Node drain

Node drain takes one TBMQ node out of service gracefully. The node reports itself as out of service so the load balancer stops routing new connections to it, refuses new MQTT connections, and then disconnects its current sessions in batches. Once the node has no sessions left, you can stop or restart it without a sudden mass disconnect.

Use it before maintenance on a single node of a cluster: a rolling upgrade, a node replacement, or scaling the cluster down.

Starting a drain on a node does the following:

1. The node’s nodeDrain health component switches to OUT_OF_SERVICE, which makes /actuator/health return 503. See Health API.
2. New MQTT CONNECT requests are refused right away. The MQTT listeners stay open, so a TCP probe on the MQTT port still succeeds and the node is not restarted by a liveness check while it drains.
3. After a grace period for the load balancer (NODE_DRAIN_LOAD_BALANCER_WAIT_MS, 20 seconds by default), the node starts disconnecting its sessions. It requests up to NODE_DRAIN_BATCH_SIZE disconnects per batch, one batch every NODE_DRAIN_BATCH_INTERVAL_MS.
4. The drain finishes as DRAINED when no sessions remain on the node, or as TIMED_OUT if sessions are still connected when NODE_DRAIN_TIMEOUT_MS has passed since the drain started. The timeout includes the load balancer grace period.

Each batch re-reads the node’s session registry once the previous pass has gone through every session, so a client whose CONNECT was already in progress when the drain started is not refused; it connects and is disconnected by a later batch.

A drain applies to the node that receives the request only. It does not touch other nodes in the cluster.

A drained session is not closed by a client-sent DISCONNECT with reason code 0x00, so the client’s Last Will is processed as for any other ungraceful disconnect.

The drain takes a node out of the load balancer only if the load balancer checks /actuator/health. Make sure the readiness probe, or the load balancer’s health check, targets /actuator/health on the HTTP port, and keep liveness on the MQTT port so the node is not restarted while it drains. See the readiness example in Health API.

The reference deployments do not do this: the Kubernetes manifests use a tcpSocket readiness probe on port 1883, and the Docker Compose HAProxy uses plain TCP checks on the MQTT ports. The MQTT listeners stay open during a drain, so with those checks the node is not taken out of the load balancer. New connections keep reaching it and are refused with the reason codes below, NODE_DRAIN_LOAD_BALANCER_WAIT_MS has no effect, and disconnected clients can be routed straight back to the same node. Switch the health check first, or use another way to stop routing traffic to the node.

The grace period must cover detection time. The first batch starts NODE_DRAIN_LOAD_BALANCER_WAIT_MS after the drain starts (20 seconds by default), whether or not the load balancer has noticed the node is unhealthy by then. Either tighten the health check or raise the wait so that it is at least the time the check needs to fail:

• Kubernetes: roughly periodSeconds × failureThreshold, plus up to timeoutSeconds for a probe that hangs. The readiness example on the Health API page (periodSeconds: 30, failureThreshold: 3) needs up to 90 seconds, so with it set NODE_DRAIN_LOAD_BALANCER_WAIT_MS to 90000 or more, or shorten the probe interval.
• Docker Compose HAProxy: inter × fall. With inter 5s and HAProxy’s default fall 3 that is 15 seconds, which fits the default wait.

In the reference haproxy.cfg, each of the four MQTT listen blocks (mqtt-in, mqtts-in, mqtt-ws-in, mqtt-wss-in) checks its backends with a plain TCP check. Add an HTTP check against the broker’s HTTP port 8083 to each block, and point the server lines at it with check port 8083. The block below is the reference mqtt-in with only those two changes; every other line stays as shipped:

listen mqtt-in
 bind *:${MQTT_PORT}
 mode tcp
 option clitcpka # For TCP keep-alive
 timeout client 3h
 timeout server 3h
 option tcplog
 balance leastconn
 option httpchk GET /actuator/health
 server tbMqtt1 tbmq1:1883 check port 8083 inter 5s resolvers docker_resolver resolve-prefer ipv4
 server tbMqtt2 tbmq2:1883 check port 8083 inter 5s resolvers docker_resolver resolve-prefer ipv4

HAProxy treats an HTTP 503 as a failed check, so a draining node is marked down and receives no new connections, while connections already open are not cut. Apply the same two changes to the other three blocks: add the option httpchk line and add port 8083 after check on each server line, keeping each line’s existing backend address. Edit the file as described in Docker Compose cluster setup, then reload HAProxy.

The drain API is part of the REST API and requires the SYS_ADMIN authority. Call the node you want to drain directly (its own address or pod address), not through the load balancer: the request is handled by whichever node receives it.

1. Read the node’s service ID. Send a GET request to the node and take serviceId from the response:

   curl -X GET \
     --header "X-Authorization: Bearer $ACCESS_TOKEN" \
     'http://NODE_ADDRESS:8083/api/node/drain'

2. Start the drain. Pass the service ID as expectedServiceId:

   curl -X POST \
     --header "X-Authorization: Bearer $ACCESS_TOKEN" \
     'http://NODE_ADDRESS:8083/api/node/drain?expectedServiceId=tbmq1'

3. Poll GET /api/node/drain until state is DRAINED. If it becomes TIMED_OUT, sessions are still connected: either cancel the drain, or continue and stop the node anyway, which disconnects the remaining sessions on shutdown.

4. Stop the node.

expectedServiceId guards against draining the wrong node, for example when a request is routed to a different node than intended. It must equal the node’s service ID, which is TB_SERVICE_ID when set and generated at startup otherwise.

POST and DELETE return 400 with error code 31 (BAD_REQUEST_PARAMS) when expectedServiceId is missing or does not match the node that received the request.

A finished drain (DRAINED or TIMED_OUT) stays in that state, and the node stays out of service, until you cancel it with DELETE or restart the node.

{
  "serviceId": "tbmq1",
  "state": "DRAINING",
  "initialSessions": 25000,
  "remainingSessions": 12000,
  "disconnectRequests": 13000,
  "startedAt": 1790000000000,
  "completedAt": 0
}

MQTT 3.1.1 has no Use another server code, so those clients get the closest available one. Clients should reconnect through the load balancer, which routes them to another node once it treats the drained node as unhealthy — see Before you drain.

The disconnects are reported in the same places as other disconnects:

• The clientDisconnects_total metric counts them, with no reason label. See Prometheus metrics.
• The CLIENT_DISCONNECTED integration event has disconnectReason USE_ANOTHER_SERVER.
• A refused connection produces a CLIENT_CONNECTION_FAILED event, with reason CONNECTION_REFUSED_USE_ANOTHER_SERVER for an MQTT 5.0 client and CONNECTION_REFUSED_SERVER_UNAVAILABLE for an MQTT 3.1.1 client.

See Integrations for the event format.

The settings are read from the service.node-drain section of thingsboard-mqtt-broker.yml. Size the batch and interval so that the reconnecting clients do not overload the rest of the cluster: the defaults disconnect up to 1000 clients per second, and all of them reconnect to the remaining nodes.