log.txt

Log call at point of error:

func BreedDragons(ctx context.Context, game *GameKey) error {
    // See 'category' in the json blob below. This adds a new category
    // to the stack, but in the log below we are able to see what methods
    // actually made the call into this method.
    ctx = logging.AddCategory(ctx, "breeddragons")
    
    // logic...
    
    // Notice that only the error and a small message are provided.
    if err != nil {
      logging.Error(ctx, err, "falling back to drg_fire")
      return err
    }
}

Logging package API (simplified):

func Error(ctx context.Context, err error, args ...interface{}) {
	category := CategoryFrom(ctx)
	if log, ok := ctx.Value(LoggerKey).(Logger); ok {
	    log.Write(ERROR, category, buildArgs(ctx, append([]interface{}{err}, args...)...))
	}
}

Data that ends up in logging backend:
{
    "build": "DV2-IP16-13",
    "category": "frontend:dv2:breeddragons",
    "cv": "1.1.0.1010-master",
    "error": {
        "message": "ServiceError({Code:ServiceErrorCode_InternalServerError CategorySource:frontend:dv2:breeddragons Msg:falling back to drg_fire ServerErrorType:*errors.errorString})",
        "type": "*core.ServiceError"
    },
    "game": "dv2",
    "host": "ab0844ae1d22",
    "level": "ERROR",
    "ref": "nBu8KzCN5GyHQftI5YT7JQ",
    "request_id": "b5c3bcae-ebea-41aa-9b1f-79328733c86b",
    "request_phase": "in-progress",
    "service": "dv2",
    "sid": "f7b7d7f1-4f44-45fe-9188-41799bcea735",
    "timestamp": "2016-05-02T12:29:55.141288Z",
    "url_path": "/service"
}