Routing questions

A router reads the question and picks the way to find sources, such as a lookup or a search.

Lesson 42 of 622 min read

The problem

Why it matters

Without it

One search method handles every question. Exact ids miss, error codes match the wrong text, and counts are never computed.

With it

The router sends each question to the method that suits it. Each method finds the sources it does best.

How it works

A rule is a pattern that matches text
A regular expression is a pattern that matches text. This pattern reads the word order, a space, a capital letter, a dash, and digits. It matches an order id such as A-1042, and the test method returns true when it matches.
```
const pattern = /order [A-Z]-\d+/;console.log(pattern.test("Where is order A-1042?"));console.log(pattern.test("Can I return an opened item?"));
```
Output
```
true
false
```

The first matching rule wins

Rules are checked from top to bottom, and the first match decides the route. Narrow rules sit at the top, so a broad rule cannot take their questions. The last route has no rule, so it catches every question left over.

Order	Rule matches	Example	Route
1	an order id like A-1042	Where is order A-1042?	exactId
2	the word error and a number	What does error 4031 mean?	keyword
3	how many or total	How many orders shipped?	count
4	no rule matches	Can I return an opened item?	meaning

The rules in the order the router checks them.

The router as code

The route function holds the rules in order. The keyword rule takes the error question, because it is the first rule that matches. The sample prints the route name for that question.

function route(question: string): string {  if (/order [A-Z]-\d+/.test(question)) return "exactId";  if (/error \d+/i.test(question)) return "keyword";  return "meaning";}console.log(route("What does error 4031 mean?"));

Output

keyword

The default route catches the rest
A question that matches no rule takes the default route, which is the meaning search. The sample sends four questions through the router, and each one gets a route name.
- Where is order A-1042?exactId
- What does error 4031 mean?keyword
- How many orders shipped last week?count
- Can I return an opened item?meaning, default
Four questions and the route each one takes.

Full code

Read the program

See the full code

Common mistakes

What goes wrong

Writing one broad rule first. It takes questions that a narrow rule should have caught.
Routing without a test set. Keep questions with known routes and check each rule against them.
Sending every question to a model route. Rules are cheaper and easier to check.

Use it when and skip it when

Is it the right tool?

Use it when

Your sources sit in different stores, such as a table and a document index.
Some questions need one exact record and others need meaning.

Skip it when

All questions go to one source, so one search is enough.

Words to know

Terms in this lesson

Router: Code that picks the route for each question.
Route: One way to find sources, such as a lookup or a search.
Regular expression: A pattern that matches a piece of text, such as an order id.
Default: The route used when no other rule matches.

Routing questions

A rule is a pattern that matches text

The first matching rule wins

The router as code

The default route catches the rest