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unleash.unleash/website/docs/reference/stickiness.md
Thomas Heartman d5fbd0b743
refactor: move docs into new structure / fix links for SEO (#2416)
## What

This (admittedly massive) PR updates the "physical" documentation
structure and fixes url inconsistencies and SEO problems reported by
marketing. The main points are:

- remove or move directories : advanced, user_guide, deploy, api
- move the files contained within to the appropriate one of topics,
how-to, tutorials, or reference
- update internal doc links and product links to the content
- create client-side redirects for all the urls that have changed.

A number of the files have been renamed in small ways to better match
their url and to make them easier to find. Additionally, the top-level
api directory has been moved to /reference/api/legacy/unleash (see the
discussion points section for more on this).

## Why

When moving our doc structure to diataxis a while back, we left the
"physical' files lying where they were, because it didn't matter much to
the new structure. However, that did introduce some inconsistencies with
where you place docs and how we organize them.

There's also the discrepancies in whether urls us underscores or hyphens
(which isn't necessarily the same as their file name), which has been
annoying me for a while, but now has also been raised by marketing as an
issue in terms of SEO.

## Discussion points

The old, hand-written API docs have been moved from /api to
/reference/api/legacy/unleash. There _is_ a /reference/api/unleash
directory, but this is being populated by the OpenAPI plugin, and mixing
those could only cause trouble. However, I'm unsure about putting
/legacy/ in the title, because the API isn't legacy, the docs are. Maybe
we could use another path? Like /old-docs/ or something? I'd appreciate
some input on this.
2022-11-22 09:05:30 +00:00

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Markdown

---
title: Stickiness
---
_Stickiness_ is how Unleash guarantees that the same user gets the same features every time. Stickiness is useful in any scenario where you want to either show a feature to only a subset of users or give users a variant of a feature.
## Calculation
By default, Unleash calculates stickiness based on the user id and the group id. If the user id is unavailable, it falls back to using the session id instead. It hashes these values to a number between 0 and 100 using the [MurmurHash hash function](https://en.wikipedia.org/wiki/MurmurHash). This number is what determines whether a user will see a specific feature or variant. Because the process is deterministic, the same user will always get the same number.
If both the user id and the session id is unavailable, the calculation returns a random value and stickiness is not guaranteed.
## Consistency
Because the number assigned to a user won't change, Unleash also guarantees that the a user will keep seeing the same features even if certain other parameters change.
For instance: When using the [gradual rollout activation strategy](../reference/activation-strategies.md#gradual-rollout), any user whose number is less than or equal to the rollout percentage will see the feature. This means that the same users will keep seeing the feature even as you increase the percentage of your user base that sees the feature.
## Custom stickiness (beta) {#custom-stickiness}
:::info
Custom stickiness is available starting from Unleash Enterprise v4.
:::
When using [the gradual rollout strategy](../reference/activation-strategies.md#gradual-rollout) or [feature toggle variants](./feature-toggle-variants.md), you can use parameters other than the user id to calculate stickiness. More specifically, you can use any field, custom or otherwise, of the [Unleash Context](../reference/unleash-context.md) as long as you have enabled custom stickiness for these fields.
:::note
This feature is currently in beta and is not yet supported by all our SDKs. Check out the [SDK compatibility table](../reference/sdks/index.md#server-side-sdk-compatibility-table) to see what SDKs support it at the moment.
:::
### Enabling custom stickiness
To enable custom stickiness on a field, navigate to the Create Context screen in the UI and find the field you want to enable. There's a "Custom stickiness" option at the bottom of the form. Enable this toggle and update the context field by pressing the "Update" button.
![The Create Context screen in the Unleash UI. There's a toggle at the bottom to control custom stickiness.](/img/enable_custom_stickiness.png)