Indexeddb 简明教程

IndexedDB - Introduction

数据库管理系统提供了一种存储和检索数据的方法。有各种类型的数据库可以使用，其中最常用的有 -

A database management system provides a mechanism for the storage and retrieval of data. There are various kinds of databases available mostly used ones among them are −

  1. Hierarchical databases

  2. Network databases

  3. Object-oriented databases

  4. Relational databases

  5. NoSQL databases

NoSQL databases

NoSQL 数据库（有时称为非 SQL）是一种提供存储和检索数据的方法的数据库，而不是关系数据库中使用的表格关系。这些数据库是无模式的，支持轻松复制，具有简单的 API，最终一致，并且可以处理大量数据（大数据）。

A NoSQL database (sometimes called Not Only SQL) is a database that provides a mechanism to store and retrieve data other than the tabular relations used in relational databases. These databases are schema-free, support easy replication, have simple API, are eventually consistent, and can handle huge amounts of data (big data).

还有不同类型的 NoSQL 数据库，例如 -

There are different types of NoSQL databases also like −

  1. Document databases.

  2. Key-value stores. Column-oriented databases. Graph databases.

What is IndexedDB

Indexed Database 是一种 NoSQL 数据库或非关系结构化查询语言。它是一个事务数据库系统，类似基于 SQL 的 RDBMS。但是，与使用固定列表的基于 SQL 的 RDBMS 不同，IndexedDB 是基于 JavaScript 面向对象数据库。

Indexed Database is a type of NoSQL database or Non−relational structured query language. It is a transactional database system, like an SQL−based RDBMS. However, unlike SQL−based RDBMSs, which use fixed-column tables, IndexedDB is a JavaScript−based object-oriented database.

当我们需要在服务器端存储大量数据并且比本地存储更快时，就会使用它。由于它将数据存储在浏览器中，因此也可以在线和离线使用。使用此功能，您可以创建一个 Web 应用程序（具有丰富的查询功能），无论是否有互联网连接都可以运行。

It is used when we need to store significant amounts of data on the server side and faster than local storage. Since it stores the data in the browser it can be used online and offline too. Using this, you can create a web application (with rich query abilities) that can run whether an internet connection is available or not.

Key Characteristics of IndexedDB

以下是 IndexedDB 数据库的主要特征 -

Following are the key characters of the IndexedDB database −

IndexedDB 是一个 NoSQL 数据库，存储键值对。它可以通过键或多种键类型存储几乎任何类型的值。

IndexedDB is a NoSQL database that stores key−value pairs. It can store almost any kind of value by keys or multiple key types.

  1. As mentioned, IndexedDB follows a transactional database model − Transaction is a wrapper class around the operation or group of operations so that data integrity is maintained. You don’t want data to be altered or missed so if the transaction is failed a callback is rolled out.

  2. IndexedDB does not use Structured Query Language − Since IndexedDB uses a NoSQL database it doesn’t use SQL, instead, it uses queries on Indexes to produce data via cursors or the getAll() method to iterate around different sets.

  3. IndexedDB uses a lot of requests − Requests are objects that receive the success or failure of DOM events (DOM - HTML DOM events allow JavaScript to register different event handlers on elements in an HTML document). DOM events are success or error which has a target property that indicates the flow of the request. Success events can’t be canceled but, error events can be canceled. There are a lot of requests in IndexedDB like on success, onerror and addEventListener(), removeEventListener() on them. For getting to know the status of the request we also have ready state, result, and error code properties.

  4. IndexedDB needs to follow the same origin − Origin is the URL of the document in which the script is being written, each origin has some databases under it and each database has its name to be identified by the origin. The security boundary imposed on IndexedDB prevents applications from accessing data with a different origin. For example, if we take a URL and take different subdirectories of it, it can retrieve data but if we change location to port 8080 and try to retrieve data from the usual URL and the changed port, we cannot retrieve the data.

Terminology

以下是 IndexedDB 中重要的各种术语，在继续之前你应该了解这些术语：

Following are various important terms in indexedDB that you should know before proceeding further −

  1. Database − In IndexedDB database is the highest level which contains the object stores that contain the data.

  2. Object Stores − Object stores are the data storage entities of IndexedDB. Think of it as tables in RDBMS where we store data based on the type of data we want to store (ex: id, name, roll no, etc).

  3. Transaction − For any database operation we do the following process. Get database objectOpen transaction on the databaseOpen object store on the transaction and then, operate on object store.So basically, a transaction is a wrapper function that is connected to every database and it ensures data integrity such that if the transaction is canceled or any error happens it will call back to where the transaction has not yet been begun.

  4. Index − Think of object store as a table and we use indexes to retrieve data of a single property from them. Ex: Name, age, etc.

  5. Cursor − In a database, if we need to traverse multiple records from object stores we use cursors.

Support for IndexedDB

IndexedDB 是浏览器中的数据库，因此我们需要检查它是否受当前／现有浏览器支持。为此，请将以下代码粘贴到文本编辑器中，将其另存为 test.html ，并在浏览器中运行它。

IndexedDB is a database in a browser so, we need to check if it is supported by the current/existing browser. To do so, paste the following code in an text editor save it as test.html and run it your browser.

const indexedDB =
   window.indexedDB ||
   window.mozIndexedDB ||
   window.webkitIndexedDB ||
   window.msIndexedDB ||
   window.shimIndexedDB;

if (!indexedDB) {
   document.write("IndexedDB could not be found in this browser.");
}
const request = indexedDB.open("MyDatabase", 1);

如果你的浏览器支持 IndexedDB，此程序将成功执行，并且将创建数据库。

If IndexedDB is supported by your browser this program gets executed successfully, an a database will be created.

mydatabase