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A dynamically-distributed, highly-available, partition-tolerant, in-memory key-value store built with Mnesia

mnesia-kv

Note: this project is just a fork of lbm_kv, created to push out some more recent releases.

About

One of the main goals of this application is to enable developers to enjoy the pleasures of distributed Mnesia without the need of exploring the complex background. Therefore, mnkv provides a primitive API along with code to handle and work around the dirty details and pitfals related to distributed Mnesia.

Why use it?

Mnesia is a powerful DBMS with support for table replication, transactions, netsplit detection and much more. So why use something on top of it? Unfortunately, as with other powerful DBMSs its use is quite complex and making a Mnesia cluster dynamic requires a lot of research and the use of undocumented features. mnkv is here to release you from this pain.

What does mnkv offer?

  • Mnesia replication management in dynamic Erlang clusters
  • automated table merges and netsplit recovery based on vector clocks and user-provided callbacks
  • a primitive and (hopefully) intuitive API
  • small, documented, fully-typed code-base
  • no additional/transitive dependencies introduced

Examples

A very simple example application/release can be found here.

How does it work?

mnkv is a simple Erlang application that gets dropped into your release. It is not necessary to know the cluster topology in advance, since mnkv can handle dynamic clusters. It listens for new node connections and replicates all its tables to the new nodes. When connected nodes go down, mnkv automatically shrinks the Mnesia cluster to the remaining nodes preserving the writability to its tables. The user decides when and what tables to create, no internal tables are created behind the scenes.

mnkv is able to merge tables automatically (based on lamport/vector clocks). This is needed when a netsplit gets resolved or when the same table gets created on several nodes independently (not a special case for mnkv). If mnkv cannot merge two table entries itself, it will look for a user-defined callback to help with the merge. This handle_conflict/3 callback is specified in the mnkv behaviour and needs to reside in a module with the same name as the table to merge values for, e.g. if your table is called my_table the callback to implement would be my_table:handle_conflict/3.

If no appropriate callback is found or the callback throws an exception during the conflict resolution, mnkv will deterministically restart one of the offending nodes using init:restart/0 (the restarted node will be the one that tried to perform the merge).

Please note that a merge cannot delete values (except for the case when the user callback gets involved). This means that if a mapping gets deleted during a netsplit, the mapping might get re-established when the netsplit gets resolved.