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Purpose

The long-term goal of this Study & Implementation Proposal is gain precise control over memory consumption in CPS and NCMP, going from linear O(N) to constant O(1) space complexity.

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

Summary of Problem

CPS and NCMP receive queries (CPS path queries and NCMP CM handle queries respectively) for which it will not be known how much data will be returned. Since internal and external APIs return Collections, these Collections containing arbitrary amounts of data are being held in memory. Additionally, these Collections undergo many transformations. Each collection cannot be garbage collected until fully processed/transformed.

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Proposed Solution

It is proposed to create an end-to-end streaming solution, from Persistence layer to Controller. A Proof of Concept will be constructed to document challenges and investigate performance characteristics.

An important observation as to why this solution will achieve O(1) constant space complexity is that the streams will not be terminated until they leave the Rest controller.

Streaming all the way

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

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// 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 -> convertToDataNode(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.

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

Additional details of current memory consumption - data conversions

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

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