1# Lightweight Data Store Development 2 3## When to Use 4 5The lightweight data store is ideal for storing lightweight and frequently used data, but not for storing a large amount of data or data with frequent changes. The application data is persistently stored on a device in the form of files. Note that the instance accessed by an application contains all data of the file. The data is always loaded to the memory of the device until the application removes it from the memory. The application can perform data operations using the **Storage** APIs. 6 7## Available APIs 8 9The lightweight data store provides applications with data processing capability and allows applications to perform lightweight data storage and query. Data is stored in key-value pairs. Keys are of the string type, and values can be of the number, string, or Boolean type. 10 11**Creating a Storage Instance** 12 13Create a **Storage** instance for data operations. A **Storage** instance is created after data is read from a specified file and loaded to the instance. 14 15**Table 1** API for creating a **Storage** instance 16 17| Package Name | Method | Description | 18| ----------------- | ------------------------------------------- | ------------------------------------------------------------ | 19| ohos.data.storage | getStorage(path: string): Promise\<Storage> | Obtains the **Storage** singleton corresponding to a file for data operations. | 20 21**Writing Data** 22 23Call the **put\(\)** method to add or modify data in a **Storage** instance. 24 25**Table 2** API for writing data 26 27| Class | Method | Description | 28| ------- | -------------------------------------------------- | ----------------------------------------------------- | 29| Storage | put(key: string, value: ValueType): Promise\<void> | Writes data of the number, string, and Boolean types. | 30 31**Reading Data** 32 33Call the **get\(\)** method to read data from a **Storage** instance. 34 35**Table 3** API for reading data 36 37| Class | Method | Description | 38| ------- | ---------------------------------------------------------- | ---------------------------------------------------- | 39| Storage | get(key: string, defValue: ValueType): Promise\<ValueType> | Reads data of the number, string, and Boolean types. | 40 41**Storing Data Persistently** 42 43Call the **flush\(\)** method to write the cached data back to its text file for persistent storage. 44 45**Table 4** API for data persistence 46 47| Class | Method | Description | 48| ------- | ----------------------- | ------------------------------------------------------------ | 49| Storage | flush(): Promise\<void> | Writes data in the **Storage** instance back to its file through an asynchronous thread. | 50 51**Observing Data Changes** 52 53Specify **StorageObserver** as the callback to subscribe to data changes. When the value of the subscribed key is changed and the **flush\(\)** method is executed, **StorageObserver** will be invoked. 54 55**Table 5** APIs for subscribing to data changes 56 57| Class | Method | Description | 58| ------- | ------------------------------------------------------------ | ------------------------------- | 59| Storage | on(type: 'change', callback: Callback\<StorageObserver>): void | Subscribes to data changes. | 60| Storage | off(type: 'change', callback: Callback\<StorageObserver>): void | Unsubscribes from data changes. | 61 62**Deleting Data** 63 64Use the following APIs to delete a **Storage** instance or data file. 65 66**Table 6** APIs for deleting data 67 68| Package Name | Method | Description | 69| ----------------- | ---------------------------------------------------- | ------------------------------------------------------------ | 70| ohos.data.storage | deleteStorage(path: string): Promise\<void> | Deletes a **Storage** instance from the cache and deletes its file from the device. | 71| ohos.data.storage | removeStorageFromCache(path: string): Promise\<void> | Deletes a **Storage** instance from the cache to release memory. | 72 73 74## How to Develop 75 761. Import **@ohos.data.storage** and related modules to the development environment. 77 78 ```js 79 import dataStorage from '@ohos.data.storage'; 80 import featureAbility from '@ohos.ability.featureAbility'; // Used to obtain the file storage path. 81 ``` 82 832. Create a **Storage** instance. 84 85 Read the specified file and load its data to the **Storage** instance for data operations. 86 87 ```js 88 var path; 89 var context = featureAbility.getContext(); 90 context.getFilesDir().then((filePath) => { 91 path = filePath; 92 console.info("======================>getFilesDirPromsie====================>"); 93 94 let promise = dataStorage.getStorage(path + '/mystore'); 95 }); 96 ``` 97 98 993. Write data. 100 101 Use the **put\(\)** method of the **Storage** class to write data to the cached **Storage** instance. 102 103 ```js 104 promise.then((storage) => { 105 let getPromise = storage.put('startup', 'auto'); // Save data to the Storage instance. 106 getPromise.then(() => { 107 console.info("Succeeded in putting the value of startup."); 108 }).catch((err) => { 109 console.info("Failed to put the value of startup with err: " + err); 110 }) 111 }).catch((err) => { 112 console.info("Failed to get the storage."); 113 }) 114 ``` 115 116 1174. Read data. 118 119 Use the **get\(\)** method of the **Storage** class to read data. 120 121 ```js 122 promise.then((storage) => { 123 let getPromise = storage.get('startup', 'default') 124 getPromise.then((value) => { 125 console.info("The value of startup is " + value); 126 }).catch((err) => { 127 console.info("Failed to get the value of startup with err: " + err); 128 }) 129 }).catch((err) => { 130 console.info("Failed to get the storage.") 131 132 }) 133 ``` 134 135 1365. Store data persistently. 137 138 Use the **flush** or **flushSync** method to flush data in the **Storage** instance to its file. 139 140 ```js 141 storage.flush(); 142 ``` 143 1446. Observe data changes. 145 146 Specify **StorageObserver** as the callback to subscribe to data changes for an application. When the value of the subscribed key is changed and the **flush\(\)** method is executed, **StorageObserver** will be invoked. Unregister the **StorageObserver** when it is no longer required. 147 148 ```js 149 promise.then((storage) => { 150 var observer = function (key) { 151 console.info("The key of " + key + " changed."); 152 } 153 storage.on('change', observer) 154 storage.putSync('startup', 'auto'); // Modify data in the Storage instance. 155 storage.flushSync(); // Trigger the StorageObserver callback. 156 157 storage.off('change', observer); // Unsubscribe from the data changes. 158 }).catch((err) => { 159 console.info("Failed to get the storage."); 160 }) 161 ``` 162 163 1647. Delete the specified file. 165 166 Use the **deleteStorage** method to delete the **Storage** singleton of the specified file from the memory, and delete the specified file, its backup file, and damaged files. After the specified files are deleted, the application cannot use that instance to perform any data operation. Otherwise, data inconsistency will occur. The deleted data and files cannot be restored. 167 168 ```js 169 let promise = dataStorage.deleteStorage(path + '/mystore'); 170 promise.then(() => { 171 console.info("Succeeded in deleting the storage."); 172 }).catch((err) => { 173 console.info("Failed to delete the storage with err: " + err); 174 175 }) 176 ``` 177