- AWS – ElastiCache: Redis in Front of Your Database
Cache-aside in fifteen lines, then the four questions that decide whether it helps: what you key on, what you invalidate, what happens on a miss storm, and what happens when Redis is down. Valkey and Redis OSS engines, cluster mode on versus off, why your client needs the configuration endpoint and not a node address, and TTL as the only invalidation strategy that never goes stale forever.
- MySQL – Server and Session Functions
The small functions that tell you where you are and what you are connected to. VERSION(), DATABASE(), USER() and CURRENT_USER() and why they differ, CONNECTION_ID(), @@variables for session and global settings, SHOW STATUS and SHOW VARIABLES, BENCHMARK() for a crude timing, and SLEEP() — useful for reproducing a lock wait on purpose.
- LeetCode 67 – Add Binary
Looks like string manipulation, is really about carries — and the parse-to-integer answer everyone writes first is exactly what the 10,000-character constraint rules out. Why the carry belongs in the loop condition, why you build and reverse instead of prepending, and XOR and AND as sum-without-carry and carry.
- LeetCode 65 – Valid Number
Not an algorithms problem at all — it tests whether you can pin down an ambiguous spec with questions and turn it into code that does not sprawl. Three flags and one pass beat the finite-state machine, and resetting seenDigit on e is the single line that rejects 1e while accepting 3e+7.
- MySQL – DATE_FORMAT and Date Functions
Formatting and calculating with dates. DATE_FORMAT and the specifier table you will keep coming back to, NOW() versus CURDATE() versus SYSDATE(), DATE_ADD and DATE_SUB, DATEDIFF and TIMESTAMPDIFF, EXTRACT, LAST_DAY, STR_TO_DATE for parsing, and why grouping by DATE_FORMAT(created_at, '%Y-%m') is convenient and quietly prevents the index on created_at from being used.