# External Storage - TypeScript SDK

> Offload large payloads to external storage using the claim check pattern in the TypeScript SDK.

> **Public Preview**
> APIs and configuration may change before General Availability. Join the
> [#large-payloads Slack channel](https://temporalio.slack.com/archives/C09VA2DE15Y) to provide feedback or ask for
> help.

The Temporal Service enforces a 2 MB per-payload limit by default. This limit is configurable on self-hosted
deployments. When your Workflows or Activities handle data larger than the limit, you can offload payloads to external
storage, such as Amazon S3, and pass a small reference token through the Event History instead. This page shows you how
to set up External Storage with Amazon S3 or Google Cloud Storage, and how to implement a custom storage driver.

For a conceptual overview of External Storage and its use cases, see [External Storage](/external-storage).

## Store and retrieve large payloads with Amazon S3 or Google Cloud Storage 

The TypeScript SDK includes storage drivers for Amazon S3 and Google Cloud Storage. Select your storage backend in the
tabs that follow. Only the driver setup differs between the two. Everything after that is the same.

### Prerequisites

- A bucket that you have read and write access to. Refer to
  [lifecycle management](/external-storage#lifecycle) to ensure that your payloads remain available for the entire
  lifetime of the Workflow. For multi-region durability, see
  [Durable External Storage](/external-storage#durable-external-storage).
- Credentials with permission to write objects on components that store payloads, and to read objects on components
  that retrieve them. Components that only retrieve payloads do not need write access, and the reverse is also true.
- Install the driver, the client adapter for your cloud provider's SDK, and that SDK:

**Amazon S3**

  ```sh
  npm install @temporalio/external-storage-s3 \
    @temporalio/external-storage-s3-aws-sdk \
    @aws-sdk/client-s3
  ```

**Google Cloud Storage**

  ```sh
  npm install @temporalio/external-storage-gcs \
    @temporalio/external-storage-gcs-google-sdk \
    @google-cloud/storage
  ```

### Procedure

1. Create a storage client, wrap it in the matching driver client, and pass the result to the driver. Each cloud SDK
   picks up your standard credentials from the environment:

**Amazon S3**

   The AWS SDK reads environment variables, an IAM role, or your AWS config file.

   <!--SNIPSTART typescript-s3-driver-create-->
   ```ts
   import { S3Client } from '@aws-sdk/client-s3';
   import { S3StorageDriver } from '@temporalio/external-storage-s3';
   import { AwsSdkS3StorageDriverClient } from '@temporalio/external-storage-s3-aws-sdk';

   const s3Client = new S3Client({ region: 'us-east-2' });

   const driver = new S3StorageDriver({
     client: new AwsSdkS3StorageDriverClient(s3Client),
     bucket: 'my-temporal-payloads',
   });
   ```
   <!--SNIPEND-->

**Google Cloud Storage**

   The Google Cloud SDK reads Application Default Credentials.

   <!--SNIPSTART typescript-gcs-driver-create-->
   ```ts
   import { Storage } from '@google-cloud/storage';
   import { GcsStorageDriver } from '@temporalio/external-storage-gcs';
   import { GoogleCloudGcsStorageDriverClient } from '@temporalio/external-storage-gcs-google-sdk';

   const storage = new Storage();

   const driver = new GcsStorageDriver({
     client: new GoogleCloudGcsStorageDriverClient(storage),
     bucket: 'my-temporal-payloads',
   });
   ```
   <!--SNIPEND-->

   To route payloads to different buckets at runtime, pass a function as `bucket` instead of a string. The function
   receives the store context and the payload, and returns a bucket name.

2. Build an `ExternalStorage` instance from the driver, set it on your Data Converter, and pass the converter to your
   Client and Worker. This step is the same for every driver. External Storage runs outside the Workflow sandbox, so
   you can pass the driver object directly rather than referencing it by path the way a custom Payload Converter
   requires:

   <!--SNIPSTART typescript-s3-external-storage-setup-->
   ```ts
   import { Client, Connection } from '@temporalio/client';
   import { ExternalStorage } from '@temporalio/common';
   import { Worker } from '@temporalio/worker';

   async function createClientAndWorker() {
     const dataConverter = {
       externalStorage: new ExternalStorage({ drivers: [driver] }),
     };

     const connection = await Connection.connect();
     const client = new Client({ connection, dataConverter });

     const worker = await Worker.create({
       workflowsPath: require.resolve('./workflows'),
       taskQueue: 'my-task-queue',
       dataConverter,
     });
   }
   ```
   <!--SNIPEND-->

By default, payloads of 256 KiB or larger are offloaded to external storage. You can adjust this with the
`payloadSizeThreshold` option, even setting it to `0` to externalize all payloads regardless of size. Refer to
[Configure payload size threshold](#configure-payload-size-threshold) for more information.

All Workflows and Activities running on the Worker use the storage driver automatically without changes to your business
logic. Both drivers upload and download payloads concurrently, address objects by a SHA-256 hash of their contents so
identical payloads are stored once, and verify that hash on retrieve. Each rejects any single payload larger than
`maxPayloadSize`, which defaults to 50 MiB, and includes diagnostic metadata in error messages to help troubleshoot
storage failures.

## Implement a custom storage driver

If you need a storage backend other than what the built-in drivers allow, you can implement your own storage driver.
Refer to [Choose a storage system](/external-storage#choose-storage) for guidance on selecting a backing store and
[Lifecycle management](/external-storage#lifecycle) for retention requirements.

A shared filesystem works for local development and for Workers that mount the same volume. For anything else, use a
storage system that every Client and Worker can reach.

The following sections walk through the key parts of a custom driver.

### 1. Implement the StorageDriver interface

A custom driver implements the `StorageDriver` interface, which has two readonly properties and two methods:

- `name` is a unique string that identifies the driver instance. The SDK stores this name in the claim check reference
  so it can route retrieval requests to the correct driver. Changing the name after payloads have been stored breaks
  retrieval. For example, two S3 drivers could be named `"s3-primary"` and `"s3-archive"`.
- `type` is a string that identifies the driver implementation, and the Worker reports it in its heartbeat. Unlike
  `name`, `type` must be the same across all instances of the same driver type regardless of configuration. Two S3
  drivers named `"s3-primary"` and `"s3-archive"` would both report `"aws.s3driver"` as their type, while the built-in
  GCS driver reports `"gcp.gcsdriver"`.
- `store()` receives an array of payloads and returns one `StorageDriverClaim` per payload. A claim wraps a set of
  string key-value pairs that the driver uses to locate the payload later.
- `retrieve()` receives the claims that `store()` produced and returns the original payloads.

### 2. Store payloads

In `store()`, serialize each Payload protobuf message to bytes and write the bytes to your storage system. The
application data has already been serialized by the
[Payload Converter](/develop/typescript/best-practices/data-handling/data-conversion) and
[Payload Codec](/develop/typescript/best-practices/data-handling/data-encryption) before it reaches the driver. See the
[data conversion pipeline](/external-storage#data-pipeline) for more details.

Return a `StorageDriverClaim` for each payload with enough information to retrieve it later. The `context.target`
provides identity information and is a discriminated union: check the `kind` property to distinguish
`"workflow"` from `"activity"`, then read `namespace`, `id`, `runId`, and `type`. Consider structuring your storage keys
to include this information so that you can identify which Workflow owns each payload. Within that scope,
content-addressable keys, such as a SHA-256 hash of the payload bytes, can help deduplicate identical payloads. The
built-in S3 and GCS drivers use this approach.

### 3. Retrieve payloads

In `retrieve()`, download the bytes using the claim data, then reconstruct the Payload protobuf message. The Payload
Converter handles deserializing the application data after the driver returns the payload.

### 4. Configure the Data Converter

Pass your driver to an `ExternalStorage` instance on the Data Converter, and use the same converter when creating your
Client and Worker. Both sides need it: a Client without External Storage configured cannot read an offloaded result. You
can also package your driver as a [plugin](/develop/plugins-guide) for easier reuse across services:

<!--SNIPSTART typescript-custom-driver-data-converter -->
```ts
export function createDataConverter(rootDir: string = STORAGE_ROOT): DataConverter {
  return {
    externalStorage: new ExternalStorage({
      drivers: [new FileSystemStorageDriver({ rootDir })],
      payloadSizeThreshold: PAYLOAD_SIZE_THRESHOLD,

      // With one driver registered, every offloaded payload goes to it. Register more
      // than one and a `driverSelector` becomes required, letting you route per
      // payload: a cheap archive tier for a known-bulky Workflow type, a driver per
      // tenant, or a per-region bucket. Returning `null` from the selector keeps that
      // payload inline, which is how you exempt specific payloads from offloading.
      //
      // driverSelector: (context, _payload) =>
      //   context.target?.type === 'processDocument' ? coldDriver : hotDriver,
    }),
  };
}
```
<!--SNIPEND-->

## Configure payload size threshold

You can configure the payload size threshold that triggers external storage. By default, payloads of 256 KiB or larger
are offloaded to external storage. You can adjust this with the `payloadSizeThreshold` option, or set it to `0` to
externalize all payloads regardless of size. Payloads smaller than the threshold stay inline in Event History. The size
compared against the threshold is that of the serialized Payload, which includes its metadata, not just your data.

<!--SNIPSTART typescript-external-storage-threshold-->
```ts
const dataConverter = {
  externalStorage: new ExternalStorage({
    drivers: [driver],
    payloadSizeThreshold: 0,
  }),
};
```
<!--SNIPEND-->

## Use multiple storage drivers

When you register multiple drivers, you must provide a `driverSelector` function that chooses which driver stores each
payload. `ExternalStorage` throws if you register more than one driver without a selector. Any driver in the list that
is not selected for storing is still available for retrieval, which is useful when migrating between storage backends.
Return `null` from the selector to keep a specific payload inline in Event History.

Multiple drivers are useful in scenarios such as:

- Driver migration. Your Worker needs to retrieve payloads created by clients that use a different driver than the one
  you prefer. Register both drivers and use the selector to always pick your preferred driver for new payloads. The old
  driver remains available for retrieving existing claims.
- Multi-cloud storage. Route payloads to different storage backends based on your cloud environment. For example, use S3
  for Workers running on AWS and GCS for Workers running on Google Cloud. The selector chooses the appropriate driver
  based on the runtime environment.

Every registered driver needs a distinct `name`. Because `S3StorageDriver` defaults its `name` to `"aws.s3driver"`,
registering two S3 drivers requires setting the `driverName` option on at least one of them.

The following example registers two drivers but always selects `preferredDriver` for new payloads. The `legacyDriver`
is only registered so the Worker can retrieve payloads that were previously stored with it:

<!--SNIPSTART typescript-external-storage-multiple-drivers-->
```ts
const preferredDriver = new S3StorageDriver({
  client: new AwsSdkS3StorageDriverClient(s3Client),
  bucket: 'my-bucket',
});
const legacyDriver = new LegacyStorageDriver();

const externalStorage = new ExternalStorage({
  drivers: [preferredDriver, legacyDriver],
  driverSelector: () => preferredDriver,
});
```
<!--SNIPEND-->

## Multi-region durability with Amazon S3

To make your S3-backed External Storage tolerant of regional failures, configure the AWS side with
[Cross-Region Replication (CRR)](https://docs.aws.amazon.com/AmazonS3/latest/userguide/replication.html) and an
[S3 Multi-Region Access Point (MRAP)](https://aws.amazon.com/s3/features/multi-region-access-points/), then point the
driver at the MRAP ARN instead of a bucket name. See
[Durable External Storage](/external-storage#durable-external-storage) for the full pattern and trade-offs.

MRAP requests are signed with [SigV4A](https://docs.aws.amazon.com/IAM/latest/UserGuide/reference_sigv-create-signed-request.html).
The AWS SDK for JavaScript does not bundle a SigV4A signer, so install one alongside your existing dependencies:

```sh
npm install @aws-sdk/signature-v4a
```

Import the signer once during application startup. The import registers the signer with the AWS SDK, so you do not
reference it directly:

```ts
import '@aws-sdk/signature-v4a';
```

`@aws-sdk/signature-v4-crt` is an alternative implementation backed by the AWS Common Runtime. When both are installed,
the AWS SDK prefers the CRT implementation.

With the signer registered, the only remaining change is the value you pass as `bucket`:

```ts
const driver = new S3StorageDriver({
  client: new AwsSdkS3StorageDriverClient(s3Client),
  bucket: 'arn:aws:s3::123456789012:accesspoint/mfzwi23gnjvgw.mrap',
});
```