Some kinds of network interations are best thought of in the context of a session that is initiated, used, and then destroyed. The states in the session are represented by data we store, usually at the “server” end, but sometimes both.

Once we have session state, however, we have to make sure that our session gets exclusive access to this session state data, and that we can get access to it even if the underlying connection changes, or even if the machines we are using as part of the session change underneath us. For example, a server might crash or be restarted for maintenance reasons in the middle of our session.

So we’d like to persist session state beyond any single machine, and yet not have this be so expensive that sessions are slow. This means we need to store session state data on more than just the server endpoint, and ideally this data is stored redundantly so that we are fault-tolerant to normal failure cases (hardware fails, at inopportune times).

And we’d even like to entertain scalability far beyond what is possible now. At the moment, sessions are typically tied to servers located in a single datacenter, but for both elasticity and for reliability, we’d like to be able to move sessions not just between servers in a data center, but between data centers.

Session State: Beyond Soft State. 2003 paper by Benjamin C. Ling, Emre Kiciman and Armando Fox, talking about a system they designed called SSM, which is highly fault-tolerant, and designed towards the needs of session state. This also includes some auto-tuning methods for detecting and handling load.

The Case For RAMClouds: Scalable High-Performance Storage Entirely in DRAM . 2011 update of 2009 paper by John Ousterhout and his group at Stanford, making a case for RAM-only data storage that is still fault-tolerant.

Fast Crash Recovery in RAMCloud.

  1. Describes RAMCloud’s recovery technique. RAMCloud doesn’t replicate data in RAM, instead it replicates logs across many servers, allowing both redundant and fast recovery.

Log-structured Memory for DRAM-based Storage.

  1. Describes RAMCloud’s main innovation, log-structured memory.

PNUTS: Yahoo!’s Hosted Data Serving Platform. 2008 paper by Brian F. Cooper and company describing Yahoo’s distributed database system. They decided that eventual consistency was sufficient.

Redis Sentinel. From Redis documentation, describes how to use Redis in a high-availability mode.

Redis Cluster. From Redis documentation, describes the new (as of 3.0) clustering system allowing for a distributed implementation of Redis.

roshi. Github repo. Distributed storage of timestamped events.


Session definition from Wikipedia.