Pouchdb 简明教程

PouchDB - Synchronization

您可以将存储在 PouchDB 中的本地数据库与存储在 CouchDB 中的数据库同步。在上一章中，我们介绍了如何使用 PouchDB 复制数据库。我们在那里使用了 PouchDB.replicate(source, destination) 方法。

You can synchronize the databases stored locally in PouchDB with those that are stored in CouchDB. In the previous chapter, we have seen how to replicate databases using PouchDB. There we have used the method PouchDB.replicate(source, destination).

此外，我们还可以使用 replicate.to() 和 replicate.from() 方法，将数据从本地数据库复制到远程数据库，并将数据从远程数据库复制到本地数据库，如下所示。

In addition to this, we can also replicate the data, from the local database to the remote database, and from the remote database to the local database using replicate.to() and replicate.from() methods as shown below.

//Replicating data from local database to remote database
localDB.replicate.to(remoteDB);

//Replicating data from remote database to local database
localDB.replicate.from(remoteDB);

其中， localDB 是存储在 PouchDB 中的数据库对象， remoteDB 是存储在 CouchDB 中的数据库对象。

Where, localDB is an object of database stored locally in PouchDB and remoteDB is an object of a database that is stored in CouchDB.

Example

假设 PouchDB 中有一个名为 local_database 的数据库，它包含 3 篇文档，doc1、doc2 和 doc3，其内容如下所示。

Suppose there is a database with the name local_database in PouchDB, and it contains 3 documents, doc1, doc2, and doc3, having contents as shown below.

doc1 = {_id: '003', name: 'Ram', age: 26, Designation: 'Programmer'}
doc2 = {_id: '004', name: 'Robert', age: 27, Designation: 'Programmer'}
doc3 = {_id: '005', name: 'Rahim', age: 28, Designation: 'Programmer'}

CouchDB 中有一个名为 Remote_Database 的数据库，其中包含 2 个文档 doc1 和 doc2，内容如下所示。

And there is a database with the name Remote_Database in CouchDB and it contains 2 documents doc1, doc2, having contents as shown below.

doc1 = {_id: '001', name: 'Geeta', age: 25, Designation: 'Programmer'}
doc2 = {_id: '002', name: 'Zara Ali', age: 24, Designation: 'Manager'}

接下来是一个同步这两个数据库的示例，其中一个存储在 PouchDB 中，另一个存储在 CouchDB 中，使用了 replicate.to() 和 replicate.from() 方法。

Following is an example of synchronizing these two databases, where one is stored in PouchDB and other is stored in CouchDB, using the replicate.to() and replicate.from() methods.

//Requiring the package
var PouchDB = require('PouchDB');

//Creating local database object
var localDB = new PouchDB('local_database');

//Creating remote database object
var remoteDB = new PouchDB('http://localhost:5984/remote_database');

//Synchronising both databases
localDB.replicate.to(remoteDB);
remoteDB.replicate.from(localDB);
console.log("Databases synchronized successfully");

将上述代码保存在一个名为 Synchronising_databases.js 的文件中。打开命令提示符并使用 node 执行 JavaScript 文件，如下所示：

Save the above code in a file with the name Synchronising_databases.js. Open the command prompt and execute the JavaScript file using node as shown below.

C:\PouchDB_Examples >node Synchronising_databases.js

这将同步这两个数据库 remoteDB 和 localDB，并在控制台上显示一条消息，如下所示：

This synchronizes the two databases remoteDB and localDB, and displays a message on the console as shown below.

Databases synchronized successfully.

同步这两个数据库后，访问 http://127.0.0.1:5984/_utils/index.html 并选择 remote_database 。你可以观察到，本地数据库的文档（003、004、005）被复制到了这个数据库中，如下所示：

After synchronizing the two databases visit the http://127.0.0.1:5984/_utils/index.html and select the remote_database. You can observe that the documents of local database (003, 004, 005) were copied in this database as shown below.

同样，如果你获取存储在 PouchDB 中 local_database 的内容，你可以观察到存储在 CouchDB 中的数据库的文档被复制到了这里。

In the same way, if you fetch the contents of the local_database stored in PouchDB you can get to observe that documents of the database that is stored in CouchDB were copied here.

[
   {
      id: '001',
      key: '001',
      value: { rev: '1-23cf3767e32a682c247053b16caecedb' },
      doc: {
         name: 'Geeta',
         age: 25,
         Designation: 'Programmer',
         _id: '001',
         _rev: '1-23cf3767e32a682c247053b16caecedb'
      }
   },
   {
      id: '002',
      key: '002',
      value: { rev: '1-d5bcfafbd4d4fae92fd7fc4fdcaa3a79' },
      doc: {
         name: 'Zara Ali',
         age: 24,
         Designation: 'Manager',
         _id: '002',
         _rev: '1-d5bcfafbd4d4fae92fd7fc4fdcaa3a79'
      }
   },
   {
      id: '003',
      key: '003',
      value: { rev: '1-bf4619471ac346fdde46cfa8fbf3587f' },
      doc: {
         name: 'Ram',
         age: 26,
         Designation: 'Programmer',
         _id: '003',
         _rev: '1-bf4619471ac346fdde46cfa8fbf3587f'
      }
   },
   {
      id: '004',
      key: '004',
      value: { rev: '1-29b8f803958c994e3eb37912a45d869c' },
      doc: {
         name: 'Robert',
         age: 27,
         Designation: 'Programmer',
         _id: '004',
         _rev: '1-29b8f803958c994e3eb37912a45d869c'
      }
   },
   {
      id: '005',
      key: '005',
      value: { rev: '1-0eb89f71998ffa8430a640fdb081abd2' },
      doc: {
         name: 'Rahim',
         age: 28,
         Designation: 'Programmer',
         _id: '005',
         _rev: '1-0eb89f71998ffa8430a640fdb081abd2'
      }
   }
]

你可以使用 sync() *method provided by PouchDB instead of the two methods *replicate.to() 和 replicate.from() 重写上述程序，如下所示：

You can rewrite the above program using the sync() *method provided by PouchDB instead of the two methods *replicate.to() and replicate.from() as shown below.

//Requiring the package
var PouchDB = require('PouchDB');

//Creating local database object
var localDB = new PouchDB('local');

//Creating remote database object
var remoteDB = new PouchDB('http://localhost:5984/remote_database');

//Synchronising Remote and local databases
localDB.sync(remoteDB, function(err, response) {
   if (err) {
      return console.log(err);
   } else {
      console.log(response);
   }
});

在执行上述程序时，它将同步这两个数据库并显示以下消息：

On executing the above program, it synchronizes the two databases displaying the following message.

{
   push: {
      ok: true,
      start_time: Fri Mar 25 2016 15:54:37 GMT+0530 (India Standard Time),
      docs_read: 6,
      docs_written: 6,
      doc_write_failures: 0,
      errors: [],
      last_seq: 10,
      status: 'complete',
      end_time: Fri Mar 25 2016 15:54:37 GMT+0530 (India Standard Time)
   },
   pull: {
      ok: true,
      start_time: Fri Mar 25 2016 15:54:37 GMT+0530 (India Standard Time),
      docs_read: 0,
      docs_written: 0,
      doc_write_failures: 0,
      errors: [],
      last_seq: 2,
      status: 'complete',
      end_time: Fri Mar 25 2016 15:54:37 GMT+0530 (India Standard Time)
   }
}