> For the complete documentation index, see [llms.txt](https://www.serverless-cqrs.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://www.serverless-cqrs.com/components/write-model/repositorybuilder.md).

# repositoryBuilder

Takes a reducer and an adapter and returns a repository. A repository is the layer of our Onion Architecture (see [Introduction](/master.md#event-sourcing-es)) which handles data persistence.

The `write-model` repository has a single method called `getById`. When invoked, it uses the adapter to load all events for given ID and runs them through the reducer to calculate the current state. It returns an object that contains `state` and also a `save` method, which is used to append new events.

## Methods

### build

`build({ adapter, reducer })` 

builds a repository 

#### Parameters

| attribute | type       | description                                                                                      |
| --------- | ---------- | ------------------------------------------------------------------------------------------------ |
| `adapter` | `object`   | Any object which implements the write-model [Adapter Interface](/advanced/repository.md).        |
| `reducer` | `function` | a function which, given a list of events, knows how to calculate the current state of an object. |

#### Returns

`{ getById }` - an object with the `getById` function.

### getById

`getById(id)` 

Loads and returns the current state of an entity. Also returns a `save` function for appending new events to the data store.

#### Parameters

| attribute | type     | description          |
| --------- | -------- | -------------------- |
| `id`      | `string` | the id of the entity |

#### Returns

`{ state, save }` - an object containing an arbitrary `state` object, and a `save` function.

### save

`save(events)` 

Appends new events to the datastore

#### Parameters

| attribute | type    | description                         |
| --------- | ------- | ----------------------------------- |
| `events`  | `array` | an array of arbitrary event objects |

#### Returns

`null`