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Current state: ["Under DiscussionDeclined"]

ISSUE: #17754

PRs: #240 #243 #18277

Keywords: IDMAP, BinaryIDMAP, brute force search, chunk

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In this MEP, we put forward an IDMAP/BinaryIDMAP Enhancement proposal that let knowhere index type IDMAP/BinaryIDMAP accept to hold an external vector data pointer instead of adding real vector data in.

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The only reasonable use scenario for IDMAP/BinaryIDMAP is for growing segment. But creating vector index is a resource consuming operation, because it involves all Milvus nodes in -- an index file is created by index node, saved by data node and loaded by query node, meanwhile proxy / if create an IDMAP/BinaryIDMAP index in a normal way, it will consume lots of resources, because it will involve index node (to create index file), data node (to save index file to S3) and rootcoord / indexcoord / datacoord / querycoord are also involved (to coordinate all these operations). 

So currently in Milvus , it uses following 2 strategies for growing segment searching:

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The advantage of this solution is resource saving, except query node, no other nodes will be involved in; while the shortcoming is code duplication.

See following "Search Flow" chart, `FloatSearchBruteForce` and `BinarySearchBruteForce` are copied from knowhere::IDMAP and knowhere::BinaryIDMAP. More code duplicated, more effort on code maintenance/BinaryIDMAP's interface Query() and modified a little. This will introduce more code maintenance effort. And when realize new feature on IDMAP/BinaryIDMAP in Knowhere, such as range search, same work need be re-done on Milvus segcorewe have to also copy these codes implementation to Milvus.

Image Modified

This is why enhanced IDMAP/BinaryIDMAP is proposed. For enhanced If we enhance IDMAP/BinaryIDMAP, not to add real vector data is not really added in, but only set hold an external vector data pointer in the index, we can use knowhere::IDMAP/BinaryIDMAP's interface Query() directly without any costs. User should need guarantee that the memory is contiguous and safe.

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• no CPU time, memory and disk consumption when creating index
• resource saving, only query node is involved in
• no code duplication for growing segment search
• unified search result for sealed segment and growing segment

Public Interfaces(optional)

Briefly list any new interfaces that will be introduced as part of this proposal or any existing interfaces that will be removed or changed.

Faiss need add a new interface "add_ex()" for IDMAP and BinaryIDMAP

//============================================================================
struct IndexBinaryFlat : IndexBinary {
    /// database vectors, size ntotal * d / 8
    std::vector<uint8_t> xb;

    /// external database vectors, size ntotal * d / 8
    uint8_t* xb_ex = nullptr;								// <==== new added
    ... ...
}

void IndexBinaryFlat::add_ex(idx_t n, const uint8_t* x) {
    xb_ex = (uint8_t*)x;
    ntotal = n;
}

//============================================================================
struct IndexFlatCodes : Index {
    ... ...
    /// encoded dataset, size ntotal * code_size
    std::vector<uint8_t> codes;

    // external encoded dataset , size ntotal * code_size
    uint8_t* codes_ex = nullptr;							// <==== new added
    ... ...
}

void IndexFlatCodes::add_ex(idx_t n, const float* x) {
    FAISS_THROW_IF_NOT(is_trained);
    FAISS_THROW_IF_NOT(codes.empty());
    codes_ex = (uint8_t*)x;
    ntotal = n;
}

Knowhere need add a new interface `AddExWithoutIds()` for IDMAP and BinaryIDMAP

    // set external data pointer instead really add data
    void
    AddExWithoutIds(const DatasetPtr&, const Config&);

For Milvus, the implementation of API "FloatSearchBruteForce()" and "BinarySearchBruteForce()" will be re-written, but the interface need not change.

Design Details(required)

Proposal 1 (only take IDMAP as an example)

Reuse index IDMAP, add new interface in both knowhere and faiss.

1. Faiss adds new field "codes_ex" and new interface "add_ex()" for structure IndexFlat. In IndexFlat, "codes" and "codes_ex" are mutual exclusive, user cannot set both of them.
2. Knowhere adds a new interface `AddExWithoutIds()` for IDMAP.
3. In Milvus, re-write API "FloatSearchBruteForce()", let it use enhanced IDMAP to search.

Advantages: Little code change

Cons: Need add new interfaces in both Faiss and Knowhere

Proposal 2 (only take IDMAP as an example)

Add new index type IDMAP_EX, add new interface in faiss.

1. Faiss adds new field "codes_ex" and new interface "add_ex()" for structure IndexFlat. In IndexFlat, "codes" and "codes_ex" are mutual exclusive, user cannot set both of them.
2. Knowhere adds a new index type IDMAP_EX, which use API "add_ex()" to insert vector data.
3. In Milvus, re-write API "FloatSearchBruteForce()", let it use new IDMAP_EX to search.

Advantages: No new interface in Knowhere

Cons: Need add new index type in Knowhere, most part of code of this index is duplicate with IDMAP

Proposal 3 (only take IDMAP as an example)

Add a new wrapper API in knowhere to call faiss brute force search for all metric types

Advantages: No code change in faiss, only one new interface in Knowhere

Cons: This change dis-obey knowhere's design concept. By now all operations in knowhere is for an index, but this API is for all metric types, not for an index.

Since both Proposal 1 and Proposal 2 need add new interface in Faiss, and Milvus team think this will break Faiss's encapsulation, we adopt Proposal 3.

Public Interfaces(optional)

Add new interface only in Knowhere

class BruteForce {
 public:
    static DatasetPtr
    Search(const DatasetPtr base_dataset,
           const DatasetPtr query_dataset,
           const Config& config,
           const faiss::BitsetView bitset);

    static DatasetPtr
    RangeSearch(const DatasetPtr base_dataset,
                const DatasetPtr query_dataset,
                const Config& config,
                const faiss::BitsetView bitset);
};

BruteForce::Search() is wrapper API to call Faiss brute force search for all metric types.

BruteForce::RangeSearch() is wrapper API to call Faiss brute force range search for all metric types. 

Design Details(required)

For growing segment in segcore, all brute force search related code can be removed, call knowhere's BruteForceSearch() and BruteForceRangeSearch() insteadDescribe the new thing you want to do in appropriate detail. This may be fairly extensive and have large subsections of its own. Or it may be a few sentences. Use judgment based on the scope of the change.

Compatibility, Deprecation, and Migration Plan(optional)

• This MEP will be transparent for users, and will not introduce any compatibility issue.

Test Plan(required)

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New testcases are added in Knowhere to test "BruteForceSearch()" and "BruteForceRangeSearch()".

• in knowhere/unittest/test_bruteforce.cpp

No extra testcases need be added in Milvus because current growing segment search testcases can cover this change.

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