private String getCacheKey(RoutingContext ctx) {
        return ctx.getRoute().getHop(0).toString();
    }
Returns a cache key for this instance of the policy. Because behaviour is based on the hop in which the policy
occurs, the cache key is the hop string itself.

@param ctx The routing context.
@return The cache key.
documentapi/src/main/java/com/yahoo/documentapi/messagebus/protocol/LocalServicePolicy.java