Interview Questions Flashcards
Explain Idempotent HTTP
In the context of REST APIs, when making multiple identical requests has the same effect as making a single request – then that REST API is called idempotent.
When you design REST APIs, you must realize that API consumers can make mistakes. They can write client code in such a way that there can be duplicate requests as well. These duplicate requests may be unintentional as well as intentional some time (e.g. due to timeout or network issues). You have to design fault-tolerant APIs in such a way that duplicate requests do not leave the system unstable.
An idempotent HTTP method is an HTTP method that can be called many times without different outcomes. It would not matter if the method is called only once, or ten times over. The result should be the same. It essentially means that the result of a successfully performed request is independent of the number of times it is executed. For example, in arithmetic, adding zero to a number is idempotent operation.
If you follow REST principles in designing API, you will have automatically idempotent REST APIs for GET, PUT, DELETE, HEAD, OPTIONS and TRACE HTTP methods. Only POST APIs will not be idempotent.
POST is NOT idempotent.
GET, PUT, DELETE, HEAD, OPTIONS and TRACE are idempotent.
Explain Fault Tolerance
Fault tolerance is the property that enables a system to continue operating properly in the event of the failure of (or one or more faults within) some of its components. If its operating quality decreases at all, the decrease is proportional to the severity of the failure, as compared to a naively designed system, in which even a small failure can cause total breakdown. Fault tolerance is particularly sought after in high-availability or life-critical systems. The ability of maintaining functionality when portions of a system break down is referred to as graceful degradation
HTTP 4xx Errors
400 Bad Request
The server could not understand the request due to invalid syntax.
401 Unauthorized
Although the HTTP standard specifies “unauthorized”, semantically this response means “unauthenticated”. That is, the client must authenticate itself to get the requested response.
402 Payment Required
This response code is reserved for future use. The initial aim for creating this code was using it for digital payment systems, however this status code is used very rarely and no standard convention exists.
403 Forbidden
The client does not have access rights to the content; that is, it is unauthorized, so the server is refusing to give the requested resource. Unlike 401, the client’s identity is known to the server.
404 Not Found
The server can not find requested resource. In the browser, this means the URL is not recognized. In an API, this can also mean that the endpoint is valid but the resource itself does not exist. Servers may also send this response instead of 403 to hide the existence of a resource from an unauthorized client. This response code is probably the most famous one due to its frequent occurrence on the web.
405 Method Not Allowed
The request method is known by the server but has been disabled and cannot be used. For example, an API may forbid DELETE-ing a resource. The two mandatory methods, GET and HEAD, must never be disabled and should not return this error code.
406 Not Acceptable
This response is sent when the web server, after performing server-driven content negotiation, doesn’t find any content that conforms to the criteria given by the user agent.
407 Proxy Authentication Required
This is similar to 401 but authentication is needed to be done by a proxy.
408 Request Timeout
This response is sent on an idle connection by some servers, even without any previous request by the client. It means that the server would like to shut down this unused connection. This response is used much more since some browsers, like Chrome, Firefox 27+, or IE9, use HTTP pre-connection mechanisms to speed up surfing. Also note that some servers merely shut down the connection without sending this message.
409 Conflict
This response is sent when a request conflicts with the current state of the server.
410 Gone
This response is sent when the requested content has been permanently deleted from server, with no forwarding address. Clients are expected to remove their caches and links to the resource. The HTTP specification intends this status code to be used for “limited-time, promotional services”. APIs should not feel compelled to indicate resources that have been deleted with this status code.
HTTP 5xx Errors
500 Internal Server Error
The server has encountered a situation it doesn’t know how to handle.
501 Not Implemented
The request method is not supported by the server and cannot be handled. The only methods that servers are required to support (and therefore that must not return this code) are GET and HEAD.
502 Bad Gateway
This error response means that the server, while working as a gateway to get a response needed to handle the request, got an invalid response.
503 Service Unavailable
The server is not ready to handle the request. Common causes are a server that is down for maintenance or that is overloaded. Note that together with this response, a user-friendly page explaining the problem should be sent. This responses should be used for temporary conditions and the Retry-After: HTTP header should, if possible, contain the estimated time before the recovery of the service. The webmaster must also take care about the caching-related headers that are sent along with this response, as these temporary condition responses should usually not be cached.
504 Gateway Timeout
This error response is given when the server is acting as a gateway and cannot get a response in time.
505 HTTP Version Not Supported
The HTTP version used in the request is not supported by the server.
506 Variant Also Negotiates
The server has an internal configuration error: the chosen variant resource is configured to engage in transparent content negotiation itself, and is therefore not a proper end point in the negotiation process.
507 Insufficient Storage (WebDAV)
The method could not be performed on the resource because the server is unable to store the representation needed to successfully complete the request.
508 Loop Detected (WebDAV)
The server detected an infinite loop while processing the request.
510 Not Extended
Further extensions to the request are required for the server to fulfil it.
511 Network Authentication Required
The 511 status code indicates that the client needs to authenticate to gain network access.
What design patterns are you familiar with?
1. Singleton The singleton pattern is used to limit creation of a class to only one object. This is beneficial when one (and only one) object is needed to coordinate actions across the system. There are several examples of where only a single instance of a class should exist, including caches, thread pools, and registries. It’s trivial to initiate an object of a class — but how do we ensure that only one object ever gets created? The answer is to make the constructor ‘private’ to the class we intend to define as a singleton. That way, only the members of the class can access the private constructor and no one else. Important consideration: It’s possible to subclass a singleton by making the constructor protected instead of private. This might be suitable under some circumstances. One approach taken in these scenarios is to create a register of singletons of the subclasses and the getInstance method can take in a parameter or use an environment variable to return the desired singleton. The registry then maintains a mapping of string names to singleton objects, which can be accessed as needed.
2. Factory Method A normal factory produces goods; a software factory produces objects. And not just that — it does so without specifying the exact class of the object to be created. To accomplish this, objects are created by calling a factory method instead of calling a constructor. The problem with the above approach is that the code using the SomeClass’s object, suddenly now becomes dependent on the concrete implementation of SomeClass. There’s nothing wrong with using new to create objects but it comes with the baggage of tightly coupling our code to the concrete implementation class, which can occasionally be problematic.
- Strategy
The strategy pattern allows grouping related algorithms under an abstraction, which allows switching out one algorithm or policy for another without modifying the client. Instead of directly implementing a single algorithm, the code receives runtime instructions specifying which of the group of algorithms to run. - Observer
This pattern is a one-to-many dependency between objects so that when one object changes state, all its dependents are notified. This is typically done by calling one of their methods.
For the sake of simplicity, think about what happens when you follow someone on Twitter. You are essentially asking Twitter to send you (the observer) tweet updates of the person (the subject) you followed. The pattern consists of two actors, the observer who is interested in the updates and the subject who generates the updates.
A subject can have many observers and is a one to many relationship. However, an observer is free to subscribe to updates from other subjects too. You can subscribe to news feed from a Facebook page, which would be the subject and whenever the page has a new post, the subscriber would see the new post.
Key consideration: In case of many subjects and few observers, if each subject stores its observers separately, it’ll increase the storage costs as some subjects will be storing the same observer multiple times. - Builder
As the name implies, a builder pattern is used to build objects. Sometimes, the objects we create can be complex, made up of several sub-objects or require an elaborate construction process. The exercise of creating complex types can be simplified by using the builder pattern. A composite or an aggregate object is what a builder generally builds.
Key consideration: The builder pattern might seem similar to the ‘abstract factory’ pattern but one difference is that the builder pattern creates an object step by step whereas the abstract factory pattern returns the object in one go. - Adapter
This allows incompatible classes to work together by converting the interface of one class into another. Think of it as a sort of translator: when two heads of states who don’t speak a common language meet, usually an interpreter sits between the two and translates the conversation, thus enabling communication.
If you have two applications, with one spitting out output as XML with the other requiring JSON input, then you’ll need an adapter between the two to make them work seamlessly. - State
The state pattern encapsulates the various states a machine can be in, and allows an object to alter its behavior when its internal state changes. The machine or the context, as it is called in pattern-speak, can have actions taken on it that propel it into different states. Without the use of the pattern, the code becomes inflexible and littered with if-else conditionals.
What is the package manager for node? Which one do you use?
A package manager or package-management system is a collection of software tools that automates the process of installing, upgrading, configuring, and removing modules and libraries. NPM and Yarn are examples.
What is the difference between var and let?
let gives you the privilege to declare variables that are limited in scope to the block, statement of expression unlike var.
let variables are usually used when there is a limited use of those variables. Say, in for loops, while loops or inside the scope of if conditions etc. Basically, where ever the scope of the variable has to be limited.
var is rather a keyword which defines a variable globally regardless of block scope.
Is node single threaded or multi threaded?
Node. js is a single threaded language which in background uses multiple threads to execute asynchronous code. Node. js is non-blocking which means that all functions ( callbacks ) are delegated to the event loop and they are ( or can be ) executed by different threads.
Difference between == vs === sign
main difference between “==” and “===” operator is that formerly compares variable by making type correction e.g. if you compare a number with a string with numeric literal, == allows that, but === doesn’t allow that, because it not only checks the value but also type of two variable, if two variables are not of the same type “===” return false, while “==” return true.
What are the differences between GraphQL and REST?
With a REST API, you would typically gather the data by accessing multiple endpoints. In the example, these could be /users/ endpoint to fetch the initial user data. Secondly, there’s likely to be a /users//posts endpoint that returns all the posts for a user. The third endpoint will then be the /users//followers that returns a list of followers per user.
In GraphQL on the other hand, you’d simply send a single query to the GraphQL server that includes the concrete data requirements. The server then responds with a JSON object where these requirements are fulfilled.
What is closure?
A closure is a function that has access to its outer function scope even after the outer function has returned. This means a closure can remember and access variables and arguments of its outer function even after the function has finished.
What are microservices?
Microservices - also known as the microservice architecture - is an architectural style that structures an application as a collection of services that are. Highly maintainable and testable. Loosely coupled. Independently deployable. Organized around business capabilities.
Prevents a single point of failure
Explain event loop.
The event loop continuously checks the call stack to see if there’s any function that needs to run. While doing so, it adds any function call it finds to the call stack and executes each one in order.
What is a singleton?
The singleton pattern is a software design pattern that restricts the instantiation of a class to one “single” instance. This is useful when exactly one object is needed to coordinate actions across the system.
What is the difference between a hash map and hash table?
Though both Hashtable and HashMap are data-structure based upon hashing and implementation of Map interface, the main difference between them is that HashMap is not thread-safe but Hashtable is thread-safe. … Another difference is HashMap allows one null key and null values but Hashtable doesn’t allow null key or values.
