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.ACCOUNT.<id>.SERVER.CONNS (connections for an account changed)
• $SYS.SERVER.<id>.CLIENT.AUTH.ERR (authentication error)
• $SYS.ACCOUNT.<id>.LEAFNODE.CONNECT (leaf node connects)
• $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)

Monitoring endpoints as listed in the table below are accessible as system services using the following subject pattern:

• $SYS.REQ.SERVER.<id>.<endpoint-name> (request server monitoring endpoint corresponding to endpoint name.)
• $SYS.REQ.SERVER.PING.<endpoint-name> (from all server, request server monitoring endpoint corresponding to endpoint name - will return multiple messages)

• "$SYS.REQ.ACCOUNT.<account-id>.<endpoint-name>(from all server, request account specific monitoring endpoint corresponding to account id and endpoint name - 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.