  type: "gae_app"
}

Imagine you containerize your App Engine app and deploy it to Cloud Run. It would be nice if logs were still correlated on Cloud Run, right? Well, they will be. If the app info as expected on App Engine is not present then gaelog will look up the info as it's expected on Cloud Run.

On Cloud Run the MonitoredResource is set as follows to match that set by Cloud Run itself on request logs. The type, "cloud_run_revision", is the literal value that Cloud Run uses (and gaelog mimics). The other strings below are placeholders for your revision's info.

resource: {
  labels: {
    configuration_name: "my-config-name"
    project_id: "my-project-id"
    revision_name: "my-revision"
    service_name: "my-service"
  }
  type: "cloud_run_revision"
}

1. Request log (aka parent) severity is not set. A nice property of google.golang.org/appengine/log is that the severity of the request log entry (aka parent entry) is set to the maximum severity of the log entries correlated with it. This makes it easy to see in the Stackdriver UI which requests have logs associated with them, and at which severity. Alas, that is not possible with this package. App Engine itself makes the request log entries and it does not know about any correlated entries created separately (such as with this package). Furthermore, entries cannot be modified after they are created. A possible remedy is for this package to emit request log entries of its own; open an issue if you'd like this and we can discuss.

2. If a request has any log entries made by google.golang.org/appengine/log or App Engine itself, then entries made by this package will not be correlated (i.e. nested) with the request in the Stackdriver Logging UI. Such logs are "embedded" logs—if you expand the request log entry you'll see the other log lines embedded there in the data structure. Embedded and correlated logs do not play nicely together. An annoying consequence is that requests that start a new instance or that time out will not be correlated with logs emitted via this package because App Engine adds embedded logs on such requests. This issue is not solvable in this package.