Chris Wen's Blog

Linear AlgebraScalars and Vectors Scalar: A single number representing magnitude only. Vector: An ordered array of scalars representing both magnitude and direction. Row Vector: $\mathbf{x} = [x_1, x_2, \dots, x_n]$ Column Vector: $\mathbf{x} = \begin{bmatrix} x_1 \ x_2 \ \vdots \ x_n \end{bmatrix}$ Vector Operations Transpose: Converts a column vector into a row vector.\mathbf{x}^T = [x_1, x_2, \dots, x_n] Addition: Element-wise addition of two vectors.\mathbf{x} + \mathbf{y} = [x_1+y_1, ...

DerivativeThe derivative is a fundamental concept in calculus. The derivative of a function at a specific point represents the rate of change of the function near that point (i.e., the slope of the tangent line to the function’s graph at that point). The essence of a derivative is to provide a local linear approximation of a function using the concept of limits. When the independent variable of function $f(x)$ produces an increment $\Delta x$ at point $x_0$, if the limit of the ratio between...

Why can a Java request hang when CPU and network look normal?When CPU and network are normal, the request is usually waiting for a resource or blocked by another operation, rather than actively executing. Here are some common causes: Slow or blocked database operations Thread deadlock or lock contention Web server thread pool exhaustion Downstream service calls without timeouts CompletableFuture or asynchronous-task starvation Transaction or distributed lock is never released Garbage collect...

1. What is Java, and what are the features of Java?Java is a high-level program language and is platform-independent.Some of the key features of Java include: Object-Oriented: Java is an object-oriented programming language, which means it allows developers to create modular programs and reusable code. Platform Independence: Java code can run on any platform that has a Java Virtual Machine (JVM), making it highly portable. Robust: Java has strong memory management and exception handling feat...

Interview QuestionsCore Java (OOPs, Collections, Concurrency Basics)Explain difference between HashMap, HashTable, and ConcurrentHashMap. Feature HashMap Hashtable ConcurrentHashMap Thread Safety Not thread-safe Thread-safe (synchronized methods) Thread-safe (lock segmentation / fine-grained locking) Null Keys/Values Allows one null key and multiple null values Does not allow any null keys/values Does not allow null keys/values Performance Faster (no synchronization overhead) Slowe...

TerraformDocumentationterraform TofuDocumentationtofu

Question 1: Concurrent Web Crawler ImplementationDesign and implement a multi-threaded web crawler that efficiently crawls a specific domain and counts unique URLs Requirements: Handle circular references and invalid URLs implement rate limiting and exponential back off Process robots.txt compliance Support different URL schemes and redirects Optimize for memory usage with large datasets Overview12345678910111213141516171819[Seed URLs] | [Frontier in-memory queue] <--- [Persistent ov...

AWS Cloud FundamentalsAWS Cli Installationcli AWS Account Creation Root login: When you first create an AWS account, you’ll have a root user login linked to your email address. The root user has full control of the account! Guard this login carefully and use it only for tasks that require it. IAM users: AWS has a system called Identity and Access Management (IAM), which allows you to set up users and roles under your account that have exactly the permissions they need to do certain work. We’l...

Why Perform Performance Tuning? An online product without performance testing is like a ticking time bomb. You don’t know when it will break down, or what its limits are. Some performance problems accumulate slowly over time and eventually explode. Many more are caused by fluctuations in traffic. Performance tuning is a continuous process that helps you identify and address performance issues before they become critical, ensuring a smooth user experience and optimal resource utilization. Perf...

What is data transformationConditional Split TransformationThe conditional split transformation routes data rows to different streams based on matching conditions The conditional split transformation is similar to a CASE decision structure in a programming language. The transformation evaluates expressions, and based on the results, directs the data row to the specified stream Exists TransformationThe exists transformation is a row filtering transformation that checks whether your data exists...