Recover the Application Database if its Replica Set Loses Majority

Overview

In certain severe Kubernetes cluster outages your Application Database's replica set deployment could lose the majority of the replica set's nodes. For example, if you have an Application Database deployment with two nodes in cluster 1 and three nodes in cluster 2, and cluster 2 undergoes an outage, your Application Database's replica set deployment will lose the node majority needed to elect a primary. Without a primary, the MongoDB Agent can't reconfigure a replica set.

To enable rescheduling replica set's nodes, the Kubernetes Operator must forcibly reconfigure the Automation Configuration for the MongoDB Agent to enable deploying replica set nodes in the remaining healthy member clusters. To achieve this, the Kubernetes Operator sets the replicaSets[n].force flag in the replica set configuration. The flag instructs the MongoDB Agent to force a replica set to use the current (latest) Automation Configuration version. Using the flag allows the Kubernetes Operator to reconfigure the replica set in case a primary node isn't elected.

Recover the Application Database through a Forced Reconfiguration

To perform a forced reconfiguration of the Application Database's nodes:

1. Change the spec.applicationDatabase.clusterSpecList configuration settings to reconfigure the Application Database's deployment on healthy Kubernetes clusters to allow the replica set to form a majority of healthy nodes.

2. Remove failed Kubernetes clusters from the spec.applicationDatabase.clusterSpecList, or scale failed Kubernetes member clusters down. This way, the replica set doesn't count the Application Database's nodes hosted on those clusters as voting members of the replica set. For example, having two healthy nodes in cluster 1 and a failed cluster 2 containing 3 nodes, you have two healthy nodes from a total of five replica set members (2/5 healthy). Adding one node to cluster 1 results in having 3/6 ratio of healthy nodes to the number of members in the replica set. To form a replica set majority, you have the following options:

   • Add at least two new replica set nodes to cluster 1, or a new healthy Kubernetes cluster. This achieves a majority (4/7), with four nodes in a seven-member replica set.

   • Scale down a failed Kubernetes cluster to zero nodes, or remove the cluster from the spec.applicationDatabase.clusterSpecList entirely, and add at least one node to cluster 1 to have 3/3 healthy nodes in the replica set's StatefulSet.

3. Add the annotation "mongodb.com/v1.forceReconfigure": "true" to the Automation Configuration for the MongoDB Agent in the metadata.annotations. Add the annotation at the top level of the MongoDBOpsManager custom resource and ensure that the value "true" is a string in quotes.

   Based on this annotation, the Kubernetes Operator performs a forced reconfiguration of the replica set in the next reconciliation process and scales the Application Database's replica set nodes according to the changed deployment configuration.

   The Kubernetes Operator has no means to determine whether the nodes in the failed Kubernetes cluster are healthy. Therefore, if the Kubernetes Operator can't connect to the failed member Kubernetes cluster's API server, the Kubernetes Operator ignores the cluster during the reconciliation process of the Application Database's replica set nodes.

   This means that scaling down of the Application Database nodes removes failed processes from the replica set configuration. In cases when only the API server is down, but the replica set's nodes are running, the Kubernetes Operator doesn't remove the Pods from the failed Kubernetes clusters.

   To indicate that it completed the forced reconfiguration, the Kubernetes Operator adds the annotation key, "mongodb.com/v1.forceReconfigurePerformed", with the current timestamp as the value.

   Important

   The Kubernetes Operator performs only one forced reconfiguration of the replica set. After the replica set reaches a running state, the Kubernetes Operator adds the "mongodb.com/v1.forceReconfigurePerformed" annotation to prevent itself from forcing the reconfiguration again in the future. Therefore, to re-trigger a new forced reconfiguration event, remove one or both of the following annotations from the resource, in the metadata.annotations for the MongoDBOpsManager custom resource.

   • "mongodb.com/v1.forceReconfigurePerformed"

   • "mongodb.com/v1.forceReconfigure"

4. Reapply the configuration for the changed MongoDBOpsManager custom resource in the Kubernetes Operator.