Microservice Decorator

legacy code + new code

living together

Tommaso Allevi

@allevo

R&D TechLeader at Mia Platform

Everytime we need to interact with a legacy

but we want to survive

HOW?

Building software near by to a legacy one is not simple.

We need to find a way!

Let's start with an example!

Microservice Decorator

A decorator is a component of your architecture
that can:

  • Be notified for a request/response
  • Change a request/response adding/removing parameters (querystring, header, body...)
  • Stop the request providing the final response to send back to the client

A simple implementation of this pattern could be using HTTP protocol on which the decorator accepts the request/response in input and reply according the the action it would make

Business Logic
API

Decoratore pre1

Decoratore pre2

Decoratore pre3

...

Decoratore post1

Decoratore post2

Decoratore post3

...

Flow

{
  "/incoming_url": {
    "GET": {
      "pre": [ "pre1", "pre2" ],
      "call": "http://business-service/proxied-url",
      "post": [ "post1" ]
    }
  },
  "decorators": {
    "pre1": {
      "call": "http://some-service2/decorators/path1",
    },
    "pre2": {
      "call": "http://other-service/decorators/path2",
    },
    "post1": {
      "call": "http://some-service3/decorators/path3",
    }
  }
}

  GET /incoming_url?foo=bar

pre1

pre2

business-service

  GET /proxied-url?bar=foo

post3

The Microservice Decorator impose an HTTP interface that each decorator has to implement

A microservice is not its API

but its Business Logic

Is this a limit?

No

Ports and Adapter Pattern

Example #1

Login

Microservice Decorator

Microservice Decorator

Legacy
Login

New Login Service

POST /login

Microservice
Decorator

POST /login

POST
/decorators/newLogin

POST /login

{ username: "aa", password: "bb" }

---> Set-Cookie: sid="abcd"

---> { userId: "1234" }

POST /decorators/newLogin

Set-Cookie: sid="abcd

{ userId: "1234" }

---> Set-Cookie: sid="abcd"; sid2="9876"

---> { userId: "1234" }

POST /login

{ username: "aa", password: "bb" }

---> Set-Cookie: sid="abcd"; sid2="9876"

---> { userId: "1234" }

{
  "/login": {
    "POST": {
      "pre": [ ],
      "call": "http://legacy-service/login",
      "post": [ "makeNewLogin" ]
    }
  },
  "decorators": {
    "makeNewLogin": {
      "call": "http://new-login-service/decorators/newLogin",
    }
  }
}

No legacy lines were changed!

We have added only the lines for the new component and change a configuration

Example #2

Login with CSRF

Microservice Decorator

Microservice Decorator

Legacy

Login

New Login Service

Microservice
Decorator

CSRF

Service

1

2

3

{
  "/login": {
    "POST": {
      "pre": [ "checkCSRF" ],
      "call": "http://legacy-service/login",
      "post": [ "makeNewLogin" ]
    }
  },
  "decorators": {
    "makeNewLogin": {
      "call": "http://new-login-service/decorators/newLogin",
    },
    "checkCSRF": {
      "call": "http://csrf-service/decorators/checkCSRF",
    }
  }
}

Thanks!

(we are hiring!)

Microservice Decorator - Eng

By Tommaso Allevi

Microservice Decorator - Eng

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