@softeq/data-mappers

@softeq/data-mappers is a lightweight library which simplifies serialization/deserialization tasks.

Motivation

One of task developer has to solve from day to day is data serialization. For example, developer needs to transform data to serializable view

• to send data via HTTP client
• to store data in local storage
• to store some data in route query parameters
• etc

In the same time if data was serialized, one day it should be deserialized.

It is not so complex task, especially when data has the following structure

{
  "firstName": "John",
  "lastName": "Smith",
  "age": 32,
  "children": ["Mary", "Ann"]
}

Developer can call JSON.stringify to serialize data and JSON.parse to deserialize data.

This approach works well while we have only primitive data (string, number, boolean, etc). But this approach will stop working as soon as we have complex objects, like Date, RegExp, BitSet, etc (other objects created by third-party libraries).

Note. Actually Date object can be transformed to string by JSON.stringify method, but anyway JSON.parse will not restore it.

For example in the following example we have two complex fields birthday and salary,

const john = {
  id: 1,
  firstName: 'John',
  lastName: 'Smith',
  birthday: new Date(1977, 3, 2),
  salary: new BigDecimal(/*some very large and precise number*/)
};

To serialize and deserialize this object we can write a pair of the following functions:

function serializeEmployee(john) {
  return JSON.stringify({
    ...john,
    birthday: john.birthday.toISOString(),
    salary: john.salary.toString(),
  });
}

function deserializeEmployee(serializedEmployee) {
  const almostDeserializedEmployee = JSON.parse(serializedEmployee);
  return {
    ...almostDeserializedJohn,
    birthday: new Date(almostDeserializedEmployee.birthday),
    salary: bigDecimalFromString(almostDeserializedEmployee.salary),
  };
}

and use them

const serializedJohn = serializeEmployee(john);
const restoredJohn = deserializeEmployee(serializedJohn);

Both these functions take too much space even for simple object and moreover they mirrors each other. Actually mirroring is a common rule especially when communication protocol follows REST practices.

Things become more complex when developer has HTTP communication and he/she should take into account headers (for example, handling of ETag, If-Match, If-None-Match headers).

Solution this library proposes

This library promotes concept of DataMapper which can both serialize and deserialize data (we will use word map instead of serialize/deserialize). For john, you can define mapper like

const employeeMapper = objectMapper({
  birthday: dateMapper(DateFormat.Date),
  salary: bigDecimalMapper,
});

You have to describe only how to map all non-primitive data.

To serialize john using employeeMapper you have to write

const serializedJohn = employeeMapper.serialize(john);
console.log(serializedJohn);

serializedJohn is an object which looks like

{
  id: 1,
  firstName: 'John',
  lastName: 'Smith',
  birthday: '1977-04-02', // birthday was serialized by dateMapper to ISO format
  salary: '12342342342', // BigDecimal converted to serialized view
}

It is important to note that serializedJohn is an Object rather than string. To serialize serializedJohn to a string developer can use JSON.stringify function. JSON.stringify is safe now, because serializedJohn contains only primitive values.

Original object can be restored from the serialized view using deserialize method

const restoredJohn = employeeMapper.deserialize(serializedJohn);
// restoredJohn is an object equal by content to original john object

Actually bigDecimalMapper used to map BigDecimal does not exist, but we can easily define it. See the following example

import { createMapper } from '@softeq/data-mappers';

const bigDecimalMapper = createMapper({
  serialize: (big) => big.toString(),
  deserialize: (serializeBig) => bigDecimalFromString(serializeBig),
});

Composition of mappers

But what if employee entity can be a part of a company entity, like in the following example?

const company = {
  name: 'Insurance Ltd',
  foundationDate: new Date(1920, 8, 23, 7, 5, 23), // This company was founded 23rd September 1920 at 7:05 AM and 23 seconds 
  employees: [{
    id: 1,
    firstName: 'John',
    lastName: 'Smith',
    birthday: new Date(1977, 3, 2),
    salary: new BigDecimal(/*some very large and precise number*/)
  }, {
    id: 2,
    firstName: 'Mary',
    lastName: 'Miller',
    birthday: new Date(1983, 2, 3),
    salary: new BigDecimal(/*also some very large and even more precise number*/)
  }]
}

In this case we can define composed mapper

const companyMapper = objectMapper({
  foundationDate: dateMapper(DateFormat.DateTime), // we have to parse date and time (timestamp),
  employees: arrayMapperOf(employeeMapper), // array of employees, here we use employeeMapper defined above
});

Here we use arrayMapperOf which defines mapper for set of employees, each of employees will be mapped by employeeMapper.

We can use companyMapper like in the following example

const serializedCompany = companyMapper.serialize(company);
const restoredCompany = companyMapper.deserialize(serializedCompany);

Mapping of HTTP Requests/Responses

HTTP communications, especially REST-based communications, actively use parts of HTTP protocol (status, method, headers) to transfer important information. While HTTP request and response has different structure, both of them support headers which sometimes play crucial role in making request and response interpretation.

For example, it is a common practice to use ETag header together with If-Match (If-Non-Match) header to avoid lost updates (this technique is called "optimistic locking"). DataMappers can also help in this field and provide reusable composable and ease to use approach to support such kind of communications.

Let's imagine we have a page where we can update employee information. From the communication point of view we need two endpoints: one to get state of resource (GET) and another one to update its state (PUT). In addition we have to avoid lost updates, for this purpose our communication protocol supports handling of ETag and If-Match headers (which is, I believe, a common practice).

To be specific, look at response of GET call

HTTP/1.1 200 OK
Content-Type: application/json
ETag: "363d707003c01515b2c627fc15b9e4d88"

{
  "id": 1,
  "firstName": "John",
  "lastName": "Smith",
  "birthday": "1977-04-02",
  "salary": "12342342342"
}

and possible request of PUT call

PUT /employees/1 HTTP/1.1
Content-Type: application/json
If-Match: "363d707003c01515b2c627fc15b9e4d88"

{
  "id": 1,
  "firstName": "John",
  "lastName": "Davis",
  "birthday": "1987-04-02",
  "salary": "92342342342"
}

Thanks to ETag/If-Match headers employee will be updated only when its state is not changed between these two requests.

Using @softeq/data-mappers library this behavior can easily be achieved. First of all you have to complement employeeMapper by ETag support

const employeeMapperWithOptimisticLock = httpEtagMapperOf(employeeMapper);

and use employeeMapperWithOptimisticLock to prepare HttpData for PUT call (read about HttpData structure here below)

const johnHttpData = employeeMapperWithOptimisticLock.serialize(john);

HttpData can be transformed back to the plain JS object using the following statement

const john = employeeMapperWithOptimisticLock.deserialize(johnHttpData);

To help with mapping of HTTP request and response @softeq/data-mappers library proposes two concepts

• HttpData
• HttpDataMapper

HttpData

HttpData is a simple data structure which has the following fields

• headers for set of headers
• body which stores serialized body.

HttpData allows to avoid dependency from specific HTTP client used for HTTP communications.

In order to use HttpData with specific HTTP client you have to use one of integration libraries described here below. For example, for Angular framework you can use @softeq/angular-http-data package.

HttpDataMapper

HttpDataMapper extends DataMapper interface and allows

• to serialize object to HttpData structure
• to deserialize object from HttpData structure

Look at our example here above.
For GET call we can use employeeMapperWithOptimisticLock.deserialize method to transform HttpData to john object. deserialize method complements john entity by etag field having value from ETag header.
For PUT call we can use employeeMapperWithOptimisticLock.serialize method to transform john object back to HttpData structure. serialize method move value from etag field back to the If-Match header.

Angular example

Although, support of HTTP mappers can sound too complex, it becomes much easier with specific library used for HTTP communications. For example, for Angular framework real world example with @softeq/angular-http-data library looks like

class EmployeeRest extends AbstractRestService {
  get(id: number): Observable<Employee> {
    return this.httpGet(`/employees/${id}`, optimisticLockingOf(employeeMapper));
  }

  update(employee: Employee): Observable<Employee> {
    return this.httpPut(`/employees/${employee.id}`, employee, optimisticLockingOf(employeeMapper));
  }
}

Chained HttpDataMappers

Several HttpDataMappers can be composed (chained) together to create more complex HttpDataMapper.

For example, back to our employee example. Imagine we need functionality to delete users. For this purpose We have to use DELETE endpoint

DELETE /employees/1 HTTP/1.1
If-Match: "363d707003c01515b2c627fc15b9e4d88"

take into account that this endpoint does not accept body (which is a common practice for DELETE endpoints).
Although employeeMapperWithOptimisticLock handles ETag/If-Match headers it does generate body. We can reset body using another HttpDataMapper: httpConstantBodyMapperOf. And construct composed HttpDataMapper like in the following example

const mapperForDelete = httpConstantBodyMapperOf(httpEtagMapperOf(employeeMapper), undefined);
// the same as
const mapperForDelete = httpConstantBodyMapperOf(httpEtagMapperOf(employeeMapper));

httpConstantBodyMapperOf always returns its second argument as HTTP body.
Now we can use result mapperForDelete for DELETE request, it behaves as httpEtagMapperOf and httpConstantBodyMapperOf simultaneously.

Construction of DataMapper handling ETag and returning empty body can be simplified using flow function from lodash.

const httpEmptyEtagMapperOf = flow(httpConstantBodyMapperOf, httpEtagMapperOf);

Angular example for chained HttpDataMapper

For @softeq/angular-http-data real world example can look like

class EmployeeRest extends AbstractRestService {
  remove(employee: Employee): Observable<void> {
    return this.httpDelete(`/employees/${employee.id}`, employee, httpEmptyEtagMapperOf(employeeMapper));
  }
}

Create custom HttpDataMapper

You can write own implementation of HttpDataMapper, look at implementation of ETagHttpDataMapper or ConstantBodyHttpDataMapper.

Integration libraries for HttpData/HttpDataMapper

• for Angular use @softeq/angular-http-data