> For the complete documentation index, see [llms.txt](https://mercure-technologies.gitbook.io/expo-open-ota/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://mercure-technologies.gitbook.io/expo-open-ota/caching/overview.md).

# Overview

The server caches data to avoid repeated storage lookups and recomputations.

Set `CACHE_MODE=redis` to use Redis, or `CACHE_MODE=redis-sentinel` for a Sentinel-monitored Redis deployment. Any other value uses the in-memory cache.

### Cached data

The cache stores:

* **Update resolution** — the latest update ID for each app, platform, runtime version, and branch.
* **Computed manifests** — generated and signed manifests.
* **Dashboard responses** — app, branch, runtime version, and update listings.
* **Metrics working sets** — unique-user sets used by [Prometheus metrics](/expo-open-ota/monitoring/prometheus-and-grafana.md).

Active-user metrics use a four-hour sliding window. Error tracking uses a ten-minute window.

### Cache invalidation

Cache keys include the release version. A new server version invalidates previous entries automatically.

You do not need to flush the cache after an upgrade.

### Choose a cache backend

#### In-memory cache

The in-memory cache is the default. It requires no configuration.

It lives in the server process. A restart clears it. Replicas do not share it.

{% hint style="warning" %}
Use the in-memory cache only for a single server instance. With multiple replicas, each instance recomputes manifests and reports only its own metric users.
{% endhint %}

#### Redis

Use Redis for any multi-replica deployment. It shares cached data and metric working sets across replicas.

For high availability with automatic master failover, use `CACHE_MODE=redis-sentinel`.

See [Redis](/expo-open-ota/caching/redis.md) for connection, TLS, ACL, Sentinel, and key-prefix configuration.


---

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