ChromaDotNet.Embeddings

ChromaDotNet.Embeddings

The default embedding function of Chroma, for .NET. It computes the vectors that the Python client of Chroma computes when a collection has no embedding function of its own. The model is all-MiniLM-L6-v2 in ONNX, with 384 dimensions. The generator is an IEmbeddingGenerator<string, Embedding<float>> of Microsoft.Extensions.AI.

Community project, not affiliated with Chroma.

Preview: the API and the package can still change.

dotnet add package ChromaDotNet.Embeddings --prerelease

With ChromaDotNet.Client

ChromaDotNet.Client 2.17.0 and later take the generator. The client of a collection computes the embeddings of the documents and of the query texts. A new collection declares the embedding function default, as the Python client does. So Python reads the collection with its default embedding function, and the other way round.

using ChromaDB.Client;
using ChromaDB.Client.Models;
using ChromaDotNet.Embeddings;

using var embeddings = await ChromaDefaultEmbeddingGenerator.CreateAsync();
using var client = new ChromaClient("http://localhost:8000/api/v2/");
var collection = await client.GetOrCreateCollectionAsync(new ChromaCollectionDefinition("fruits") { EmbeddingGenerator = embeddings });
var fruits = client.GetCollectionClient(collection).WithEmbeddingGenerator(embeddings);

await fruits.AddAsync(["apple", "lemon", "banana"], documents: ["A crunchy apple", "A sour lemon", "A sweet banana"]);
var results = await fruits.QueryAsync("something sour", nResults: 2, include: ChromaQueryInclude.Documents | ChromaQueryInclude.Distances);

Try it in your browser: SearchByText stores and searches texts with this package and ChromaDotNet.Client, on a Chroma started by ChromaDotNet.Local. Open it in a Codespace.

Without the client

using ChromaDotNet.Embeddings;
using Microsoft.Extensions.AI;

using var embeddings = await ChromaDefaultEmbeddingGenerator.CreateAsync();
var vectors = await embeddings.GenerateAsync(["A crunchy apple", "A sour lemon"]);
ReadOnlyMemory<float> query = await embeddings.GenerateVectorAsync("something sour");

Each vector has 384 dimensions and an L2 norm of 1. With an earlier version of ChromaDotNet.Client, pass these vectors to AddAsync and QueryAsync.

Platforms

Intel Macs are not supported. ONNX Runtime has no osx-x64 library from version 1.24 on, and this package uses 1.24.

32-bit processes are not supported. ONNX Runtime has no win-x86 library from version 1.23 on.

On .NET Framework, set PlatformTarget to x64 or ARM64, or keep AnyCPU without Prefer32Bit. An AnyCPU application runs as a 64-bit process on 64-bit Windows. The build of a 32-bit application fails with a message that says so, on .NET Framework and on .NET 8 and later. In a 32-bit process, CreateAsync throws a PlatformNotSupportedException before any download. .NET Framework does not load native libraries from runtimes/. So the package copies hf_tokenizers.dll and onnxruntime.dll next to the application at the build.

On .NET 8, the library is compatible with trimming and NativeAOT.

The model and its cache

The model is not in the package. The first CreateAsync downloads the archive that Python downloads: https://chroma-onnx-models.s3.amazonaws.com/all-MiniLM-L6-v2/onnx.tar.gz, 83 MB. It checks the SHA-256 hash of the archive, and does not keep a file with another hash. It extracts the six files of onnx/, by their exact names.

The cache is the folder of Python: ~/.cache/chroma/onnx_models/all-MiniLM-L6-v2. On Windows it is %USERPROFILE%\.cache\chroma\onnx_models\all-MiniLM-L6-v2. A model that Python downloaded is used as it is. Python uses a model downloaded here.

As in Python, the six files of onnx/ are used when they are all there. Otherwise onnx.tar.gz is used when its hash matches, or downloaded again. Two processes that start at the same time download it once, through a lock file next to the archive. tokenizer.json truncates and pads to 128 tokens; Python changes both to 256. So a copy without them, tokenizer.untruncated.json, goes next to it.

Without the network and without the model in the cache, CreateAsync throws an InvalidOperationException. Its message has the address it tried, and the options to work offline.

What it computes

It does what ONNXMiniLM_L6_V2 of chromadb 1.5.9 does:

• the tokenizer of tokenizer.json, with [CLS] and [SEP];
• truncation and padding to 256 tokens;
• token_type_ids of zeros;
• the mean of last_hidden_state over the tokens of the attention mask;
• L2 normalization, in float32;
• batches of 32. A batch gives the same vectors as each text alone.

The tests compare 135 texts with Python: empty texts, accents, emoji, CJK and other scripts, symbols, control characters, texts of 254 to 1000 tokens, and short documents. The token ids are the same for all of them. On these texts, against chromadb 1.5.9 with onnxruntime 1.31.0, the largest difference of a component is 1.9e-7 on x64 and 2.4e-7 on arm64. The tests allow 1e-6: other texts can differ more than these. The five nearest neighbours of each text are the same. The reference of Python is in tests/ChromaDotNet.Embeddings.Tests/TestData.

The constants of ChromaDefaultEmbeddingGenerator are those of Python for a collection created without an embedding function:

• EmbeddingFunctionName is default, the name in the configuration of the collection;
• DefaultSpace is l2;
• Dimensions is 384, MaxTokens is 256.

GetService(typeof(EmbeddingGeneratorMetadata)) gives the provider chroma and the model all-MiniLM-L6-v2.

The computation runs on the CPU, in the calling thread. Concurrent calls run one after the other. ONNX Runtime logs errors only, so Linux without a GPU does not warn at every start that the discovery of the GPU devices failed. An application that creates OrtEnv before keeps its own level.

Built on

• Tokenizers.DotNet, by Taein Kim: the tokenizers library of Hugging Face, the one of Python. GitHub
• ElBruno.LocalEmbeddings, by Bruno Capuano, on .NET 8 and later. Its OnnxEmbeddingModel runs the model, with the mean pooling and the L2 normalization. GitHub
• ONNX Runtime, directly on .NET Framework, which ElBruno.LocalEmbeddings does not target. GitHub

Licenses

The code of this repository is under the MIT License. The model is downloaded, not distributed here: all-MiniLM-L6-v2 of sentence-transformers is under the Apache License 2.0. THIRD-PARTY-NOTICES.md lists the model and the packages that NuGet installs, with their licenses.

Build and tests

bash build/fetch-model.sh
dotnet test tests/ChromaDotNet.Embeddings.Tests

build/fetch-model.sh puts onnx.tar.gz in artifacts/model, checked against its hash. The tests of the download serve it from a local mirror; without it they are skipped. The tests run on .NET 8 and on .NET Framework 4.8, and put their caches in artifacts/test-tmp.

build/python-reference.py, with chromadb 1.5.9, computes the vectors of the test texts in Python again, and compares them with the reference. dotnet run -c Release --project tests/ChromaDotNet.Embeddings.Bench measures the speed.