# Redis semantic cache with redis-rb

```json metadata
{
  "title": "Redis semantic cache with redis-rb",
  "description": "Build a Redis-backed semantic cache for LLM responses in Ruby with redis-rb and informers",
  "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 Ruby with [`redis-rb`](https://redis.io/docs/latest/develop/clients/ruby) and the [`informers`](https://github.com/ankane/informers) gem, a Ruby port of Hugging Face transformers that runs the ONNX-exported [`sentence-transformers/all-MiniLM-L6-v2`](https://huggingface.co/sentence-transformers/all-MiniLM-L6-v2) encoder locally on CPU. It includes a local web server built with the standard-library [`WEBrick`](https://github.com/ruby/webrick) HTTP server 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 `distance_threshold`. 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 [`informers`](https://github.com/ankane/informers) running the ONNX-exported [`sentence-transformers/all-MiniLM-L6-v2`](https://huggingface.co/sentence-transformers/all-MiniLM-L6-v2) model — 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), the [Go example](https://redis.io/docs/latest/develop/use-cases/semantic-cache/go), and the [Jedis example](https://redis.io/docs/latest/develop/use-cases/semantic-cache/java-jedis) use. Embeddings produced by the Ruby ONNX path are semantically equivalent to the PyTorch ones — paraphrase distances differ by ~0.01, the same drift the Node.js Xenova ONNX path sees — 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.encode_one(prompt)` to turn the incoming text into a 384-element `Array<Float>`.
2. `cache.lookup(query_vec, tenant:, locale:, model_version:)` 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` 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), 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` 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:, model_version:)`. 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/ruby/lib/cache.rb)):

```ruby
require 'redis'
require_relative 'lib/cache'
require_relative 'lib/embeddings'

client = Redis.new(host: 'localhost', port: 6379)

cache = SemCache::RedisSemanticCache.new(
  redis_client: client,
  index_name: 'semcache:idx',
  distance_threshold: 0.5,    # cosine distance, lower = stricter
  default_ttl_seconds: 3600   # one hour
)
embedder = SemCache::LocalEmbedder.new   # sentence-transformers/all-MiniLM-L6-v2

# One-time index setup (idempotent).
cache.create_index

# 1) Embed the prompt.
prompt = 'How do I return an item?'
query_vec = embedder.encode_one(prompt)

# 2) Look up under a metadata scope. The TAG filter and the KNN
#    travel together in one FT.SEARCH.
result = cache.lookup(
  query_vec,
  tenant: 'acme',
  locale: 'en',
  model_version: 'gpt-4.5-2026'
)

if result.is_a?(SemCache::CacheHit)
  puts "hit (#{format('%.3f', result.distance)}): #{result.response}"
else
  # 3a) Miss — call the LLM. (Use your real client here.)
  response = call_llm(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: query_vec,
    tenant: 'acme',
    locale: 'en',
    model_version: 'gpt-4.5-2026'
  )
end
```

### 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 `Array<Float>` with Ruby's [`Array#pack`](https://docs.ruby-lang.org/en/master/packed_data_rdoc.html) directive `'e*'`, which is little-endian single-precision float; the resulting `String` is ASCII-8BIT (binary) so `redis-rb` ships the exact bytes without any UTF-8 transcoding.

```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. `redis-rb` doesn't ship dedicated `FT.*` bindings — it exposes a generic `#call` method that lets you send any command, so the helper builds the argument list directly:

```ruby
args = [
  'FT.SEARCH', 'semcache:idx',
  '(@tenant:{acme} @locale:{en} @model_version:{gpt\-4\.5\-2026} @safety:{ok})' \
    '=>[KNN 1 @embedding $vec AS distance]',
  'PARAMS', '2', 'vec', query_vec.pack('e*'),
  'SORTBY', 'distance', 'ASC',
  'RETURN', '7',
  'prompt', 'response', 'tenant', 'locale',
  'model_version', 'hit_count', 'distance',
  'LIMIT', '0', '1',
  'DIALECT', '2'
]
reply = client.call(*args)
```

`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.

`FT.SEARCH` returns a flat array — `[total, key1, [field1, value1, ...], key2, [...], ...]` — that the helper parses into per-document hashes; binary fields like `embedding` and the encoded vector come back as ASCII-8BIT Strings so a future tooling change that wants the raw bytes doesn't have to re-encode.

## The mock LLM

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

```ruby
require_relative 'lib/mock_llm'

llm = SemCache::MockLLM.new(latency_ms: 1500.0)
response = llm.complete('What is your return policy?')
# response.response       — the templated answer text
# response.latency_ms     — wall-clock time the call took
# response.total_tokens   — 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 — `openai`, `anthropic`, an internal 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, `seed_cache.rb` 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/ruby/lib/seed_cache.rb)):

```ruby
cache.create_index
SemCache::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 seed phase batches the encoder calls into a single `encode_many` so the model dispatch is paid once across the whole seed list.

## The interactive demo

`demo_server.rb` runs a [`WEBrick`](https://github.com/ruby/webrick) HTTP server. 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 Jedis demos and is loaded from `index.html` next to `demo_server.rb`. 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/ruby
    ```

2.  Make sure you have Ruby 3.2 or newer and [Bundler](https://bundler.io/), then install the gems:

    ```bash
    bundle install
    ```

    `informers` pulls in [`onnxruntime`](https://rubygems.org/gems/onnxruntime) (which ships the ONNX Runtime shared library as a native extension) and [`tokenizers`](https://rubygems.org/gems/tokenizers) (a Hugging Face fast-tokenizer Rust binding). Both come as pre-built binary gems for `arm64-darwin`, `x86_64-darwin`, `aarch64-linux`, and `x86_64-linux`, so there is no system ONNX install step on those platforms.

3.  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.

4.  Start the demo server. The first run downloads the ONNX-exported
    `sentence-transformers/all-MiniLM-L6-v2` model into the local Hugging Face
    cache (~22 MB):

    ```bash
    bundle exec ruby demo_server.rb
    ```

5.  Open <http://localhost:8094> 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 at the default threshold 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.js, Go, and Jedis 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.