Delete data on expiration in memory backend

[fdegiuli]

The memory backend lazily deletes expired values on get:

MemoryBackend.prototype.get = function get(key) {
  var wrappedValue = this.cache[key] || null;
  if (util.isExpired(wrappedValue && wrappedValue.e)) {
    wrappedValue = null;
    delete this.cache[key];
  }
  return Bluebird.resolve(wrappedValue ? wrappedValue.d : null);
};

This causes a memory leak if the data isn't accessed again, which is epecially a problem with versioned data.

Is there any interest on your end in deleting the data via timeout?
E.g.:

MemoryBackend.prototype.set = function set(key, value, options) {
  this.cache[key] = {
    d: value,
  };
  if (options.expire) {
    setTimeout(this.unset.bind(this, key), options.expire * 1000);
  }
  return Bluebird.resolve(value);
};

I know you probably do the lazy expire for a reason. So if you're not into it, would you take a look at #31 so I we can do it from the outside?

[jkrems]

I think the manual deletion is more in line with what we had for the memory backend. Maybe we should also clarify in the docs that the memory backend isn't well-suited for general purpose caching in production. E.g. even without active pruning of old values it might run out of memory if you try to use it as anything as a "better" Map since there's no cache eviction etc..

[Juraj-Masiar]

The docs should have been indeed updated. I was tracking down a slow memory leak for past several days and after checking the source code here, I see the expired items are never removed :(.