An implementation of stale-while-revalidate for Cloudflare Workers

swr.ts

export const CACHE_STALE_AT_HEADER = 'x-edge-cache-stale-at';
export const CACHE_STATUS_HEADER = 'x-edge-cache-status';
export const CACHE_CONTROL_HEADER = 'Cache-Control';
export const CLIENT_CACHE_CONTROL_HEADER = 'x-client-cache-control';
export const ORIGIN_CACHE_CONTROL_HEADER = 'x-edge-origin-cache-control';

enum CacheStatus {
  HIT = 'HIT',
  MISS = 'MISS',
  REVALIDATING = 'REVALIDATING',
}

const swr = async ({
  request,
  event,
}: {
  request: Request;
  event: FetchEvent;
}) => {
  const cache = caches.default;
  const cacheKey = toCacheKey(request);
  const cachedRes = await cache.match(cacheKey);

  if (cachedRes) {
    let cacheStatus = cachedRes.headers.get(CACHE_STATUS_HEADER);

    if (shouldRevalidate(cachedRes)) {
      cacheStatus = CacheStatus.REVALIDATING;

      // update cached entry to show it's 'updating'
      // and thus shouldn't be re-fetched again
      await cache.put(
        cacheKey,
        addHeaders(cachedRes, {
          [CACHE_STATUS_HEADER]: CacheStatus.REVALIDATING,
        })
      );

      event.waitUntil(
        fetchAndCache({
          cacheKey,
          request,
          event,
        })
      );
    }

    return addHeaders(cachedRes, {
      [CACHE_STATUS_HEADER]: cacheStatus,
      [CACHE_CONTROL_HEADER]: cachedRes.headers.get(
        CLIENT_CACHE_CONTROL_HEADER
      ),
    });
  }

  return fetchAndCache({
    cacheKey,
    request,
    event,
  });
};

const fetchAndCache = async ({
  cacheKey,
  request,
  event,
}: {
  request: Request;
  event: FetchEvent;
  cacheKey: Request;
}) => {
  const cache = caches.default;

  // we add a cache busting query param here to ensure that
  // we hit the origin and no other upstream cf caches
  const originRes = await fetch(addCacheBustParam(request));

  const cacheControl = resolveCacheControlHeaders(request, originRes);

  const headers = {
    [ORIGIN_CACHE_CONTROL_HEADER]: originRes.headers.get('cache-control'),
    [CACHE_STALE_AT_HEADER]: cacheControl?.edge?.staleAt?.toString(),
    'x-origin-cf-cache-status': originRes.headers.get('cf-cache-status'),
  };

  if (cacheControl?.edge) {
    // store the cache response w/o blocking response
    event.waitUntil(
      cache.put(
        cacheKey,
        addHeaders(originRes, {
          ...headers,

          [CACHE_STATUS_HEADER]: CacheStatus.HIT,
          [CACHE_CONTROL_HEADER]: cacheControl.edge.value,

          // Store the client cache-control header separately as the main
          // cache-control header is being used as an api for cf worker cache api.
          // When the request is pulled from the cache we switch this client
          // cache-control value in place.
          [CLIENT_CACHE_CONTROL_HEADER]: cacheControl?.client,

          // remove headers we don't want to be cached
          'set-cookie': null,
          'cf-cache-status': null,
          vary: null,
        })
      )
    );
  }

  return addHeaders(originRes, {
    ...headers,
    [CACHE_STATUS_HEADER]: CacheStatus.MISS,
    [CACHE_CONTROL_HEADER]: cacheControl?.client,
    // 'x-cache-api-cache-control': cacheControl?.edge?.value,
    // 'x-origin-res-header': JSON.stringify(toObject(originRes.headers)),
  });
};

const resolveCacheControlHeaders = (req: Request, res: Response) => {
  // don't cache error or POST/PUT/DELETE
  const shouldCache = res.ok && req.method === 'GET';

  if (!shouldCache) {
    return {
      client: 'public, max-age=0, must-revalidate',
    };
  }

  const cacheControl = res.headers.get(CACHE_CONTROL_HEADER);

  // never cache anything that doesn't have a cache-control header
  if (!cacheControl) return;

  const parsedCacheControl = parseCacheControl(cacheControl);

  return {
    edge: resolveEdgeCacheControl(parsedCacheControl),
    client: resolveClientCacheControl(parsedCacheControl),
  };
};

const resolveEdgeCacheControl = ({
  sMaxage,
  staleWhileRevalidate,
}: ParsedCacheControl) => {
  // never edge-cache anything that doesn't have an s-maxage
  if (!sMaxage) return;

  const staleAt = Date.now() + sMaxage * 1000;

  // cache forever when no swr window defined meaning the stale
  // content can be served indefinitely while fresh stuff is re-fetched
  if (staleWhileRevalidate === 0) {
    return {
      value: 'immutable',
      staleAt,
    };
  }

  // when no swr defined only cache for the s-maxage
  if (!staleWhileRevalidate) {
    return {
      value: `max-age=${sMaxage}`,
      staleAt,
    };
  }

  // when both are defined we extend the cache time by the swr window
  // so that we can respond with the 'stale' content whilst fetching the fresh
  return {
    value: `max-age=${sMaxage + staleWhileRevalidate}`,
    staleAt,
  };
};

const resolveClientCacheControl = ({ maxAge }: ParsedCacheControl) => {
  if (!maxAge) return 'public, max-age=0, must-revalidate';

  return `max-age=${maxAge}`;
};

interface ParsedCacheControl {
  maxAge?: number;
  sMaxage?: number;
  staleWhileRevalidate?: number;
}

const parseCacheControl = (value = ''): ParsedCacheControl => {
  const parts = value.replace(/ +/g, '').split(',');

  return parts.reduce((result, part) => {
    const [key, value] = part.split('=');
    result[toCamelCase(key)] = Number(value) || 0;
    return result;
  }, {} as Record<string, number | undefined>);
};

const addHeaders = (
  response: Response,
  headers: { [key: string]: string | undefined | null }
) => {
  const response2 = new Response(response.clone().body, {
    status: response.status,
    headers: response.headers,
  });

  for (const key in headers) {
    const value = headers[key];

    // only truthy
    if (value !== undefined) {
      if (value === null) response2.headers.delete(key);
      else {
        response2.headers.delete(key);
        response2.headers.append(key, value);
      }
    }
  }

  return response2;
};

const toCamelCase = (string: string) =>
  string.replace(/-./g, (x) => x[1].toUpperCase());

/**
 * Create a normalized cache-key from the inbound request.
 *
 * Cloudflare is fussy. If we pass the original request it
 * won't find cache matches perhaps due to subtle differences
 * in headers, or the presence of some blacklisted headers
 * (eg. Authorization or Cookie).
 *
 * This method strips down the cache key to only contain:
 *  - url
 *  - method
 *
 * We currently don't cache POST/PUT/DELETE requests, but if we
 * wanted to in the future the cache key could contain req.body,
 * but this is probably not ever a good idea.
 */
const toCacheKey = (req: Request) =>
  new Request(req.url, {
    method: req.method,
  });

const shouldRevalidate = (res: Response) => {
  // if the cache is already revalidating then we shouldn't trigger another
  const cacheStatus = res.headers.get(CACHE_STATUS_HEADER);
  if (cacheStatus === CacheStatus.REVALIDATING) return false;

  const staleAtHeader = res.headers.get(CACHE_STALE_AT_HEADER);

  // if we can't resolve an x-cached-at header => revalidate
  if (!staleAtHeader) return true;

  const staleAt = Number(staleAtHeader);
  const isStale = Date.now() > staleAt;

  // if the cached response is stale => revalidate
  return isStale;
};

const addCacheBustParam = (request: Request) => {
  const url = new URL(request.url);
  url.searchParams.append('t', Date.now().toString());
  return new Request(url.toString(), request);
};

export default swr;

test.ts

import { Request, Response } from 'node-fetch';
import mockDate from 'mockdate';

import swr, { CACHE_CONTROL_HEADER } from './swr';

const STATIC_DATE = new Date('2000-01-01');

const fetchMock = jest.fn();
const cachesMock = {
  match: jest.fn(),
  put: jest.fn(),
};

// @ts-ignore
global.caches = { default: cachesMock };
global.fetch = fetchMock;

beforeEach(() => {
  mockDate.set(STATIC_DATE);
  cachesMock.match.mockReset();
  cachesMock.put.mockReset();
  global.fetch = fetchMock.mockReset();
});

describe('swr', () => {
  describe('s-maxage=60, stale-while-revalidate', () => {
    describe('first request', () => {
      let cachesPutCall: [Request, Response];
      let response;
      let request;

      beforeEach(async () => {
        request = new Request('https://example.com');

        fetchMock.mockResolvedValue(
          new Response('', {
            headers: {
              'cache-control': 's-maxage=60, stale-while-revalidate',
            },
          })
        );

        response = await swr({
          request,
          event: mockFetchEvent(),
        });

        cachesPutCall = cachesMock.put.mock.calls[0];
      });

      it('calls fetch as expected', () => {
        const [request] = fetchMock.mock.calls[0];

        expect(fetchMock).toHaveBeenCalledTimes(1);

        expect(request.url).toBe(
          `https://example.com/?t=${STATIC_DATE.getTime()}`
        );
      });

      it('has the expected cache-control header', () => {
        expect(response.headers.get(CACHE_CONTROL_HEADER)).toBe(
          'public, max-age=0, must-revalidate'
        );
      });

      it('caches the response', () => {
        const [request, response] = cachesMock.put.mock.calls[0];

        expect(cachesMock.put).toHaveBeenCalledTimes(1);
        expect(request.url).toBe('https://example.com/');

        // caches forever until revalidate
        expect(response.headers.get('cache-control')).toBe('immutable');
      });

      describe('… then second request (immediate)', () => {
        let response: Response;

        beforeEach(async () => {
          request = new Request('https://example.com');

          cachesMock.match.mockResolvedValueOnce(cachesPutCall[1]);

          response = ((await swr({
            request,
            event: mockFetchEvent(),
          })) as unknown) as Response;
        });

        it('returns the expected cached response', () => {
          expect(response.headers.get('x-edge-cache-status')).toBe('HIT');
        });

        it('has the expected cache-control header', () => {
          expect(response.headers.get('cache-control')).toBe(
            'public, max-age=0, must-revalidate'
          );
        });
      });

      describe('… then second request (+7 days)', () => {
        let response: Response;

        beforeEach(async () => {
          request = new Request('https://example.com');

          // mock clock forward 7 days
          mockDate.set(
            new Date(STATIC_DATE.getTime() + 1000 * 60 * 60 * 24 * 7)
          );

          cachesMock.match.mockResolvedValueOnce(cachesPutCall[1]);

          response = ((await swr({
            request,
            event: mockFetchEvent(),
          })) as unknown) as Response;
        });

        it('returns the expected cached response', () => {
          expect(response.headers.get('x-edge-cache-status')).toBe(
            'REVALIDATING'
          );
        });
      });
    });
  });

  describe('max-age=10, s-maxage=60, stale-while-revalidate=60', () => {
    let cachesPutCall: [Request, Response];
    let request;
    let response;

    beforeEach(async () => {
      request = new Request('https://example.com');

      fetchMock.mockResolvedValueOnce(
        new Response('', {
          headers: {
            'cache-control':
              'max-age=10, s-maxage=60, stale-while-revalidate=60',
          },
        })
      );

      response = await swr({
        request,
        event: mockFetchEvent(),
      });

      cachesPutCall = cachesMock.put.mock.calls[0];
    });

    it('calls fetch as expected', () => {
      const [request] = fetchMock.mock.calls[0];

      expect(fetchMock).toHaveBeenCalledTimes(1);

      expect(request.url).toBe(
        `https://example.com/?t=${STATIC_DATE.getTime()}`
      );
    });

    it('has the expected response', () => {
      expect(response.headers.get('cache-control')).toBe('max-age=10');
    });

    it('caches the response', () => {
      const [request, response] = cachesPutCall;

      expect(cachesMock.put).toHaveBeenCalledTimes(1);
      expect(request.url).toBe('https://example.com/');

      // stores the cached response for the additional swr window
      expect(response.headers.get('cache-control')).toBe('max-age=120');
    });

    describe('… then second request', () => {
      let res: Response;

      beforeEach(async () => {
        request = new Request('https://example.com');

        cachesMock.match.mockResolvedValueOnce(cachesPutCall[1]);

        res = ((await swr({
          request,
          event: mockFetchEvent(),
        })) as unknown) as Response;
      });

      it('returns the expected cached response', () => {
        expect(res.headers.get('x-edge-cache-status')).toBe('HIT');
      });

      it('has the expected cache-control header', () => {
        expect(res.headers.get('Cache-Control')).toBe('max-age=10');
      });

      describe('… then + 61s', () => {
        let response: Response;
        let revalidateFetchDeferred;

        beforeEach(async () => {
          // move clock forward 61 seconds
          mockDate.set(new Date(STATIC_DATE.getTime() + 61 * 1000));

          request = new Request('https://example.com');

          // reset mock state
          fetchMock.mockReset();
          cachesMock.put.mockReset();

          cachesMock.match.mockResolvedValueOnce(cachesPutCall[1]);

          revalidateFetchDeferred = deferred();

          // mock the revalidated request
          fetchMock.mockImplementationOnce(() => {
            revalidateFetchDeferred.resolve(
              new Response('', {
                headers: {
                  'cache-control': 's-maxage=60, stale-while-revalidate=60',
                },
              })
            );

            return revalidateFetchDeferred.promise;
          });

          response = ((await swr({
            request,
            event: mockFetchEvent(),
          })) as unknown) as Response;
        });

        it('returns the STALE response', () => {
          expect(response.headers.get('x-edge-cache-status')).toBe(
            'REVALIDATING'
          );
        });

        it('updates the cache entry state to REVALIDATING', () => {
          const [, response] = cachesMock.put.mock.calls[0];

          expect(response.headers.get('x-edge-cache-status')).toBe(
            'REVALIDATING'
          );
        });

        it('fetches to revalidate', () => {
          expect(fetchMock).toHaveBeenCalled();
        });

        it('updates the cache with the fresh response', async () => {
          await revalidateFetchDeferred.promise;

          const [, response] = cachesMock.put.mock.calls[1];

          expect(response.headers.get('x-edge-cache-status')).toBe('HIT');
        });
      });
    });
  });

  describe('max-age=60', () => {
    let request;

    beforeEach(async () => {
      request = new Request('https://example.com');

      fetchMock.mockResolvedValueOnce(
        new Response('', {
          headers: {
            'cache-control': 'max-age=60',
          },
        })
      );

      await swr({
        request,
        event: mockFetchEvent(),
      });
    });

    it('calls fetch as expected', () => {
      const [request] = fetchMock.mock.calls[0];

      expect(fetchMock).toHaveBeenCalledTimes(1);

      expect(request.url).toBe(
        `https://example.com/?t=${STATIC_DATE.getTime()}`
      );
    });

    it('does NOT cache the response', () => {
      expect(cachesMock.put).not.toHaveBeenCalled();
    });
  });

  describe('s-maxage=60', () => {
    let response;
    let request;

    beforeEach(async () => {
      request = new Request('https://example.com');

      fetchMock.mockResolvedValueOnce(
        new Response('', {
          headers: {
            'cache-control': 's-maxage=60',
          },
        })
      );

      response = await swr({
        request,
        event: mockFetchEvent(),
      });
    });

    it('caches the response', () => {
      expect(cachesMock.put).toHaveBeenCalled();
    });

    it('has the expected response', () => {
      expect(response.headers.get(CACHE_CONTROL_HEADER)).toBe(
        'public, max-age=0, must-revalidate'
      );
    });
  });

  describe('no-store, no-cache, max-age=0', () => {
    let response;
    let request;

    beforeEach(async () => {
      request = new Request('https://example.com');

      fetchMock.mockResolvedValueOnce(
        new Response('', {
          headers: {
            'cache-control': 'no-store, no-cache, max-age=0',
          },
        })
      );

      response = await swr({
        request,
        event: mockFetchEvent(),
      });
    });

    it('does NOT cache the response', () => {
      expect(cachesMock.put).not.toHaveBeenCalled();
    });

    it('has the expected response', () => {
      expect(response.headers.get(CACHE_CONTROL_HEADER)).toBe(
        'public, max-age=0, must-revalidate'
      );
    });
  });

  describe('404', () => {
    let response;
    let request;

    beforeEach(async () => {
      request = new Request('https://example.com');

      fetchMock.mockResolvedValueOnce(
        new Response('error', {
          status: 404,
          headers: {
            'cache-control': 's-maxage=100',
          },
        })
      );

      response = await swr({
        request,
        event: mockFetchEvent(),
      });
    });

    it('does NOT cache the response', () => {
      expect(cachesMock.put).not.toHaveBeenCalled();
    });
  });

  describe('POST', () => {
    let response;
    let request;

    beforeEach(async () => {
      request = new Request('https://example.com', {
        method: 'POST',
      });

      fetchMock.mockResolvedValueOnce(
        new Response('error', {
          headers: {
            'cache-control': 's-maxage=100',
          },
        })
      );

      response = await swr({
        request,
        event: mockFetchEvent(),
      });
    });

    it('does NOT cache the response', () => {
      expect(cachesMock.put).not.toHaveBeenCalled();
    });
  });

  describe('max-age=60', () => {
    let request;

    beforeEach(async () => {
      request = new Request('https://example.com');

      fetchMock.mockResolvedValueOnce(
        new Response('', {
          headers: {
            'cache-control': 'max-age=60',
          },
        })
      );

      await swr({
        request,
        event: mockFetchEvent(),
      });
    });

    it('calls fetch as expected', () => {
      const [request] = fetchMock.mock.calls[0];

      expect(fetchMock).toHaveBeenCalledTimes(1);
      expect(request.url).toBe(`https://example.com/?t=${Date.now()}`);
    });

    it('does NOT cache the response', () => {
      expect(cachesMock.put).not.toHaveBeenCalled();
    });
  });
});

describe('s-maxage=1800, stale-while-revalidate=86400', () => {
  let request;

  beforeEach(async () => {
    request = new Request('https://example.com');

    fetchMock.mockResolvedValueOnce(
      new Response('', {
        headers: {
          'cache-control': 's-maxage=1800, stale-while-revalidate=86400',
        },
      })
    );

    await swr({
      request,
      event: mockFetchEvent(),
    });
  });

  // it('calls fetch as expected', () => {
  //   const [request] = fetchMock.mock.calls[0];

  //   expect(fetchMock).toHaveBeenCalledTimes(1);
  //   expect(request.url).toBe(`https://example.com/?t=${Date.now()}`);
  // });

  it('does cache the response', () => {
    expect(cachesMock.put).toHaveBeenCalled();
  });
});

const mockFetchEvent = () =>
  (({
    waitUntil: jest.fn(),
  } as unknown) as FetchEvent);

const deferred = () => {
  let resolve;
  let reject;

  const promise = new Promise((_resolve, _reject) => {
    resolve = _resolve;
    reject = _reject;
  });

  return {
    promise,
    resolve,
    reject,
  };
};

@wilsonpage I'm interested in using some of your implementation in our codebase, would it be possible for you to confirm a license the code could be used under? i.e (MIT, BSD, Apache, etc)

@wilsonpage I'm interested in using some of your implementation in our codebase, would it be possible for you to confirm a license the code could be used under? i.e (MIT, BSD, Apache, etc)

Haha! Yeah feel free to use whatever :) Shall we say it's 'MIT'? 🤷

MIT is great. Will add attribution for anything I use, thanks!

Haha! Yeah feel free to use whatever :) Shall we say it's 'MIT'? 🤷

👏 👏

This works great with worker + hono + trpc!

Thanks 👍