What is loopback?

Main parts

JSON definition

CRUD REST endpoints

ACL

Datasource

Client lib

• each model has one JS file
• API endpoint
• model method
• operation hook

Custom logic

• auto-migrate
  • drop all tables and recreate them
  • => useless
• auto-migrate
  • looks for changes in model definitions
  • can't handle: rename fields, indices
  • => useless
• no real support

Migrations

Transaction management

• manually handle opening, committing and rollbacking
• manually use the transaction when performing DB operations
• nested transactions?

...

MyModel.beginTransaction(tx => {
  MyModel.findById(42, {}, {transaction: tx})
    .then(modelInstance => {
      return modelInstance.update(
        {attribute: newValue}, 
        {transaction: tx}
      );
    })
    .then(result => {
      return tx.commit().then(() => result);
    })
    .catch(err) => {
      console.error(err.stack);
      tx.rollback();
    });
    
});

function fancyFind(criteria, transaction = null) {
  if (transaction) {
    return FancyModel.find(criteria);
  } else {
    return FancyModel.find(criteria, {transaction});
  }
}

Consequence

Support for promises

The (in)famous Contex

• update a model in a transaction
• any model validations need to run in THE SAME transaction

...
return modelInstance.update(
  {attribute: newValue}, 
  {transaction: tx}
);
...

MyModel.validate('new value is ok', () => {
  AnotherModel.find(
    {where: {aValue: this.attribute}},
    {transaction: tx}
  )
});

• how does tx get from left to right?
  • the CONTEXT (manually)

• what else uses the context?
  • the loopback way of getting the current user

The datasource juggler

• allows you to connect the models to any datasource:
  • MySQL
  • Memory
  • PostgreSQL
  • MongoDB
  • REST API
  • ...

• however:
  • various inconsistencies in storage providers make this very hard to achieve
  • not all datasources use the same code (the juggler)

The API interface

MyModel.find({
  "filter": {
    "where": {
      "id": {
        "inq": [1, 2, 3]
      },
      "startDate": {
        "gte": "2016-01-02"
      }
    },
    "include": {
      "child": {
        "include": ["grandchild"]
      }
    }
  }
});

http://localhost:3010/api/action-plan-items?filter=%7B%22where%22%3A%7B%22id%22%3A%7B%22
inq%22%3A%5B1%2C2%2C3%5D%7D%2C%22startDate%22%3A%7B%22gte%22%3A%222016-01-02%22%7D%7D
%2C%22include%22%3A%7B%22child%22%3A%7B%22include%22%3A%5B%22grandchild%22%5D%7D%7D%7D

http://localhost:3010/api/action-plan-items?filter={"where":{"id":{"inq":[1,2,3]},
"startDate":{"gte":"2016-01-02"}},"include":{"child":{"include":["grandchild"]}}}

• very flexible
• one API invocation to get everything you want
• no need to implement a custom API endpoint

-------------

• easy to go over the url length limit
• exposing a lot of internal structure (but that is REST; 1-1 mapping between Model - REST Resource)

• the correctness relies on using query params with empty string and empty array
• JS querystring library removes the empty array

Query params

http://localhost:3010/api/action-plan-items?filter={"where":{"id":{"inq":[]}}}

http://localhost:3010/api/action-plan-items

Take aways

• Very good for rapid prototyping
• Very good integration with Angular
• Decent community where you can find resources
• Multitude of github repos some actively maintained, some not so much
• Pretty nice to do unit tests

Take aways

• Doesn't encapsulate really well
• A lot of manual work when working with transactions
  • Also, hard to test without a MySQL datasource
• in the long run, you will move away from the generic REST API
• No migration support
• Once you get past a certain level, you will get hit by bugs
• Could be better and more actively maintained

Q & A

Thanks!

Resources

• TODO