Implement BM25

Milvus.Client/AnnSearchRequest.cs

@@ -116,3 +116,42 @@ public SparseVectorAnnSearchRequest(
     /// </summary>
     public IReadOnlyList<MilvusSparseVector<T>> Vectors { get; }
 }
+
+/// <summary>
+/// Represents an ANN search request for full-text search using BM25.
+/// </summary>
+/// <remarks>
+/// This request type is used for hybrid search that combines BM25 full-text search
+/// with dense vector search. The text queries are automatically converted to sparse
+/// vectors by the Milvus server using the BM25 function defined in the collection schema.
+/// </remarks>
+public sealed class TextAnnSearchRequest : AnnSearchRequest
+{
+    /// <summary>
+    /// Creates a new ANN search request for BM25 full-text search.
+    /// </summary>
+    /// <param name="vectorFieldName">
+    /// The name of the sparse vector field (output field of the BM25 function) to search in.
+    /// </param>
+    /// <param name="texts">The text queries to search for.</param>
+    /// <param name="limit">The maximum number of results to return.</param>
+    public TextAnnSearchRequest(
+        string vectorFieldName,
+        IReadOnlyList<string> texts,
+        int limit)
+        : base(vectorFieldName, SimilarityMetricType.Bm25, limit)
+    {
+        Verify.NotNull(texts);
+        if (texts.Count == 0)
+        {
+            throw new ArgumentException("At least one text query must be provided", nameof(texts));
+        }
+
+        Texts = texts;
+    }
+
+    /// <summary>
+    /// The text queries to search for.
+    /// </summary>
+    public IReadOnlyList<string> Texts { get; }
+}

Milvus.Client/CollectionSchema.cs

@@ -10,6 +10,7 @@ namespace Milvus.Client;
 public sealed class CollectionSchema
 {
     private readonly List<FieldSchema> _fields = new();
+    private readonly List<FunctionSchema> _functions = new();
 
     /// <summary>
     /// Instantiates a new <see cref="CollectionSchema" />.
@@ -19,7 +20,15 @@ public CollectionSchema()
     }
 
     internal CollectionSchema(IReadOnlyList<FieldSchema> fields)
-        => _fields.AddRange(fields);
+        : this(fields, Array.Empty<FunctionSchema>())
+    {
+    }
+
+    internal CollectionSchema(IReadOnlyList<FieldSchema> fields, IReadOnlyList<FunctionSchema> functions)
+    {
+        _fields.AddRange(fields);
+        _functions.AddRange(functions);
+    }
 
     /// <summary>
     /// The name of the collection.
@@ -36,6 +45,15 @@ internal CollectionSchema(IReadOnlyList<FieldSchema> fields)
     /// </summary>
     public IList<FieldSchema> Fields => _fields;
 
+    /// <summary>
+    /// The functions defined in the schema for automatic data transformation.
+    /// </summary>
+    /// <remarks>
+    /// Functions enable automatic data transformations during insertion, such as BM25 full-text
+    /// search indexing where text fields are automatically converted to sparse vectors.
+    /// </remarks>
+    public IList<FunctionSchema> Functions => _functions;
+
     /// <summary>
     /// Whether to enable dynamic fields for this schema. Defaults to <c>false</c>.
     /// </summary>

Milvus.Client/Constants.cs

@@ -181,4 +181,14 @@ internal static class Constants
     /// to instruct server execute query/search after all DML operations finished.
     /// </summary>
     internal const long GuaranteeStrongTs = 0L;
+
+    /// <summary>
+    /// Key name in type_params. Indicates whether analyzer is enabled for a varchar field.
+    /// </summary>
+    internal const string EnableAnalyzer = "enable_analyzer";
+
+    /// <summary>
+    /// Key name in type_params. Contains analyzer parameters as JSON.
+    /// </summary>
+    internal const string AnalyzerParams = "analyzer_params";
 }

Milvus.Client/FieldSchema.cs

@@ -112,6 +112,12 @@ public static FieldSchema Create<TData>(
     /// <param name="defaultValue">
     /// The default value for the field. Available since Milvus v2.5.
     /// </param>
+    /// <param name="enableAnalyzer">
+    /// Whether to enable the analyzer for this field. Required for BM25 full-text search input fields.
+    /// </param>
+    /// <param name="analyzerParams">
+    /// Optional analyzer parameters. For example: <c>new Dictionary&lt;string, object&gt; { ["type"] = "english" }</c>.
+    /// </param>
     public static FieldSchema CreateVarchar(
         string name,
         int maxLength,
@@ -120,10 +126,14 @@ public static FieldSchema CreateVarchar(
         bool isPartitionKey = false,
         string description = "",
         bool nullable = false,
-        string? defaultValue = null)
+        string? defaultValue = null,
+        bool enableAnalyzer = false,
+        IReadOnlyDictionary<string, object>? analyzerParams = null)
         => new(name, MilvusDataType.VarChar, isPrimaryKey, autoId, isPartitionKey, description, nullable, defaultValue)
         {
-            MaxLength = maxLength
+            MaxLength = maxLength,
+            EnableAnalyzer = enableAnalyzer,
+            AnalyzerParams = analyzerParams
         };
 
     /// <summary>
@@ -362,6 +372,28 @@ internal FieldSchema(
     /// </summary>
     public int? Dimension { get; set; }
 
+    /// <summary>
+    /// Whether to enable the analyzer for this field. Required for BM25 full-text search input fields.
+    /// </summary>
+    /// <remarks>
+    /// When enabled, the field can be used as an input to a BM25 function for full-text search.
+    /// This property is only applicable for <see cref="MilvusDataType.VarChar" /> fields.
+    /// </remarks>
+    public bool EnableAnalyzer { get; set; }
+
+    /// <summary>
+    /// Optional analyzer parameters. For example: <c>new Dictionary&lt;string, object&gt; { ["type"] = "english" }</c>.
+    /// </summary>
+    /// <remarks>
+    /// This property is only applicable when <see cref="EnableAnalyzer" /> is set to true.
+    /// </remarks>
+    public IReadOnlyDictionary<string, object>? AnalyzerParams { get; set; }
+
+    /// <summary>
+    /// Whether this field is the output of a function and should not be provided during insertion.
+    /// </summary>
+    public bool IsFunctionOutput { get; internal set; }
+
     /// <summary>
     /// The state of the field.
     /// </summary>

Milvus.Client/FunctionSchema.cs

@@ -0,0 +1,117 @@
+using System.Diagnostics;
+
+namespace Milvus.Client;
+
+/// <summary>
+/// Defines a function within a <see cref="CollectionSchema" /> that automatically transforms data during insertion.
+/// </summary>
+/// <remarks>
+/// Functions enable automatic data transformations such as BM25 full-text search indexing,
+/// where text fields are automatically converted to sparse vectors.
+/// </remarks>
+[DebuggerDisplay("{DebuggerDisplay,nq}")]
+public sealed class FunctionSchema
+{
+    private readonly List<string> _inputFieldNames = new();
+    private readonly List<string> _outputFieldNames = new();
+    private readonly Dictionary<string, string> _params = new();
+
+    /// <summary>
+    /// Creates a new function schema.
+    /// </summary>
+    /// <param name="name">The name of the function.</param>
+    /// <param name="type">The type of function.</param>
+    /// <param name="inputFieldNames">The names of the input fields for this function.</param>
+    /// <param name="outputFieldNames">The names of the output fields for this function.</param>
+    /// <param name="description">An optional description for the function.</param>
+    public FunctionSchema(
+        string name,
+        FunctionType type,
+        IEnumerable<string> inputFieldNames,
+        IEnumerable<string> outputFieldNames,
+        string description = "")
+    {
+        Name = name;
+        Type = type;
+        _inputFieldNames.AddRange(inputFieldNames);
+        _outputFieldNames.AddRange(outputFieldNames);
+        Description = description;
+    }
+
+    /// <summary>
+    /// Creates a BM25 function schema for full-text search.
+    /// </summary>
+    /// <param name="name">The name of the function.</param>
+    /// <param name="inputFieldName">
+    /// The name of the VARCHAR input field. This field must have <see cref="FieldSchema.EnableAnalyzer"/> set to true.
+    /// </param>
+    /// <param name="outputFieldName">
+    /// The name of the SPARSE_FLOAT_VECTOR output field that will be automatically populated.
+    /// </param>
+    /// <param name="description">An optional description for the function.</param>
+    /// <returns>A new BM25 function schema.</returns>
+    public static FunctionSchema CreateBm25(
+        string name,
+        string inputFieldName,
+        string outputFieldName,
+        string description = "")
+        => new(name, FunctionType.Bm25, new[] { inputFieldName }, new[] { outputFieldName }, description);
+
+    internal FunctionSchema(
+        long id,
+        string name,
+        FunctionType type,
+        IEnumerable<string> inputFieldNames,
+        IEnumerable<string> outputFieldNames,
+        string description,
+        IEnumerable<KeyValuePair<string, string>> parameters)
+    {
+        Id = id;
+        Name = name;
+        Type = type;
+        _inputFieldNames.AddRange(inputFieldNames);
+        _outputFieldNames.AddRange(outputFieldNames);
+        Description = description;
+        foreach (var param in parameters)
+        {
+            _params[param.Key] = param.Value;
+        }
+    }
+
+    /// <summary>
+    /// The internal Milvus ID assigned to this function.
+    /// </summary>
+    public long Id { get; }
+
+    /// <summary>
+    /// The name of the function.
+    /// </summary>
+    public string Name { get; }
+
+    /// <summary>
+    /// The type of function.
+    /// </summary>
+    public FunctionType Type { get; }
+
+    /// <summary>
+    /// The names of the input fields for this function.
+    /// </summary>
+    public IList<string> InputFieldNames => _inputFieldNames;
+
+    /// <summary>
+    /// The names of the output fields for this function.
+    /// </summary>
+    public IList<string> OutputFieldNames => _outputFieldNames;
+
+    /// <summary>
+    /// An optional description for the function.
+    /// </summary>
+    public string Description { get; }
+
+    /// <summary>
+    /// Additional parameters for the function.
+    /// </summary>
+    public IDictionary<string, string> Params => _params;
+
+    private string DebuggerDisplay => $"{Name} ({Type})";
+}

Milvus.Client/FunctionType.cs

@@ -0,0 +1,37 @@
+namespace Milvus.Client;
+
+/// <summary>
+/// The type of function to use in a collection schema.
+/// </summary>
+public enum FunctionType
+{
+    /// <summary>
+    /// Unknown function type.
+    /// </summary>
+    Unknown = 0,
+
+    /// <summary>
+    /// BM25 function for full-text search. Automatically generates sparse vectors from text input.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// The BM25 function requires exactly one VARCHAR input field (with analyzer enabled) and
+    /// exactly one SPARSE_FLOAT_VECTOR output field.
+    /// </para>
+    /// <para>
+    /// When using BM25, the sparse vector field is automatically populated during insertion based
+    /// on the text content of the input field.
+    /// </para>
+    /// </remarks>
+    Bm25 = 1,
+
+    /// <summary>
+    /// Text embedding function for generating dense vectors from text.
+    /// </summary>
+    TextEmbedding = 2,
+
+    /// <summary>
+    /// Rerank function for reordering search results.
+    /// </summary>
+    Rerank = 3
+}

Milvus.Client/IndexType.cs

@@ -231,9 +231,23 @@ public enum IndexType
     /// with low-dimensional non-zero values.
     /// </para>
     /// <para>
-    /// Build parameters: <c>drop_ratio_build</c> (the proportion of small vector values excluded during indexing,
-    /// range [0, 1), default 0).
+    /// Build parameters (passed via <c>extraParams</c>):
     /// </para>
+    /// <list type="bullet">
+    /// <item>
+    /// <c>inverted_index_algo</c>: The query algorithm. Values: <c>"DAAT_MAXSCORE"</c> (default, best for high k
+    /// or many terms), <c>"DAAT_WAND"</c> (best for small k or short queries), <c>"TAAT_NAIVE"</c> (adapts to
+    /// collection changes). String values must be quoted in JSON, e.g. <c>"\"DAAT_WAND\""</c>.
+    /// </item>
+    /// <item>
+    /// <c>bm25_k1</c>: Controls term frequency saturation for BM25 scoring. Range [1.2, 2.0].
+    /// Only applicable when metric type is <see cref="SimilarityMetricType.Bm25" />.
+    /// </item>
+    /// <item>
+    /// <c>bm25_b</c>: Controls document length normalization for BM25 scoring. Range [0, 1], default 0.75.
+    /// Only applicable when metric type is <see cref="SimilarityMetricType.Bm25" />.
+    /// </item>
+    /// </list>
     /// <para>
     /// Search parameters: <c>drop_ratio_search</c> (the proportion of small vector values excluded during search,
     /// range [0, 1), default 0).

Milvus.Client/MilvusClient.Collection.cs

@@ -1,4 +1,5 @@
 using System.Globalization;
+using System.Text.Json;
 
 namespace Milvus.Client;
 
@@ -127,9 +128,51 @@ public async Task<MilvusCollection> CreateCollectionAsync(
                 grpcField.DefaultValue = ConvertToValueField(field.DefaultValue, field.DataType);
             }
 
+            if (field.EnableAnalyzer)
+            {
+                grpcField.TypeParams.Add(new Grpc.KeyValuePair
+                {
+                    Key = Constants.EnableAnalyzer,
+                    Value = "true"
+                });
+            }
+
+            if (field.AnalyzerParams is not null)
+            {
+                grpcField.TypeParams.Add(new Grpc.KeyValuePair
+                {
+                    Key = Constants.AnalyzerParams,
+                    Value = JsonSerializer.Serialize(field.AnalyzerParams)
+                });
+            }
+
             grpcCollectionSchema.Fields.Add(grpcField);
         }
 
+        foreach (FunctionSchema function in schema.Functions)
+        {
+            Grpc.FunctionSchema grpcFunction = new()
+            {
+                Name = function.Name,
+                Description = function.Description,
+                Type = (Grpc.FunctionType)(int)function.Type
+            };
+
+            grpcFunction.InputFieldNames.AddRange(function.InputFieldNames);
+            grpcFunction.OutputFieldNames.AddRange(function.OutputFieldNames);
+
+            foreach (var param in function.Params)
+            {
+                grpcFunction.Params.Add(new Grpc.KeyValuePair
+                {
+                    Key = param.Key,
+                    Value = param.Value
+                });
+            }
+
+            grpcCollectionSchema.Functions.Add(grpcFunction);
+        }
+
 #pragma warning disable CS0612 // Schema-level AutoID is obsolete, but still there in pymilvus
         grpcCollectionSchema.AutoID = schema.Fields.Any(static p => p.AutoId);
 #pragma warning restore CS0612 // Type or member is obsolete