In order to connect your application to Unleash you will need a client SDK (software developer kit) for your programming language and an [API token](../user_guide/api-token). The SDK will handle connecting to the Unleash server instance and retrieving feature toggles 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 language. Additionally, our community have 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).
For security and performance reasons, the front-end SDKs do not communicate directly with your Unleash instance. Instead, they go via the [Unleash Proxy](unleash-proxy.md).
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://join.slack.com/t/unleash-community/shared_invite/zt-8b6l1uut-LL67kLpIXm9bcN3~6RVaRQ), 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!
If you can't find an SDK that fits your need, you can also develop your own SDK. To make implementation easier, check out these resources:
- [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.
Once they have been initialised, 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 cache their feature toggle states locally and only communicate with the Unleash server (in the case of the server-side SDKs and the Proxy) or the Proxy (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 toggle data set. If the SDK doesn't have a feature toggle data set available, all toggles 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 toggle 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 toggle configuration from a file, the environment, or other local resources. These SDKs can work without any network connection whatsoever.
Bootstrapping is also supported by the following front-end client SDKs:
