Inline Handlers

We saw Inline Handlers first in the Basics guides to introduce adding handlers to the TAO. As the first part of understanding the 3 different Types or Modes of handlers, it's best to start with Inline Handlers which most closely resemble what you're used to with traditional Event-based handlers.

Inline = Ordered

Inline Handlers operate like standard Event listeners in other areas of JavaScript in that the handlers are expected to be called in the order in which they were registered (added) to the TAO for a given trigram ("Event").

This expectation is modified when it comes to using Wildcard Handlers because internally each trigram is managing it's own set of handlers. While all matching Wildcard Handlers will be called along with all of the Concrete Handlers, they will be called in the order that the trigrams themselves were added to the TAO, and the order per trigram that each handler was added as this will be the traversal order used by the TAO.

Ordering of Inline Handler execution is not a guarantee provided by the TAO, and the underlying implementation may change, causing any code written that relies on this explanation of ordering to be executed in a different order.

Suffice to say, it's best not to rely on exact ordering for handlers in the TAO, even Inline Handlers, and write them as if they can be called at any point in the Set of Inline Handlers.

Trying this again…Inline = Ordered?^?^?

So what is meant by "Inline = Ordered?"

Good question.

- exchange with everyone who read the above section

The reason behind the addition of the other 2 types or modes of handlers is to provide specific guarantees about when handlers will be called and how they will affect our Systems and Applications.

The meaning behind "Inline = Ordered" is that each Inline Handler will complete before the next is called. In other words, when adding 2 Inline Handlers to the TAO for the same Application Context, whether the trigram for both is Concrete, Wildcard or they are added to a combination of the two, only one Inline Handler will be executed to completion before the next is called.

This guarantee is true for all Inline Handlers even if the handler itself is an async function or returns a Promise.

Example of ordering:

TAO.addInlineHandler({ t: 'User', a: 'Find', o: 'Portal' }, async (tao, data) => {
  console.log('First Inline Handler in, so I will be called first');
  const users = await findUsersSomehow(data.Find);
  return new AppCon('User', 'List', 'Portal', users); // <--- will execute before the TAO calls another handler

TAO.addInlineHandler({}, (tao, data) => {
  console.log('If no other total Wildcard handlers have been added, I will run third, after all handlers from the previously added trigram');

TAO.addInlineHandler({ t: 'User', a: 'Find', o: 'Portal' }, (tao, data) => {
  console.log('I have something else to do for this AppCon, and will run second'); // <--- b/c the {User,Find,Portal} trigram was added to the TAO first, and this handler added for it second

Special Note: In all likelihood, our App will register some type of Handler on the total Wildcard trigram ({}) first during bootstrapping of the App, so the above scenario of it being called 3rd is unlikely and only here for illustrative purposes.

Chaining Inline Handlers

If you remember Chaining Handlers from the Basics guide, we can return an AppCon from our Inline Handler in order to "chain" Application Contexts to automatically trigger the next Application Context on the TAO.

When Inline Handlers are chaining (returning AppCons), the TAO will spool up all returned AppCons so that all Inline Handlers are completed per the above :point_up_2: guarantee before it sets the Application Context to the returned AppCons.

If more than one Inline Handler returns an AppCon then each AppCon will set the Application Context on the TAO in the order in which they were received, mirroring the order in which the Inline Handlers were called.

Chaining = Fanout

Because the TAO is designed so that calls to setCtx and setAppCtx do not block the caller, in the process of chaining Inline Handlers (either intially or for any single downstream AppCon handling), if the TAO receives multiple AppCons, it will not block on each AppCon before setting the Application Context to a second or more AppCon.

This means that registering multiple Inline Handlers to react to an Application Context and then returning AppCons from more than one will operate as a Fan Out operation and not a Sequential set of setting Application Contexts on the TAO.

async Functions as Inline Handlers

Because a lot of what is done in JavaScript relies on asynchronous operation, the TAO allows you to add async functions or functions that return a Promise (both referred to as async functions) as Inline Handlers for Application Contexts.

When we add an async function as an Inline Handler to the TAO, the TAO will await for our handler to fully complete (resolve or reject) before moving onto the next handler.

This ensures that all Inline Handlers operate with the same ordering guarantees outlined above regardless of whether the function needs to block on waiting for some other resource to complete.

There is no difference in the way the TAO operates between an async function versus a fuction that just returns a Promise.

Error Handling

As mentioned in the Basics guide about Handlers throwing Errors, intial Inline Handlers that throw an Error will bubble the Error (not catch) to the caller that is setting the Application Context on the TAO, e.g.:

TAO.addInlineHandler({ t: 'User', a: 'Find', o: 'Portal' }, (tao, data) => {
  throw new Error('I can\'t find Users now!');

TAO.setCtx('User', 'Find', 'Portal'); // <---- will have uncaught Error

Downstream Errors are Swallowed

However, when an Inline Handler chains by returning an AppCon, the inner call to setting the downstream Application Context using the chained AppCon will swallow any Errors that are raised. This is provided by the TAO to ensure that downstream calls that may be unanticipated will not blow up your app, or stated otherwise, the TAO provides a guarantee of graceful degradation.

This is a specific design choice around a Functional Programming Principle to decouple knowledge and responsibility within Apps and Systems built using the TAO.

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