voretaq7's questions

My company has a customer-facing web application distributed across several servers for the purpose of load balancing and fault tolerance. The application is written in Ruby (Rack, running under Passenger), and authentication to the application is handled via HTTP session cookies.

We currently use a SQL database to store the session data (replicating it as part of our standard database replication), however this solution is not ideal as our SQL database is Postgres, and does not support multi-master operations (during a maintenance outage on the master database logged-in users can check their sessions against a slave, but new users cannot log in). The overhead of SQL queries for every page hit is also not optimal.

I would like to know what practical solutions folks are currently using in production.
Ideally we're looking for:

• A shared session store
  Users logged in to Server A should be able to transparently move to Server B without having to log back in.

• Good redundancy
  Losing a single server shouldn't lose any session state.

• Low overhead
  At a minimum "less intensive than a SQL query for each page hit".

This is a canonical question about capacity planning

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I have a question regarding capacity planning. Can the Server Fault community please help with the following:

• What kind of server do I need to handle some number of users?
• How many users can a server with some specifications handle?
• Will some server configuration be fast enough for my use case?
• I'm building a social networking site: what kind of hardware do I need?
• How much bandwidth do I need for some project?
• How much bandwidth will some number of users use in some application?

For a more comprehensive list of monitoring tools and their features, check out this Wikipedia page.

As the question states, what are the most commonly used tools used for this task and what are their strengths and weaknesses?