CDF Async Developer's Guide

CDF now supports proper, asynchronous, AJAX calls for all its querying. The following is a guide to converting old components and dashboards to the new async style, and developing new ones based on asynchronous querying.

Rationale

The first step to understanding the changes in the async patch is understanding the CDF component lifecycle. When a component is updated, the basic update lifecycle looks like this:

preExecution -> update -> postExecution

Usually, though, there will be a call to a data source, with a subsequent call to postFetch, and only then is the component rendered:

preExecution -> update -> query -> postFetch -> redraw -> postExecution

This is a more typical lifecycle, and one that has some important limitations. First, preExeuction and postExecution are entirely the responsibility of CDF itself, rather than the component. Because CDF has no control over the contents of the update method, it has no way of ensuring that, should the component execute an asynchronous query, postExecution only runs after redraw. In this case, you're likely to see this instead:

preExecution -> update -> postExecution -> query -> postFetch -> redraw

Which breaks the contract for postExecution running after the component is done updating. The solution here is that the component itself must take control of postExecution, while keeping the burden of implementing the lifecycle in CDF rather than passing it to the component developer. On a related topic, postFetch has become a de facto standard part of the lifecycle, yet its implementation was left to the component implenters, which leads to a fairly large amount of boilerplate code.

Our objective here was to retool the base component so as to deal with both of these issues, thus allowing queries to be performed asynchronously while reducing the developer effort involved in creating a component.

Component execution order and Priority

There are no major changes in the way components behave. There is, however an important caveat - since all components (that have been converted) will be executed simultaneously, we can no longer rely on the order of execution.

There's now an additional property named priority. The priority of component execution, defaulting to 5. The lower the number, the higher priority the component has. Components with same priority with be executed simultaneously. Useful in places where we need to give higher priority to filters or other components that need to be executed before other components.

This way there's no longer the need to use dummy parameters and postChange tricks to do, for instance, cascade prompts.

Backward compatibility and changes

We did a big effort in order to maintain backward compatibility, but some care has to be taken. What we do is assume that if components have no priority, we give them a sequential value, trying to emulate the old behavior. It's recommended that proper priorities are set in order to take advantage of the new improvements.

If using CDE, please note that if you edit a dashboard and save it, all components will have a default priority of 5. This may break the old behavior. If you need to change a dashboard, make sure you tweak the priorities, if needed.

Developing Components

Components desiring to use asynchronous queries should inherit from the new UnmanagedComponent, instead of BaseComponent. The UnmanagedComponent base class provides pre-composed methods that implement the core lifecycle, for a variety of different scenarios:

Since all these lifecycle methods expect a callback that handles the actual component rendering, it's conventional style to have that callback as a method of the Component, called redraw. It's also considered standard practice to use Function#bind or _.bind to ensure that, inside the redraw callback, this points to the component itself.

Use synchronous If Your Component Doesn't Use External Data

Components that don't use any external data at all can continue subclassing BaseComponent without any change of functionality. However, for the sake of consistency (or because you want querying to be optional -- see the section for details), your can use subclass UnmanagedComponent and use the synchronous lifecycle method to emulate BaseComponent's behaviour:

update: function() {
		this.synchronous(this.redraw);
	}

If you want to pass parameters to redraw, you can pass them as an array to synchronous:


update: function() {
		/* Will call this.redraw(1,2,3) */
		this.synchronous(this.redraw, [1,2,3]);
}

Use triggerQuery When You Want Your Component To Use CDA/Query Objects

If you're using a CDA data source, you probably want to use triggerQuery to handle the component lifecycle for you. triggerQuery expects at a minimum a query definition and a redraw callback to process the query results. The query definition is an object of the form:


{
  dataAccessId: 'myQuery',
	file: '/path/to/my/datasourceDefinition.cda'
}

Typically, if you're using CDE, these properties will be added to one of either this.queryDefinition or this.chartDefinition so you can just use this pattern:


update: function() {
	var redraw = _.bind(this.redraw,this);
	this.triggerQuery(this.queryDefinition, redraw);
}

Alternating Between Static And Query-Based Data

As the lifecycle methods are completely self-contained, you can switch between them at will, deciding on an appropriate lifecycle at runtime. A common pattern (used e.g. in SelectComponent, and the CccComponent family) is exposing a valuesArray property, and using static data if valuesArray is provided, or a query if it is not. Using UnmanagedComponent, this convention would like like this:


update: function() {
	var redraw = _.bind(this.redraw,this);
	if(this.valuesArray && this.valuesArray.length > 0) {
		this.synchronous(redraw,this.valuesArray);
	} else {
		this.triggerQuery(this.queryDefinition,redraw);
	}
}

Rolling Your Own

If you prefer having absolute control over your component, you can eschew the use of any of the lifecycle methods. Instead, you're expected to follow these guidelines:

New and Changed Features

Component Cloning

If your component holds any references to other components, you need to override the clone method so as to ensure that you don't accidentally clone the target component. For example, if your component has a property named otherComponent pointing at another component, you should override clone using this general template:

clone: function(parameterRemap,componentRemap,htmlRemap) {
	var other = this.otherComponent;
	delete this.otherComponent;
	var that = this.base(parameterRemap,componentRemap,htmlRemap);
	this.otherComponent = that.otherComponent = other;
	return that;
}

New Base Component Class: UnmanagedComponent

UnmanagedComponent is a new base class for components. It provides the base on which all asynchronous components should be built.

Per-Component isManaged Flag

Each component should have a member property with isManaged, indicating whether CDF should managed the component's lifecycle. Components where isManaged is false need to implement it's own calls to the lifecycle

Component Stub

Here's an example of a stub to be used whenever you need to use a new component. Just override the redraw function to what you need and define the component as you'd usually do