CDNs Put copies of your static content near users, so most requests never reach you. +
Rate limiting Say no cheaply, before the expensive part of your system finds out. +
Consistent hashing Add or remove a machine and move only a small slice of the keys, not all of them. +
Bloom filters A tiny structure that says "definitely not here" or "probably here", and never the wrong way round. +
Distributed counters Counting is easy until everyone counts the same thing at once. +
Geospatial indexing Turn "near me" into a range query, because databases cannot sort by two dimensions at once. +
Search indexing basics An index from words to documents, so "find me things containing this" is not a full scan. +
Long polling vs WebSockets vs SSE Three ways to push data to a browser, with very different costs at scale. +
Circuit breakers and retries with jitter Stop calling something that is already failing, and never retry all at the same moment. +