express-lembro

It's not meant to scale and it doesn't leak memory.

$ npm install express-lembro

If you are looking at this module, you are likely looking for an answer to the question: what if I want server-side in-memory session storage that doesn't need to scale past a single process?

You've come to the right place.

Usage

When you require it in, you'll need to pass it the session.

var session = require('express-session');
var lembro = require('express-lembro')(session);

If you want to configure any event listeners, you'll want to create a store first.

var store = new lembro();
store.on('error', function(error)
{
  assert.ifError(error);
  assert.ok(false);
});

Then pass that store to the session middle-ware.

app.use(session({ store : store }));

Or you can just new one up on the fly.

app.use(session({ store : new lembro() }));

Options

The constructor takes an options object and a callback that will be executed once the store has been set up.

new lembro({ interval: 360000, maxSize : '4M' }, function ()
{
    console.log('session store ready, captain.');
});

This example will purge expired sessions every hour and allow a cache size of 4 megabytes.

Option	Default	Description
interval	360000	Number of milliseconds between purging expired sessions
maxSize	4M	Maximum (approximate) cache size

The sizeomatic module is used to determine the cache size and parse the value of the maxSize property - hence the approximate caveat.

Methods

In addition to the methods below, express-lembro exposes all required and optional methods for a session store implementation.

store.reduce(callback)

This method call will reduce the amount of memory being utilized to 75% of the configured maximum (options.maxSize). It will be called automatically whenever the session store cache size exceeds the configured maximum, so it is not necessary to call it directly.

This is the only method instrumented for debugging using the debug module. Messages displayed include a message with the starting cache size and another with the ending cache size.

Memory Usage and Scaling

The MemoryStore Warning

If you've used express-session, then you've likely already seen this warning from their documentation:

Warning The default server-side session storage, MemoryStore, is purposely not designed for a production environment. It will leak memory under most conditions, does not scale past a single process, and is meant for debugging and developing.

And if you tried to run it production anyway, you saw an error like this:

Warning: connection.session() MemoryStore is not
designed for a production environment, as it will leak
memory, and obviously only work within a single process.

That sounds ominous, right?

Except MemoryStore Doesn't Actually Leak Memory

The reality is that MemoryStore doesn't leak memory in the way you might expect, but rather has the same effect as a memory leak in that it can diminish the performance of your application by reducing the amount of available memory. This happens in two ways:

1. Expired sessions won't get purged unless you restart your application. Until then, data that will almost certainly never get used again continues taking up valuable memory.
2. There is no limit on the amount of memory MemoryStore can consume. So it continues to grow in size until your application eventually crashes. How long this takes will depend on a number of factors, but left running long enough it will eventually happen.

Pedantic? Perhaps. But the distinction is important if we want to solve the problem.

And Sometimes You Don't Really Need To Scale

I know we'd all like to think that our apps are going to run the entire world, but the reality is that there are many cases when our apps will never scale beyond a single process. And that's okay.

The most common case I run across is internal LOB applications that utilize LDAP for authentication and authorization, and often have less than 100 users. In these cases, a session is generated without the user ever seeing a login screen, so if their session gets dropped on the server side, that's okay. We'll simply generate a new session for them without them ever noticing.

And That's The Niche express-lembro Fills

This module aims to fill the niche where you want production-ready in-memory session storage that doesn't need to scale - which is admittedly probably a small niche. Use this comparison chart to help determine if you are okay being in that niche.

MemoryStore	express-lembro	Other Session Stores
Will not scale past a single process	Will not scale past a single process	Will most likely scale
Sessions lost on server restart	Sessions lost on server restart	Might persists sessions across application restarts
Leaks memory by not purging expired sessions	Purges expired sessions at a configurable interval	Probably purges sessions
Unbounded cache size	Configurable cache size restrictions	Manages it's own session store size

• If you need something that will scale beyond a single process, use a database.
• If you really need to scale and store sessions in memory, take a look at connect-redis.

Acknowledgments

I used MemoryStore as the basis for this module, referenced connect-mongo-session and this stackoverflow.com question to produce this solution.