how redis/memcached caching is different from Etag caching ?

Question

i can see that restful has a inbuilt caching mechanism called Etags then why do we need to cache the end points using redis or memcached . adding to that with etags we can check if the resource has modified or not.

here is the sample redis caching code

const getBook = (req, res) => {
  let isbn = req.query.isbn;
  let url = `https://www.googleapis.com/books/v1/volumes?q=isbn:${isbn}`;
  return axios.get(url)
    .then(response => {
      let book = response.data.items;
      // Set the string-key:isbn in our cache. With he contents of the cache : title
      // Set cache expiration to 1 hour (60 minutes)
      client.setex(isbn, 3600, JSON.stringify(book));

      res.send(book);
    })
    .catch(err => {
      res.send('The book you are looking for is not found !!!');
    });
};

const getCache = (req, res) => {
  let isbn = req.query.isbn;
  //Check the cache data from the server redis
  client.get(isbn, (err, result) => {
    if (result) {
      res.send(result);
    } else {
      getBook(req, res);
    }
  });
}

app.get('/book', getCache);

Answer 1

Redis is not caching your endpoint. It is caching the work done inside your endpoint. In this example, Redis is being used as an internal datastore to save the result of Google to query for an ISBN. This is a type of memoization technique to speed up your /book endpoint.

Once you do the work for this query and generate a resource (your custom book response), then you can create ETags for the final resources that you are serving to your client.

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Redis typically uses a timestamp to serve smaller, dynamic content (calculations, objects, templates) for use in near- and real-time applications. It's best utilized as a cache if you know when your resources change. Then you can evacuate the cache, and prime it with the new values. It can set an expires header to prevent future requests for the same resource. Only cache misses are expensive.

Entity Tags (ETags) are not a cache. They prevent your server from wasting processing and bandwidth. ETags are MD5 hashes of a resource, and typically used for larger, static content: graphics, stylesheets, scripts, rendered pages and video. ETags are best used if you don't know when the resource changes, or you don't have control over the caching. Your resource server will set an ETag header in the first response. When the client asks for a resource with the same hash, the server can response with headers only: 304 Not Modified. Only the first call is expensive.