code: private Set<String> getActiveCacheKeys(final List<Node> nodes) {
          Set<String> cacheKeys = new HashSet<>(nodes.size());
          for (Node node : nodes) {
              // can't use node.getRootPath() cause won't work with disconnected agents.
              String cacheKey = Util.getDigestOf(node.getNodeName() + ":" + ((hudson.model.Slave)node).getRemoteFS());
              LOGGER.log(Level.FINEST, "cacheKey {0} is active", cacheKey);
              cacheKeys.add(StringUtils.right(cacheKey, 8));
          }
          return cacheKeys;
      }
docs: Build a Set including the cacheKeys associated to every agent in the instance