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System Events

NATS servers leverage Accounts support and generate events such as:

  • account connect/disconnect
  • authentication errors
  • server shutdown
  • server stat summary

In addition the server supports a limited number of requests that can be used to query for account connections, server stat summaries, and pinging servers in the cluster.

These events are enabled by configuring system_account and subscribing/requesting using a system account user.

Accounts are used so that subscriptions from your applications, say >, do not receive system events and vice versa. Using accounts requires either:

  • Configuring authentication locally and listing one of the accounts in system_account
  • Or by using decentralized authentication and authorization via jwt as shown in this Tutorial. In this case system_account contains the account public key.

Available Events and Services

The system account publishes messages under well known subject patterns.

Server initiated events:

  • $SYS.ACCOUNT.<id>.CONNECT (client connects)
  • $SYS.ACCOUNT.<id>.DISCONNECT (client disconnects)
  • $SYS.SERVER.ACCOUNT.<id>.CONNS (connections for an account changed)
  • $SYS.SERVER.<id>.CLIENT.AUTH.ERR (authentication error)
  • $SYS.ACCOUNT.<id>.LEAFNODE.CONNECT (leaf node connnects)
  • $SYS.ACCOUNT.<id>.LEAFNODE.DISCONNECT (leaf node disconnects)
  • $SYS.SERVER.<id>.STATSZ (stats summary)

In addition other tools with system account privileges, can initiate requests (Examples can be found here):

  • $SYS.REQ.SERVER.<id>.STATSZ (request server stat summary)
  • $SYS.REQ.SERVER.PING (discover servers - will return multiple messages)

Servers like nats-account-server publish system account messages when a claim is updated, the nats-server listens for them, and updates its account information accordingly:

  • $SYS.ACCOUNT.<id>.CLAIMS.UPDATE

With these few messages you can build useful monitoring tools:

  • health/load of your servers
  • client connects/disconnects
  • account connections
  • authentication errors

Local Configuration

To make use of System events, just using accounts, your configuration can look like this:

accounts: {
    USERS: {
        users: [
            {user: a, password: a}
        ]
    },
    SYS: { 
        users: [
            {user: admin, password: changeit}
           ]
    },
}
system_account: SYS

Please note that applications now have to authenticate such that a connection can be associated with an account. In this example username and password were chosen for simplicity of the demonstration. Subscribe to all system events like this nats-sub -s nats://admin:changeit@localhost:4222 ">" and observe what happens when you do something like nats-pub -s "nats://a:a@localhost:4222" foo bar. Examples on how to use system services can be found here.