Repository pattern for data access in Express - Step-by-Step Execution

Concept Flow - Repository pattern for data access

Start

↓

Controller calls Repository

↓

Repository handles data logic

↓

Repository calls Database

↓

Database returns data

↓

Repository returns data to Controller

↓

Controller sends response

↓

End

The controller asks the repository for data. The repository talks to the database and returns data back. The controller then sends the response.

Execution Sample

Express

class UserRepository {
  async getUser(id) {
    return db.findUserById(id);
  }
}

const repo = new UserRepository();

app.get('/user/:id', async (req, res) => {
  const user = await repo.getUser(req.params.id);
  res.json(user);
});

This code shows a repository class fetching a user by ID and the Express route using it to send JSON response.

Execution Table

Step	Action	Input	Repository Method Called	Database Call	Returned Data	Response Sent
1	HTTP GET /user/5 received	id=5	No	No	No	No
2	Controller calls repo.getUser(5)	id=5	getUser(5)	Yes: db.findUserById(5)	No	No
3	Database returns user data	id=5	getUser(5)	db.findUserById(5)	{id:5, name:'Anna'}	No
4	Repository returns data to controller	id=5	getUser(5)	No	{id:5, name:'Anna'}	No
5	Controller sends JSON response	id=5	No	No	No	{id:5, name:'Anna'}
6	Request complete	id=5	No	No	No	Response sent

💡 Request ends after controller sends JSON response with user data.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
id	undefined	5	5	5	5	5
user	undefined	undefined	undefined	{id:5, name:'Anna'}	{id:5, name:'Anna'}	{id:5, name:'Anna'}

Key Moments - 3 Insights

Why does the controller not access the database directly?

The controller calls the repository (see step 2 in execution_table) to keep data logic separate and easier to manage.

What happens if the database call fails?

The repository would handle errors before returning data to the controller, preventing direct database errors from reaching the controller.

Why is the repository method async?

Because database calls take time, the repository uses async to wait for data before returning it (step 3 in execution_table).

Visual Quiz - 3 Questions

Look at the execution_table, what data does the repository return to the controller at step 4?

A {id:5, name:'Anna'}

B undefined

C db.findUserById(5)

D No data

💡 Hint

Check the 'Returned Data' column at step 4 in execution_table.

At which step does the controller send the JSON response?

A Step 3

B Step 2

C Step 5

D Step 6

💡 Hint

Look at the 'Response Sent' column in execution_table.

If the repository method was not async, what would happen?

A Controller would wait correctly for data

B Controller might get undefined before data arrives

C Database call would be faster

D No change in behavior

💡 Hint

Refer to key_moments about async behavior and step 3 in execution_table.

Concept Snapshot

Repository pattern separates data logic from controllers.
Controller calls repository methods.
Repository handles database calls.
Repository methods are async to wait for data.
This keeps code clean and easier to maintain.

Full Transcript

The repository pattern in Express means the controller does not talk directly to the database. Instead, it calls a repository class that handles all data access. When a request comes in, the controller calls a repository method with needed parameters. The repository then calls the database asynchronously to get data. Once the data returns, the repository sends it back to the controller. The controller then sends the data as a JSON response. This separation helps keep code organized and easier to manage. The repository methods are async because database calls take time. The controller waits for the repository to finish before sending the response.