How do you set payload budgets and reduce bytes: field selection, deltas, compression, image sizes, and what fan-out on write versus read means for the client?

Bytes are latency, battery and (for some users) money, so give every important screen a payload budget (a target for the data needed before the screen is usable, agreed with the backend team and enforced by a test or a dashboard) and measure it with the network instruments and URLSessionTaskMetrics (Q456). The main tools. Ask for less: field selection (GraphQL or sparse fieldsets, Q325), pagination with a sensible page size (Q318), and summaries for lists with the detail fetched on demand. Send changes, not copies: incremental sync by cursor or updated_since, conditional requests with ETag so unchanged data costs a header (Q319), and delta payloads, as the check below shows a five-item delta is a small fraction of the full list. Compress: JSON and text compress very well (the same list shrinks more than fivefold), and URLSession negotiates standard HTTP compression for you, so make sure the server enables it; for images use modern formats and the right pixel size for the screen scale (Q327), never full-resolution originals for a thumbnail. Remember small payload ceilings: a push notification carries only a few kilobytes (Q375), so it carries an id and the app or a notification service extension fetches the rest.

Fan-out is how an event reaches many readers: fan-out on write pushes a new post into each follower’s precomputed timeline at write time (fast reads, expensive writes for accounts with millions of followers), while fan-out on read assembles the timeline when asked (cheap writes, slower reads), and real systems mix them. The client consequences are what an iOS engineer should say: a timeline is a cursor-paginated, partly precomputed list that can have gaps and reordering; new items arrive by push or poll and are merged into the local database without jumping the scroll position (“N new posts” pill); deletions and edits need tombstones and versions; and a celebrity-scale event can cause a thundering herd on refresh, so jitter background refresh and use ETag/304 to keep it cheap. Include cost awareness: respect Low Data Mode (allowsConstrainedNetworkAccess, Q437), use waitsForConnectivity, and avoid autoplaying video on cellular without consent.