A quick tour of Arangojs

I’ve been using ArangoDB for a while now, but for most of that time I’ve been using it from Ruby. I’ve dabbled with the Guacamole library and even took a crack at writing my own, but switching to Javascript has led me to get to know Arangojs.

Given that Arangojs is talking to ArangoDB via its HTTP API, basically everything you do is asynchronous. There are a few ways of dealing with async code in Javascript, and Arangojs has been written to support basically all of them.

Arangojs’s flexibility and my inexperience with the new Javascript syntax combined to give me bit of an awkward start, so with a little learning under my belt I thought I would write up some examples that would have saved me some time.

My most common use case is running an AQL query, so lets use that as an example. First up, I’ve been saving my config in a separate file:

// arango_config.js
//Using auth your url would look like:
// "http://uname:passwd@127.0.0.1:8529"
module.exports = {
  "production" : {
    "databaseName": process.env.PROD_DB_NAME,
    "url": process.env.PROD_DB_HOST,
  },
  "development" : {
    "databaseName": process.env.DEVELOPMENT_DB_NAME,
    "url": process.env.DEVELOPMENT_URL
  },
  "test" : {
    "databaseName": "test",
    "url": "http://127.0.0.1:8529",
  },
}

With that I can connect to one of my existing databases like so:

var config = require('../arangodb_config')[process.env.NODE_ENV]
var db = require('arangojs')(config)

This keeps my test database nicely separated from everything else and all my db credentials in the environment and out of my project code.

Assuming that our test db has a collection called “documents” containing a single document, we can use Arangojs to go get it:

db.query('FOR doc IN documents RETURN doc', function(err, cursor) {
  cursor.all(function(err, result) {
    console.log(result)
  })
})

Which returns:

[ { foo: 'bar',
    _id: 'documents/206191358605',
    _rev: '206192931469',
    _key: '206191358605' } ]

While this is perfectly valid Javascript, its pretty old-school at this point since ECMAScript 2015 is now standard in both Node.js and any browser worth having. This means we can get rid of the “function” keyword and replace it with the “fat arrow” syntax and get the same result:

db.query('FOR doc IN documents RETURN doc', (err, cursor) => {
  cursor.all((err, result) => {
    console.log(result)
  })
})

So far so good but the callback style (and the callback-hell it brings) is definitely an anti-pattern. The widely cited antidote to this is promises:

db.query('FOR doc IN documents RETURN doc')
  .then((cursor) => { return cursor.all() })
  .then((doc) => { console.log(doc) });

While this code is functionally equivalent, it operates by chaining promises together. While it’s an improvement over callback-hell, after writing a bunch of this type of code, I ended up feeling like I had replaced callback hell with promise hell.

what-fresh-hell-is-this

The path back to sanity lies in ECMAScript 2016 aka ES7 and the new async/await keywords. Inside a function marked as async, you have access to an await keyword which allows you to write code that looks synchronous but does not block the event loop.

Using the babel transpiler lets us use the new ES7 syntax right now by compiling it all down to ES5/6 equivalents. Installing with npm install -g babel and running your project with babel-node is all that you need to be able to write this:

async () => {
    let cursor = await db.query('FOR doc IN documents RETURN doc')
    let result = await cursor.all()
    console.log(result)
}()

Once again we get the same result but without all the extra cruft that we would normally have to write.

One thing that is notably absent in these examples is the use of bound variables in our queries to avoid SQL injection (technically parameter injection since this is NoSQL).

So what does that look like?

async () => {
    let bindvars = {foo: "bar"}
    let cursor = await db.query('FOR doc IN documents FILTER doc.foo == @foo RETURN doc', bindvars)
    let result = await cursor.all()
    console.log(result)
}()

But Arangojs lets you go further, giving you a nice aqlQuery function based on ES6 template strings:

async () => {
    let foo = "bar"
    let aql = aqlQuery`
      FOR doc IN documents
        FILTER doc.foo == ${foo}
          RETURN doc
    `
    let cursor = await db.query(aql)
    let result = await cursor.all()
    console.log(result)
}()

Its pretty astounding how far that simple example has come. It’s hard to believe that it’s even the same language.
With Javascript (the language and the community) clearly in transition, Arangojs (and likely every other JS library) is compelled to support both the callback style and promises. It’s a bit surprising to see how much leeway that gives me to write some pretty sub-optimal code.

With all the above in mind, suddenly Arangojs’s async heavy API no longer feels intimidating.

The documentation for Arangojs is simple (just a long readme file) but comprehensive and there is lots more it can do. Hopefully this little exploration will help people get started with Arangojs a little more smoothly than I did.

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