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Traditional Databases in Kubernetes Kunal Kushwaha @kunalkushwaha

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About Me ● Senior Engineer @ NTT Open Source Software Center ● Contributes to OSS container projects like podman, KubeVirt etc ● Docker Community Leader & Meetup Organizer. ● I did detailed evaluation on legacy application migration on Kubernetes using KubeVirt. ○ Presented at Open Source Summit, Japan (2019) ○ ● Currently working on fixing issues in KubeVirt for smooth migration of traditional DB and HA application to Kubernetes.

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Major bottlenecks for DB migration in Kubernetes ● Lack of Virtual IP on Custom Networks. ○ Traditionally, in HA architecture data path, control path have its own network. ○ K8s service cannot be binded to secondary networks created through meta CNI like Multus. ● Lack of Fencing mechanism for data consistency ○ Shared storage is quite popular in traditional HA architecture. ○ Still fencing mechanism is not yet available in Kubernetes, which makes difficult for traditional HA application to migrate without modification. ● Lack of static IP ○ Static IPs were heavily used in traditional Infrastructure. ○ It is hard to get especially in non-cloud infrastructure and its very costly in cloud too.

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Alternative solutions for Kubernetes ● Replace shared disk with streaming replication. ○ Due to lack of reliable fencing mechanism, avoid shared disk. ○ Better scale factor (even to multi-region level) ● HA packages in side-car. ○ Move HA packages out of DB containers to side-car containers within pod. ○ Run along with each db instance and update the DB state to external key-value store. ○ Participate in Leader election with help of state stored in external key-value store. ● Store cluster state outside DB clusters. ○ Storing cluster state outside DB cluster helps to deal with split-brain conditions. ○ Cluster state can be stored in distributed and strongly consistent key-value store, so can be accessed independent of db pods.

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Building Blocks ● Persistent Volume ● StatefulSet ● Service ● ConfigMaps ● Secrets ● Operator Model

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Popular open source DB Operators ● zalando/postgres-operator ○ Build by an european e-commerce company Zalando. ○ Based on zalando/patroni, a template for postgresql HA with Zookeeper, etc or consul ○ Production quality. ● sorintlab/stolon ○ Developed by sorintlab for running PostgreSQL in cloud native environment ○ Similar design like patroni, but independent implementation. ● CrunchyData/postgres-operator ○ Developed by team of PostgreSQL veterans. ○ Uses zalando/paroni for HA functionality since latest release.

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Zalando PG Operator Operator Creates & manage postgreSQL HA Cluster. Build using open source projects ● Patroni: A python daemon that manage one postgres instance. ○ It keeps cluster state in distributed & strongly consistent key value store. ● Spilo: Spilo is a Docker image that provides PostgreSQL and Patroni bundled together.

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Operator Actions

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HA Implementation As PostgreSQL cannot talk to Kubernetes directly, So Patroni manage PostgreSQL. ● Patroni run alongside PostgreSQL & keeps cluster state in distributed & strongly consistent key value store like etcd. ● A leader node name is set as a value of the leader key that expires at predefined TTL ● The leader node update leader key more often than expiration TTL, preventing expiration. ● A non-leader node is not allowed to update the leader key with its name. ● Each instance watches the leader key. ● Once the leader key expires, each remaining instance decides if it is “healthy enough” * to become leader. ● The first “healthy”* instance that creates leader key with its name, becomes leader. ★ Member is never healthy if old master is still running. ★ Member is never healthy if its WAL is behind some other member or its too far behind the last known master position.

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Failover simulation

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Failover simulation

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Failover simulation

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Failover simulation

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Failover simulation

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Failover simulation

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Credits & References Diagrams Credits PostgreSQL and Kubernetes: DBaaS without a vendor-lock Referenced materials ● Blue elephant on-demand: PostgreSQL + Kubernetes FOSDEM 2018, Brussels ● zalando/patroni: A template for PostgreSQL High Availability with ZooKeeper, etcd, or Consul ● An introduction to stolon: cloud native PostgreSQL high availability ● sorintlab/stolon: PostgreSQL cloud native High Availability and more. ● ● Crunchy Data Container Suite Documentation

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Thank You

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