> ## Documentation Index
> Fetch the complete documentation index at: https://docs.getmembrane.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Custom Pull

Custom Pull Strategy provides a customized approach for retrieving lists of created, updated, or deleted records.

This strategy consists of two methods: `subscribe` and `pull`.

## How It Works

1. When a subscription is created, a `state` is generated and stored. It allows pulling events starting from the subscription time.
2. When events are pulled, the `state` is used to generate events since the state was created, as well as the new state for the next pull.

## Configuration Example

```yaml theme={null}
# spec.yml
name: Collection Name
events:
  created:
    implementationType: custom-pull
    methods:
      subscribe:
        implementationType: javascript
        filePath: events/custom-subscribe.js
      pull:
        implementationType: javascript
        filePath: events/custom-pull.js
  updated:
    implementationType: custom-pull
    methods:
      subscribe:
        implementationType: javascript
        filePath: events/custom-subscribe.js
      pull:
        implementationType: javascript
        filePath: events/custom-pull.js
  deleted:
    implementationType: custom-pull
    methods:
      subscribe:
        implementationType: javascript
        filePath: events/custom-subscribe.js
      pull:
        implementationType: javascript
        filePath: events/custom-pull.js
```

## Functions

### subscribe

**File:** `<event-type>-subscribe.<ext>`, for example `created-subscribe.js`

The subscribe method initializes the subscription to events and retrieves a unique identifier for future pulls.

**Arguments:**

* `parameters` – object, parameters (matching `parametersSchema`) of this particular data collection.

**Result:**

* `state` – object, subscription state. This state will be saved and passed as `state` to the `pull` function.

**Example:**

```javascript theme={null}
// Example: Subscribe to delta changes and return the deltaLink.
export default async function subscribe({ apiClient, parameters }) {
  let cursor = false
  const path = `${parameters.path}/delta`

  while (true) {
    const response = await apiClient.get(`${cursor ? cursor : path}`)

    if (response.deltaLink) {
      break
    } else {
      cursor = response.nextPageLink
    }
  }

  return { state: { deltaLink: response.deltaLink } }
}
```

### pull

**File:** `<event-type>-pull.<ext>`, for example `created-pull.js`

The pull method fetches records based on the subscription and stores them in the state.\
It also generates events for each created, updated, or deleted record.

**Arguments:**

* `state` – object, state returned from `subscribe` function.
* `parameters` – object, parameters (matching `parametersSchema`) of this particular data collection.

**Result:**

* `events` – list of objects representing events with `type` and `record` fields.
* `state` – updated state information, including the new information for subsequent pulls.

**Example:**

```javascript theme={null}
// Example: Pull records using the deltaLink and generate events.
export default async function pull({ apiClient, state, parameters }) {
  const events = []
  let cursor = false

  while (true) {
    const response = await apiClient.get(`${cursor ? cursor : state?.deltaLink}`)

    response.value.forEach((record) => {
      events.push({
        type: 'created',
        record: record,
      })
    })

    if (response.deltaLink) {
      break
    } else {
      cursor = response.nextPageLink
    }
  }

  return {
    events,
    state: {
      deltaLink: response.deltaLink,
    },
  }
}
```