writeonce/docs/examples/porch/middleware/idempotent.wo

-- porch/middleware/idempotent.wo — idempotency middleware backed by @table.
-- Stores successful responses keyed by Idempotency-Key header (+ optional body digest).
-- Replays stored response on subsequent requests with same key.
-- Iteration 1 of the porch track.

use time
use http
use json

-- Idempotent middleware: before (check/replay) + after (store on miss).
-- Key composition: "idem:${header}" or "idem:${header}:${sha256(method|path|body)}"
-- Only stores status, body, content-type (allowlist). Never replays Set-Cookie, Date, etc.
-- In-flight collision: returns 409 if key exists but response not yet stored.
-- Lazy expiry: deletes expired keys on access (TTL default 24h).
pub class Idempotent {
  key_header:   Text        -- e.g., "Idempotency-Key"
  include_body: Bool = true -- digest method+path+body into key
  ttl:          Int = 86_400_000_000  -- 24h in µs

  fn before(mut req: Req) -> ?Resp {
    let header_val = req.headers[self.key_header];
    if header_val == nil { return nil; }

    let key = idempotent_key(self, header_val, req);
    let now = time.ticks();

    -- Look up existing key
    let hits = from k in IdempotencyKey where k.key == key take 1 select k;

    if len(hits) > 0 {
      let stored = hits[0];
      -- Check expiry
      if now - stored.created_at > self.ttl {
        -- Expired: delete and treat as miss
        delete stored;
      } else {
        -- Check if response is stored (created_at within last 10s = in-flight)
        if now - stored.created_at < 10_000_000 {
          -- In-flight collision: another request with same key is being processed
          let r = Resp { status: 409, headers: {}, body: "{\"error\":\"idempotency key in flight\"}" };
          set_header(r, "content-type", "application/json");
          return r;
        }
        -- Valid stored response: decode and replay
        let resp_json = stored.response;
        -- Parse JSON response (status, headers, body)
        let resp = json.decode(resp_json) as IdempotentStoredResp;
        if resp != nil {
          let r = Resp { status: resp.status, headers: resp.headers, body: resp.body };
          return r;
        }
        -- Corrupted stored response: delete and fall through to miss
        delete stored;
      }
    }

    -- Miss: mark request so after() knows to store the response
    req.ctx["idem_miss"] = "true";
    req.ctx["idem_key"] = key;
    return nil;
  }

  fn after(req: Req, mut resp: Resp) {
    -- Only store on successful responses (2xx/3xx) and only if before() was a miss
    if req.ctx["idem_miss"] != "true" { return; }
    if resp.status < 200 { return; }
    if resp.status >= 400 { return; }

    let key = req.ctx["idem_key"];
    if key == nil { return; }

    -- Allowlist headers for replay: only content-type
    let hdrs: map<Text, Text> = {};
    let ct = resp.headers["content-type"];
    if ct != nil { hdrs["content-type"] = ct; }

    let stored = IdempotentStoredResp {
      status:  resp.status,
      headers: hdrs,
      body:    resp.body
    };
    let resp_json = json.encode(stored);

    let now = time.ticks();
    -- Compute digest of method|path|body
    let digest_input = "${req.method}|${req.path}|${req.body}";
    let digest = sha256(bytes_of_text(digest_input));
    let digest_hex = base64_encode(bytes_slice(digest, 0, 16));
    try insert IdempotencyKey { key: key, response: resp_json, created_at: now, digest: digest_hex } catch (e) nil;
  }
}

-- Internal typedef for JSON decode of stored response
typedef IdempotentStoredResp = {
  status:  Int,
  headers: map<Text, Text>,
  body:    Text
}

-- Key composition function
pub fn idempotent_key(self: Idempotent, header_val: Text, req: Req) -> Text {
  if self.key_header == "" { return "idem:${header_val}"; }
  if self.include_body == false { return "idem:${self.key_header}:${header_val}"; }
  -- Include body digest: sha256(method|path|body)
  let digest_input = "${req.method}|${req.path}|${req.body}";
  let digest = sha256(bytes_of_text(digest_input));
  -- Take first 16 chars of hex digest for brevity (base64_encode of bytes)
  let short_digest = base64_encode(bytes_slice(digest, 0, 16));
  return "idem:${self.key_header}:${header_val}:${short_digest}";
}