gestalt

gestalt

Gestalt is a library for managing configuration information for node.js applications. The main premise of gestalt is that the underlying configuration for an application may change while the application is still running. Gestalt gives you a framework detecting and reacting to these changes without having to completely restart your application.

There are a couple of motivations for gestalt. Configuration of a large software system is often complicated - there are of course many tools out there for gathering configuration information from a bunch of different sources. nconf for node is a good one, and gestalt is to some extent based upon it, ( but also influenced by configliere for ruby and the configuration node structure of chef). There are a couple of things that many of these tools do not do. First, configuration files (and other sources) can change, and it would be nice to be able to react to these changes on-the-fly. Second, when you have a sufficiently complicated system of default and override configuration sources, it can become difficult to figure out exactly where a particular setting came from. Gestalt solves both of these problems. It has a per-value event change tracking system so that you can track changes to individual settings to your configuration. It also rigorously keeps track of where the values for particular settings came from.

Basics

The basic object is a Configuration:

var gestalt = require('gestalt');
var options = {source: 'Source'};
var config = new gestalt.Configuration( options );

// config.set( name, value, source )
config.set("x", 'stuff');   // source defaults to the config source
config.set("y", 6, 'HERE'); // but source can be set per-value

var x = config.get("x");  // returns 'stuff'
var y = config.get("y");  // returns 6

Configuration names are hierarchical - use a ':' to delimit namespaces. The namespaces become nested Configuration objects.

config.set("owner:name", 'Joe');
config.set("owner:phone", '5551234');

var ownerConfig = config.get('owner'); // another Configuration object
var owner = ownerConfig.toObject();    // convert to a plain javascript object:
                                       // { name: 'Joe', phone: '5551234' }
 

Values can be primative values (numbers, strings, booleans, etc.). Assignments of structured objects get destructured into nested Configuration objects.

config.set("neighbor", {name: 'Fred', phone: '5559876'} );
var fred_phone = config.get( 'neighbor:phone' );

In many cases (not quite all...this is not yet supported for RemapConfig objects...) it is possible to turn a configuration object back into a regular object. In fact, if a configuration object looks like an array (all integer keys...) toObject will in fact return an array.

Events

Configuration objects are EventEmitters. When a value of a configuration object changes, it emits a 'change' event.

config.on('change', function( change) {
    console.log("name: %s, value: %s, old_value: %s, source: %s ",
        change.name, change.value, change.old_value, change.source);
});

config.set("owner:phone", "5554444", "phone book");
// listener prints
// name: owner:phone, value: 5554444, old_value: 5551234, source: phone book

You can also listen to events on the nested configuration objects. Note that configuration names in the events are reported relative to the configuration object you are listening to.

Containers

The ConfigContainer class gives you a way to set up a system of defaults and overrides of configuration information that comes from different sources. For instance, if you allow configuration parameters to be set at the command line, in environment variables, or from a configuration file, this class can help you out.

var override = new gestalt.Configuration({source: "Override"});
var def = new gestalt.Configuration({source: "Default"});
var container = new gestalt.ConfigContainer({source: "Container"});

container.addOverride( override );
container.addDefault(def);

container.on( 'change',function(change) { 
    console.log("%j", change ); 
});

def.set("a",1);
// logs {"name":"a", "value":1, "source":"Default" }

container.set("a",2);
// logs {"name":"a", "value":2, "old_value": 1, "source":"Container" }

override.set("a",3);
// logs {"name":"a", "value":3, "old_value": 2, "source":"Override" }

container.set("a",4);
// logs nothing - overall value does not change

def.set("a",5);
// also logs nothing

container.remove("a");
// logs nothing

override.remove("a");
// logs {"name":"a", "value":5, "old_value":3, "source":"Default" }

Remap Configs

Configuration files are often structured, while environment variables and command line arguments are usually not structured. However, it is often the case that a program will use an environment variable or a command line argument to override a setting in a structured document. RemapConfig objects give you a formal way to show exactly what part of the structure a given command line argument will override.

The constructor for RemapConfig expects to see a couple of options in the options argument. First, it needs a reference to the original configuration object that is being remapped. Second, it needs a function that will map names from the original object into the new object space.

var gestalt       = require('../lib/gestalt'),
    Configuration = gestalt.Configuration,
    RemapConfig   = gestalt.RemapConfig; 


function mapper(old) {
    // map names that start with "f" to
    // new:<old_name>
    if( old.match(/^f/ )) {
	return "new:" + old;	
    } else {
	// ignore everything else
	return undefined;
    }
}

var c = new Configuration();
c.set("foo",1);
c.set("gak",4);

var r = new RemapConfig( { mapper: mapper, original: c } );

console.log( r.get('new:foo') );
// prints out "1"

Not surprisingly, there are a couple of restrictions on this type of configuration object. First, it is read only. Second, the remapper function can show that it ignores part of the object space by returning undefined for some values. For the rest of the values, it must make sure to return unique new names for different old names. Third, the toObject function does not try to detect array-like objects.

Remapped objects do pass on events, and can be used as overrides or defaults in a config container.

API

Configuration

A Configuration object is a container of name value pairs. The names can include colon separated hierarchical namespaces.

• constructor Configuration( options )

Creates a configuration object. Options include

• source

The default source for changes to this object. If no source is given for a set operation on the Configuration, the default is used instead. Note that only the initial value of this option is important. If it is changed with the options() method, it will not change the default source for an object.

• destructure_assignments

By default, this option is on. When turned on, if you try to assign an object or an array to a name in the configuration, it will destructure the object into nested namespace structures.

• initial_state

State to transition to after initialization. Defaults to 'ready' for many configuration objects. Defaults to 'not_ready' for files, zookeeper, and other objects that load asynchonously.

• get(name)

Returns the value assigned to name, or undefined if not present. Namespaces are separated by colons. If name is an array, it is treated the same as if it were a single string joined together by colons.

• getValSource(name)

The same as get, only instead of the assigned value, it returns an object that contains a value and a source field. The value is the same as the value one would expect from get. The source field contains the source of the value.

• set(name, value, source)

Sets the value of name. If no source is given, the default source for the Configuration is used. If the configuration object does not have a default source, then the source becomes a reference to the line number and file of the function calling set.

• update(name, value, source)

For standard Configuration objects, update works the same as set, except that if no source is provided in the update call, the configuration's default source is not used. For ConfigContainers, update and set have different behaviors.

• has(name)

Returns true if there is a value defined for the given name.

• keys()

Returns an array containing all of the keys that have assigned values in the configuration.

• each( function( value, name, configuration ) )

Iterates through the keys and values of the configuration, calling the supplied function once for each setting.

• remove(name)

Deletes the name from the configuration.

• report()

Generates a detailed report of all of the names.

• toObject()

Builds a javascript object representing the current state of the Configuration. Namespaces are converted to nested objects. If a namespace has the appearance of an array in that its internal names are sequential numbers starting with 0, it will be converted into an array instead of a regular object.

• options( options )

Returns the options object for the Configuration. If passed an object, it will use it to override the existing options. Note that changing the options of a Configuration does not change the options of existing nested namespace configurations, but it will affect any namespaces created after the change.

• addPatternListener(pattern, callback )

Calls the callback whenever there is a change to the configuration that matches the pattern. Pattern could be a string, or a regex, in which cases match means that the name of the changed value either equals the string or matches the regex respectively. Pattern could also be a function which return a truthy value if the change matches its criteria.

Events

• change

Change events are emitted whenever the Configuration object detects that something has changed in the data. The handlers to these events are passed an object describing the change:

 change = { name: "a:b:c",        // name of the value that changed
            value: 5,             // the new value
            old_value: undefined, // old value
            source: "X"};         // the source of the change or the last value (for deletions)

• ready

A ready event is emitted when the object becomes ready for use. For ConfigFile objects, this means that the file has been loaded. For other objects, it might mean something different. By default, a configuration object will emit a 'ready' event in the next tick after it was initialized. This behavior can be changed by setting the 'initial_state' option to 'not ready' and then changing the state to 'ready' through some other means (e.g. upon successfully loading a file).

• invalid

An invalid event is fired when something goes wrong - for instance if there is a problem parsing a configuration file. While a configuration is in an invalid state, you should not trust its data until it becomes ready again.