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Ogorod

A toy key-value storage based on CASPaxos. Gryadka's code base was used as the starting point.

Note that Ogorod is an educational project and not meant to be used in production.

Implementation Details

  • Vanilla CASPaxos without any optimizations.
  • Uses a primitive in-memory storage for acceptor data.
  • Each node contains both proposer (available via public HTTP API) and acceptor (available via private HTTP API).
  • Supports static configuration only.
  • Supports basic operations over HTTP API:
    • read (GET /api/{key});
    • write (PUT /api/{key});
    • compare-and-swap, aka CAS (POST /api/{key}/cas).

As the consequence, the following technical restrictions take place:

  • Each operation require at least 2RTT for each accessor.
  • Nodes can't be added to a running cluster.
  • Proposer configuration is not versioned and not persisted, as well as acceptor data. So, when a node was stopped, it shouldn't be restarted. Otherwise, the linearizability guarantee will be violated.
  • The primitive in-memory storage isn't optimized by any means. Read operations for non-existing keys leave some "garbage" in the storage.
  • Proposer and accessor running in the same Node.js instance communicate over the network.

Running Ogorod

The most simple way to run Ogorod is the following:

docker-compose up

This runs a 3 nodes cluster locally and maps one of proposers on localhost:8080.

Once the cluster is started, you can write values:

$ curl -X PUT http://localhost:8080/api/test \
  -H "Content-Type: application/json" \
  -d '{"foo":"bar"}'

{"version":0,"value":{"foo":"bar"}}

And read them back:

$ curl http://localhost:8080/api/test
{"version":0,"value":{"foo":"bar"}}

Now let's do some magic CAS:

$ curl -X POST http://localhost:8080/api/test/cas \
  -H "Content-Type: application/json" \
  -d '{"version":0,"value":{"bar":"baz"}}'

{"version":1,"value":{"bar":"baz"}}

Note: pass version: null to initialize value during CAS operation.

Next Steps

  • Implement CAS operation - DONE
  • Implement some Jepsen tests

Potential Enhancements

Being a toy implementation, Ogorod has many flaws. Here are some enhancements that could be done in order to make it a bit more serious:

  • Add applicable *Paxos optimizations, like CASPaxos 1RTT optimization.
  • Dynamic configuration support, including proposer configuration versioning and persistence.
  • Implement delete operation support. It will require a GC-like background process (see the CASPaxos paper for more details).
  • Use an embedded key-value storage, like LevelDB, for data persistence.
  • Swap HTTP with plain TCP communication for the internal API.
  • Implement proper error handling and logging.
  • And many-many more.