An application that helps managing a cluster of Erlang nodes.

• Site ID: a random unique identifier of the node that persists between the restarts and host name changes.
• Cluster ID: a random unique identifier of the cluster.
• Run level: global system state derived from the configuration and the number of peers. There are the following run levels:
  • stopped: classy is stopped
  • single: classy application is running and exchanging membership information
  • cluster: number of known peers is >= n_sites configuration parameter.
  • quorum: number of running peers is >= quorum configuration parameter.

classy is configured via OTP application environment variables and callbacks.

Type: mfa().

A callback that is classy executes during startup. It allows business application to set up other hooks using a more type-safe API.

Type: file:filename().

Default: "."

Directory where persistent data is stored. It should be writable.

Type: non_neg_integer().

Unit: ms.

Maximum interval of time that can pass between the membership CRDT server receiving an update and the moment it propagates it to the peers.

Type: timeout().

Unit: ms.

Default: 5s.

Default timeout for remote procedure calls.

Type: pos_integer()

Unit: s.

Default: 1w.

Forget information about inactive (kicked) sites after this period of time.

Note: cleanup procedure may lead to the following situation:

1. Site A goes down
2. Site B kicks A
3. Information about event 2 propagates through out the cluster.
4. Cleanup. All active peers delete data about A.
5. A goes back up

Since at step 4 we removed the data about event 2., A will reappear in the cluster.

So forget_timeout should be set to a fairly large value to make sure it doesn't cover nodes that can go back online.

Type: pos_integer(),

Default: 1.

Minimum number of running members necessary to advance run_stage from single to cluster.

Type: pos_integer(),

Default: 1.

Minimum number of running members necessary to advance run_stage from cluster to quorum.

By default, classy initializes site to a random value, and the same value is used for the cluster ID.

Business applications can override this behavior by registering on_node_init hook containing a call to classy_node:maybe_init_the_cluster:

classy:on_node_init(
  fun() ->
      classy_node:maybe_init_the_site(SiteId)
  end,
  0)