# Redis semantic cache with NRedisStack

```json metadata
{
  "title": "Redis semantic cache with NRedisStack",
  "description": "Build a Redis-backed semantic cache for LLM responses in C# with NRedisStack and ONNX Runtime",
  "categories": ["docs","develop","stack","oss","rs","rc"],
  "tableOfContents": {"sections":[{"id":"overview","title":"Overview"},{"children":[{"id":"hit-path-the-goal","title":"Hit path (the goal)"},{"id":"miss-path","title":"Miss path"}],"id":"how-it-works","title":"How it works"},{"children":[{"id":"data-model","title":"Data model"},{"id":"the-query","title":"The query"}],"id":"the-cache-helper","title":"The cache helper"},{"id":"the-mock-llm","title":"The mock LLM"},{"id":"pre-seeding-the-cache","title":"Pre-seeding the cache"},{"id":"the-interactive-demo","title":"The interactive demo"},{"id":"run-the-demo-locally","title":"Run the demo locally"}]}

,
  "codeExamples": []
}
```
This guide shows you how to build a small Redis-backed semantic cache for LLM responses in C# / .NET with [`NRedisStack`](https://redis.io/docs/latest/develop/clients/dotnet/nredisstack) and [ONNX Runtime](https://onnxruntime.ai/) running the [`sentence-transformers/all-MiniLM-L6-v2`](https://huggingface.co/sentence-transformers/all-MiniLM-L6-v2) encoder locally. It includes a local web server built with the BCL's standard `System.Net.HttpListener` so you can send paraphrased prompts at a mock LLM, watch the cache decide hit or miss, sweep the cosine-distance threshold, and see the cumulative latency and token savings build up.

## Overview

Each cache entry is stored as a single Redis [Hash](https://redis.io/docs/latest/develop/data-types/hashes) at `cache:<id>`. The hash holds the original prompt, the LLM's response, the raw `float32` bytes of a 384-dimensional embedding of the prompt, and metadata fields — tenant, locale, model version, safety flag — plus a `created_ts` and a `hit_count`. A single [Redis Search](https://redis.io/docs/latest/develop/ai/search-and-query) index covers the embedding field and every metadata field, so one [`FT.SEARCH`](https://redis.io/docs/latest/commands/ft.search) call with a `KNN` clause does the vector lookup *and* the TAG pre-filter in the same round trip — no cross-store joins.

The lookup is thresholded: [`FT.SEARCH`](https://redis.io/docs/latest/commands/ft.search) always returns the nearest entry that satisfies the filters, but the application only serves it as a hit when the reported cosine distance is at or below `DistanceThreshold`. Anything further away is treated as a miss; the caller runs the LLM and writes the new prompt, response, and embedding back to the same key pattern with a TTL.

The embedder is [ONNX Runtime](https://onnxruntime.ai/) loading the ONNX export of [`sentence-transformers/all-MiniLM-L6-v2`](https://huggingface.co/sentence-transformers/all-MiniLM-L6-v2) (mirrored at [`Xenova/all-MiniLM-L6-v2`](https://huggingface.co/Xenova/all-MiniLM-L6-v2) on Hugging Face) paired with the `BertTokenizer` from [`Microsoft.ML.Tokenizers`](https://www.nuget.org/packages/Microsoft.ML.Tokenizers). This is the same 384-dimensional encoder the [Python example](https://redis.io/docs/latest/develop/use-cases/semantic-cache/redis-py), the [Node.js example](https://redis.io/docs/latest/develop/use-cases/semantic-cache/nodejs), and the [Java/Jedis example](https://redis.io/docs/latest/develop/use-cases/semantic-cache/java-jedis) use. Embeddings produced by the implementations are semantically equivalent — paraphrase distances differ only at the second or third decimal place — so a cache populated by one demo can be queried by another against the same Redis instance.

That gives you:

* A single round trip for lookup — vector KNN + metadata pre-filter in one [`FT.SEARCH`](https://redis.io/docs/latest/commands/ft.search).
* Tens of milliseconds on a hit vs. a multi-second LLM call on a miss; the embedding step is the bottleneck either way, and that's a model-side cost, not a Redis one.
* Tenant, locale, and model-version isolation enforced inside the query, not in application code — a write under one tenant cannot be served to another.
* Bounded memory: every entry has an [`EXPIRE`](https://redis.io/docs/latest/commands/expire) TTL, and a database-level [eviction policy](https://redis.io/docs/latest/develop/reference/eviction) (LRU / LFU) caps the cache size under pressure.

## How it works

A query goes through three stages: **embed**, **lookup**, and (on a miss) **call the LLM and write back**.

### Hit path (the goal)

1. The application calls `embedder.EncodeOne(prompt)` to turn the incoming text into a 384-element `float[]`.
2. `cache.Lookup(queryVec, tenant, locale, modelVersion, "ok", threshold)` runs [`FT.SEARCH`](https://redis.io/docs/latest/commands/ft.search) with a TAG pre-filter and a `KNN 1` clause. Redis returns the closest cached prompt that satisfies the filters along with its cosine distance.
3. If the distance is at or below the threshold, the cache returns a `CacheHit` record containing the cached response. The helper also runs an [`HINCRBY`](https://redis.io/docs/latest/commands/hincrby) on `hit_count` and an [`EXPIRE`](https://redis.io/docs/latest/commands/expire) refresh inside a [`MULTI/EXEC`](https://redis.io/docs/latest/commands/multi) (built with `IDatabase.CreateTransaction()`), so a frequently used answer keeps its TTL and the demo UI can see which entries are load-bearing.
4. The LLM is not called at all. The application returns the cached response to the user.

### Miss path

When the distance is above the threshold — or there is no candidate in scope at all — the helper returns a `CacheMiss` record instead, carrying the distance of the nearest candidate (if any) for logging. The application then:

1. Calls the LLM with the prompt.
2. Calls `cache.Put(prompt, response, embedding, tenant, locale, modelVersion, ...)`. The same embedding the lookup used is reused — no re-encode. The helper writes the Hash with [`HSET`](https://redis.io/docs/latest/commands/hset) and an [`EXPIRE`](https://redis.io/docs/latest/commands/expire) TTL inside a single [`MULTI/EXEC`](https://redis.io/docs/latest/commands/multi) so the entry never lands without a TTL on a partial failure.
3. Returns the LLM's response to the user. The next semantically similar prompt under the same metadata scope will be a hit.

## The cache helper

The `RedisSemanticCache` class wraps the Redis Search index and the lookup / write flow
([source](https://github.com/redis/docs/blob/main/content/develop/use-cases/semantic-cache/dotnet/RedisSemanticCache.cs)):

```csharp
using NRedisStack.RedisStackCommands;
using StackExchange.Redis;
using SemanticCacheDemo;

var mux = ConnectionMultiplexer.Connect("localhost:6379");
var db = mux.GetDatabase();

var embedder = await LocalEmbedder.CreateAsync();   // sentence-transformers/all-MiniLM-L6-v2

var cache = new RedisSemanticCache(
    db,
    indexName: "semcache:idx",
    keyPrefix: "cache:",
    vectorDim: 384,
    distanceThreshold: 0.5,    // cosine distance, lower = stricter
    defaultTtlSeconds: 3600);

// One-time index setup (idempotent).
cache.CreateIndex();

// 1) Embed the prompt.
string prompt = "How do I return an item?";
float[] queryVec = embedder.EncodeOne(prompt);

// 2) Look up under a metadata scope. The TAG filter and the KNN
//    travel together in one FT.SEARCH.
var result = cache.Lookup(
    queryVec, tenant: "acme", locale: "en",
    modelVersion: "gpt-4.5-2026");

string response;
if (result is CacheHit hit)
{
    response = hit.Response;
    Console.WriteLine($"hit ({hit.Distance:F3}): {response}");
}
else
{
    // 3a) Miss — call the LLM. (Use your real client here.)
    response = CallLlm(prompt);

    // 3b) Cache the new entry. Reuses the same embedding bytes the
    //     lookup used, so we don't pay the encoder twice.
    cache.Put(
        prompt: prompt,
        response: response,
        embedding: queryVec,
        tenant: "acme",
        locale: "en",
        modelVersion: "gpt-4.5-2026");
}
```

### Data model

Each cache entry is one Redis Hash. The vector field is raw little-endian `float32` bytes — no JSON wrapping — because the Redis Search vector encoding expects exactly that. The helper packs the `float[]` with `BinaryPrimitives.WriteSingleLittleEndian` so the byte order is pinned to little-endian regardless of host architecture; every supported .NET runtime today is little-endian, and the explicit encoding matches the bytes the Python, Node, Go, and Java ports write.

```text
cache:7c3f8a1b9e02
  prompt=How do I return an item?
  response=You can return any unworn item within 30 days...
  tenant=acme
  locale=en
  model_version=gpt-4.5-2026
  safety=ok
  created_ts=1715990400.123
  hit_count=4
  embedding=<384 × float32 little-endian bytes>
```

The Redis Search index schema treats every field as queryable in its natural type:

```text
FT.CREATE semcache:idx
  ON HASH PREFIX 1 cache:
  SCHEMA
    prompt         TEXT
    response       TEXT
    tenant         TAG
    locale         TAG
    model_version  TAG
    safety         TAG
    created_ts     NUMERIC SORTABLE
    hit_count      NUMERIC SORTABLE
    embedding      VECTOR HNSW 6 TYPE FLOAT32 DIM 384 DISTANCE_METRIC COSINE
```

### The query

The lookup is a hybrid query: a TAG pre-filter expression in parentheses, then `=>[KNN 1 @embedding $vec]`. With `DIALECT 2`, Redis applies the filter first and KNN-ranks only the matching documents. In NRedisStack:

```csharp
// `.` and `-` (and other punctuation) are TAG-value syntax in Redis
// Search and must be backslash-escaped — `gpt-4.5-2026` raw would
// be parsed as three tokens. The helper's EscapeTagValue does this
// for every TAG value; the literal below shows what the parser
// actually sees on the wire.
var query = new Query(
        "(@tenant:{acme} @locale:{en} @model_version:{gpt\\-4\\.5\\-2026} @safety:{ok})"
        + "=>[KNN 1 @embedding $vec AS distance]")
    .ReturnFields(
        "prompt", "response", "tenant", "locale",
        "model_version", "hit_count", "distance")
    .SetSortBy("distance", ascending: true)
    .Limit(0, 1)
    .AddParam("vec", LocalEmbedder.ToBytes(queryVec))
    .Dialect(2);

var result = db.FT().Search("semcache:idx", query);
```

`distance` is the cosine *distance* (0 means identical, 2 means opposite). The result is sorted ascending, so the top row is the closest candidate. The application inspects `distance` against the threshold and decides hit or miss in user code — Redis returns the row either way, and treating it as a hit or a miss is a policy decision the cache helper owns, not a server-side filter.

## The mock LLM

To make the latency and token savings visible without requiring an API key, `MockLLM.cs` provides a deterministic stand-in
([source](https://github.com/redis/docs/blob/main/content/develop/use-cases/semantic-cache/dotnet/MockLLM.cs)):

```csharp
using SemanticCacheDemo;

var llm = new MockLLM(modelVersion: "gpt-4.5-2026", latencyMs: 1500.0);
var response = llm.Complete("What is your return policy?");
// response.Text         — the templated answer text
// response.LatencyMs    — wall-clock time the call took
// response.TotalTokens  — estimated prompt + completion tokens
```

The mock sleeps for the configured latency, then keyword-matches against a small FAQ table to produce an answer. The deliberate slowness is what makes a hit visibly cheaper than a miss in the demo. In production code, you would replace `MockLLM` with your real client of choice — an HTTP call to OpenAI, Anthropic, an Azure OpenAI deployment, a self-hosted vLLM endpoint, anything — without changing the cache helper.

## Pre-seeding the cache

In a real deployment the cache fills up organically: a first-time question is a miss, the LLM answers, and the response is written back. For the demo, `SeedCache.cs` pre-loads a small set of canonical FAQ prompts so the very first query lands on a hit
([source](https://github.com/redis/docs/blob/main/content/develop/use-cases/semantic-cache/dotnet/SeedCache.cs)):

```csharp
cache.CreateIndex();
SeedCache.Seed(cache, embedder, tenant: "acme", locale: "en");
```

The seed list stores the canonical phrasing of each question ("What is your return policy?"). Paraphrases of any of these prompts ("How do I return an item?", "Can I get a refund?") embed close to the canonical entry, so the cache lookup serves the stored response without ever calling the model.

## The interactive demo

`Program.cs` runs an HTTP server built on the BCL's `System.Net.HttpListener` — no ASP.NET Core, no Kestrel, no Minimal API. The HTML page lets you:

* Type a prompt and toggle metadata: tenant, locale, model version. Each combination is a separate cache namespace inside the same index.
* Slide the cosine-distance threshold and see hits flip to misses (and back) on the same prompt, with the actual distance reported on each query.
* Submit with **Ask** to run the full hit-or-miss path (calls the LLM on a miss, writes the answer back). Submit with **Lookup only (no LLM)** to sweep the threshold against a fixed prompt without polluting the cache.
* Watch the cumulative panel build up: total queries, cache hits, cache misses, hit ratio, tokens not spent, LLM milliseconds not waited.
* Inspect every cached entry, including remaining TTL and total hit count, and drop individual entries to simulate eviction.

The server holds one `LocalEmbedder`, one `RedisSemanticCache`, and one `MockLLM` for the lifetime of the process. The HTML page is shared with the Python, Node.js, Go, and Java demos; `index.html` ships next to the binary via a `<None CopyToOutputDirectory>` entry in the `.csproj`, so `dotnet bin/Release/net8.0/SemanticCacheDemo.dll` works from any working directory. Endpoints:

| Endpoint        | What it does                                                                  |
|-----------------|-------------------------------------------------------------------------------|
| `GET  /state`   | Index info and the full list of cached entries.                               |
| `POST /query`   | Embed the prompt, run `FT.SEARCH`, on miss call the LLM and write back.       |
| `POST /reset`   | Drop every cached entry and re-seed from the FAQ list.                        |
| `POST /drop`    | Delete a single cached entry by id.                                           |

## Run the demo locally

1.  Clone the [`redis/docs`](https://github.com/redis/docs) repository and change into the example
    directory:

    ```bash
    git clone https://github.com/redis/docs.git
    cd docs/content/develop/use-cases/semantic-cache/dotnet
    ```

2.  Make sure a Redis instance with the Redis Search module is running locally on
    port 6379. [Redis Stack](https://redis.io/docs/latest/operate/oss_and_stack/install/install-stack) or
    [Redis 8 with Search](https://redis.io/docs/latest/develop/ai/search-and-query) both work.

3.  Build the project. This pulls `NRedisStack`, `Microsoft.ML.OnnxRuntime`, and
    `Microsoft.ML.Tokenizers` from NuGet. The first build takes a couple of minutes:

    ```bash
    dotnet build -c Release
    ```

4.  Run the demo. The first run downloads the ONNX export of
    `sentence-transformers/all-MiniLM-L6-v2` (~90 MB) and the matching BERT
    `vocab.txt` into a local `model_cache/` directory next to the binary; every
    subsequent run is offline:

    ```bash
    dotnet run -c Release
    ```

    Or run the built binary directly:

    ```bash
    dotnet bin/Release/net8.0/SemanticCacheDemo.dll
    ```

5.  Open <http://localhost:8092> and try some queries:

    * **"What is your return policy?"** — exact match against the seed, distance ≈ 0,
      hit at any threshold.
    * **"How fast is delivery?"** — paraphrase of the shipping seed; distance
      around 0.30, hit at the default threshold of 0.5.
    * **"How do I return an item?"** — slightly looser paraphrase of the returns
      seed; distance around 0.49, still a hit at the default threshold. Slide
      the threshold down to 0.4 to see this one flip to a miss.
    * **"What payment methods do you accept?"** — unrelated to anything in the
      seed; distance > 0.6, so you'll see a miss, the mock LLM kicks in for
      ~1.5 s, the new answer is cached, and a follow-up of the same question
      is now an immediate hit.
    * Switch the **Tenant** dropdown to `globex` or `initech` and re-ask any
      seeded question — the result flips to a miss because the cache entries
      live under `acme`. That's the metadata pre-filter at work inside `FT.SEARCH`.

The server is read/write against your local Redis. The default index name is `semcache:idx` and entry keys live under `cache:`. Flags mirror the Python, Node, Go, and Java demos: `--no-reset` to keep an existing cache across restarts, `--threshold` to change the default cosine-distance cutoff, `--llm-latency-ms` to make the mock LLM faster or slower for the demo, or `--port` to listen on a different port.