Low-Level Design Interview Questions and Answers for 2025

If you run into a problem in LLD that you can't solve, there are a few things you can do:  

• Try to find out as much as you can about the problem: This may include information about the error message you're getting, the steps you took before the error, and any code or data that's important.  
• Look online for an answer: People talk about programming problems and share ways to solve them in many online forums and communities. If you search for the error message or problem you are having, you might find a way to fix it. 
• Check the documentation: This is for the programming language or library you're using. There may be information in the documentation about common problems and how to fix them.  
• Think about asking a coworker or a mentor for help: Based on their experience, another developer might be able to come up with a solution.

If you still can't figure out how to solve the problem, you may need to ask the community or a professional service for help. You could post on an online forum or hire a consultant to help you figure out what's wrong. 

A customer asked me to change an LLD that was already in place. First, I got together all the important information, like the original LLD design and specs. Once I had that information, I started to look at it to see if there were any obvious changes that needed to be made. Once I knew what needed to be changed, I made a new LLD design based on what the customer needed. Then, I put the new design through its paces to make sure it met all the customer's needs.

When writing up and making an easy-to-understand LLD, there are a few important things to keep in mind:

• Use clear, concise language.
• Set up your paperwork in a way that makes sense.
• Give examples when you need to.
• Use diagrams and charts when it makes sense to do so.
• Use formatting to make your document look nice to look at. 

You can make sure that your LLD is well-documented and easy to understand in a few keyways. One important way is to use diagrams and charts as much as possible to show how the different parts of your system fit together. Using clear, concise language will also help make your documentation easier to understand. Lastly, it's important to keep your documentation up to date so that everyone who needs it has the most accurate information. 

I'll give you two examples of a low-level design I'm especially happy with.  

One, I came up with the design for a secure communication protocol a few years ago. It was my first big cryptography project, and I had to learn a lot about how to make systems that are safe. The protocol uses a new method of encryption that I came up with based on the ideas of group theory. This plan works very well and gives strong guarantees of safety. I also made a custom authentication protocol that can't be hacked by someone in the middle. I am very happy with how the protocol turned out as a whole. It's strong and works well, and it meets all of the security requirements I set. 

Second, I was working on a project for a client about two years ago that involved making a web app using the Laravel framework. The app had to be able to handle a lot of requests, so I had to find a way to make the code as efficient as possible. I ended up making a custom caching system that used Memcached to store data in memory. This meant that the application could handle more requests without having to wait for the database to respond. The result was an application that worked much better than any of the other Laravel applications my team had made. 

In software engineering, flexibility is how well a system can handle changes and adapt to new needs. Complexity is the number of parts that depend on each other, or the amount of work needed to understand and change the system. When making a low-level design (LLD), it's important to think about the trade-off between flexibility and complexity to make sure the design is easy to understand, easy to maintain, and scalable.  

Using modular design is one way to balance the trade-off between flexibility and complexity in an LLD. By breaking the design into smaller, more independent parts, it's easier to change one part without affecting the others. This makes the system more flexible. But this can make the overall design more complicated because there may be more interfaces and dependencies between the modules.  

Design patterns, which are well-known ways to solve common design problems, are another way to make the trade-off work. Design patterns can help reduce complexity by giving a standard way to solve a problem, but they can also make a project less flexible if they are hard to change to fit the needs of a specific project.  

In the end, the specific needs and limits of the project will determine the right balance between flexibility and complexity. It's important to think about the trade-offs carefully and decide which is more important in the given situation. 

A good low-level design interview questions, don't miss this one. There are a few things you can do to make sure your low-level design (LLD) fits with the system's overall architecture:  

• Understand the high-level design: Before you start the LLD, make sure you fully understand the overall goals, requirements, and constraints of the system's high-level design.  
• Follow the rules for building that have already been set: When making the LLD, make sure to follow any architectural rules that the system already has. Design patterns, coding standards, and naming conventions may be part of these rules.  
• Use architectural views: Architectural views, such as the logical view, the process view, and the deployment view, show the architecture of the system from different points of view. Use these views to make sure that the LLD fits in with the rest of the system's architecture.  
• Talk to the architecture team about the LLD: Before you sign off on the LLD, go over it with the architecture team to make sure it fits in with the system's overall architecture. They can give useful feedback and make sure the LLD fits in with the rest of the system.  
• Update the high-level design if needed: If the low-level design (LLD) shows any differences or inconsistencies with the high-level design, update the high-level design as needed to make sure the two are in sync.

By following these steps, you can make sure that your LLD fits in with the system's overall architecture and helps meet the project's goals and needs. 

You can find and fix possible problems or conflicts in a low-level design (LLD) in a few different ways:

• Look over the LLD in detail: Look over the LLD carefully to find any possible problems or conflicts. This could mean looking for things like inconsistencies, requirements that contradict each other, or areas that might be hard to implement.
• Involve relevant stakeholders: Find the people who might be affected by the possible problems or conflicts and get them involved in solving them. This could include the project team, the architecture team, and any other relevant parties.
• Look at the effects of the problems: Carefully look at how the possible problems or conflicts will affect the project. This could mean considering things like the amount of time and money needed to fix them, the risks and consequences of not fixing them, and the benefits of fixing them.
• Identify potential solutions: Think of ways to solve the problems or issues that have been identified. Think about different options and weigh their pros and cons to find the best thing to do.
• Implement the solution and test it: Once you've found a solution, you should put it into action and test it to make sure it solves the problem or conflict. If the solution doesn't work like you thought it would, go back to step 4 and try again until the problem is fixed.

By following these steps, you can find and fix any potential problems or conflicts in an LLD, making sure that the design is consistent, easy to maintain, and meets the project's needs. 

There are a few things you can do to make sure your low-level design (LLD) works well and is efficient:

• Set performance goals: Before you start the LLD, you need to set the system's performance goals, such as response time, throughput, and scalability. This will help you design the system with the most important parts of performance in mind.
• Think about how the design will work: As you design the system, think about how your choices might affect how well it works. This could mean using algorithms and data structures that are known to work well, using as little memory and processing power as possible, and making the system work best in certain hardware or software environments.
• Use tools to test performance: Use tools for performance testing to measure and evaluate how well the system works as it is being built. These tools can help you find performance problems and bottlenecks so that you can fix the design as needed.
• Monitor and improve performance in production: Once the system is up and running, it should still be monitored and improved. This could mean using performance monitoring tools to track key performance indicators and making changes to the system as needed to improve performance.

By following these steps, you can make sure that your LLD works well and is efficient, meeting the project's performance goals and giving users a good experience.

Let's say you're working on a project to make a new app for a client. You have been asked to make the LLD for the database design of the app. To work with the rest of your team, you could take these steps:  

• Communicate the LLD's purpose and scope: Make sure that other team members understand the LLD's purpose and scope, as well as how it fits into the system's overall architecture.  
• Get the right people involved in the design process: Include other team members in the design process who will be affected by the LLD, like the front-end developers and the testing team. This will help make sure that the LLD meets their needs and addresses any worries they may have.  
• Tell the team about the LLD: Share the LLD with the rest of the team by writing it down or giving a presentation. Make sure to explain the design choices and defend any trade-offs or decisions that might be controversial.  
• Ask the team for feedback and ideas: Ask other team members for their thoughts and feedback on the LLD. Think about what they say and make any changes that will make the design better.  

Tell the team about changes and progress: Keep the team up to date on the LLD's progress and status and let them know if there are any changes to the design. 

During low-level design (LLD), you can acquire stakeholder feedback in numerous ways.  

• Identify: Identify stakeholders, including project team members, the architecture team, and external parties like clients or users.  
• Design: Design with stakeholders: Early stakeholder involvement is key. This may involve organizing meetings or workshops to discuss the LLD or sharing draught designs for comments.  
• Stakeholder feedback: Consult LLD stakeholders. Encourage them to give creative suggestions and listen to their concerns.  
• Assess stakeholder feedback: Evaluate each suggestion's benefits and cons to determine if it can be implemented.  
• Incorporate comments within the LLD: Document and share team changes. 

Low-level design (LLD) alignment with business goals and objectives can be achieved through several means.

• Get a firm grasp of the project's business objectives: The project charter, interviews with important players, and an examination of the market and the competitors are all potential steps in this direction.
• Think about the repercussions your design choices will have on the bottom line: The business ramifications of your design choices should be taken into account throughout the system's development. A design that speeds up development may save money in the long run, while one that improves the system's ability to scale may attract more customers.
• Integrate the LLD into the core of the company: Verify that the LLD fits in with the project's business strategy. The LLD should provide features and functionality that are appropriate for the business model being used.
• Discuss the LLD with those who have a stake in it: Make sure the LLD is in line with the project's business goals and objectives by reviewing it with the project manager and the business owner before finishing it.
• Incorporate necessary changes into the LLD: Maintain the LLD's relevance to the project's current course by revising it as necessary as the project develops and as corporate goals and objectives change. 

Let's say you're hired to create a brand-new mobile app for a company's use. You have developed an LLD in response to the client's initial set of requirements. However, as development continues, the client becomes aware of the need for more functionality.

Consider taking the following actions to accommodate these shifting needs:

• Take a look at the new norms: Learn how the new rules will affect the LLD by reading through the requirements. Think about how this might affect the system's overarching design and rollout schedule.
• Find out what needs to be altered in the LLD: Find out what has to be altered in the LLD in order to meet the new standards. This could necessitate the introduction of brand-new parts, or the alteration of preexisting ones.
• Check the effects of the adjustments: Analyze how the LLD will be affected by the alterations, taking into account potential risks and difficulties. Think about how much effort and what kinds of resources will be needed to implement the changes, as well as any necessary compromises.
• Changes to the LLD: Modify the LLD as required to meet the new standards. Be sure to keep track of and share this information with the rest of the team as soon as possible.
• See if the adjustments work: and make sure they do. Validate and test the modifications to make sure they function as expected and do not have unintended consequences for the system as a whole.

By adhering to these procedures, you may easily adjust to shifting conditions and update the LLD as necessary to meet the developing needs of the project. 

You can take a number of measures to make sure your low-level design (LLD) is testable and that you can successfully validate it:  

• Design for testability: Take testing into account when you create the system. To achieve this, the system may need to be designed in a modular manner, with clear and consistent naming standards, and with the right hooks and interfaces for testing.  
• Define the test standards: Define the test standards that will be applied to the LLD validation. Performance, dependability, and usability are a few examples of functional and non-functional requirements that may be included.  
• Make a testing strategy: Create a testing strategy that details the precise tests that will be carried out to confirm the LLD. Unit tests, integration tests, and system tests could be included in this.  
• Execute the tests: Execute the tests in accordance with the testing strategy. Make careful to record the outcomes of each test as well as any problems or flaws found.  
• Examine the test outcomes: Examine the test results to see if the LLD satisfies the established test requirements. If any problems or flaws are found, make the necessary changes to the LLD and retest it to make sure it is completely validated.

You can take a number of actions to find and mitigate any security issues in a low-level design (LLD), including:  

• Recognize the security requirements: Before beginning the LLD, confirm that you are completely aware of the system's security requirements. This could entail examining any applicable security standards or laws and assessing the threats and vulnerabilities the system might encounter.  
• Think about security when designing: Think about potential security hazards as you develop the system and how to handle them. This may entail applying secure coding standards, design patterns, and security measures like authentication and authorization.  
• Utilize security testing software: To find potential security holes in the LLD, employ security testing techniques. You can use these tools to find widespread vulnerabilities, including SQL injection and cross-site scripting (XSS) attacks.  
• With the security team, go over the LLD: Review the LLD with the security team before it is put into its final form to make sure it satisfies the system's security standards and takes care of any concerns that have been pointed out.  
• Refresh the LLD as necessary: Update the LLD as necessary to address any security risks or vulnerabilities that the LLD identifies. Test the modifications to make sure they effectively reduce the risks that have been identified. 

You can estimate LLD time and resources in numerous ways:  

• Identify LLD tasks and deliverables: This may require chunking the design process.  
• Time and resource estimates for each task: Estimate time and resources for each task. Consider the task's complexity, team members' skill levels, and available resources.  
• Constraints and dependencies or limitations: This may affect LLD time and resources. If the LLD depends on other tasks or resources, this must be considered in the estimate.  
• Verify estimates: Compare estimates to similar projects or ask team members or specialists for input. Adjust estimations depending on comments.  
• Update estimations as needed: Update estimates as needed during design. As the project proceeds and additional information is available, estimates may need to be amended to reflect scope or demand changes. 

Let's say you're working on a project to make a new web application for a client. You made an LLD that uses a certain set of technologies and platforms. But halfway through the project, the client decides they want to switch to a different set of technologies and platforms.

You could take these steps to adjust to these changes:

• Look over the new technologies and platforms to see how they differ from what was originally planned. Think about how this might affect the LLD and how long it will take to implement.
• Find out what changes need to be made to the LLD so that it can work with the new technologies and platforms. This could mean changing or replacing parts or modules that are already there or adding new ones.
• Think about how the changes will affect the LLD, including any risks or problems that might come up. Think about how much time and money it will take to make the changes, as well as any choices that may need to be made.
• Change the LLD as needed to work with the new technologies and platforms. Make sure to write down and tell the rest of the team about these changes. Check and make sure the changes work.
• Test and verify the changes to make sure they work as planned and don't hurt the rest of the system.

A low-level design (LLD) can help with testing and validating a system in several ways:  

• Testability: A system can be made easier to test and validate if it is built with clear and consistent testing practices and a strong set of automated tests. This could mean designing the system with testing in mind, like by making sure interfaces to system parts are clear and well-defined or by using design patterns that make testing easier.  
• Test data: Creating a system that makes it easy to use test data, such as by making it easy to load and change test data or by making it part of the system, can help with testing and validating the system.  
• Test environment: Designing the system to support the use of test environments, such as by making it easy to set up and configure test environments or by including tools for managing test environments, can help with testing and validating the system.  
• Test automation: Designing the system to support the use of test automation, such as by making it easy to automate test cases or by including tools for managing test automation, can help with testing and validating the system.

By taking these and other things into account, you can make an LLD that helps test and validate the system. This can help make sure the system works the way it's supposed to and meets the project's needs. 

• Interfaces and APIs: Including interfaces and APIs, such as RESTful APIs or message-based APIs, can aid with system integration. These interfaces and APIs allow systems to exchange data with other platforms.  
• Designing the system to assist data integration: such as by allowing easy import or export of data from other systems or by integrating data mapping and transformation tools, can aid with system integration with other systems or platforms.  
• System integration: This can be supported by designing the system to readily link to other systems or platforms or by introducing tools for managing system-level connections.  
• Designing the system to allow integration testing: These systems such as by offering a mechanism to readily test and validate integrations with other systems or platforms, can help with system integration.  

You can develop an LLD that facilitates system integration by considering these and other considerations. This can assist the system converse and exchange data with other systems or platforms.

A low-level design (LLD) can help a system change over time in many ways, such as by making it possible to add new features or capabilities:  

• Extensibility: Making the system "extendable" by giving it well-defined "extension points" and a clear plan for adding new features or functions can help the system change over time. This could mean making the system modular, using design patterns that make it easy to add on, and making sure the interfaces between components are clear and well-defined.  
• Reusability: Components and modules that can be easily used for different purposes can help the system change over time. This is made possible by designing the system to be reusable. This can make it possible for the system to be changed to meet new needs or requirements without having to be redesigned a lot.  
• Maintainability: Making sure the system is easy to keep up to date by using clear and consistent coding, documentation, and testing practices can help it change over time. This can make it easier to understand and change the system as needed, which can help the system continue to grow and get better.  
• Flexibility: Making the system flexible, so it can easily change or adapt to new needs or requirements, can help the system grow and change over time. This could be done by building the system with options for configuration or customization, or by using design patterns that make it easier to be flexible.  

By taking these and other things into account, you can design an LLD that allows the system to change over time and add new features or capabilities. This can help make sure that the system can be changed to meet new needs as they come up.

YAGNI, which stands for "You Ain't Gonna Need It," is a software development principle that tells developers not to add features or functions to a system that won't be used. The idea behind YAGNI is that it's hard to know what a system will need in the future, and adding features or functions that it doesn't need can make it harder to use, riskier, and cost more. Instead, YAGNI says that developers should focus on building only the features that are needed right now and put off building other features until they are actually needed.  

DRY, which stands for "Don't Repeat Yourself," is another software development principle that tells developers not to use the same code or other information more than once in a system. DRY is based on the idea that code or information that is duplicated can make a system harder to maintain and increase the chance of mistakes or inconsistencies. Instead, DRY says that developers should try to make components that can be used more than once and are easy to share and use elsewhere in the system. 

Design patterns are solutions that can be used again and again to solve common design problems that come up when making software. They aren't specific to any one programming language or platform. Instead, they offer a general way to solve common design problems. Design patterns are usually explained with a set of standard terms and a notation that software developers all over the world can understand.  

There are many different kinds of design patterns, and each one solves a different problem or issue with design. Some common design patterns include:  

• Creational patterns deal with how objects are made, and they try to make things in a way that fits the situation.  
• Structural patterns deal with how objects are put together, making connections between them to make bigger structures.  
• Behavior patterns are about how objects talk to each other, what goes on between them, and how they work together.  

The main advantage of design patterns is that they give software developers a common language and set of best practices to use when designing systems. By using design patterns, developers can talk to each other and other people involved in the project better. They can also build systems that are easier to maintain, scale, and change. Design patterns can also help developers avoid common design pitfalls and mistakes and make better design decisions. 

UML, or Unified Modeling Language, is a standard language for software engineering that is used to model things visually. UML gives you a set of graphical notations and elements that you can use to show the design and structure of a system, as well as how the different parts of the system relate to each other and work together.  

UML diagrams are pictures of the elements and notation of UML. There are different kinds of UML diagrams, and each one is meant to show a different part of a system. The following types of UML diagrams can be made:  

• Structural diagrams show the static structure of a system, including how different parts relate to each other and depend on each other. Examples of structure diagrams include class diagrams, component diagrams, and deployment diagrams.  
• Behavioral diagrams show how different parts of a system interact and talk to each other. Activity diagrams, sequence diagrams, and state diagrams are all kinds of behavioural diagrams.  
• Use case diagrams show how a system works and how its users interact with it.  

Software engineers can make models of a system that are easier to understand and explain to others by using UML diagrams. This can help make the design and development process better and make systems that work better and are easier to keep up. 

Design patterns are a popular and well-known way to design software, but not everyone agrees with them. Some of the most common complaints about design patterns are:  

• They can be used too much or in the wrong way: Some critics say that developers who don't fully understand design patterns can use them too much or in the wrong way. In these situations, design patterns may be used to solve problems they weren't made to solve, which can lead to solutions that are too complicated or rigid.  
• They may not be specific enough: Some critics say that design patterns are too general to be useful in all situations, and that they may not give enough direction or context to be used effectively in certain situations. In these situations, developers may need to use design patterns along with other ways of making designs.  
• They can be hard to understand: Some critics say that design patterns can be too hard for some developers, especially those who are new to software design, to understand or use. In these situations, developers may need more training or help to use design patterns effectively.  
• They can be hard to change: Some critics say that design patterns can be hard to change and may not adapt well to new needs or requirements. In these situations, developers may have to change or add to the way design patterns are used to meet the needs of the project.  

Overall, design patterns can be a useful tool for designing software, but developers need to know how to use them right and think about the needs and circumstances of their projects when deciding whether and how to use them. 

There are several ways to stop reflection, serialization, and cloning from making a singleton pattern vulnerable:

• Reflection: To keep a singleton pattern from being broken through reflection, you can use the java.lang.reflect.Constructor.setAccessible method to make the function Object() { [native code] } of the singleton class private and inaccessible. This will stop reflection from being used to make new singleton class instances.
  Also, enums are utilized because intrinsic Java checks make sure an enum value is only ever instantiated once. Java Enums can be used for singletons because they are globally accessible. It's one limitation is that it is rigid and does not support lazy initialization. 
• Serialization: To keep a singleton pattern from being broken through serialization, implement the java.io.Serializable interface in the singleton class and add a readResolve method that returns the singleton instance. This will make sure that the singleton instance is returned instead of a new instance being made when deserialization is done.
• Cloning: To keep cloning from breaking a singleton pattern, you can replace the clone method in the singleton class and throw a CloneNotSupportedException. This will stop clones from being made of the singleton instance.
• By taking these steps, you can help make sure that reflection, serialization, or cloning can't be used to break the singleton pattern. 

There are a few key parts and things to think about when making a web crawler system or a search engine:  

The web crawler part of the system will be in charge of getting information from the web, such as HTML pages, images, and other types of content. This part will need to be able to store and process a lot of data as it is collected. It will also need to be able to handle a large amount of data.  

• Indexing: The search engine part of the system will be in charge of indexing the collected data. This will create a searchable index that can be used to quickly find relevant results for user queries. This part will need to be able to store and process the index quickly and handle a large amount of data.  
• Search: It will be up to the search part of the system to handle user requests and give them relevant search results. This part will need to be able to search the index quickly and put results in order based on how relevant they are and other factors.  
• User interface: The system will also need a way for people to enter search terms and see the results. This part will need to be easy to understand and use.  
• Scalability and performance: The system will need to be able to handle a lot of data and a lot of user queries, so it will need to be able to grow and work well. This could mean using load balancing, caching, and other methods to make the system run better.

By taking these and other things into account, you can make a web crawler system or search engine that collects, indexes, and searches web content quickly and gives users relevant search results. 

"Before I can make a safe and effective traffic light system, I need to know how many cars and people are in the area. To do this, I look at traffic footage, which also helps me make a list of traffic flow needs, such as how long each light cycle should be and when pedestrians should be able to cross. I also look at how long it takes for a group of cars to get through an intersection completely. This helps me figure out how long to set the timer for before switching to a new signal.  

When I program a light, I first put in the right traffic signals that respond to feedback from trigger points. Then, I set a timer so that the light changes every 20 to 30 seconds, depending on how big the intersection is and how long it takes a car to cross it. Before putting in a traffic light system at an intersection, I make sure to choose a spot that gives pedestrians enough space to cross the street and gives drivers a clear view of the system." 

To make a traffic control system, you need to think about and take into account the following:  

• Traffic sensors: The traffic control system will need to be able to collect information about the flow of traffic, the speeds of vehicles, and where they are. Traffic sensors, like cameras, radar, or loop detectors, can be set up at different points along the road network to do this.  
• Traffic control devices: To manage the flow of traffic, the traffic control system will need to be able to control things like traffic lights, road signs, and variable message signs. This could mean using sensors to find out what's going on with the traffic and changing how and when the control devices work based on what they find.  
• Central control: The traffic control system will need a part that is in charge of gathering and analysing data from the traffic sensors and controlling the traffic control devices. This part is called the central control. This part could be a computer or some other kind of system that can process data and decide what to do based on that data.  
• Communication: The traffic control system will need to be able to talk to the traffic sensors and control devices, as well as other systems and stakeholders. This could be done with Ethernet, Bluetooth, Wi-Fi, or other wired or wireless communication technologies.
• User interface: The traffic control system will also need a user interface that lets operators see how the traffic is moving and control the traffic control devices as needed. This part will need to be easy to understand and use. 

When using low-level design to make a Snakes and Ladders game, there are a few key parts and things to think about:  

• Board: The game board will be a grid of squares, and each square will be a possible place for a player's game piece. There will also be snakes and ladders on the board, which will let players move their game pieces to different spots.  
• Pieces of the game: Each player will have a piece of the game, which will be represented on the board by a marker or token. The game pieces will move based on how the dice are rolled.  
• Dice: The game will come with dice, which will be used to decide how far each player's piece moves on their turn. The dice will have six sides, and each side will show a different number from 1 to 6.  
• Rules: The game will have a set of rules that explain how to play, such as how to roll the dice, move the game pieces, and use the snakes and ladders.  
• User interface: The game will also have a user interface that lets players do things like move their game pieces, roll the dice, and look at the game board. The user interface could be a physical board game or a digital version that can be played on a computer or a mobile device.   

By taking these and other things into account, you can make a game of snakes and ladders that is fun, easy to play, and has clear rules.
 
START
 
Initialize game board
 
Initialize game pieces for each player
 
Initialize dice
 
Set current player to player 1
 
LOOP
 
Roll dice
 
Move current player's game piece according to dice roll
 
Check if game piece has landed on a snake or ladder
 
If yes, move game piece to new position
 
Check if game piece has reached the end of the board
 
If yes, declare current player the winner and exit loop
 
Switch current player
 
END LOOP 

This flow diagram shows the basic steps involved in playing a snake and ladder game, including initializing the game board and components, rolling the dice, moving the game pieces, and checking for snakes and ladders. The loop structure represents the continuous play of the game, with each iteration representing a single turn for a player. The flow diagram ends when a player reaches the end of the board, at which point the game is declared over and the current player is declared the winner. 

To make data structures and algorithms for an in-memory file system, there are a few important things to keep in mind:  

• Structure of the files: The file system will need to store and organize files and directories in a way that makes them easy to find and use. A hierarchical tree structure is one type of data structure that could be used for this purpose. In this structure, directories are shown as nodes and files are shown as leaf nodes.  
• Metadata about files: The file system will need to store information about each file, like its name, size, and date of creation. This information can be kept in a separate data structure, like a hash table or dictionary, that links file names to records of metadata.  
• File contents: The file system will have to store each file's actual contents. A dynamic array or a linked list, which can be used to store the contents of the file in a single block of memory, could be used for this.  
• File access: The file system will need to let you do things like read and write to files, create and delete files, and move up and down the file hierarchy. Several algorithms can be used to do these things, such as tree traversal algorithms to move through the file hierarchy and I/O algorithms to read and write to files. 

Algorithm: 

START 
Define file hierarchy data structure 
Define file metadata data structure 
Define file contents data structure 
Define file access operations 
Read operation 
Write operation 
Create operation 
Delete operation 
Navigate operation 
Implement file access algorithms using defined data structures and operations 
END 

This Algorithm shows the basic steps for making data structures and algorithms for a file system that works in memory. The first step is to define the data structures that will be used to store the file hierarchy, metadata, and content. The next step is to define the file access operations that the file system will support, like reading and writing to files, making, and deleting files, and so on. Lastly, the file access algorithms can be put into place using the data structures and operations that have already been defined. 

To make a global app for sharing and storing files, there are a few key parts and things to think about:  

• User accounts: The app will need to let users create and manage their own accounts so they can sign up and sign in to get to their files. This could mean putting in place authentication and permission systems, like hashing passwords and two-factor authentication.  
• File storage: The app will need to store and manage user files, both the metadata about the files and the files themselves. This could mean using a distributed storage system, like a distributed file system or object storage, to store the files on multiple servers.  
• File sharing: The app will need to let users share files with each other, either by letting them see the contents of the files or by letting them download copies of the files. This could mean putting in place ways to share, like link sharing or access control lists, to control who can access the shared files.  
• User interface: The app will need a user interface that lets people do things like upload and download files, create and manage user accounts, and share files with other people. The user interface can be a web interface or a native app for a specific platform, like iOS or Android.  
• Performance and scalability: The app will need to be able to store and transfer a lot of files and handle a lot of users. This could mean putting in place performance and scalability techniques like load balancing and caching to make sure the app can keep up with the demand. 

Algorithm: 

START 
Define user accounts data structure 
Define file storage system 
Define file sharing mechanisms 
Implement user interface 
Web-based interface 
Native app for specific platform 
Implement performance and scalability techniques 
END 

This Algorithm shows the main steps that go into making an app for sharing and storing files globally. The first step is to decide on the data structure that will be used to store and manage user accounts. The next step is to decide on the file storage system that will be used to store the actual files and file metadata. Then, the ways to share files can be set up, and finally, the user interface can be made. Lastly, performance and scalability techniques can be used to make sure the app can handle a large number of users and a large number of file transfers and storage. 

To make an e-commerce site that uses microservices to handle transactions, there are a few important parts and things to think about:  

• Product catalogue: The e-commerce site will need to store and manage a list of all the things that can be bought. This could mean putting in place a microservice that manages the product catalogue by adding and removing products, updating product details, and getting information about products.  
• Shopping cart: The e-commerce website will need to be able to create and manage shopping carts. This means that users will be able to add and remove items from their carts and see what's in them. This could mean putting in place a microservice that manages the shopping carts, including adding and removing items, updating the cart's contents, and figuring out how much the cart's items cost in total.  
• Order management: The e-commerce website will need to be able to process payments, figure out shipping costs, and let customers know where their orders are in the process. This could mean putting in place a microservice that handles orders, such as making and changing orders, processing payments, and talking to fulfilment partners.  
• User accounts: The e-commerce website will need to be able to create and manage user accounts, so people can sign up and sign in to see their orders and account information. This could mean putting in place a microservice that handles user accounts, including authentication and authorization, as well as storing and retrieving account information.  
• User interface: The e-commerce website will need a user interface that lets people look at and buy products, create and manage shopping carts and orders, and get to their account information. The user interface can be a web interface or a native app for a specific platform, like iOS or Android. 

Algorithm: 

START 
Define product catalog microservice 
Define shopping cart microservice 
Define order management microservice 
Define user accounts microservice 
Implement user interface 
Web-based interface 
Native app for specific platform 
Implement performance and scalability techniques 
END 

This algorithm shows the basic steps for making an e-commerce website that uses microservices to handle transactions. The first step is to define the microservices that will be in charge of managing the product catalogue, shopping carts, orders, and user accounts. After that, the user interface can be set up, and then performance and scalability techniques can be put in place to make sure the website can handle a lot of traffic and transactions. 

When making a URL shortening service, there are a few important parts and things to think about:  

• URL database: The service that shortens URLs will need to keep a list of the original URLs and the shortened versions of those URLs. This could mean putting in place a data structure, like a hash table or dictionary, that maps long URLs to short ones.  
• Shortened URLs: The URL shortening service will need to create unique shortened URLs that can be used to send users back to the original URL. This could mean putting in place an algorithm that creates unique IDs or strings that can be used as shortened URLs.  
• URL redirection: The service that shortens URLs will need to let users go from the shortened URL to the original URL. This could mean putting in place a mechanism for redirection, like a server-side redirect or a client-side redirect using JavaScript.  
• URL statistics: The URL shortening service may also be able to keep track of statistics for shortened URLs, like the number of clicks, the website that sent the user there, and where the user is located. This could mean putting in place a system for storing data, like a database, to hold the statistics data.  
• User interface: The URL shortening service will need a user interface that lets people enter original URLs, make shortened URLs, and see statistics about shortened URLs. The user interface can be a web interface or a native app for a specific platform, like iOS or Android. 

Algorithm: 

1. Accept a long URL as input. 
2. Check if the service has already shortened the URL before by searching the database for an entry with a matching original URL. 
3. If an entry is found, retrieve the short identifier and skip to step 6. 
4. If no entry is found, generate a short, unique identifier for the URL. 
5. Initialize a counter to 1. 
6. Check if the counter has been used as a short identifier before by searching the database for an entry with a matching short identifier. 
7. If the counter has not been used, store the long URL and the counter as a short identifier in the database and return the short URL (consisting of the service's domain and the counter) to the user. 
8. If the counter has been used, increment the counter by 1 and go back to step b. 
9. Return the short URL to the user. 
10. When a user visits the short URL, search the database for an entry with a matching short identifier. 
11. If an entry is found, retrieve the original URL. 
12. Redirect the user's browser to the original URL. 

Keep a large list of words and phrases in a database or cache (such as a distributed cache like Memcached or Redis). This set of data should have the most common words and phrases that people search for.  

Send a request to the server with the current search query when a user starts to type a search query.  

Get the current search query from the server and use it to look for words and phrases that start with the same letters in the database or cache.  

Send the user's browser a list of words and phrases that match the search.  

Below the search field in the user's browser, show a list of the matching words and phrases.  

As the user types, update the search query and send another request to the server to get updated suggestions.  

A few more things may need to be thought about if this design is to be used by millions of people:  

• For the server infrastructure to be able to handle the large number of search queries, horizontal scaling should be built in. This could be done by spreading requests across multiple servers with a load balancer or by running a distributed cache on a group of servers.  
• So that the server doesn't have to work as hard, the database or cache should be set up for quick lookups. This could mean using an in-memory cache like Memcached or Redis, or it could mean using a database with indexing and sharding strategies that are optimised for fast search queries.  
• Caching on the client side could help to reduce the load on the server even more. This could be done by storing recently searched words and phrases in the user's browser and showing them as suggestions without making a request to the server. 

Here is some example code that shows how the autocomplete feature could work on the server side:

function getSuggestions(query):
 
// Search the database or cache for words and phrases that begin with the same letters as the query.
 
results = search(query) 
 
// Return the list of matching words and phrases.
 
return results
 
On the client side, the feature could be implemented using JavaScript and an HTML form element:
 
// Listen for changes to the search field.
 
document.getElementById("search").addEventListener("input", function() {
 
// Get the current search query.
 
query = this.value
 
// Send a request to the server with the search query.
 
fetch("/suggestions?q=" + query)
 
.then(function(response) {
 
// When the server responds, display the list of suggestions.
 
response.json().then(function(suggestions) {
 
displaySuggestions(suggestions)
 
})
 
})
 
})
 
function displaySuggestions(suggestions) {
 
// Clear the list of suggestions.
 
document.getElementById("suggestions").innerHTML = ""
 
// For each suggestion, create a list item and append it to the list of suggestions.
 
for (suggestion of suggestions) {
 
li = document.createElement("li")
 
li.textContent = suggestion
 
document.getElementById("suggestions").appendChild(li)
 
}
 
}