A component test that renders markup and asserts on the output is only half a test. The other half is the interaction: the user types into a field, clicks a button, presses Escape, tabs through a form, or holds a modifier key while clicking. React Testing Library’s userEvent library exists to simulate those interactions with…
React Testing Library (RTL) is not a test runner. It is a set of utilities that render a React component into a DOM and provide queries for finding elements the way a user would find them—by role, by label, by text—rather than by implementation details like component state or class names . The library’s guiding…
A React component’s DOM output normally lands inside its parent’s DOM element. The component tree and the DOM tree are assumed to mirror each other. Portals break that assumption deliberately. createPortal(children, domNode) renders the children into a DOM node that exists anywhere in the document, while keeping the children in the same position in the…
The control props pattern is the mechanism that powers React’s own <input>. When you pass a value prop to an input, React stops managing the input’s state internally and defers to you. It still tells you what it would have done via onChange, but the decision is yours. The pattern generalizes that behavior to any…
The compound component pattern is the React equivalent of HTML’s <select> and <option>. A native <select> manages its own state internally, and the <option> elements are configuration—they do not need to know how the dropdown works, only that they belong to it. The parent controls the behavior, the children declare the content, and the two…
Lock escalation, latching, and two-phase locking are three mechanisms that operate at different layers of the database engine, and confusing them is the source of many operational misunderstandings. Lock escalation is a memory optimization: when a single statement accumulates too many row-level locks, the engine converts them into a single table-level lock to reduce the…
Multi-Version Concurrency Control is the mechanism that lets a database serve readers and writers simultaneously without locking. A reader sees a consistent snapshot of the data as it existed when its query or transaction started, while a writer modifies the current version. The two never block each other because they are looking at different versions…
The buffer pool is the database’s cache of disk pages in memory. Every query that reads a row, every index lookup, and every update touches a page in the buffer pool before it touches the disk. When the requested page is already in memory, the read is served without I/O. When it is not, the…
Every relational database that claims ACID compliance has to solve the same problem: how do you guarantee that a committed transaction survives a crash, and how do you undo a transaction that was interrupted mid-flight? The answer, across every major engine, is the same: write the change to a log before you write it to…
The storage engine is the component of a database that decides how data is physically written to disk, how it is read back, and how concurrent access is managed. The query planner and the SQL parser sit above it; the operating system and the disk sit below it. When you run SELECT * FROM users…