Save persistent data in app

It's very common to store persistent data in native apps, people usually do it by embedding database libraries or manipulate plain text files. In node-webkit, you have much better choices than that, you can use Web SQL Database, Web Storage or Application Cache without headaches of any extra dependencies.

Web SQL Database

The Web SQL Database API isn't actually part of the HTML5 specification but it is a separate specification which introduces a set of APIs to manipulate client-side databases using SQL. I'll assume you're familiar with basic database operations and SQL language in following guides.

The Web SQL Database API in node-webkit is implemented with sqlite, and operations are basically the same:

• openDatabase: This method creates the database object either using existing database or creating new one.
• transaction: This method give us the ability to control a transaction and performing either commit or rollback based on the situation.
• executeSql: This method is used to execute actual SQL query.

To create and open a database, use the following code:

var db = openDatabase('mydb', '1.0', 'my first database', 2 * 1024 * 1024);

I’ve passed four arguments to the openDatabase method. These are:

• Database name
• Version number
• Text description
• Estimated size of database

If you try to open a database that doesn’t exist, the API will create it on the fly for you. You also don’t have to worry about closing databases.

To create table, insert data and query data, just executeSql in transaction:

// Create table and insert one line
db.transaction(function (tx) {
  tx.executeSql('CREATE TABLE IF NOT EXISTS foo (id unique, text)');
  tx.executeSql('INSERT INTO foo (id, text) VALUES (1, "synergies")');
  tx.executeSql('INSERT INTO foo (id, text) VALUES (2, "luyao")');
});

// Query out the data
tx.executeSql('SELECT * FROM foo', [], function (tx, results) {
  var len = results.rows.length, i;
  for (i = 0; i < len; i++) {
    alert(results.rows.item(i).text);
  }
});

For more information, you can read tutorials like Introducing Web SQL Databases.

IndexedDB

IndexedDB is considered the NoSQL successor of WebSQL. It is supported by the latest versions of Chromium and therefore node-webkit - the implementation is based on the key-value storage LevelDB. IndexedDB's API is asynchronous and relatively low-level and verbose, so you might prefer using an abstraction, like PouchDB.

PouchDB

PouchDB is an embedded database engine inspired by CouchDB. In node-webkit, it can be used as an abstraction over IndexedDB (via the Chromium implementation) or directly on LevelDB (via the node.js module). As with CouchDB, there are no dynamic index-based queries, but you can dynamically aggregate a view via map/reduce. Unlike CouchDB, you cannot have incremental views, which ultimately makes retrieving data from it slow with large data sets (always execute map/reduce to query). It can directly replicate to/from CouchDB, which gives it an advantage if you are building an application that has to sync with the cloud.

EJDB

EJDB (Embedded JSON Database engine) is a simple & fast database engine based on Tokyo Cabinet. It's usage copies MongoDB - you can easily make dynamic queries and sort/paginate the result. The fast queries and easy-to-use API make it the best choice for node-webkit.

Web Storage

Web storage is a easy to use key-value database, you can use it like normal js objects but everything will be saved to disk for you.

There are two types of web storage:

• localStorage - stores data with no expiration date
• sessionStorage - stores data for one session

The localStorage object stores the data with no expiration date. The data will not be deleted when the browser is closed, and will be available the next day, week, or year.

localStorage.love = "luyao";

// Love lasts forever
console.log(localStorage.love);

The sessionStorage object is equal to the localStorage object, except that it stores the data for only one session. The data is deleted when the user closes the window.

sessionStorage.life = "";

// But life will have an end
console.log(sessionStorage.life);

Be warned that for large data sets web storage is incredibly impractical, since the API is synchronous, there is no advanced indexing/queries (only one key-value store) and the value can only be a string.

Application Cache

HTML5 introduces application cache, which means that a web application is cached, and accessible without an internet connection.

Application cache gives an application three advantages:

• Offline browsing - users can use the application when they're offline
• Speed - cached resources load faster
• Reduced server load - the browser will only download updated/changed resources from the server

However, application cache is designed for browser use, for apps using node-webkit, it's less useful than the other two method, read HTML5 Application Cache if you want to use it.