To connect your application to Unleash you need a [client SDK](#official-sdks) for your programming language and an [API token](../how-to/how-to-create-api-tokens). The SDK handles connecting to the Unleash server instance and retrieving feature flags based on your configuration. All versions of Unleash (OSS, Pro, and Enterprise) use the same client SDKs.
Unleash provides official client SDKs for a number of programming languages. Additionally, our community has developed and contributed SDKs for other languages. So if you can't find your favorite language in the list of official SDKs, check out the [list of clients written by our fantastic community](#community-sdks).
Server-side clients run on your server and communicate directly with your Unleash instance to evaluate your feature flags in their respective language. We provide these official clients:
Client-side SDKs can connect to [Unleash Edge](/reference/unleash-edge) or to the [Unleash front-end API](../front-end-api.md), but _not_ to the regular Unleash client API.
The below table shows what features the various server-side SDKs support. Note that certain features make sense only for some clients due to how the programming language works or due to how the client works.
If you see an item marked with a ❌ that you would find useful, feel free to reach out to us ([on Slack](https://slack.unleash.run/), for instance) with your use case. It may not be something we can prioritize right now, but if you'd like to contribute it back to the community, we'd love to help you build it.
Here's some of the fantastic work our community has done to make Unleash work in even more contexts. If you still can't find your favorite language, let us know and we'd love to help you create the client for it!
- [Unleash Client Specifications](https://github.com/Unleash/client-specification) - Used by all official SDKs to make sure they behave correctly across different language implementations. This lets us verify that a gradual rollout to 10% of the users would affect the same users regardless of which SDK you're using.
- [Client SDK overview](./client-specification) - A brief, overall guide of the _Unleash Architecture_ and important aspects of the SDK role in it all.
When the SDK is initialized in the application, an in-memory repository is set up and synchronized against the frontend API using the configured token and context. Note that the frontend API is hosted by either the Unleash Proxy/Edge or the upstream Unleash instance directly.
1. All feature flag evaluation is performed by the Proxy/Edge or Unleash instance. A payload of all enabled flags and their variants (if applicable) is returned as a single request. Disabled flags are not included.
2. When a page inside the application requests a feature flag, the SDK will return the flag state from memory. No network connection to the frontend API is performed.
Once they have been initialized, all Unleash clients will continue to work perfectly well without an internet connection or in the event that the Unleash Server has an outage.
Because the SDKs and the Unleash Proxy/Edge cache their feature flag states locally and only communicate with the Unleash server (in the case of the server-side SDKs and the Proxy) or the Proxy/Edge (in the case of front-end SDKs) at predetermined intervals, a broken connection only means that they won't get any new updates.
Unless the SDK supports [bootstrapping](#bootstrapping), it _will_ need to connect to Unleash at startup to get its initial feature flag data set. If the SDK doesn't have a feature flag data set available, all flags will fall back to evaluating as disabled or as the specified default value (in SDKs that support that).
By default, all SDKs reach out to the Unleash Server at startup to fetch their flag configuration. Additionally, some of the server-side SDKs and the Proxy (see the above [compatibility table](#server-side-sdk-compatibility-table)) also support _bootstrapping_, which allows them to get their flag configuration from a file, the environment, or other local resources. These SDKs can work without any network connection whatsoever.
