CPS-2146 CPS Core Java Streams API

References

CPS-2146 - Getting issue details... STATUS

Purpose

The long-term goal of this Study & Implementation Proposal is reduce overall memory consumption in CPS and NCMP by over 99% - actually from linear O(N) to constant O(1).

The immediate objective is to fix Out Of Memory Errors encountered in NCMP while performing CM handles searches.

Proposed Solution

It is proposed to create an end-to-end streaming solution, from Repository (database) to Rest interface. It is speculated to achieve more than 99% reduction in memory usage. A POC will be constructed to investigate.

This will require adding Stream versions of CPS Core read operations, e.g.

Stream<DataNode> queryDataNodesAsStream(String dataspaceName, String anchorName, String cpsPath, FetchDescendantsOption fetchDescendantsOption);

This Stream will implement pagination when fetching data from the FragmentRepository. (This may be implemented in a variety of ways, using Spring JpaRepository Pageable interface, or alternately Spring Data supports streaming from a repository directly, but this needs to be investigated for suitability.)

Here is a proposed end-to-end flow - a simplified example to show how greatly this can reduce memory usage:

// In CPS core
Stream<DataNode> queryDataNodesAsStream(String dataspaceName, String anchorName, String cpsPath, FetchDescendantsOption fetchDescendantsOption) {
	return fragmentRepository.streamByAnchorAndCpsPath(getAnchor(dataspaceName, anchorName), cpsPath)
			.map(fragment -> fetchDescendants(fragment, fetchDescendantsOption))
			.map(fragment -> toDataNode(fragment));
}

// In NCMP
private YangModelCmHandle getAnyReadyCmHandleByModuleSetTag(final String moduleSetTag) {
    return cmHandleQueries.queryNcmpRegistryByCpsPath("/dmi-registry/cm-handles[@module-set-tag='" + moduleSetTag + "']", DIRECT_CHILDREN_ONLY)
			.map(YangDataConverter::convertCmHandleToYangModel)
            .filter(cmHandle -> cmHandle.getCompositeState().getCmHandleState() == CmHandleState.READY)
            .findFirst()
            .orElse(null);
}

CPS and NCMP Rest APIs

Instead of returning Collections from Rest APIs, a Stream may be returned, reducing memory pressure on the server.

Current code looks like this:

    @Override
    public ResponseEntity<List<String>> searchCmHandleIds(final CmHandleQueryParameters cmHandleQueryParameters) {
        final CmHandleQueryServiceParameters cmHandleQueryServiceParameters = ncmpRestInputMapper.toCmHandleQueryServiceParameters(cmHandleQueryParameters);
        final Collection<String> cmHandleIds = networkCmProxyDataService.executeCmHandleIdSearchForInventory(cmHandleQueryServiceParameters);
        return ResponseEntity.ok(List.copyOf(cmHandleIds));
    }

It is proposed to stream out of the Rest API, something like this:

    @Override
    public Stream<String> searchCmHandleIds(final CmHandleQueryParameters cmHandleQueryParameters) {
        final CmHandleQueryServiceParameters cmHandleQueryServiceParameters = ncmpRestInputMapper.toCmHandleQueryServiceParameters(cmHandleQueryParameters);
        return networkCmProxyDataService.executeCmHandleIdSearchForInventory(cmHandleQueryServiceParameters);
    }

Details of current memory consumption

The read APIs in CPS Core (cps-service and cps-ri) return Collection<DataNode>:

Collection<DataNode> queryDataNodes(String dataspaceName, String anchorName, String cpsPath, FetchDescendantsOption fetchDescendantsOption);
Collection<DataNode> getDataNodes(String dataspaceName, String anchorName, String xpath, FetchDescendantsOption fetchDescendantsOption);
Collection<DataNode> getDataNodesForMultipleXpaths(String dataspaceName, String anchorName, Collection<String> xpaths, FetchDescendantsOption fetchDescendantsOption);

Additionally, internal APIs in CPS Reference Implementation (cps-ri) use List<FragmentEntity>, e.g.

List<FragmentEntity> findByAnchorAndCpsPath(AnchorEntity anchorEntity, CpsPathQuery cpsPathQuery);

When a CPS path query is run, this will result in a List<FragmentEntity> which needs to be converted to a Collection<DataNode>. Thus, the Fragment Entities cannot be garbage collected until the list is converted to Data Nodes. This doubles the memory usage.

Additionally, NCMP uses CPS path queries, e.g. to find CM handles in a given state. NCMP will then convert Collection<DataNode> to Collection<YangModelCmHandle>. Again, the Collection<DataNode> cannot be garbage collected until fully converted to YangModelCmHandles. This again results in doubling of memory usage.

Similar applies when converting to NcmpServiceCmHandle.

NCMP also contains many queries where only partial results are needed, making a Streams API ideal.

Additionally, all Rest APIs returning query results return Lists. Spring framework allows returning Streams, eliminating memory overhead.