apollo-biz/src/main/java/com/ctrip/framework/apollo/biz/service/ReleaseService.java/ReleaseService/getBranchReleaseKeys
class ReleaseService:  private Collection<String> getBranchReleaseKeys(long releaseId) {
    Page<ReleaseHistory> releaseHistories = releaseHistoryService
        .findByReleaseIdAndOperationInOrderByIdDesc(releaseId, BRANCH_RELEASE_OPERATIONS, FIRST_ITEM);

    if (!releaseHistories.hasContent()) {
      return null;
    }

    Map<String, Object> operationContext = GSON
        .fromJson(releaseHistories.getContent().get(0).getOperationContext(), OPERATION_CONTEXT_TYPE_REFERENCE);

    if (operationContext == null || !operationContext.containsKey(ReleaseOperationContext.BRANCH_RELEASE_KEYS)) {
      return null;
    }

    return (Collection<String>) operationContext.get(ReleaseOperationContext.BRANCH_RELEASE_KEYS);
  }