code: @VisibleForTesting
      protected Mesos startInternal() {
          String version = System.getenv("MESOS_API_VERSION");
          if (version == null) {
              version = "V0";
          }
  
          LOGGER.info("Using Mesos API version: {}", version);
  
          if (version.equals("V0")) {
              if (credential == null) {
                  return new V0Mesos(this, frameworkInfo, master);
              } else {
                  return new V0Mesos(this, frameworkInfo, master, credential);
              }
          } else if (version.equals("V1")) {
              if (credential == null) {
                  return new V1Mesos(this, master);
              } else {
                  return new V1Mesos(this, master, credential);
              }
          } else {
              throw new IllegalArgumentException("Unsupported API version: " + version);
          }
      }
docs: Broken out into a separate function to allow testing with custom `Mesos` implementations.