# Lightweight Storage

Lightweight storage provides applications with data processing capability and allows applications to perform lightweight data storage and query. Data is stored in key-value (KV) pairs. Keys are of the string type, and values can be of the number, string, or Boolean type.


> **NOTE**
>
> - The initial APIs of this module are supported since API version 6. Newly added APIs will be marked with a superscript to indicate their earliest API version.
>
> - The APIs of this module are no longer maintained since API Version 9. You are advised to use [`@ohos.data.preferences`](js-apis-data-preferences.md).


## Modules to Import

```js
import data_storage from '@ohos.data.storage';
```

## Constants

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

| Name             | Type   | Readable | Writable | Description                                                 |
| ---------------- | ------ | -------- | -------- | ----------------------------------------------------------- |
| MAX_KEY_LENGTH   | string | Yes      | No       | Maximum length of a key. It must be less than 80 bytes.     |
| MAX_VALUE_LENGTH | string | Yes      | No       | Maximum length of a value. It must be less than 8192 bytes. |


## data_storage.getStorageSync

getStorageSync(path: string): Storage

Reads the specified file and loads its data to the **Storage** instance for data operations.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Parameters**

| Name | Type   | Mandatory | Description              |
| ---- | ------ | --------- | ------------------------ |
| path | string | Yes       | Path of the target file. |

**Return value**

| Type                | Description                                            |
| ------------------- | ------------------------------------------------------ |
| [Storage](#storage) | **Storage** instance used for data storage operations. |

**Example**

```js
import featureAbility from '@ohos.ability.featureAbility';

var path;
var context = featureAbility.getContext();
context.getFilesDir().then((filePath) => {
    path = filePath;
    console.info("======================>getFilesDirPromise====================>");
});  

let storage = data_storage.getStorageSync(path + '/mystore');
storage.putSync('startup', 'auto');
storage.flushSync();
```


## data_storage.getStorage

getStorage(path: string, callback: AsyncCallback<Storage>): void

Reads the specified file and loads its data to the **Storage** instance for data operations. This API uses an asynchronous callback to return the result.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Parameters**

| Name     | Type                                     | Mandatory | Description                                   |
| -------- | ---------------------------------------- | --------- | --------------------------------------------- |
| path     | string                                   | Yes       | Path of the target file.                      |
| callback | AsyncCallback<[Storage](#storage)> | Yes       | Callback used to return the execution result. |

**Example**

```js
import featureAbility from '@ohos.ability.featureAbility';

var path;
var context = featureAbility.getContext();
context.getFilesDir().then((filePath) => {
    path = filePath;
    console.info("======================>getFilesDirPromise====================>");
});  

data_storage.getStorage(path + '/mystore', function (err, storage) {
    if (err) {
        console.info("Failed to get the storage. path: " + path + '/mystore');
        return;
    }
    storage.putSync('startup', 'auto');
    storage.flushSync();
})
```


## data_storage.getStorage

getStorage(path: string): Promise<Storage>

Reads the specified file and loads its data to the **Storage** instance for data operations. This API uses a promise to return the result.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Parameters**

| Name | Type   | Mandatory | Description              |
| ---- | ------ | --------- | ------------------------ |
| path | string | Yes       | Path of the target file. |

**Return value**

| Type                               | Description                        |
| ---------------------------------- | ---------------------------------- |
| Promise<[Storage](#storage)> | Promise used to return the result. |

**Example**

```js
import featureAbility from '@ohos.ability.featureAbility';

var path;
var context = featureAbility.getContext();
context.getFilesDir().then((filePath) => {
    path = filePath;
    console.info("======================>getFilesDirPromise====================>");
});  

let getPromise = data_storage.getStorage(path + '/mystore');
getPromise.then((storage) => {
    storage.putSync('startup', 'auto');
    storage.flushSync();
}).catch((err) => {
    console.info("Failed to get the storage. path: " + path + '/mystore');
})
```


## data_storage.deleteStorageSync

deleteStorageSync(path: string): void

Deletes the singleton **Storage** instance of a file from the memory, and deletes the specified file, its backup file, and damaged files. After the specified files are deleted, the **Storage** instance cannot be used for data operations. Otherwise, data inconsistency will occur.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Parameters**

| Name | Type   | Mandatory | Description              |
| ---- | ------ | --------- | ------------------------ |
| path | string | Yes       | Path of the target file. |

**Example**

```js
import featureAbility from '@ohos.ability.featureAbility';

var path;
var context = featureAbility.getContext();
context.getFilesDir().then((filePath) => {
    path = filePath;
    console.info("======================>getFilesDirPromise====================>");
});  

data_storage.deleteStorageSync(path + '/mystore');
```


## data_storage.deleteStorage

deleteStorage(path: string, callback: AsyncCallback<void>): void

Deletes the singleton **Storage** instance of a file from the memory, and deletes the specified file, its backup file, and damaged files. After the specified files are deleted, the **Storage** instance cannot be used for data operations. Otherwise, data inconsistency will occur. This API uses an asynchronous callback to return the result.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Parameters**

| Name     | Type                      | Mandatory | Description                     |
| -------- | ------------------------- | --------- | ------------------------------- |
| path     | string                    | Yes       | Path of the target file.        |
| callback | AsyncCallback<void> | Yes       | Callback that returns no value. |

**Example**

```js
import featureAbility from '@ohos.ability.featureAbility';

var path;
var context = featureAbility.getContext();
context.getFilesDir().then((filePath) => {
    path = filePath;
    console.info("======================>getFilesDirPromise====================>");
});  

data_storage.deleteStorage(path + '/mystore', function (err) {
    if (err) {
        console.info("Failed to delete the storage with err: " + err);
        return;
    }
    console.info("Succeeded in deleting the storage.");
})
```


## data_storage.deleteStorage

deleteStorage(path: string): Promise<void>

Deletes the singleton **Storage** instance of a file from the memory, and deletes the specified file, its backup file, and damaged files. After the specified files are deleted, the **Storage** instance cannot be used for data operations. Otherwise, data inconsistency will occur. This API uses a promise to return the result.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Parameters**

| Name | Type   | Mandatory | Description              |
| ---- | ------ | --------- | ------------------------ |
| path | string | Yes       | Path of the target file. |

**Return value**
| Type                | Description                    |
| ------------------- | ------------------------------ |
| Promise<void> | Promise that returns no value. |

**Example**

```js
import featureAbility from '@ohos.ability.featureAbility';

var path;
var context = featureAbility.getContext();
context.getFilesDir().then((filePath) => {
    path = filePath;
    console.info("======================>getFilesDirPromise====================>");
});  

let promisedelSt = data_storage.deleteStorage(path + '/mystore');
promisedelSt.then(() => {
    console.info("Succeeded in deleting the storage.");
}).catch((err) => {
    console.info("Failed to delete the storage with err: " + err);
})
```


## data_storage.removeStorageFromCacheSync

removeStorageFromCacheSync(path: string): void

Removes the singleton **Storage** instance of a file from the cache. The removed instance cannot be used for data operations. Otherwise, data inconsistency will occur.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Parameters**

| Name | Type   | Mandatory | Description              |
| ---- | ------ | --------- | ------------------------ |
| path | string | Yes       | Path of the target file. |

**Example**

```js
import featureAbility from '@ohos.ability.featureAbility';

var path;
var context = featureAbility.getContext();
context.getFilesDir().then((filePath) => {
    path = filePath;
    console.info("======================>getFilesDirPromise====================>");
});  

data_storage.removeStorageFromCacheSync(path + '/mystore');
```


## data_storage.removeStorageFromCache

removeStorageFromCache(path: string, callback: AsyncCallback<void>): void

Removes the singleton **Storage** instance of a file from the cache. The removed instance cannot be used for data operations. Otherwise, data inconsistency will occur. This API uses an asynchronous callback to return the result.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Parameters**

| Name     | Type                      | Mandatory | Description                     |
| -------- | ------------------------- | --------- | ------------------------------- |
| path     | string                    | Yes       | Path of the target file.        |
| callback | AsyncCallback<void> | Yes       | Callback that returns no value. |

**Example**

```js
import featureAbility from '@ohos.ability.featureAbility';

var path;
var context = featureAbility.getContext();
context.getFilesDir().then((filePath) => {
    path = filePath;
    console.info("======================>getFilesDirPromise====================>");
});  

data_storage.removeStorageFromCache(path + '/mystore', function (err) {
    if (err) {
        console.info("Failed to remove storage from cache with err: " + err);
        return;
    }
    console.info("Succeeded in removing storage from cache.");
})
```


## data_storage.removeStorageFromCache

removeStorageFromCache(path: string): Promise<void>

Removes the singleton **Storage** instance of a file from the cache. The removed instance cannot be used for data operations. Otherwise, data inconsistency will occur. This API uses a promise to return the result.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Parameters**

| Name | Type   | Mandatory | Description              |
| ---- | ------ | --------- | ------------------------ |
| path | string | Yes       | Path of the target file. |

**Return value**

| Type                | Description                    |
| ------------------- | ------------------------------ |
| Promise<void> | Promise that returns no value. |

**Example**

```js
import featureAbility from '@ohos.ability.featureAbility';

var path;
var context = featureAbility.getContext();
context.getFilesDir().then((filePath) => {
    path = filePath;
    console.info("======================>getFilesDirPromise====================>");
});  

let promiserevSt = data_storage.removeStorageFromCache(path + '/mystore')
promiserevSt.then(() => {
    console.info("Succeeded in removing storage from cache.");
}).catch((err) => {
    console.info("Failed to remove storage from cache with err: " + err);
})
```


## Storage

Provides APIs for obtaining and modifying storage data.


### getSync

getSync(key: string, defValue: ValueType): ValueType

Obtains the value corresponding to a key. If the value is null or not in the default value format, the default value is returned.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Parameters**

| Name     | Type                    | Mandatory | Description                                                  |
| -------- | ----------------------- | --------- | ------------------------------------------------------------ |
| key      | string                  | Yes       | Key of the data. It cannot be empty.                         |
| defValue | [ValueType](#valuetype) | Yes       | Default value to be returned if the value of the specified key does not exist. It can be a number, string, or Boolean value. |

**Return value**

| Type      | Description                                                  |
| --------- | ------------------------------------------------------------ |
| ValueType | Value corresponding to the specified key. If the value is null or not in the default value format, the default value is returned. |

**Example**

```js
let value = storage.getSync('startup', 'default');
console.info("The value of startup is " + value);
```


### get

get(key: string, defValue: ValueType, callback: AsyncCallback<ValueType>): void

Obtains the value corresponding to a key. If the value is null or not in the default value format, the default value is returned. This API uses an asynchronous callback to return the result.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Parameters**

| Name     | Type                           | Mandatory | Description                                                  |
| -------- | ------------------------------ | --------- | ------------------------------------------------------------ |
| key      | string                         | Yes       | Key of the data. It cannot be empty.                         |
| defValue | [ValueType](#valuetype)        | Yes       | Default value to be returned. It can be a number, string, or Boolean value. |
| callback | AsyncCallback<ValueType> | Yes       | Callback used to return the execution result.                |

**Example**

```js
storage.get('startup', 'default', function(err, value) {
    if (err) {
        console.info("Failed to get the value of startup with err: " + err);
        return;
      }
    console.info("The value of startup is " + value);
})
```


### get

get(key: string, defValue: ValueType): Promise<ValueType>

Obtains the value corresponding to a key. If the value is null or not in the default value format, the default value is returned. This API uses a promise to return the result.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Parameters**

| Name     | Type                    | Mandatory | Description                                                  |
| -------- | ----------------------- | --------- | ------------------------------------------------------------ |
| key      | string                  | Yes       | Key of the data. It cannot be empty.                         |
| defValue | [ValueType](#valuetype) | Yes       | Default value to be returned. It can be a number, string, or Boolean value. |

**Return value**

| Type                     | Description                        |
| ------------------------ | ---------------------------------- |
| Promise<ValueType> | Promise used to return the result. |

**Example**
```js
let promiseget = storage.get('startup', 'default');
promiseget.then((value) => {
    console.info("The value of startup is " + value)
}).catch((err) => {
    console.info("Failed to get the value of startup with err: " + err);
})
```


### putSync

putSync(key: string, value: ValueType): void

Obtains the **Storage** instance corresponding to the specified file, writes data to the **Storage** instance using a **Storage** API, and saves the modification using **flush()** or **flushSync()**.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Parameters**

| Name  | Type                    | Mandatory | Description                                                  |
| ----- | ----------------------- | --------- | ------------------------------------------------------------ |
| key   | string                  | Yes       | Key of the data. It cannot be empty.                         |
| value | [ValueType](#valuetype) | Yes       | New value to store. It can be a number, string, or Boolean value. |

**Example**

```js
storage.putSync('startup', 'auto')
```


### put

put(key: string, value: ValueType, callback: AsyncCallback<void>): void

Obtains the **Storage** instance corresponding to the specified file, writes data to the **Storage** instance using a **Storage** API, and saves the modification using **flush()** or **flushSync()**. This API uses an asynchronous callback to return the result.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Parameters**

| Name     | Type                      | Mandatory | Description                                                  |
| -------- | ------------------------- | --------- | ------------------------------------------------------------ |
| key      | string                    | Yes       | Key of the data. It cannot be empty.                         |
| value    | [ValueType](#valuetype)   | Yes       | New value to store. It can be a number, string, or Boolean value. |
| callback | AsyncCallback<void> | Yes       | Callback that returns no value.                              |

**Example**

```js
storage.put('startup', 'auto', function (err) {
    if (err) {
        console.info("Failed to put the value of startup with err: " + err);
        return;
    }
    console.info("Succeeded in putting the value of startup.");
})
```


### put

put(key: string, value: ValueType): Promise<void>

Obtains the **Storage** instance corresponding to the specified file, writes data to the **Storage** instance using a **Storage** API, and saves the modification using **flush()** or **flushSync()**. This API uses a promise to return the result.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Parameters**

| Name  | Type                    | Mandatory | Description                                                  |
| ----- | ----------------------- | --------- | ------------------------------------------------------------ |
| key   | string                  | Yes       | Key of the data. It cannot be empty.                         |
| value | [ValueType](#valuetype) | Yes       | New value to store. It can be a number, string, or Boolean value. |

**Return value**

| Type                | Description                    |
| ------------------- | ------------------------------ |
| Promise<void> | Promise that returns no value. |

**Example**
```js
let promiseput = storage.put('startup', 'auto');
promiseput.then(() => {
    console.info("Succeeded in putting the value of startup.");
}).catch((err) => {
    console.info("Failed to put the value of startup with err: " + err);
})
```


### hasSync

hasSync(key: string): boolean

Checks whether the storage object contains data with a given key.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Parameters**

| Name | Type   | Mandatory | Description                          |
| ---- | ------ | --------- | ------------------------------------ |
| key  | string | Yes       | Key of the data. It cannot be empty. |

**Return value**

| Type    | Description                                                  |
| ------- | ------------------------------------------------------------ |
| boolean | Returns **true** if the storage object contains data with the specified key; returns **false** otherwise. |

**Example**

```js
let isExist = storage.hasSync('startup');
if (isExist) {
    console.info("The key of startup is contained.");
}
```


### has

has(key: string, callback: AsyncCallback<boolean>): boolean

Checks whether the storage object contains data with a given key. This API uses an asynchronous callback to return the result.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Parameters**

| Name     | Type                         | Mandatory | Description                                   |
| -------- | ---------------------------- | --------- | --------------------------------------------- |
| key      | string                       | Yes       | Key of the data. It cannot be empty.          |
| callback | AsyncCallback<boolean> | Yes       | Callback used to return the execution result. |

**Return value**

| Type    | Description                                                  |
| ------- | ------------------------------------------------------------ |
| boolean | Returns **true** if the storage object contains data with the specified key; returns **false** otherwise. |

**Example**

```js
storage.has('startup', function (err, isExist) {
    if (err) {
        console.info("Failed to check the key of startup with err: " + err);
        return;
    }
    if (isExist) {
        console.info("The key of startup is contained.");
    }
})
```


### has

has(key: string): Promise<boolean>

Checks whether the storage object contains data with a given key. This API uses a promise to return the result.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Parameters**

| Name | Type   | Mandatory | Description                          |
| ---- | ------ | --------- | ------------------------------------ |
| key  | string | Yes       | Key of the data. It cannot be empty. |

**Return value**

| Type                   | Description                        |
| ---------------------- | ---------------------------------- |
| Promise<boolean> | Promise used to return the result. |

**Example**

```js
let promisehas = storage.has('startup')
promisehas.then((isExist) => {
    if (isExist) {
        console.info("The key of startup is contained.");
    }
}).catch((err) => {
    console.info("Failed to check the key of startup with err: " + err);
})
```


### deleteSync

deleteSync(key: string): void

Deletes data with the specified key from this storage object.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Parameters**

| Name | Type   | Mandatory | Description                          |
| ---- | ------ | --------- | ------------------------------------ |
| key  | string | Yes       | Key of the data. It cannot be empty. |

**Example**

```js
storage.deleteSync('startup')
```


### delete

delete(key: string, callback: AsyncCallback<void>): void

Deletes data with the specified key from this storage object. This API uses an asynchronous callback to return the result.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Parameters**

| Name     | Type                      | Mandatory | Description                          |
| -------- | ------------------------- | --------- | ------------------------------------ |
| key      | string                    | Yes       | Key of the data. It cannot be empty. |
| callback | AsyncCallback<void> | Yes       | Callback that returns no value.      |

**Example**

```js
storage.delete('startup', function (err) {
    if (err) {
        console.info("Failed to delete startup key failed err: " + err);
        return;
    }
    console.info("Succeeded in deleting startup key.");
})
```


### delete

delete(key: string): Promise<void>

Deletes data with the specified key from this storage object. This API uses a promise to return the result.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Parameters**

| Name | Type   | Mandatory | Description      |
| ---- | ------ | --------- | ---------------- |
| key  | string | Yes       | Key of the data. |

**Return value**

| Type                | Description                    |
| ------------------- | ------------------------------ |
| Promise<void> | Promise that returns no value. |

**Example**

```js
let promisedel = storage.delete('startup')
promisedel.then(() => {
    console.info("Succeeded in deleting startup key.");
}).catch((err) => {
    console.info("Failed to delete startup key failed err: " + err);
})
```


### flushSync

flushSync(): void

Saves the modification of this object to the **Storage** instance and synchronizes the modification to the file.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Example**

```js
storage.flushSync()
```


### flush

flush(callback: AsyncCallback<void>): void

Saves the modification of this object to the **Storage** instance and synchronizes the modification to the file. This API uses an asynchronous callback to return the result.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Parameters**

| Name     | Type                      | Mandatory | Description                     |
| -------- | ------------------------- | --------- | ------------------------------- |
| callback | AsyncCallback<void> | Yes       | Callback that returns no value. |

**Example**

```js
storage.flush(function (err) {
    if (err) {
        console.info("Failed to flush to file with err: " + err);
        return;
    }
    console.info("Succeeded in flushing to file.");
})
```


### flush

flush(): Promise<void>

Saves the modification of this object to the **Storage** instance and synchronizes the modification to the file. This API uses a promise to return the result.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Return value**

| Type                | Description                    |
| ------------------- | ------------------------------ |
| Promise<void> | Promise that returns no value. |

**Example**

```js
let promiseflush = storage.flush();
promiseflush.then(() => {
    console.info("Succeeded in flushing to file.");
}).catch((err) => {
    console.info("Failed to flush to file with err: " + err);
})
```


### clearSync

clearSync(): void

Clears this **Storage** object.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Example**

```js
storage.clearSync()
```


### clear

clear(callback: AsyncCallback<void>): void

Clears this **Storage** object. This API uses an asynchronous callback to return the result.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Parameters**

| Name     | Type                      | Mandatory | Description                     |
| -------- | ------------------------- | --------- | ------------------------------- |
| callback | AsyncCallback<void> | Yes       | Callback that returns no value. |

**Example**

```js
storage.clear(function (err) {
    if (err) {
        console.info("Failed to clear the storage with err: " + err);
        return;
    }
    console.info("Succeeded in clearing the storage.");
})
```


### clear

clear(): Promise<void>

Clears this **Storage** object. This API uses a promise to return the result.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Return value**

| Type                | Description                    |
| ------------------- | ------------------------------ |
| Promise<void> | Promise that returns no value. |

**Example**

```js
let promiseclear = storage.clear();
promiseclear.then(() => {
    console.info("Succeeded in clearing the storage.");
}).catch((err) => {
    console.info("Failed to clear the storage with err: " + err);
})
```


### on('change')

on(type: 'change', callback: Callback<StorageObserver>): void

Subscribes to data changes. The **StorageObserver** needs to be implemented. When the value of the key subscribed to is changed, a callback will be invoked after **flush()** or **flushSync()** is executed.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Parameters**

| Name     | Type                                                | Description                                                  |
| -------- | --------------------------------------------------- | ------------------------------------------------------------ |
| type     | string                                              | Event type. The value **change** indicates data change events. |
| callback | Callback<[StorageObserver](#storageobserver)> | Callback used to return data changes.                        |

**Example**

```js
var observer = function (key) {
    console.info("The key of " + key + " changed.");
}
storage.on('change', observer);
storage.putSync('startup', 'auto');
storage.flushSync();  // observer will be called.
```


### off('change')

off(type: 'change', callback: Callback<StorageObserver>): void

Unsubscribes from data changes.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

**Parameters**

| Name     | Type                                                | Description                                                  |
| -------- | --------------------------------------------------- | ------------------------------------------------------------ |
| type     | string                                              | Event type. The value **change** indicates data change events. |
| callback | Callback<[StorageObserver](#storageobserver)> | Callback used to return data changes.                        |

**Example**

```js
var observer = function (key) {
    console.info("The key of " + key + " changed.");
}
storage.off('change', observer);
```


## StorageObserver

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

| Name | Type   | Mandatory | Description   |
| ---- | ------ | --------- | ------------- |
| key  | string | No        | Data changed. |

## ValueType

Enumerates the value types.

**System capability**: SystemCapability.DistributedDataManager.Preferences.Core

| Type    | Description                   |
| ------- | ----------------------------- |
| number  | The value is a number.        |
| string  | The value is a string.        |
| boolean | The value is of Boolean type. |