Akka

Question 1

Akka is made up of

Answer 1

Modules that are distributed as JAR files

Question 2

What does the Akka concept of ‘let it crash?’ anticipate

Answer 2

There will be cases of individual handlers failing to carry out their tasks, and it is prepared to reroute that work to avoid a catastrophic failure.

Question 3

What happens if the failure occurs up the chain?

Answer 3

Underlings can carry on until a new supervisor arrives. There is no broken chain, and most importantly, it is no longer the duty of the developer to find all the ways that chain might break catastrophically and provide specialized handling for each case

Question 4

Actors are lightweight because

Answer 4

They run on top of dispatchers

Question 5

How many threads fit in 1GB of memory and how many actors

Answer 5

4096

2.7 Million

Question 6

How do you get an actor reference to an actor in the hierarchy?

Answer 6

You use the actor path

Question 7

actors are a lot like ___________ without the configuration and ________ ________
installation overhead

Answer 7

message queues

message broker

Question 8

Messages are simple data structures that …

Answer 8

can’t be changed after they’ve been created,

or in a single word, they’re immutable.

Question 9

Actors can receive messages one at a time and execute some behaviour whenever a
message is received. Unlike queues, they can

Answer 9

also send messages

Question 10

What are the points made in the reactive manifesto

Answer 10

 Blocking I/O limits opportunities for parallelism, so nonblocking I/O is preferred.
 Synchronous interaction limits opportunities for parallelism, so asynchronous interaction is preferred.
 Polling reduces opportunity to use fewer resources, so an event-driven style is preferred.
 If one node can bring down all other nodes, that’s a waste of resources. So you need isolation of errors (resilience) to avoid losing all your work.

Question 11

What two goals should we aim for in attempting to make applications scalable?

Answer 11

Complexity must stay as low as possible.

Resources must be used as efficiently as possible while you scale the application

Question 12

Scaling out with ____ and scaling up with ____ are not good ideas

Answer 12

RPC

Low level threading

Question 13

Why is scaling out with RPC not a good idea?

Answer 13

Every RPC call needs to bloc the current thread and wait for a response from the network. Impeded the goal of using resources efficiently.

Question 14

Why is using a combination of RPC and threading not a good idea?

Answer 14

You need to know exactly where you scale up or scale out. (Increase in complexity)

Question 15

Spinning up thousands of servers is simple today, but

Answer 15

the same cannot be said for programming them

Question 16

Actors are nothing new at all, in and of themselves. It’s … that make them unique

Answer 16

It is the way that actors are provided in Akka to scale applications both up and out on the JVM

Question 17

This type of database is often called a journal

Answer 17

when changes are kept as a sequence of events

Question 18

The technique of storing changes as a sequence of events is called

Answer 18

event sourcing

Question 19

How can event sourcing be kept consistent across two servers?

Answer 19

sharding or partitioning

Question 20

Most general purpose programming languages are written in sequence. A ________ ______ ____ is required to bridge the gap between sequential definition and parallel execution

Answer 20

concurrent programming model

Question 21

Whereas parallelization is all about executing processes simultaneously, concurrency concerns itself with

Answer 21

defining processes that can function simultaneously, or can overlap in time, but don’t necessarily need to run simultaneously.

Question 22

An actor is a lightweight process that has only four core operations:

Answer 22

create, send, become, and supervise

Question 23

Any actor can be a supervisor but

Answer 23

only for actors that it creates itself

Question 24

How are actors decoupled?

Answer 24

on three axes - namely

• Space / location
• Time
• Interface

Question 25

What happens when an actor is decoupled in space

Answer 25

An actor gives no guarantee and has no expectation about where another actor is located

Question 26

What happens when an actor is decoupled in time

Answer 26

An actor gives no guarantee and has no expectation about when its work will be done

Question 27

What happens when an actor is decoupled from interface - that is it has no defined interface

Answer 27

The actor has no expectation about which messages other components can understand.

Question 28

The first thing that every Akka application does is ...

Answer 28

Create the Actor System

Question 29

Which actors can the actor system create

Answer 29

The so called top-level actors

Question 30

The actor system can create so called top-level actors, and it’s a common pattern to

Answer 30

Create only one top-level actor for all actors in the application. The Supervisor actor that monitors everything

Question 31

An ActorSystem returns an address to create the top-level actor instead of the actor itself. This is called an

Answer 31

ActorRef

Question 32

An ActorSystem returns an address to the created top-level actor instead of the actor itself. This address is called an ActorRef. The ActorRef can be used to send messages to the actor. This makes sense when you think about the fact that

Answer 32

the actor could be on another server

Question 33

Sometimes you would like to look up an actor in the actor system this is where _____ _____ come in.

Answer 33

Actor Paths come in

Question 34

You could compare the hierarchy of actors to a URL path structure. Every actor has a name. This name needs to be ...

Answer 34

unique per level in the hierarchy: two sibling actors can’t have the same name

Question 35

All actor references can be located directly by

Answer 35

an actor path (absolute or relative)

Question 36

What does a dispatcher do?

Answer 36

pushes the messages in the mailbox through the actors

Question 37

The type of dispatcher determines ...

Answer 37

which threading model is used to push the messages through

Question 38

All kinds of dispatchers can be configured in some way, and you can allocate a dispatcher to an

Answer 38

- actor - a specific group of actors - all of the actors in a system

Question 39

When you send a message to an actor, all you are really doing is

Answer 39

leaving a message behind in its mailbox. Eventually a dispatcher will push it through the actor

Question 40

Per the REST conventions, a GET whose URL ends with an entity type should return

Answer 40

a list of known instances of that entity

Question 41

Why is implicit val ec = system.dispatcher needed in order to call Http().bindAndHandle

Answer 41

bindAndHandle is asynchronous and requires an implicit ExecutionContext

Question 42

what happens after | implicit val system = ActorSystem()

Answer 42

the actor system is immediately available

Question 43

When you define an implicit val with execution context it does what?

Answer 43

determines which execution context will be used by Future(s) in scope of its definition

Question 44

Dispatchers implement the ExecutionContext interface and can thus be

Answer 44

used to run Future invocations etc.

Question 45

an asynchronous method returns a ______ before it is completed

Answer 45

future

Question 46

How would you program an actor to respond to a message assuming there were three types of message namely Larry, Curly and Mo

Answer 46

``` def receive = { case Larry(punch) => case Curly() => case Mo(something) => } ```

Question 47

actors created with the context of another actor are

Answer 47

its children and subject to the parent actor’s supervision

Question 48

what does | Http().bindAndHandle(api, host, port) do?

Answer 48

``` binds the routes defined in the RestApi (api) to the HTTP server It is asynchronous so it returns a 'future' before it is completed ```

Question 49

Services interfaces, as they grow, need

Answer 49

more sophisticated routing of requests

Question 50

Actors exist in a supervision hierarchy - what does this mean

Answer 50

each actor has a supervisor and can have one or several child actors for which it is responsible

Question 51

An actor can send and receive messages of any kind using the _______ method

Answer 51

receive

Question 52

An actor can send and receive messages of any kind using the receive method, which is a so-called partial function that uses ...

Answer 52

Scala’s pattern matching to figure out which case statement will deal with the incoming message.

Question 53

What is a partial function?

Answer 53

A partial function is a function that does not provide an answer for every possible input value it can be given

Question 54

Since the ’90s, nothing much has changed in how programming languages support networking, either. Many technologies still essentially use ____________ to communicate over the network

Answer 54

RPC (remote procedure calls)

Question 55

One doomsday scenario is where you’d need to pay increasingly more for more underutilized resources. Another nightmare scenario is

Answer 55

where the complexity of the application shoots through the roof when more resources are added.

Question 56

Spinning up thousands of servers is simple today, but

Answer 56

the same cannot be said for programming them

Question 57

It shouldn’t matter if we send a message locally on one server or remotely to another. So we need _______________________________ It will also need to ________________________________. This is one of the things that _______ does for you.

Answer 57

1. some service that takes care of sending the messages to actors on other servers if necessary. 2. It will also need to keep track of where actors live and be able to provide references so other servers can communicate with the actors. 3. Akka

Question 58

This is the biggest impediment to building applications that can recover from failure and scale according to demand.

Answer 58

Coupling components in location, time, and interface is the biggest impediment to building applications that can recover from failure and scale according to demand.

Question 59

The messages that an actor can receive or send back on a request are bundled together in

Answer 59

the actors’ companion object

Question 60

If an actor is created using the system - as in | createBoxOffice = system.actorOf(BoxOffice.props, BoxOffice.name) how are the child actors created?

Answer 60

Using their context | CreateTicketSeller = context.actorOf(TicketSeller.props(name), name)

Question 61

What does a materializer do?

Answer 61

makes actors execute a graph to produce results

Question 62

A graph, in its simplest form, consists of

Answer 62

a source that provides elements, and a sink that consumes elements.