Realtime Systems.

WebSockets, SSE, CRDTs, local-first architecture, presence, collaborative editing, and realtime databases. Covers 9 sub-skills.

• Realtime Systems · 1 of 3

  WebSockets

  What transport protocol does WebSocket use underneath?

  AQUICBHTTPCTCPDUDP

• Realtime Systems · 2 of 3

  Server-Sent Events (SSE)

  What header does the browser send on SSE reconnection to indicate where it left off?

  AX-Last-EventBSSE-Resume-FromCLast-Event-IDDIf-None-Match

• Realtime Systems · 3 of 3

  CRDTs & Local-First

  What mathematical structure must a CRDT's merge function form to guarantee convergence?

  AA join-semilattice: merge is commutative, associative, and idempotentBA total order: every pair of states can be linearly ordered by mergeCA ring: merge supports both an addition and a multiplication operationDA group: merge has an identity element and an inverse for every state