It is powerful to be able to turn a feature on and off instantaneously, without redeploying the application. Activation strategies let you enable a feature only for a specified audience. Different strategies use different parameters. Predefined strategies are bundled with Unleash. The recommended strategy is the gradual rollout strategy with 100% rollout, which basically means that the feature should be enabled to everyone.
Unleash comes with a number of built-in strategies (described below) that can be enhanced with [constraints](https://docs.getunleash.io/reference/strategy-constraints) for fine-grained control. For more advanced use cases, where constraints do not fulfill your needs, you can add your own [custom activation strategies](../reference/custom-activation-strategies.md). However, while activation strategies are _defined_ on the server, the server does not _implement_ the strategies. Instead, activation strategy implementation is done client-side. This means that it is _the client_ that decides whether a feature should be enabled or not.
All [server-side client SDKs](../reference/sdks/index.md#server-side-sdks) and the [Unleash Proxy](../generated/unleash-proxy.md) implement the default strategies (and allow you to add your own [custom strategy implementations](../reference/custom-activation-strategies.md#implementation)). The [front-end client SDKs](../reference/sdks/index.md#front-end-sdks) do not do the evaluation themselves, instead relying on the [Unleash Proxy](../generated/unleash-proxy.md) to take care of the implementation and evaluation.
Some activation strategies require the client to provide the current [Unleash context](unleash-context.md) to the flag evaluation function for the evaluation to be done correctly.
Active for users with a `userId` defined in the `userIds` list. A typical use case is to enable a feature for a few specific devs or key persons before enabling the feature for everyone else. This strategy allows you to specify a list of user IDs that you want to expose the new feature for. (A user id may, of course, be an email if that is more appropriate in your system.)
A flexible rollout strategy which combines all gradual rollout strategies in to a single strategy. This strategy allows you to customize what parameter should be sticky, and defaults to userId or sessionId.
- **stickiness** is used to define how we guarantee consistency for a gradual rollout. The same userId and the same rollout percentage should give predictable results. Configuration that should be supported:
- **groupId** is used to ensure that different flags will **hash differently** for the same user. The groupId defaults to _feature flag name_, but the user can override it to _correlate rollout_ of multiple feature flags.
- **rollout** The percentage (0-100) you want to enable the feature flag for.
Custom stickiness is supported by all of our SDKs except for the Rust SDK. You can always refer to the [SDK compatibility table](../reference/sdks/index.md#server-side-sdk-compatibility-table) for the full overview.
By enabling the stickiness option on a custom context field you can use the custom context field to group users with the gradual rollout strategy. This will guarantee a consistent behavior for specific values of this context field.
The remote address strategy activates a feature flag for remote addresses defined in the IP list. We occasionally use this strategy to enable a feature only for IPs in our office network.
You can apply as many activation strategies to a flag as you want. When a flag has multiple strategies, Unleash will check each strategy in isolation. If any one of the strategies would enable the flag for the current user, then the flag is enabled.
As an example, consider a case where you want to roll a feature out to 75% of your users. However, you also want to make sure that you and your product lead get access to the feature. To achieve this, you would apply a **gradual rollout** strategy and set it to 75%. Additionally, you would add a **user IDs** strategy and add `engineer@mycompany.com` and `productlead@mycompany.com`.
The `gradualRolloutUserId` strategy gradually activates a feature flag for logged-in users. Stickiness is based on the user ID. The strategy guarantees that the same user gets the same experience every time across devices. It also assures that a user which is among the first 10% will also be among the first 20% of the users. That way, we ensure the users get the same experience, even if we gradually increase the number of users exposed to a particular feature. To achieve this, we hash the user ID and normalize the hash value to a number between 1 and 100 with a simple modulo operator.
Starting from v3.x all clients should use the 32-bit [MurmurHash3](https://en.wikipedia.org/wiki/MurmurHash) algorithm to normalize values. ([issue 247](https://github.com/Unleash/unleash/issues/247))
Similar to `gradualRolloutUserId` strategy, this strategy gradually activates a feature flag, with the exception being that the stickiness is based on the session IDs. This makes it possible to target all users (not just logged-in users), guaranteeing that a user will get the same experience within a session.
The `gradualRolloutRandom` strategy randomly activates a feature flag and has no stickiness. We have found this rollout strategy very useful in some scenarios, especially when we enable a feature which is not visible to the user. It is also the strategy we use to sample metrics and error reports.
