apollo-configservice/src/main/java/com/ctrip/framework/apollo/configservice/controller/ConfigController.java/ConfigController/queryConfig
class ConfigController:  @GetMapping(value = "/{appId}/{clusterName}/{namespace:.+}")
  public ApolloConfig queryConfig(@PathVariable String appId, @PathVariable String clusterName,
                                  @PathVariable String namespace,
                                  @RequestParam(value = "dataCenter", required = false) String dataCenter,
                                  @RequestParam(value = "releaseKey", defaultValue = "-1") String clientSideReleaseKey,
                                  @RequestParam(value = "ip", required = false) String clientIp,
                                  @RequestParam(value = "label", required = false) String clientLabel,
                                  @RequestParam(value = "messages", required = false) String messagesAsString,
                                  HttpServletRequest request, HttpServletResponse response) throws IOException {
    String originalNamespace = namespace;
    //strip out .properties suffix
    namespace = namespaceUtil.filterNamespaceName(namespace);
    //fix the character case issue, such as FX.apollo <-> fx.apollo
    namespace = namespaceUtil.normalizeNamespace(appId, namespace);

    if (Strings.isNullOrEmpty(clientIp)) {
      clientIp = tryToGetClientIp(request);
    }

    ApolloNotificationMessages clientMessages = transformMessages(messagesAsString);

    List<Release> releases = Lists.newLinkedList();

    String appClusterNameLoaded = clusterName;
    if (!ConfigConsts.NO_APPID_PLACEHOLDER.equalsIgnoreCase(appId)) {
      Release currentAppRelease = configService.loadConfig(appId, clientIp, clientLabel, appId, clusterName, namespace,
          dataCenter, clientMessages);

      if (currentAppRelease != null) {
        releases.add(currentAppRelease);
        //we have cluster search process, so the cluster name might be overridden
        appClusterNameLoaded = currentAppRelease.getClusterName();
      }
    }

    //if namespace does not belong to this appId, should check if there is a public configuration
    if (!namespaceBelongsToAppId(appId, namespace)) {
      Release publicRelease = this.findPublicConfig(appId, clientIp, clientLabel, clusterName, namespace,
          dataCenter, clientMessages);
      if (Objects.nonNull(publicRelease)) {
        releases.add(publicRelease);
      }
    }

    if (releases.isEmpty()) {
      response.sendError(HttpServletResponse.SC_NOT_FOUND,
          String.format(
              "Could not load configurations with appId: %s, clusterName: %s, namespace: %s",
              appId, clusterName, originalNamespace));
      Tracer.logEvent("Apollo.Config.NotFound",
          assembleKey(appId, clusterName, originalNamespace, dataCenter));
      return null;
    }

    auditReleases(appId, clusterName, dataCenter, clientIp, releases);

    String mergedReleaseKey = releases.stream().map(Release::getReleaseKey)
            .collect(Collectors.joining(ConfigConsts.CLUSTER_NAMESPACE_SEPARATOR));

    if (mergedReleaseKey.equals(clientSideReleaseKey)) {
      // Client side configuration is the same with server side, return 304
      response.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
      Tracer.logEvent("Apollo.Config.NotModified",
          assembleKey(appId, appClusterNameLoaded, originalNamespace, dataCenter));
      return null;
    }

    ApolloConfig apolloConfig = new ApolloConfig(appId, appClusterNameLoaded, originalNamespace,
        mergedReleaseKey);
    apolloConfig.setConfigurations(mergeReleaseConfigurations(releases));

    Tracer.logEvent("Apollo.Config.Found", assembleKey(appId, appClusterNameLoaded,
        originalNamespace, dataCenter));
    return apolloConfig;
  }