seneca

seneca - Node.js module

A Node.js toolkit for startups building Minimum Viable Products

Seneca is a toolkit for organizing the business logic of your app. You can break down your app into "stuff that happens", rather than focusing on data models or managing dependencies.

For a gentle introduction to this module, see the senecajs.org site.

If you're using this module, feel free to contact me on twitter if you have any questions! :) @rjrodger

Current Version: 0.5.16

Tested on: Node 0.10.24, and 0.11 on Travis

Use this module to define commands that work by taking in some JSON, and, optionally, returning some JSON. The command to run is selected by pattern-matching on the the input JSON. There are built-in and optional sets of commands that help you build Minimum Viable Products: data storage, user management, distributed logic, caching, logging, etc. And you can define your own product by breaking it into a set of commands - "stuff that happens".

That's pretty much it.

Why do this?

It doesn't matter,

• who provides the functionality,
• where it lives (on the network),
• what it depends on,
• it's easy to define blocks of functionality (plugins!).

So long as some command can handle a given JSON document, you're good.

Here's an example:

var seneca = require('seneca')()

seneca.add( {cmd:'salestax'}, function(args,callback){
  var rate  = 0.23
  var total = args.net * (1+rate)
  callback(null,{total:total})
})

seneca.act( {cmd:'salestax', net:100}, function(err,result){
  console.log( result.total )
})

In this code, whenever seneca sees the pattern {cmd:'salestax'}, it executes the function associated with this pattern, which calculates sales tax. Yah!

The seneca.add method adds a new pattern, and the function to execute whenever that pattern occurs.

The seneca.act method accepts an object, and runs the command, if any, that matches.

Where does the sales tax rate come from? Let's try it again:

seneca.add( {cmd:'config'}, function(args,callback){
  var config = {
    rate: 0.23
  }
  var value = config[args.prop]
  callback(null,{value:value})
})

seneca.add( {cmd:'salestax'}, function(args,callback){
  seneca.act( {cmd:'config', prop:'rate'}, function(err,result){
    var rate  = parseFloat(result.value)
    var total = args.net * (1+rate)
    callback(null,{total:total})
  })
})

seneca.act( {cmd:'salestax', net:100}, function(err,result){
  console.log( result.total )
})

The config command provides you with your configuration. This is cool because it doesn't matter where it gets the configuration from

• hard-coded, file system, database, network service, whatever. Did you have to define an abstraction API to make this work? Nope.

There's a little but too much verbosity here, don't you think? Let's fix that:

var shop = seneca.pin({cmd:'*'})

shop.salestax({net:100}, function(err,result){
  console.log( result.total )
})

By pinning a pattern, you get a little API of matching function calls. The shop object gets a set of methods that match the pattern: shop.salestax and shop.config.

Programmer Anarchy

The way to build Node.js systems, is to build lots of little processes. Here's a great talk explaining why you should do this: Programmer Anarchy.

Seneca makes this really easy. Let's put configuration out on the network into its own process:

seneca.add( {cmd:'config'}, function(args,callback){
  var config = {
    rate: 0.23
  }
  var value = config[args.prop]
  callback(null,{value:value})
})

seneca.listen()

The listen method starts a web server that listens for JSON messages. When these arrive, they are submitted to the local Seneca instance, and executed as actions in the normal way. The result is then returned to the client as the response to the HTTP request. Seneca can also listen for actions via a message bus.

Your implementation of the configuration code stays the same.

The client code looks like this:

seneca.add( {cmd:'salestax'}, function(args,callback){
  seneca.act( {cmd:'config', prop:'rate'}, function(err,result){
    var rate  = parseFloat(result.value)
    var total = args.net * (1+rate)
    callback(null,{total:total})
  })
})

seneca.client()

var shop = seneca.pin({cmd:'*'})

shop.salestax({net:100}, function(err,result){
  console.log( result.total )
})

On the client-side, calling seneca.client() means that Seneca will send any actions it cannot match locally out over the network. In this case, the configuration server will match the cmd:config pattern and return the configuratin data.

Again, notice that your sales tax code does not change. It does not need to know where the configuration comes from, who provides it, or how.

You can do this with every command.

Keeping the Business Happy

The thing about business requirements is that have no respect for common sense, logic or orderly structure. The real world is messy.

In our example, let's say some countries have single sales tax rate, and others have a variable rate, which depends either on locality, or product category.

Here's the code. We'll rip out the configuration code for this example.

// fixed rate
seneca.add( {cmd:'salestax'}, function(args,callback){
  var rate  = 0.23
  var total = args.net * (1+rate)
  callback(null,{total:total})
})


// local rates
seneca.add( {cmd:'salestax',country:'US'}, function(args,callback){
  var state = {
    'NY': 0.04,
    'CA': 0.0625
    // ...
  }
  var rate = state[args.state]
  var total = args.net * (1+rate)
  callback(null,{total:total})
})


// categories
seneca.add( {cmd:'salestax',country:'IE'}, function(args,callback){
  var category = {
    'top': 0.23,
    'reduced': 0.135
    // ...
  }
  var rate = category[args.category]
  var total = args.net * (1+rate)
  callback(null,{total:total})
})


var shop = seneca.pin({cmd:'*'})

shop.salestax({net:100,country:'DE'}, function(err,result){
  console.log( 'DE: '+result.total )
})

shop.salestax({net:100,country:'US',state:'NY'}, function(err,result){
  console.log( 'US,NY: '+result.total )
})

shop.salestax({net:100,country:'IE',category:'reduced'}, function(err,result){
  console.log( 'IE: '+result.total )
})

In this case, you provide different implementations for different patterns. This lets you isolate complexity into well-defined places. It also means you can deal with special cases very easily.

Changelog

0.5.16: 2014-04-09

• use norma for api function arguments
  • parambulator arg checking moved into arg specification
  • stats moved outside to rolling-stats module
  • seneca.fix sets fixed args
  • started annotating source code
  • small improvement to error handling and logging