Draining Kubernetes nodes

For maintenance work in k8s it is sometimes necessary to drain a k8s node, which means removing all pods from it. Kubernetes offers a standard API for this and our operator supports this - to the best of its ability.

Draining nodes is easy enough for stateless services, which can simply be re-launched on any other node. However, for a stateful service this operation is more difficult, and as a consequence more costly and there are certain risks involved, if the operation is not done carefully enough. To put it simply, the operator must first move all the data stored on the node (which could be in a locally attached disk) to another machine, before it can shut down the pod gracefully. Moving data takes time, and even after the move, the distributed system ArangoDB has to recover from this change, for example by ensuring data synchronicity between the replicas in their new location.

Therefore, a systematic drain of all k8s nodes in sequence has to follow a careful procedure, in particular to ensure that ArangoDB is ready to move to the next step. This is necessary to avoid catastrophic data loss, and is simply the price one pays for running a stateful service.

Anatomy of a drain procedure in k8s: the grace period

When a kubectl drain operation is triggered for a node, k8s first checks if there are any pods with local data on disk. Our ArangoDB pods have this property (the Coordinators do use EmptyDir volumes, and Agents and DB-Servers could have persistent volumes which are actually stored on a locally attached disk), so one has to override this with the --delete-local-data=true option.

Furthermore, quite often, the node will contain pods which are managed by a DaemonSet (which is not the case for ArangoDB), which makes it necessary to override this check with the --ignore-daemonsets=true option.

Finally, it is checked if the node has any pods which are not managed by anything, either by k8s itself (ReplicationController, ReplicaSet, Job, DaemonSet or StatefulSet) or by an operator. If this is the case, the drain operation will be refused, unless one uses the option --force=true. Since the ArangoDB operator manages our pods, we do not have to use this option for ArangoDB, but you might have to use it for other pods.

If all these checks have been overcome, k8s proceeds as follows: All pods are notified about this event and are put into a Terminating state. During this time, they have a chance to take action, or indeed the operator managing them has. In particular, although the pods get termination notices, they can keep running until the operator has removed all finalizers. This gives the operator a chance to sort out things, for example in our case to move data away from the pod.

However, there is a limit to this tolerance by k8s, and that is the grace period. If the grace period has passed but the pod has not actually terminated, then it is killed the hard way. If this happens, the operator has no chance but to remove the pod, drop its persistent volume claim and persistent volume. This will obviously lead to a failure incident in ArangoDB and must be handled by fail-over management. Therefore, this event should be avoided.

Things to check in ArangoDB before a node drain

There are basically two things one should check in an ArangoDB cluster before a node drain operation can be started:

1. All cluster nodes are up and running and healthy.
2. For all collections and shards all configured replicas are in sync.

One possibility to verify these two things is via the ArangoDB web interface. Node health can be monitored in the Overview tab under NODES:

Check that all nodes are green and that there is no node error in the top right corner.

As to the shards being in sync, see the Shards tab under NODES:

Check that all collections have a green check mark on the right side. If any collection does not have such a check mark, you can click on the collection and see the details about shards. Please keep in mind that this has to be done for each database separately!

Obviously, this might be tedious and calls for automation. Therefore, there are APIs for this. The first one is Cluster Health:

POST /_admin/cluster/health

… which returns a JSON document looking like this:

{
  "Health": {
    "CRDN-rxtu5pku": {
      "Endpoint": "ssl://my-arangodb-cluster-coordinator-rxtu5pku.my-arangodb-cluster-int.default.svc:8529",
      "LastAckedTime": "2019-02-20T08:09:22Z",
      "SyncTime": "2019-02-20T08:09:21Z",
      "Version": "3.4.2-1",
      "Engine": "rocksdb",
      "ShortName": "Coordinator0002",
      "Timestamp": "2019-02-20T08:09:22Z",
      "Status": "GOOD",
      "SyncStatus": "SERVING",
      "Host": "my-arangodb-cluster-coordinator-rxtu5pku.my-arangodb-cluster-int.default.svc",
      "Role": "Coordinator",
      "CanBeDeleted": false
    },
    "PRMR-wbsq47rz": {
      "LastAckedTime": "2019-02-21T09:14:24Z",
      "Endpoint": "ssl://my-arangodb-cluster-dbserver-wbsq47rz.my-arangodb-cluster-int.default.svc:8529",
      "SyncTime": "2019-02-21T09:14:24Z",
      "Version": "3.4.2-1",
      "Host": "my-arangodb-cluster-dbserver-wbsq47rz.my-arangodb-cluster-int.default.svc",
      "Timestamp": "2019-02-21T09:14:24Z",
      "Status": "GOOD",
      "SyncStatus": "SERVING",
      "Engine": "rocksdb",
      "ShortName": "DBServer0006",
      "Role": "DBServer",
      "CanBeDeleted": false
    },
    "AGNT-wrqmwpuw": {
      "Endpoint": "ssl://my-arangodb-cluster-agent-wrqmwpuw.my-arangodb-cluster-int.default.svc:8529",
      "Role": "Agent",
      "CanBeDeleted": false,
      "Version": "3.4.2-1",
      "Engine": "rocksdb",
      "Leader": "AGNT-oqohp3od",
      "Status": "GOOD",
      "LastAckedTime": 0.312
    },
    ... [some more entries, one for each instance]
  },
  "ClusterId": "210a0536-fd28-46de-b77f-e8882d6d7078",
  "error": false,
  "code": 200
}

Check that each instance has a Status field with the value "GOOD". Here is a shell command which makes this check easy, using the jq JSON pretty printer:

curl -k https://arangodb.9hoeffer.de:8529/_admin/cluster/health --user root: | jq . | grep '"Status"' | grep -v '"GOOD"'

For the shards being in sync there is the Cluster Inventory API call:

POST /_db/_system/_api/replication/clusterInventory

… which returns a JSON body like this:

{
  "collections": [
    {
      "parameters": {
        "cacheEnabled": false,
        "deleted": false,
        "globallyUniqueId": "c2010061/",
        "id": "2010061",
        "isSmart": false,
        "isSystem": false,
        "keyOptions": {
          "allowUserKeys": true,
          "type": "traditional"
        },
        "name": "c",
        "numberOfShards": 6,
        "planId": "2010061",
        "replicationFactor": 2,
        "shardKeys": [
          "_key"
        ],
        "shardingStrategy": "hash",
        "shards": {
          "s2010066": [
            "PRMR-vzeebvwf",
            "PRMR-e6hbjob1"
          ],
          "s2010062": [
            "PRMR-e6hbjob1",
            "PRMR-vzeebvwf"
          ],
          "s2010065": [
            "PRMR-e6hbjob1",
            "PRMR-vzeebvwf"
          ],
          "s2010067": [
            "PRMR-vzeebvwf",
            "PRMR-e6hbjob1"
          ],
          "s2010064": [
            "PRMR-vzeebvwf",
            "PRMR-e6hbjob1"
          ],
          "s2010063": [
            "PRMR-e6hbjob1",
            "PRMR-vzeebvwf"
          ]
        },
        "status": 3,
        "type": 2,
        "waitForSync": false
      },
      "indexes": [],
      "planVersion": 132,
      "isReady": true,
      "allInSync": true
    },
    ... [more collections following]
  ],
  "views": [],
  "tick": "38139421",
  "state": "unused"
}

Check that for all collections the attributes "isReady" and "allInSync" both have the value true. Note that it is necessary to do this for all databases!

Here is a shell command which makes this check easy:

curl -k https://arangodb.9hoeffer.de:8529/_db/_system/_api/replication/clusterInventory --user root: | jq . | grep '"isReady"\|"allInSync"' | sort | uniq -c

If all these checks are performed and are okay, then it is safe to continue with the clean out and drain procedure as described below.

If all replicationFactors of all collections are at least 2, then the system can tolerate the failure of a single DB-Server. If you have set the Environment to Production in the specs of the ArangoDB deployment, you will only ever have one DB-Server on each k8s node and therefore the drain operation is relatively safe, even if the grace period is chosen too small.

Furthermore, we recommend to have one k8s node more than DB-Servers in you cluster, such that the deployment of a replacement DB-Server can happen quickly and not only after the maintenance work on the drained node has been completed. However, with the necessary care described below, the procedure should also work without this.

Finally, one should not run a rolling upgrade or restart operation at the time of a node drain.

Clean out a DB-Server manually

In this step we clean out a DB-Server manually, before issuing the kubectl drain command. Previously, we have denoted this step as optional, but for safety reasons, we consider it mandatory now, since it is near impossible to choose the grace period long enough in a reliable way.

Furthermore, if this step is not performed, we must choose the grace period long enough to avoid any risk, as explained in the previous section. However, this has a disadvantage which has nothing to do with ArangoDB: We have observed, that some k8s internal services like fluentd and some DNS services will always wait for the full grace period to finish a node drain. Therefore, the node drain operation will always take as long as the grace period. Since we have to choose this grace period long enough for ArangoDB to move all data on the DB-Server pod away to some other node, this can take a considerable amount of time, depending on the size of the data you keep in ArangoDB.

Therefore it is more time-efficient to perform the clean-out operation beforehand. One can observe completion and as soon as it is completed successfully, we can then issue the drain command with a relatively small grace period and still have a nearly risk-free procedure.

To clean out a DB-Server manually, we have to use this API:

POST /_admin/cluster/cleanOutServer

… and send as body a JSON document like this:

{"server":"DBServer0006"}

The value of the "server" attribute should be the name of the DB-Server which is the one in the pod which resides on the node that shall be drained next. This uses the UI short name (ShortName in the /_admin/cluster/health API), alternatively one can use the internal name, which corresponds to the pod name. In our example, the pod name is:

my-arangodb-cluster-prmr-wbsq47rz-5676ed

… where my-arangodb-cluster is the ArangoDB deployment name, therefore the internal name of the DB-Server is PRMR-wbsq47rz. Note that PRMR must be all capitals since pod names are always all lower case. So, we could use the body:

{"server":"PRMR-wbsq47rz"}

You can use this command line to achieve this:

curl -k https://arangodb.9hoeffer.de:8529/_admin/cluster/cleanOutServer --user root: -d '{"server":"PRMR-wbsq47rz"}'

The API call will return immediately with a body like this:

{"error":false,"id":"38029195","code":202}

The given id in this response can be used to query the outcome or completion status of the clean out server job with this API:

GET /_admin/cluster/queryAgencyJob?id=38029195

… which will return a body like this:

{
  "error": false,
  "id": "38029195",
  "status": "Pending",
  "job": {
    "timeCreated": "2019-02-21T10:42:14.727Z",
    "server": "PRMR-wbsq47rz",
    "timeStarted": "2019-02-21T10:42:15Z",
    "type": "cleanOutServer",
    "creator": "CRDN-rxtu5pku",
    "jobId": "38029195"
  },
  "code": 200
}

Use this command line to check progress:

curl -k https://arangodb.9hoeffer.de:8529/_admin/cluster/queryAgencyJob?id=38029195 --user root:

It indicates that the job is still ongoing ("Pending"). As soon as the job has completed, the answer will be:

{
  "error": false,
  "id": "38029195",
  "status": "Finished",
  "job": {
    "timeCreated": "2019-02-21T10:42:14.727Z",
    "server": "PRMR-e6hbjob1",
    "jobId": "38029195",
    "timeStarted": "2019-02-21T10:42:15Z",
    "timeFinished": "2019-02-21T10:45:39Z",
    "type": "cleanOutServer",
    "creator": "CRDN-rxtu5pku"
  },
  "code": 200
}

From this moment on the DB-Server can no longer be used to move shards to. At the same time, it will no longer hold any data of the cluster.

Now the drain operation involving a node with this pod on it is completely risk-free, even with a small grace period.

Performing the drain

After all above checks before a node drain and the manual clean out of the DB-Server have been done successfully, it is safe to perform the drain operation, similar to this command:

kubectl drain gke-draintest-default-pool-394fe601-glts --delete-local-data --ignore-daemonsets --grace-period=300

As described above, the options --delete-local-data for ArangoDB and --ignore-daemonsets for other services have been added. A --grace-period of 300 seconds has been chosen because for this example we are confident that all the data on our DB-Server pod can be moved to a different server within 5 minutes. Note that this is not saying that 300 seconds will always be enough. Regardless of how much data is stored in the pod, your mileage may vary, moving a terabyte of data can take considerably longer!

If the highly recommended step of cleaning out a DB-Server manually has been performed beforehand, the grace period can easily be reduced to 60 seconds - at least from the perspective of ArangoDB, since the server is already cleaned out, so it can be dropped readily and there is still no risk.

At the same time, this guarantees now that the drain is completed approximately within a minute.

Things to check after a node drain

After a node has been drained, there will usually be one of the DB-Servers gone from the cluster. As a replacement, another DB-Server has been deployed on a different node, if there is a different node available. If not, the replacement can only be deployed when the maintenance work on the drained node has been completed and it is uncordoned again. In this latter case, one should wait until the node is back up and the replacement pod has been deployed there.

After that, one should perform the same checks as described in things to check before a node drain above.

Finally, it is likely that the shard distribution in the “new” cluster is not balanced out. In particular, the new DB-Server is not automatically used to store shards. We recommend to re-balance the shard distribution, either manually by moving shards or by using the Rebalance Shards button in the Shards tab under NODES in the web interface. This redistribution can take some time again and progress can be monitored in the UI.

After all this has been done, another round of checks should be done before proceeding to drain the next node.