10 posts tagged with "Backend"

Abstract. Test-Driven Development (TDD) is a technique that repeats three steps: write a failing test (Red), write the minimum code that makes it pass (Green), then restructure the code while keeping all tests passing (Refactor). Empirical studies from before the AI era are mixed: a case study of four industrial teams recorded a 40–90% reduction in defect density at the price of 15–35% more initial development time, while a meta-analysis of 27 studies found only a small quality improvement. In the AI-Driven Development Life Cycle (AI-DLC), where source code is increasingly generated by large language models (LLMs), tests acquire an additional function: they act as executable specifications that clarify intent and verify AI output. Studies of test-driven code generation from 2023 to 2026 report that giving tests to an LLM improves results on benchmarks, and the newest approach lets the model generate and then jointly refine both tests and code. However, the evidence comes mainly from small-scale benchmark problems, the quality of the test suite itself caps any claim of correctness, and no evaluation yet exists at the level of the whole AI-DLC.

Indexes speed up reads by charging you on every write, and the charge is larger than most people picture. Measured on PostgreSQL 16 with the same 200,000 rows: a table with no indexes finished inserting in 287.8 ms and occupied 10.2 MB; a table with four indexes took 1,722.3 ms and occupied 27 MB. That is 6× slower and 2.6× bigger — just to carry four structures that no query may ever touch. The right question is not "should this column have an index?" but "does the read time saved cover the write time paid?"

GROUP BY collapses rows; a window function keeps every row while still computing the group total. Measured: on the same 200,000-row table, GROUP BY returns 200 rows and the window returns 200,000 rows. It is also faster than the old approach — replacing a self-join with a window took the query from 79.2 ms down to 22.7 ms. But there is one default that quietly corrupts numbers and almost nobody reads it: OVER (ORDER BY ...) uses a RANGE frame, so every peer row receives the same running total. If you want a row-by-row cumulative sum, you must spell out ROWS.

MailKit là thư viện mail mã nguồn mở cho .NET (giấy phép MIT, tác giả Jeffrey Stedfast), nói được cả SMTP, IMAP và POP3. Tài liệu chính thức của Microsoft về System.Net.Mail.SmtpClient khuyến nghị thẳng việc dùng MailKit thay thế. Phần khó của việc gửi email không nằm ở chỗ gọi SendAsync — nó nằm ở cái HTML bên trong: Outlook classic render bằng engine của Word nên không có flex hay grid, Gmail cắt thư dài quá 102KB, và một thư chỉ có HTML mà thiếu bản plain-text thì điểm spam tăng ngay.

Isolation level quyết định một giao dịch nhìn thấy gì khi có giao dịch khác chạy song song. Ở mức mặc định của hầu hết database là READ COMMITTED, hai phiên cùng đọc số dư 1000 rồi cùng ghi 1100 sẽ cho kết quả cuối là 1100 — một lần cộng biến mất, không có lỗi nào được ném. Nâng lên REPEATABLE READ thì phiên thứ hai nhận ERROR: could not serialize access due to concurrent update: câu trả lời sai âm thầm biến thành một lỗi rõ ràng mà ứng dụng phải thử lại. Toàn bộ số liệu dưới đây đo bằng hai session psql song song trên PostgreSQL 16.11.