# Glossary

The terms the [KV Store](/en/documentation/platform/kv-store/) documentation uses in its own way, each defined once and linked to the page that carries it in full.

| Term                 | Definition                                                                                                                                                                                                                                                                                                                                         |
| -------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `Azion.KV`           | The runtime global that reaches KV Store from inside a [function](/en/documentation/platform/functions/). Its constructor is private, so the only way to a client is the awaited `Azion.KV.open`, and there is no import line to write. [KV client](/en/documentation/devtools/runtime/api-reference/kv-store/) documents every method it carries. |
| `Azion.KV.open`      | The static method that opens a namespace by name and returns the client for it. It is asynchronous, and it is the only entry point to the data. [KV client](/en/documentation/devtools/runtime/api-reference/kv-store/) carries its signature.                                                                                                     |
| `cacheTtl`           | The option a single-key `get` or `getWithMetadata` accepts to set how many seconds the result stays cached where the function that read it ran. [Best practices](/en/documentation/platform/kv-store/best-practices/) says when raising it helps and when it costs you.                                                                            |
| cold read            | A read whose result is not already cached where the function runs, so KV Store fetches the value before answering. A hot read is one the cache answers. A read of a key that does not exist is cached the same way, so `cacheTtl` governs it too.                                                                                                  |
| default namespace    | It does not exist: every client names the namespace it opens, and [KV client](/en/documentation/devtools/runtime/api-reference/kv-store/) shows the only form that opens one.                                                                                                                                                                      |
| eventual consistency | The consistency model KV Store applies. A write is readable where it was made before it is readable everywhere else, and concurrent writes to one key resolve to the last write. [How KV Store works](/en/documentation/platform/kv-store/how-it-works/) describes what a read can return in between.                                              |
| `expiration`         | The `put` option that expires a key at a Unix timestamp given in seconds. `expirationTtl` expresses the same expiry as a count of seconds instead. [KV client](/en/documentation/devtools/runtime/api-reference/kv-store/) documents both.                                                                                                         |
| `expirationTtl`      | The `put` option that expires a key after a number of seconds, counted rather than given as a point in time. `expiration` is the same expiry given as a timestamp. [KV client](/en/documentation/devtools/runtime/api-reference/kv-store/) documents both.                                                                                         |
| keys read            | The billing meter that counts every key read from KV Store. It is one of three, with keys written and storage, and [KV Store limits](/en/documentation/platform/kv-store/limits/) carries the amount included.                                                                                                                                     |
| keys written         | The billing meter that counts every key a write affects, including a `put` and a `delete`. [KV Store limits](/en/documentation/platform/kv-store/limits/) carries the amount included.                                                                                                                                                             |
| metadata             | A serializable value stored beside a key's value and set through the `put` option of the same name. `getWithMetadata` returns it next to the value, and it is `null` when the key carries none. [KV client](/en/documentation/devtools/runtime/api-reference/kv-store/) documents both calls.                                                      |
| multi-key get        | A `get` or `getWithMetadata` given an array of keys rather than one. It returns a plain object, accepts only the `text` and `json` return types, and duplicate keys in the array are removed. [KV client](/en/documentation/devtools/runtime/api-reference/kv-store/) documents the shape and what an absent key returns.                          |
| namespace            | An isolated key space, addressed by the name it was created with, which is also its identifier. A namespace is permanent: it cannot be renamed, deactivated, emptied, or deleted on any interface. [Namespaces](/en/documentation/platform/kv-store/namespaces/) documents its fields and the API that creates it.                                 |
| Preview              | The release stage KV Store ships at. Preview access is granted on request and subject to approval, so an account does not hold it until [technical support](/en/documentation/support/) enables it.                                                                                                                                                |
| `returnType`         | The second argument to `get` and `getWithMetadata`, which decides the form the value comes back in. It defaults to `text`, and [KV client](/en/documentation/devtools/runtime/api-reference/kv-store/) lists every value it accepts.                                                                                                               |
| storage              | The billing meter for the data your namespaces hold. It is one of three, with keys read and keys written, and [KV Store limits](/en/documentation/platform/kv-store/limits/) carries the amount included.                                                                                                                                          |