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MongoDB-in-Kubernetes

MongoDB Sharded cluster deployed in K8s cluster

Deployment of pods for Shard and Config server:

  • Here the shard replicaset stores actual data or chunks (collection of documents) and config servers replicaset stores metadata and config information of the mongodb cluster.

  • In k8s cluster we create a headless service and statefulsets for the deployment of shards and config server pods. When headless service combined with statefulsets, k8s provides a unique DNS name for each pods which resolves to IP address of pods and these names doesn't change even when the pods are rescheduled, so all the replica sets within the shards or config server can still communicate with each other.

  • There are other configuration that are done in yaml definitions like specifying the pod affinity, defining readiness/liveness probes, volume claim templates for persistent volumes and other container specific config for mongodb cluster.

#shards
kubectl create -f mongodbShardReplicaset.yaml
kubectl get pods -w -l role=mongoshard
kubectl describe pods mongodbshardstateful-0
 
#configserver
kubectl create -f mongodbConfigReplicaset.yaml
kubectl get pods -w -l role=mongoconfig
kubectl describe pods mongodbconfigstateful-0
  • Once the deployment of pods are completed , Initiate the shard and config replicaset with rs.initiate() [run this script replicasetInitiate.sh] which will assign primary/secondary state to replica sets. Here each shard and config server are deployed with 3 replicas(1 primary +2 secondary/Arbiter) to maintain the quorum when one of the node goes down, the mongodb should be able to vote and pick another primary shard within the replicaset.
#Run
replicasetInitiate.sh 
#Check the status of replicas set
kubectl exec mongodbshardstateful-0 -c mongodshardcontainer -- mongo --eval 'rs.status()' 
kubectl exec mongodbconfigstateful-0 -c mongodconfigcontainer -- mongo --eval 'rs.status()'

Deployment of Router pod in k8s:

#Start the routers
kubectl create -f mongodbRouter.yaml
#Get the status
kubectl get pods -w -l role=mongorouter
kubectl describe pods mongosrouter-0
--------------------
#Add Shard to router
#Run this script
addShardsRouter.sh
#Get the status of the shard on routers
kubectl exec mongosrouter-0 -c mongosroutercontainer -- mongo --eval 'sh.status()'

Insert documents and validate shard distribution:

  • Create DB/Collection and Insert some 1000 record
#Access the router
kubectl exec -it mongosrouter-0 -c mongosroutercontainer bash
#Create DB/collection and enable hash key sharding
mongo
use nbxdb
sh.enableSharding("nbxdb")
sh.shardCollection("nbxdb.nbximage", {"_id" : "hashed"})
show dbs
show collections
#INSERT 1000 documents
for (var i = 1; i <= 1000; i++) {db.nbximage.insert([{client:"nbximageclient1dokershard"+i, master:"11111111111111111"+i, data: "ytyyytytyyytytyyyytytyyytyytyt"+i }]) }
#VERIFY
db.nbximage.find({})
  • Get the shard distribution status with 1 shard replicaset
#Access the router
kubectl exec -it mongosrouter-0 -c mongosroutercontainer bash
#Create DB/collection and enable hash key sharding
mongo
#Get the shard distribution status
db.nbximage.getShardDistribution()
 
mongos> db.nbximage.getShardDistribution()
Shard mongoreplicaset1shard at mongoreplicaset1shard/mongodbshardstateful-0.mongodbshardservice.default.svc.cluster.local:27017,mongodbshardstateful-1.mongodbshardservice.default.svc.cluster.local:27017,mongodbshardstateful-2.mongodbshardservice.default.svc.cluster.local:27017
 data : 136KiB docs : 1000 chunks : 2
 estimated data per chunk : 68KiB
 estimated docs per chunk : 500
Totals
 data : 136KiB docs : 1000 chunks : 2
 Shard mongoreplicaset1shard contains 100% data, 100% docs in cluster, avg obj size on shard : 139B
  • From the above shard distribution status, the single shard replicaset holds the two chunks where documents are split based on the shard hash key.

Scale the Shards horiznotally:

  • In order to scale, we can add more sharded replicaset so the chunks gets distributed across these shards based on the hashed key.
#Deploy another shard replicaset to scale
kubectl create -f scalemongodbShards.yaml
kubectl get pods -w -l role=mongoshard
kubectl describe pods mongodbshardstateful2-0
# Run this script to perform rs.initiate and sh.addshard()
addShardsRouter_scale.sh
#Get the status
kubectl exec mongodbshardstateful2-0 -c mongodshardcontainer -- mongo --eval 'rs.status()'
kubectl exec mongosrouter-0 -c mongosroutercontainer -- mongo --eval 'sh.status()'
  • Now observe the chunk distribution after adding new shard replicaset
#Access the router
kubectl exec -it mongosrouter-0 -c mongosroutercontainer bash
#Create DB/collection and enable hash key sharding
mongo
#Get the shard distribution status
db.nbximage.getShardDistribution()
 
mongos> db.nbximage.getShardDistribution()
Shard mongoreplicaset1shard at mongoreplicaset1shard/mongodbshardstateful-0.mongodbshardservice.default.svc.cluster.local:27017,mongodbshardstateful-1.mongodbshardservice.default.svc.cluster.local:27017,mongodbshardstateful-2.mongodbshardservice.default.svc.cluster.local:27017
 data : 136KiB docs : 1000 chunks : 1
 estimated data per chunk : 136KiB
 estimated docs per chunk : 1000
Shard mongoreplicaset2shard at mongoreplicaset2shard/mongodbshardstateful2-0.mongodbshardservice2.default.svc.cluster.local:27017
 data : 66KiB docs : 484 chunks : 1
 estimated data per chunk : 66KiB
 estimated docs per chunk : 484
Totals
 data : 202KiB docs : 1484 chunks : 2
 Shard mongoreplicaset1shard contains 67.36% data, 67.38% docs in cluster, avg obj size on shard : 139B
 Shard mongoreplicaset2shard contains 32.63% data, 32.61% docs in cluster, avg obj size on shard : 139B

Scale the Replicaset:

  • We can also scale the number of replicas within the shard replicaset to acheive better read performance or resiliency within the sharded replicas.
#scale the replicas within the shard replicaset (need not create new service/statefulset here)
kubectl scale sts mongodbshardstateful --replicas=4
#Run this script to make sure replica is in ready/secondary state (modify with vote:0 to disable the vote )
scaleShardReplicas.sh
#Get the status
kubectl exec mongodbshardstateful-0 -c mongodshardcontainer -- mongo --eval 'rs.status()'
kubectl exec mongosrouter-0 -c mongosroutercontainer -- mongo --eval 'sh.status()'

Resiliency on Sharded replicas:

  • Test the fault tolerance or liveness probe when one of primary shard goes down or when mongod/mongos service is crashed. During the split brain scenario and if the quorum is manitained within the shard replicaset(1 primary + 2 secondary/Arbiter) , the other two nodes which are alive can vote and one of the shard gets elected as primary.
#Get the shard status with 3 node replicaset
kubectl exec mongodbshardstateful-0 -c mongodshardcontainer -- mongo --eval 'rs.status()'
#kill the mongod service
kubectl exec mongodbshardstateful-0 -- kill 1
#watch the restart of pods
kubectl get pod -w -l role=mongoshard  (another terminal)
#Get the shard status after primary pod get restarted and notice another pod will claim the primary state based on the votes
kubectl exec mongodbshardstateful-0 -c mongodshardcontainer -- mongo --eval 'rs.status()'