系統設計

1const SIGNING_SECRET = "storage-secret", MAX_TRIES = 20;
2 
3type Transfer = (mb: number) => { ok: boolean; sent: number };
4 
5// Send a big file in parts; a failure costs one part, not the whole file.
6function uploadMultipart(sizeMb: number, partMb: number, transfer: Transfer) {
7  const parts: { attempts: number; mbSent: number; ok: boolean }[] = [];
8  const totalParts = Math.ceil(sizeMb / partMb);
9  for (let offset = 0; offset < sizeMb; offset += partMb) {
10    const size = Math.min(partMb, sizeMb - offset);
11    const part = { attempts: 0, mbSent: 0, ok: false };
12    for (let tries = 0; tries < MAX_TRIES; tries++) {
13      const result = transfer(size);
14      part.attempts++;
15      part.mbSent += result.sent;
16      if (result.ok) { part.ok = true; break; }
17    }
18    parts.push(part);
19    if (!part.ok) return { parts, ok: false, failedPart: parts.length - 1, totalParts };
20  }
21  return { parts, ok: true, failedPart: null, totalParts }; // only success may complete
22}
23 
24function fnv1a(text: string): number {
25  let h = 0x811c9dc5;
26  for (const byte of new TextEncoder().encode(text)) h = Math.imul(h ^ byte, 0x01000193);
27  return h >>> 0;
28}
29 
30// Real services use HMAC-SHA256; the idea is identical.
31function signature(key: string, expires: number): string {
32  return fnv1a(`${key}|${expires}|${SIGNING_SECRET}`).toString(16);
33}
34 
35// The app server hands this out; the client uploads straight to storage.
36function presign(key: string, expires: number): string {
37  return `/${key}?expires=${expires}&sig=${signature(key, expires)}`;
38}
39 
40// Storage checks the URL without asking the app server anything.
41function verify(key: string, expires: number, sig: string, now: number): string {
42  if (now > expires) return "expired";
43  if (sig !== signature(key, expires)) return "bad-signature";
44  return "ok";
45}
46 
47// One parity part lets any single lost part be rebuilt.
48function xorParity(parts: number[][]): number[] {
49  const parity = new Array(parts[0].length).fill(0);
50  for (const part of parts) {
51    for (let i = 0; i < part.length; i++) parity[i] ^= part[i];
52  }
53  return parity;
54}
55 
56function recoverPart(parts: (number[] | null)[], parity: number[]): number[] {
57  const rebuilt = [...parity];
58  for (const part of parts) {
59    if (part === null) continue;
60    for (let i = 0; i < part.length; i++) rebuilt[i] ^= part[i];
61  }
62  return rebuilt;
63}