populateComponentsByTagWithStormInternals.java

private void populateComponentsByTagWithStormInternals(
        Map<String, ArrayList<String>> componentsByTag,
        Set<String> components
) {
    // Storm uses some internal components, like __acker.
    // These components are topology-agnostic and are therefore not accessible through a StormTopology object.
    // While a bit hacky, this is a way to make sure that we schedule those components along with our topology ones:
    // we treat these internal components as regular untagged components and add them to the componentsByTag map.

    for (String componentID : components) {
        if (componentID.startsWith("__")) {
            if (componentsByTag.containsKey("untagged")) {
                // If we've already seen untagged components, then just add the component to the existing ArrayList.
                componentsByTag.get("untagged").add(componentID);
            } else {
                // If this is the first untagged component we see, then create a new ArrayList,
                // add the current component, and populate the map's untagged entry with it.
                ArrayList<String> newComponentList = new ArrayList<String>();
                newComponentList.add(componentID);
                componentsByTag.put("untagged", newComponentList);
            }
        }
    }
}