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# JavaScript Database (JSDB)
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__Work in progress:__ A transparent, in-memory, streaming write-on-update JavaScript database for Small Web applications that persists to a JavaScript transaction log.
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__Needless to say, this is not ready for use yet. But feel free to take a look around.__

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## Roadmap

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  - [x] Implement persistence (15 Sept)
  - [x] Add unit tests for persistence (19 Sept)
  - [x] Document persistence (19 Sept)
  - [x] Add persistence example (19 Sept)
  - [x] Implement queries (22 Sept)
  - [x] Refactor to implement persistence as append-only JavaScript transaction log and use streaming writes (29 Sept)
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  - [ ] Update documentation to reflect new persistence engine.
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  - [ ] Add unit tests for queries
  - [ ] Document queries
  - [ ] Add queries example
  - [ ] Implement indices
  - [ ] Add unit tests for indices
  - [ ] Document indices
  - [ ] Add indices example
  - [ ] Use/test on upcoming small-web.org site
  - [ ] Release version 1.0

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## To install

Currently, you need to clone the repo as this is a work-in-progress and no releases have been made yet.

## Usage

Here’s a quick example to whet your appetite:

```js
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const JSDB = require('.')
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// Create your database in the test folder.
// (This is where your JSON files – “tables” – will be saved.)
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const db = new JSDB('db')
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// Create test/people.json with some data.
db.people = [
  {name: 'Aral', age: 43},
  {name: 'Laura', age: 34}
]

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// Correct Laura’s age. (This will automatically update db/people.js)
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db.people[1].age = 33

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// Add Oskar to the family. (This will automatically update db/people.js)
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db.people.push({name: 'Oskar', age: 8})
```

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After running the above script, take a look at the resulting database table in the `./db/people.js` file.

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## JavaScript Data Format (JSDF)

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JSDB tables are written into JavaScript Data Format (JSDF) files. A JSDF file is a plain JavaScript file that comprises an append-only transaction log that creates the table in memory. For our example, it looks like this:

```js
globalThis._ = [];
(function () { if (typeof define === 'function' && define.amd) { define([], globalThis._); } else if (typeof module === 'object' && module.exports) { module.exports = globalThis._ } else { globalThis.people = globalThis._ } })();
_[0] = JSON.parse(`{"name":"Aral","age":43}`);
_[1] = JSON.parse(`{"name":"Laura","age":34}`);
_[1]['age'] = 33;
_[2] = JSON.parse(`{"name":"Oskar","age":8}`);
_['length'] = 3;
_[2]['name'] = `Osky`;
```

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(Note: the format is a work-in-progress like the rest of the project at the moment. I am considering cleaning up the superfluous length statements and weighing up the performance hit of maintaining state to enable that versus the potential use cases of a cleaner log – like history replay for example – and file size/initial load speed, which is really not too much of a concern given that they occur at server start for our use cases).

## It’s just JavaScript!

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Given that a JSDF file is just JavaScript, and includes a [UMD](https://github.com/umdjs/umd)-like declaration in its header (the first two lines), you can simply `require()` it as a module in Node.js or even load it in a script tag.

For example, create an _index.html_ file with the following content in the same folder as the other script and serve it locally using [Site.js](https://sitejs.org) and you will see the data printed out in your browser:

```html
<script src="db/people.js"></script>
<h1>People</h1>
<ul>
<script>
  people.forEach(person => {
    document.write(`<li>${person.name} (${person.age} years old)</li>`)
  })
</script>
</ul>
```

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## JavaScript Query Language (JSQL)

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Of course, when you load the data in directly, you are not running it inside JSDB so you cannot update the data or use the JavaScript Query Language (JSQL) to query it.

To test that out, open a Node.js command-line interface (run `node`) from the directory that your scripts are in and enter the following commands:
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```js
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const JSDB = require('.')
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// This will load test database with the people table we created earlier.
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const db = new JSDB('db')
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// Let’s carry out a query that should find us Osky.
console.log(db.people.where('age').isLessThan(21).get())
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```
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## Compaction

When you load in a JSDB table, by default JSDB will compact the JSDF file.

Compaction is important for two reasons:

  - It is when deleted data is actually deleted from disk. (Privacy.)
  - It is when old version of updated data are actually removed. (Again, privacy.)

Compaction will also reduce the size of your tables.

You do have the option to override the default behaviour and keep all history. You might want to do this, for example, if you’re creating a web app that lets you create a drawing and you want to play the drawing back stroke by stroke, etc.

Now that you’ve loaded the file back, look at the `./db/people.js` JSDF file again to see how it looks after compaction:

```js
globalThis._ = [];
(function () { if (typeof define === 'function' && define.amd) { define([], globalThis._); } else if (typeof module === 'object' && module.exports) { module.exports = globalThis._ } else { globalThis.people = globalThis._ } })();
_[0] = JSON.parse(`{"name":"Aral","age":43}`);
_[1] = JSON.parse(`{"name":"Laura","age":33}`);
_[2] = JSON.parse(`{"name":"Osky","age":8}`);
```

Ah, that is neater. You can see that Laura’s record is created with the correct age from the outset and Oskar’s name is set at Osky from the outset also.

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(You can find these examples in the `examples/basic` folder of the source code.)
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## Use case

A data layer for simple [Small Web](https://ar.al/2020/08/07/what-is-the-small-web/) sites for basic public (e.g., anonymous comments on articles) or configuration data. Built for use in [Site.js](https://sitejs.org).

## Features

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  - __Transparent:__ if you know how to work with arrays and objects and call methods in JavaScript, you already know how to use JSDB? It’s not called JavaScript Database for nothing.
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  - __Automatic:__ it just works. No configuration.

## Limitations

  - __Small Data:__ this is for small data, not Big Data™.

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  - __For Node.js:__ will not work in the browser. (Although the data table can be loaded in the browser.)
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  - __Runs on untrusted nodes:__ this is for data kept on untrusted (server) nodes. Use it judiciously if you must for public data, configuration data, etc. If you want to store personal data or model human communication, consider end-to-end encrypted and peer-to-peer replicating data structures instead to protect privacy and freedom of speech. Keep an eye on the work taking place around the [Hypercore Protocol](https://hypercore-protocol.org/).

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  - __In-memory:__ all data is kept in memory and, [without tweaks, cannot exceed 1.4GB in size](https://www.the-data-wrangler.com/nodejs-memory-limits/). While JSDB will work with large datasets, that’s not its primary purpose and it’s definitely not here to help you farm people for their data, so please don’t use it for that. (If that’s what you want, quite literally every other database out there is for your use case so please use one of those instead.)
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  - __Streaming writes on update:__ writes are streamed to disk to an append-only transaction log as JavaScript statements and are both quick (in the single-digit miliseconds region on my development laptop with an SSD drive) and as safe as we can make them (synchronous as the kernel level).
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  - __No schema, no migrations__: again, this is meant to be a very simple persistence, query, and observation layer for local server-side data. If you want schemas and migrations, take a look at nearly every other database out there.
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## Events

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Given that a core goal for JSDB is to be transparent, you will mostly feel like you’re working with regular JavaScript collections (objects and arrays). At times, however, it might be useful to have access to the underlying abstractions like the table object. One of those instances is if you want to be notified of events.
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To listen for an event, access the special `__table__` property of your collection. e.g.,

```js
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db.people.__table__.addListener('persist', (table, change) => {
  console.log(`Table ${table.tableName} persisted change ${change.replace('\n', '')} to disk.`)
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})
```

### Table events

| Event name | Description                           |
| ---------- | ------------------------------------- |
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| persist    | The table has been persisted to disk. |
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## Performance characteristics

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  - Reads are fast (take fraction of a milisecond and are about an order of magnitude slower than direct memory reads).
  - Writes are fast (in the order of a couple of miliseconds on tests on my dev machine).
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## Limits
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  - Your database size is limited by available memory.
  - If your database size is larger than > 1GB, you should start your node process with a larger heap size than the default (~1.4GB). E.g., to set aside 8GB of heap space:
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  ```
  node --max-old-space-size=8192 why-is-my-database-so-large-i-hope-im-not-doing-anything-shady.js
  ```
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## Memory Usage

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__TODO: THIS SECTION NEEDS TO BE RE-WRITTEN WITH STATS FOR THE NEW STREAMING TRANSACTION LOG__

<strike>The reason JSDB is fast is because it keeps the whole database in memory. Also, to provide a transparent persistence and query API, it maintains a parallel object structure of proxies. This means that the amount of memory used will be multiples of the size of your database on disk.
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For example, using the simple performance example above, we clock:

| Number of records | Table size on disk | Memory used |
| ----------------- | ------------------ | ----------- |
| 1,000             | 183K               | 6.62MB      |
| 10,000            | 1.8MB              | 15.67MB     |
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| 100,000           | 18MB               | 74.50MB     |

</strike>
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## Developing

Please open an issue before starting to work on pull requests.

1. Clone this repository.
2. `npm i`
3. `npm test`

For code coverage, run `npm run coverage`.
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## Related projects, inspiration, etc.

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  - [Initial brainstorming (query language)](https://gist.github.com/aral/fc4115fdf338e02d735ae58e245817ce)
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  - [proxy-fun](https://github.com/mikaelbr/awesome-es2015-proxy)
  - [filejson](https://github.com/bchr02/filejson)
  - [Declaraoids](https://github.com/Matsemann/Declaraoids/blob/master/src/declaraoids.js)
  - [ScunMEngine](https://github.com/jlvaquero/SCUNM/blob/master/SCUNMEngine/SCUNMEngine.js)
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## Copyright

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&copy; 2020 [Aral Balkan](https://ar.al), [Small Technology Foundation](https://small-tech.org).