May 12, 2016

Building a distributed in-memory key value store in Clojure.

In this series we are going to build a distributed in-memory key value store, from scratch, using Clojure. Why? For learning and fun!

The distributed in-memory store will be called memdi and will have the following semantics.

  • There will be a single master node and N follower nodes
  • Any node will accept the write request and write the key-value to the master
  • A write is considered successful if the data is stored in-memory on the master
  • Follower nodes can heal by requesting keys from the master
  • New follower nodes are manually added to a cluster (no discovery) and can request all keys from the master
  • All follower nodes perform a heartbeat with the master and can invoke a leader election, promoting themselves to a candidate

I'm going to break the series into the following posts:

1. Building out a skeleton

We will get the basic of the application together, starting from a lein new app memdi.

2. Handling our first, in process, replicated write

We will create enough of the application to have the core concepts and will be able to handle a write to master node with in process replication.

3. Handling in process heartbeats

Still working just in process we add the code required to handle heartbeats between master and follower nodes.

4. Handling in process elections

Here we add code required to manage an election process initiated by a follower node losing contact with the master.

5. Handling our first "real" replicated write

We will be able to send a write request to the cluster and have the key-value replicate across "real" nodes as apposed to our in process nodes.

6. Handling heart beats across nodes

We now add the functionality for follower nodes to monitor the masters heartbeat across process/machine boundaries.

7. Handling elections

Making use of the logic created previously we enable a follower node to initiate a leader elections across process/machine boundaries.

For all of the development above we will be using the repl as the client to our cluster. At some point we will want to develop a real client implementation. It would also be cool if we can design the store in such a way that it can optionally use asynchronous file IO to support durable writes and that we can configure it to treat a successful write as either in-memory or to-disk.

Tags: memdi clojure