Week 2 - Monoliths, Microservices and Communication

Question 1

What is a monolith?

Answer 1

Where products are programs from a collection of modules, run by a single process. All communication is by procedural calls.

Question 2

Name 3 advantages of monoliths.

Answer 2

Modules can be made to correspond directly to business functions identified by domain-driven design.

Modules can be developed by teams working to agreed interfaces through permissionless innovation.

The program as a whole can be easily deployed from version control automatically.

Question 3

What is the microservice architectural style?

Answer 3

Where products are applications from a collection of microservices, each a separate program run by a separate process. All communication between microservices is by message passing.

Question 4

What is Conway’s Law?

Answer 4

Any organisation that designs a system (defined more broadly here than just information systems) will inevitably produce a design whose structure is a copy of the organisation’s communication structure.

Question 5

What are the two approaches to building a software product MVP?

Answer 5

Microservices early and microservices late.

Question 6

What is the microservices late approach?

Answer 6

When building a system, first build a single large monolith before (if successful) migrating to many small microservices.

Question 7

What is the idea of microservices late?

Answer 7

To enter the marketplace early with a (perhaps poorly-structured) product, ready to learn from the customer.

Question 8

What is the microservices early approach?

Answer 8

To begin with microservices when building a system.

Question 9

What is the idea of microservices early?

Answer 9

To enter the marketplace later with a better-structured product, ready to make the customer segment pivot.

Question 10

What do the best microservice designs have?

Answer 10

High cohesion and loose coupling.

Question 11

What is high cohesion and how does it help microservice design?

Answer 11

Related functionality is to be found in one component, while unrelated functionality is to be found in others.

This helps because, should functionality need changing, only one component needs changing.

Question 12

What is loose coupling and how does it help microservice design?

Answer 12

Changing one component does not necessitate changing others.

This helps because, should the functionality of one component be changed, no other components need changing.

Question 13

What are advantages of microservices?

Answer 13

Development is easier because each microservice can use whatever technology is best for it.

Deployment is easier because each microservice can be released just when it is ready.

Robustness is increased because the failure of one microservice doesn’t cause the failure of another.

Scaling is improved because the resources for one microservice don’t need to be the same as those for another.

Question 14

How does the Strangler pattern manage migrations from monoliths to microservices?

Answer 14

The Strangler pattern manages migrations by putting an intermediary facade in front of the monolith.

Initially, the facade provides access to modules.

As modules are replaced by microservices behind the facade, it provides access to them instead.

In this way, the monolith is slowly “strangled” by microservices, which eventually kill it.

Question 15

What does the the Least Knowledge principle say?

Answer 15

Components should only know about the structure of components they deal with directly.

Question 16

What does the Don’t Repeat Yourself (DRY) principle say?

Answer 16

That things that are duplicated should be merged.

Question 17

What does the Single Responsibility (SRP) principle say?

Answer 17

Components should only have one responsibility, and so only one reason to change.

Question 18

What does the microservice bus do?

Answer 18

Connects microservices together with an open, pluggable framework.

Question 19

The microservice bus has a single communication protocol, data format and interface convention. What are they?

Answer 19

HTTP, JSON and REST

Question 20

What is HTTP?

Answer 20

Hypertext Transfer Protocol - a synchronous protocol where the client sends a request and then blocks until a server sends a response.

Question 21

What does a HTTP request consist of?

Answer 21

A request line, some headers and an optional body.

Question 22

What are the HTTP request methods and what do they do?

Answer 22

POST, GET, PUT and DELETE operate on resources described by URIs (Universal Resource Identifiers).

Question 23

What does a HTTP response consist of?

Answer 23

A status line, some headers and an optional body.

Question 24

What do these HTTP statuses mean?
200
400
500

Answer 24

200 - OK (everything went well)
400 - Bad Request (client went wrong)
500 - Internal Server Error (server went wrong)

Question 25

What is the data format of the microservice bus?

Answer 25

JSON (JavaScript Object Notation)

Question 26

What is the interface convention for the microservice bus?

Answer 26

REST (Representational State Transfer)

Question 27

What are the four resource archetypes?

Answer 27

Document, Collection, Store, Controller

Question 28

What do microservices provide?

Answer 28

One or more resources (larger resources made by plugging smaller resources together).

Question 29

Describe the document resource archetype.

Answer 29

Describes operations on file-like resources:
- GET reads the file and returns its contents
- PUT updates the file with its given contents
- DELETE deletes the file

Question 30

Describe the collection resource archetype.

Answer 30

Describes operations on directory-like resources:
- GET reads the directory and returns a list of the resources it contains
- POST creates a new resource in the directory with given contents, choosing the resource name and returning it (it chooses the name)

Question 31

Describe the store resource archetype.

Answer 31

Describes possible operations on directory-like resources:
- GET reads the directory and returns a list of the resources it contains
- PUT either creates a new resource in the directory using the given resource name and contents, or updates an existing one (you choose the name)

Question 32

Describe the controller resource archetype.

Answer 32

Described possible operations on external resources:
- POST causes the external resource to carry out some task and returns any result

Question 33

What is gRPC?

Answer 33

Google Remote Procedure Call - an alternative to REST. Services connected using gRPC are defined using protocol buffers (for typed safety) and exchange binary data (for efficient transfer).