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Deep Persistent Proxy: Persistent Objects in your Browser


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DPP: Deep Persistent Proxy Objects for JavaScript

Rob Tweed rtweed@mgateway.com
19 May 2023, MGateway Ltd https://www.mgateway.com

Twitter: @rtweed

Google Group for discussions, support, advice etc: http://groups.google.co.uk/group/enterprise-web-developer-community

Thanks to @mpen for his original deep proxy logic on which this module depends.

What is DPP?

Deep Persistent Proxy (DPP) is a JavaScript module that allows you to create and maintain JavaScript objects/JSON that will persist automatically between browser sessions.

Next time you start a browser session, any persistent objects you define will be restored to their previous state automatically.

Quick Example

The following creates a persistent object named myObj which adds to its content every time you reload the page:

    const {createDPP} = await import('https://cdn.jsdelivr.net/gh/robtweed/DPP/src/dpp_browser.min.js');
    let dpp = await createDPP({storeName: 'demo'});
    let myObj = await dpp.start();

    if (!myObj.counter) myObj.counter = 0;
    if (!myObj.arr) myObj.arr = [];

    myObj.counter++;

    myObj.arr.push({
      time: Date.now()
    });

    console.log('myObj: ' + JSON.stringify(myObj));

Output on each page load/reload

    myObj: {"counter":1,"arr":[{"time":1691913947097}]}

    myObj: {"arr":[{"time":1691913947097},{"time":1691913998671}],"counter":2}

    myObj: {"arr":[{"time":1691913947097},{"time":1691913998671}],{"time":1691914072961}],"counter":3}

    etc...

How does DPP Manage to Persist JavaScript Objects?

DPP uses the browser's built-in indexedDB database to maintain a persistent image of your JavaScript object(s).

DPP also makes use of JavaScript Proxy Objects, allowing changes to your object(s) to be trapped and recorded to indexedDB.

Because indexedDB's APIs are asynchronous, a key challenge was how to keep the database copy of the object in synchronisation with the local proxy object, particularly if rapid sequences of sets and deletes occur.

The solution is provided by the unique characteristics of our queue-based QOper8 WebWorker management module.

DPP establishes a single QOper8 worker process, and any changes to the local object are added by DPP to the Qoper8 queue. By using a single persistent QOper8 WebWorker process, the corresponding changes to the indexedDB copy of the object can be guaranteed to be handled in strict chronological sequence within the WebWorker.

In summary, DPP decouples your local object from the indexedDB copy. The indexedDB copy is only used by the WebWorker. All you have to do is manipulate your local object and let DPP do the rest!

Do you need to understand the indexedDB API or WebWorkers in order to use DPP?

No. DPP implements all the necessary indexedDB, Qoper8 and WebWorker mechanics for you behind the scenes.

You just use a couple of simple DPP APIs to specify a persistent object, and DPP looks after everything else for you automatically.

When using a DPP-defined Persistent Object, you are accessing its Proxy Object and not the indexedDB database. The indexedDB database image of it is maintained automatically behind the scenes for you within its own WebWorker process.

The only time the indexedDB version is directly used by DPP is to automatically restore the contents of your Persistent Object(s) whenever you start a new browser session.

Will DPP Work on All Browsers?

DPP should work on all modern browsers, but not on older ones. It is usable on any browser that supports all the following:

  • indexedDB
  • Proxy Objects
  • async/await
  • ES6 Modules
  • WebWorkers

Try It Out

Basic Demo

Try out this live example, running directly from the code you'll find in the /examples folder of this repo.

Each time you load/reload the page, a persistent object named myObj is extended.

It includes a button that clears down the object and reloads the page again.

Note that you can also view the persistent version of this object and and manually clear it down in the indexedDB database: use the browser's Developer Tools.

ToDo Application Demo

If you're wanting to see DPP in use in a more realistic application environment, try running this live ToDo application.

This demonstrates the use of DPP to provide both persistent state data and reactive application control.

NOTE: You must use a modern browser that supports WebComponents to run this example!

For details about and source code for this application, see the Golgi /examples/todo folder.

Installing

DPP is designed so that it can be used whether you build your front-end code manually, or build it using Node.js and WebPack. The way you install and use DPP varies depending on which approach you use.

Using a CDN Copy

You can use DPP directly from the Github CDN linked to this repository. In your main browser application module, load it using:

  const {createDPP} = await import('https://cdn.jsdelivr.net/gh/robtweed/DPP/src/dpp_browser.min.js');

Using a Local Copy

Alternatively, clone or copy the folder /src to an appropriate directory on your web server (eg /var/www/dpp).

You should now have the following files in your web server folder:

    - dpp.js
    - dpp.min.js
    - dpp_browser.js
    - dpp_browser.min.js
    - dpp_node.js
    - idb_handlers
      - delete.js
      - instantiate.js
      - put.js
      - restore.js

You can now load DPP directly from your Web Server, eg:

  const {createDPP} = await import('/dpp/dpp_browser.min.js');

Note that if you're going to use local copies of both DPP and its dependent QOper8 modules, you can bypass this step and just use the DPP class modules itself:

  const {DPP} = await import('/dpp/dpp.min.js');

See below on how to use this latter approach.

From NPM

DPP is available on NPM:

    npm install dpp-db

You can now load DPP using:

    import {createDPP} from "dpp-db/node";

Starting DPP

Using the CDN Version

You need to decide on a store name: this is the name of the store that will be used by indexedDB to maintain your object. The name is entirely for you to decide, eg

    let storeName = 'myObjCopy';

Optionally you can specify an indexedDB database name. By default, DPP will apply a database name of "DPP". Otherwise we can do the following:

    let idb_name = 'MY-DB';

Now create an instance of DPP using these as inputs:

    const {createDPP} = await import('https://cdn.jsdelivr.net/gh/robtweed/DPP/src/dpp_browser.min.js');

    let dpp = await createDPP({
      storeName: storeName,
      idb_name: idb_name
    });

You can now start DPP and attach your local object to its indexedDB copy:

    let myObj = await dpp.start();

You're ready to start using your object!

Note that this approach will use CDN versions of QOper8's resources.

Using a Local Copy of DPP, but a CDN Copy of QOper8

You can use DPP with your own local copy of DPP, but allow it to use a CDN copy of QOper8 behind the scenes.

Installing a local copy of DPP is described earlier above.

You need to decide on a store name: this is the name of the store that will be used by indexedDB to maintain your object. The name is entirely for you to decide, eg:

    let storeName = 'myObjCopy';

Optionally you can specify an indexedDB database name. By default, DPP will apply a database name of "DPP". Otherwise we can do the following:

    let idb_name = 'MY-DB';

Now create an instance of DPP using these as inputs:

    const {createDPP} = await import('/dpp/dpp_browser.min.js');
    const {DPP} = await import('/dpp/dpp.min.js');

    let dpp = await createDPP({
      storeName: storeName,
      idb_name: idb_name,
      DPP: DPP
    });

You can now start DPP and attach your local object to its indexedDB copy:

    let myObj = await dpp.start();

You're ready to start using your object!

Using a Local Copies of DPP and QOper8

if you're using a local copy of DPP, it's probably most sensible to also use a local copy of QOper8.

Installing a local copy of DPP is described earlier above.

Installing a local copy of Qoper8 is very similar: copy/clone it from its Github repository to your own web server. For example, let's say you installed it in the folder /var/www/qoper8 on your Web Server.

You need to decide on a store name: this is the name of the store that will be used by indexedDB to maintain your object. The name is entirely for you to decide, eg:

    let storeName = 'myObjCopy';

Optionally you can specify an indexedDB database name. By default, DPP will apply a database name of "DPP". Otherwise we can do the following:

    let idb_name = 'MY-DB';

You can now just use the DPP module itself and create an instance of DPP using its constructor directly:

    const {DPP} = await import('/dpp/dpp.min.js');
    const {QOper8} = await import('/qoper8/QOper8.min.js');

    let dpp = new DPP({
      storeName: storeName,
      idb_name: idb_name,
      QOper8: QOper8
    });

You can now start DPP and attach your local object to its indexedDB copy:

    let myObj = await dpp.start();

You're ready to start using your object!

Using DPP Installed Using NPM

If you're using a bundler such as WebPack to create your front-end code, you'll need to use the NPM/Node.js-based approach described below.

If you installed DPP using NPM (see above), you use it as follows:

You need to decide on a store name: this is the name of the store that will be used by indexedDB to maintain your object. The name is entirely for you to decide, eg:

    let storeName = 'myObjCopy';

Optionally you can specify an indexedDB database name. By default, DPP will apply a database name of "DPP". Otherwise we can do the following:

    let idb_name = 'MY-DB';

Now import the createDPP function and use it to create an instance of DPP. Then link the local object of your choice (eg myObj) to its corresponding copy in indexedDB:

    import {createDPP} from "dpp-db/node";

    let dpp = createDPP({
      storeName: storeName,
      idb_name: idb_name
    });

    let myObj = await dpp.start();

You're ready to start using your object!


Using Your Object

You can now add properties to your local object, and subsequently change any content within those properties: as far as you're concerned, you just use your object as normal. Every change you make to your object will automatically be mirrored in the decoupled indexedDB copy.

if you restart your script in your browser, you'll find that the previous contents of your object are recovered automatically!

Worked Example

This simple example adds to a persistent object named myObj each time the page is loaded/reloaded.

It includes a button to clear the object down.

Here's the module file:

app.js

    (async () => {
      const {createDPP} = await import('https://cdn.jsdelivr.net/gh/robtweed/DPP/src/dpp_browser.min.js');
      let dpp = await createDPP({storeName: 'demo'});

      // recover myObj from indexedDB if it exists

      let myObj = await dpp.start();

      // if it didn't previously exist in indexedDB, 
      //  instantiate its counter and arr properties

      if (!myObj.counter) myObj.counter = 0;
      if (!myObj.arr) myObj.arr = [];

      // increment the counter and add a new element to the array

      myObj.counter++;

      myObj.arr.push({
        time: Date.now()
      });

      // display the results in the web page

      document.getElementById('content').textContent = JSON.stringify(myObj, null, 2);

      // add the clear-down button handler

      document.getElementById('cleardown').addEventListener("click", async () => {

        // use the DPP object's clear() method to clear down all content,
        // but leaving the DPP object instantiated
        //  await its completion and then reload the page

        await myObj.clear();
        document.location.reload();
      });
    })();

Here's the web page that loads the module and displays the results:

index.html

    <!DOCTYPE html>
    <html lang="en">
      <head>
        <title>DPP Demo</title>
      </head>
      <body>
        <script type="module" src="./app.js"></script>

        <button id="cleardown">Click Me to Clear Down Object</button>
        <br />
        <br />

        <div>Contents of the myObj Object:</div>
        <br />
        <pre id="content"></pre>

      </body>
    </html>

You'll also be able to see the persisted copy of the myObj object in the indexedDB database by using the Application tab in your browser's development tools panel. Remember that you don't normally need to access the indexedDB database yourself, but it's interesting to inspect its contents.

DPP APIs

  • Loading the DPP Creation Module into your browser:

    const {createDPP} = await import(/path/to/dpp_browser.js);
    

or, if building using local copies of DPP and QOper8, load the DPP module itself:

  const {DPP} = await import(/path/to/dpp.min.js);

or, if building using Node.js/NPM

  import {createDPP} from 'dpp-db/node';
  • Starting DPP:

    • Using DPP and QOper8 from CDN

      let dpp = await createDPP({
        storeName: storeName,
        idb_name: idbName,           // optional, 'DPP' used if omitted
      });
      
    • Using a local copy of DPP (but CDN copy of QOper8):

      const {DPP} = await import('/path/to/dpp.min.js');
      
      let dpp = await createDPP({
        storeName: storeName,
        idb_name: idbName, 
        DPP: DPP
      });
      
    • Using local copies of both DPP and QOper8:

      const {DPP} = await import('/path/to/dpp.min.js');
      const {QOper8} = await import('path/to/qoper8.min.js');
      
      let dpp = new DPP({
        storeName: storeName,
        idb_name: idbName,
        DPP: DPP,
        QOper8: QOper8
      });
      
  • creating a local object and attaching it to its DPP-managed copy in indexedDB:

    let local_object = await dpp.start(mode);
    
    • If mode is not specified, the Proxy Object specified by local_object will be restored with any existing data from the specified indexedDB ObjectStore

    • If you specify a value of new for mode, then the specified indexedDB ObjectStore is cleared down and the Proxy Object will be empty. For example:

      let local_object = await dpp.start('new');

  • deleting all content within a local object:

      await local_object.clear();
    

    As this method can take time to complete, you should always use await when invoking it.

Limitations to DPP

  • Your Persistent Object(s) are Proxy Objects (actually deep proxies, with Proxies at every key level).

  • A DPP Proxy Object can be as deeply-nested as you want, and can contain any mixture of:

    • simple key/value pairs
    • objects
    • arrays
  • A DPP Proxy Object cannot contain:

    • methods
    • functions
    • classes
  • You must not create circular references within your Proxy Object.

  • Note that the base Proxy Object variable cannot be used as a simple variable, ie the following will not be persisted, because you'll overwrite the Proxy Object:

      let a = await createDPP({
        storeName: 'a-copy',
        mode: 'new'
      });
    
      a = 'hello world';
    

    Instead, you must define at least one property of the Proxy Object. ie the following will be persisted:

    a.value = 'hello world';
    
    a.arr = [1, 2, 3];
    
    a.obj = {hello: 'world};    
    
  • Although indexedDB is capable of storing very large volumes of data, you need to be aware that the first thing that happens when you re-load a DPP script is that the entire contents of the indexedDB store that holds the copy of your local object must be traversed in order to be retrieved. Whilst this occurs very quickly for small or reasonably-sized objects, clearly the larger your object, the more time DPP will take to recover your object from indexedDB.

    We therefore recommend that DPP is used for small persistent objects. You should probably run some benchmark tests to confirm the speed of retrieval of typical instances of the objects you wish to use with DPP.

  • You need to be aware that different browsers have different policies for the retention times of data held in indexedDB. This document from the authors of the Dexie indexedDB wrapper is quite helpful, and applies equally to data you store in indexedDB using DPP.

Reactive Use of DPP

An additionally powerful feature of DPP is that it can be used to implement reactive behaviour within your applications, simply by listening for two events:

  • Save
  • Delete

These events are emitted whenever any changes are made to your persistent object.

Save Event

To handle the Save event:

    dpp.on('save', (data) => {
      // your handler
    });

The data object structure is:

    {
      key: {{key array}},
      value: {{ new value at this key }}
    }

For example, if your persistent object was todos and you saved:

    todos.byId[1] = {
      text: 'Start Project',
      completed: false
    }

then two save events would be emitted with data being:

    {
      key: ['byId', 1, 'text'],
      value: 'Start Project'
    }

and

    {
      key: ['byId', 1, 'completed'],
      value: false
    }

Delete Event

To handle the Delete event:

    dpp.on('delete', (key) => {
      // your handler
    });

The key argument is an array representing the keys for the deleted item.

For example, if you invoked:

    delete todos.byId[1]

then the key argument of the Delete event would be:

    ['byId', 1]

You'll probably find that you don't actually need the key and data arguments in your reactive logic: you'll simply want to trigger a state update to your User Interface whenever either event is fired.

Committed Event

The committed event is emitted whenever DPP receives back an ok response message from the QOper8 WebWorker. On receipt of this event you can be sure that a save or delete change to the DPP proxy object has been committed into its persistent copy within indexedDB.

To handle the Committed event:

    dpp.on('committed', () => {
      // your handler
    });

Note that it has no argument.

Secure/Encrypted Persistent Storage

Background

You've probably realised that by default, DPP stores its keys and data into indexedDB as clear text. This will often be quite satisfactory for many needs.

In terms of data security, you need to also understand that like all local storage within a browser, indexedDB is completely accessible by the user of the browser: it can be inspected using the browser's Developer Tools and can also be deleted using the Developer Tools.

The APIs for accessing and maintaining each indexedDB database and the stores within it are only accessible to scripts that run in the same origin. In other words, an indexedDB database created in your browser when accessing one web application at one endpoint will not be accessible to scripts running in another session on your browser that accesses and uses a different web application and endpoint.

However, if your front-end code is using third-party JavaScript scripts, you need to be aware that those scripts potentially have full and unfettered access to any indexedDB databases, even if you loaded those scripts from a different origin. This means that a rogue third-party script could potentially access, use and even modify all the data in your indexedDB database at that origin!

DPP's Optional Encryption

As a means of helping to mitigate at least the latter risk, DPP includes a mechanism for encrypting the data (though not the keys) for records held in indexedDB.

To use DPP in this way, you need to supply two secret credentials to DPP's start() API. These are ostensibly known as username and password, but are really just two pieces of secret information that you supply when starting DPP, eg:

    let my_local_object = await dpp.start({
      auth: {
        username: secret_1,
        password: secret_2
      }
    });

Of course you don't want these secret credentials to be available to any third-party scripts, so you should invoke this authenticated startup of DPP within a closure. You'd also not want to hard-code the values of the credentials into your code, but ask for them from the user or acquire them in some other secure way that was also unavailable to a third-party script. If you hard code the secret values into a script used in your front-end code, then they will be visible to the user simply by viewing the script code.

When started with authentication, DPP has privileged access to these credentials, but does not expose them outside of its own internal closures (both within the main DPP process and its QOper8 Worker process).

You'll discover that that indexedDB datastore you use for mirroring your local object will now have a record stored with a key of ['signature']. This is an HMAC-SHA256 hash value derived from the username and password secrets and is used only to denote ownership to DPP of the database. It is NOT used as an encryption/decryption key.

The actual encryption/decryption key is known only to DPP and the DPP QOper8 Worker handler method, and is derived from the specified authentication credentials. The indexedDB database does not, therefore, hold any information that can be used as a clue for decrytping its contents.

Once started in this way, the data values of any records you create in your local object are AES-GCM encrypted before they are stored in indexedDB.

When you invoke the start() API with authentication, any data stored in the indexedDB store will be decrypted before retrieval, so your local object will be in plain text. Once again, the key to protection from a rogue third-party script will be to maintain your local object from within a closure to which any third-party scripts do not have access.

Notices and Limitations

Notices

M/Gateway Developments Ltd does not provide any warranty with respect to the security provisions implemented in DPP. You use its authenticated access entirely at your own risk, and it is your responsibility to conduct your own security audit of its operations to confirm its fitness for purpose in your applications. Your use of DPP is subject to the terms and conditions of its Apache 2.0 License (see below).

With respect to any audit you conduct, you should note that all of the source code used by DPP and QOper8 is provided in full within their respective Github repositories, and you should also note that neither DPP nor QOper8 use or rely upon any third-party code. They only rely upon the native capabilities provided by the browser's JavaScript run-time environment.

Limitations

You need to be aware of a number of limitations to the security provided by DPP.

  • if you allow your local object or the security credentials to be accessible to any third-party scripts, then they will potentially have full access to its data. Make use of JavaScript closures to prevent this. If you do not know how to use JavaScript closures, please seek advice before attempting to use DPP in a secure way.

  • The user of the browser will still be able to delete the indexedDB store via its Developer Tools. They will not, however, be able to see anything other than the encrypted data values within the store and should not be able to decrypt its contents.

  • DPP only encrypts the data values stored for each key in your local object. If you create indices from any data values and store them as keys in your local object, those values will be visible in the indexedDB store.

  • OWnership of, and therefore access by DPP to, an authenticated/encrypted store is determined by the two secret values supplied when it was first started (ie username and password). You cannot change either the username or password: if you do so, you will lose access via DPP to the store.

  • If you started DPP without authentication, the specified indexedDB store can only be re-accessed without authentication and DPP will store its data in indexedDB in the clear.

  • Conversely, if you started DPP with authentication, the specified indexedDB store will be encrypted, and cannot be re-accessed without providing the correct authentication credentials.

The JSON Database

With DPP, you now, in effect, have a JSON database at your disposal within your browser!

With DPP, JavaScript Objects are no longer an ephemeral, in-memory data structure. DPP will persist them in the indexedDB database for you, but, as far as you're concerned, you're just using plain old standard JavaScript Objects/JSON.

Their hierarchical nature means that you can, in effect, model any other kind of database using it. By way of example, you'll find included in this repository's /db folder pre-built examples of two such NoSQL database models using DPP:

You can read more about these NoSQL stores that you can use within a browser here:

Of course, these are just two simple examples. There's nothing to stop people using DPP to design other, more complex database models on top of plain old JavaScript Objects/JSON, eg a Graph database or even a Relational one.

Enjoy DPP!.

License

Copyright (c) 2023 MGateway Ltd,
Banstead, Surrey UK.
All rights reserved.

https://www.mgateway.com
Email: rtweed@mgateway.com

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

  http://www.apache.org/licenses/LICENSE-2.0                           
                                                                       

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and
limitations under the License.

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Last updated on 09 Nov 2023

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