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High Availability in Kubernetes

Redpanda is designed to ensure data integrity and high availability (HA), even at high-throughput levels.

Kubernetes offers an ideal environment for managing and scaling Redpanda. By deploying Redpanda on Kubernetes, you can ensure HA through Kubernetes-native features, benefit from automated rollouts and rollbacks, and take advantage of the platform’s self-healing properties.

Deployment strategies

Consider the following Redpanda deployment strategies for the most common types of failures.

Failure Impact Mitigation strategy

Broker failure

Loss of function for an individual broker or for any virtual machine (VM) that hosts the broker

Multi-broker deployment

Rack or switch failure

Loss of brokers/VMs hosted within that rack, or loss of connectivity to them

Multi-broker deployment spread across multiple racks or network failure domains

Data center failure

Loss of brokers/VMs hosted within that data center, or loss of connectivity to them

Multi-AZ or replicated deployment

Region failure

Loss of brokers/VMs hosted within that region, or loss of connectivity to them

Geo-stretch (latency dependent) or replicated deployment

Global, systemic outage (DNS failures, routing failures)

Complete outage for all systems and services impacting customers and staff

Offline backups (Tiered Storage), replicas in 3rd-party domains

Data loss or corruption (accidental or malicious)

Corrupt or unavailable data that also affects synchronous replicas

Offline backups (Tiered Storage)

HA deployment options

This section explains the trade-offs with different HA configurations.

Multi-broker deployment

Redpanda is designed to be deployed in a cluster that consists of at least three brokers. Although clusters with a single broker are convenient for development and testing, they aren’t resilient to failure. Adding brokers to a cluster provides a way to handle individual broker failures. You can also use Rack awareness to assign brokers to different racks, which allows Redpanda to tolerate the loss of a rack or failure domain.

By default, the Redpanda Helm chart deploys three Redpanda brokers in separate Pods that are all managed by a StatefulSet. If the Redpanda broker inside a Pod crashes, Kubernetes automatically restarts the Pod.

The Redpanda Helm chart also uses podAntiAffinity rules to stop the Kubernetes scheduler from placing multiple Redpanda brokers on the same node. These rules offer two benefits:

  • To minimize the risk of data loss by ensuring that a node’s failure results in the loss of only one Redpanda broker.

  • To prevent resource contention between brokers by ensuring they are never co-located on the same node.

Single-AZ deployment

See also: Single-AZ deployments

Multi-AZ deployment

An availability zone (AZ) consists of one or more data centers served by high-bandwidth links with low latency (and typically within a close distance of one another). All AZs have discrete failure domains (power, cooling, fire, and network), but they also have common-cause failure domains, such as catastrophic events, that affect their geographical location. To safeguard against such possibilities, a cluster can be deployed across multiple AZs by configuring each AZ as a rack using rack awareness.

Redpanda’s internal implementation of Raft lets it tolerate losing a minority of replicas for a given topic or for controller groups. For this to translate to a multi-AZ deployment, however, it’s necessary to deploy to at least three AZs (affording the loss of one zone). In a typical multi-AZ deployment, cluster performance is constrained by inter-AZ bandwidth and latency.

For greater resilience in Kubernetes:

  • Use nodeAffinity rules to control the placement of Pods based on node labels indicating zones or regions.

    Kubernetes provides a scheduling feature known as nodeAffinity that allows you to specify conditions on which a Pod can or cannot be scheduled based on the node labels. This is particularly useful for deploying workloads in a multi-zone or multi-region setup, ensuring high availability and resilience.

  • If you use the Redpanda Operator, deploy multiple instances of it across multiple AZs on multiple nodes.

    The Redpanda Operator has built-in leader election. When the leader (the active Redpanda Operator instance) fails or becomes unreachable, one of the other instances automatically takes over the leadership role, ensuring uninterrupted management of Redpanda instances.

    By distributing instances across multiple AZs, you protect against the failure of a whole availability zone. Even if one zone goes down, an instance from another zone can take over.

Multi-AZ deployment

See also:

Multi-region deployment

A multi-region deployment is similar to a multi-AZ deployment, in that it needs at least three regions to counter the loss of a single region. Note that this deployment strategy increases latency due to the physical distance between regions. Consider the following strategies to mitigate this problem:

  • Manually configure leadership of each partition to ensure that leaders are congregated in the primary region (closest to the producers and consumers).

  • Configure producers to have acks=1 instead of acks=all; however, this introduces the possibility of losing messages if the primary region becomes lost or unavailable.

Multi-cluster deployment

In a multi-cluster deployment, each cluster is configured using one of the other HA deployments, along with standby clusters or Remote Read Replica clusters in one or more remote locations. A standby cluster is a fully functional cluster that can handle producers and consumers. A remote read replica is a read-only cluster that can act as a backup for topics. To replicate data across clusters in a multi-cluster deployment, use one of the following options:

Alternatively, you could dual-feed clusters in multiple regions. Dual feeding is the process of having producers connect to your cluster across multiple regions. However, this introduces additional complexity onto the producing application. It also requires consumers that have sufficient deduplication logic built in to handle offsets, since they won’t be the same across each cluster.

HA features in Kubernetes

Kubernetes includes the following high-availability features:

  • Automatic failovers: Kubernetes has built-in controllers to detect Pod failures and recreate them, ensuring service continuity.

  • Scaling: Use Kubernetes Horizontal Pod Autoscaler (HPA) to scale Redpanda Pods based on metrics like CPU and memory usage.

  • Network policies: Use Kubernetes network policies to control the communication between Redpanda Pods, ensuring secure data transfer.

Node pools

If you use managed Kubernetes services such as Amazon EKS, consider creating dedicated node pools for your Redpanda deployment. These node pools can have machines optimized for the workload with high I/O capacity or more CPU and memory resources. Dedicated node pools ensure that Redpanda brokers do not compete for resources with other workloads in your Kubernetes cluster. This is particularly useful when you need to guarantee specific performance for your Redpanda deployment. Node pools are also beneficial during scaling operations. By having dedicated node pools, you can scale your Redpanda cluster without affecting other applications.

Storage

Data persistence and protection are crucial for high availability:

  • Persistent Volume Claims (PVCs): By default, the Redpanda Helm chart uses the default StorageClass in your Kubernetes cluster to create one PVC for each Redpanda broker, ensuring data is stored persistently. These PVCs ensure that if a Pod is terminated or crashes, the data isn’t lost, and when a new Pod starts, it reattaches to the existing PVC.

  • Storage Class: The dynamic nature of Kubernetes deployments benefits from StorageClasses that automatically provision new PersistentVolumes (PVs) for each PVC. For Redpanda, choose a StorageClass that supports high I/O operations, especially if backed by high-performance NVMe storage.

See also:

HA features in Redpanda

Redpanda includes the following high-availability features:

Replica synchronization

A cluster’s availability is directly tied to replica synchronization. Brokers can be either leaders or replicas (followers) for a partition. A cluster’s replica brokers must be consistent with the leader to be available for consumers and producers.

  1. The leader writes data to the disk. It then dispatches append entry requests to the followers in parallel with the disk flush.

  2. The replicas receive messages written to the partition of the leader. They send acknowledgments to the leader after successfully replicating the message to their internal partition.

  3. The leader sends an acknowledgment to the producer of the message, as determined by that producer’s acks value. Redpanda considers the group consistent after a majority has formed consensus; that is, a majority of participants acknowledged the write.

While Apache Kafka® uses in-sync replicas, Redpanda uses a quorum-based majority with the Raft replication protocol. Kafka performance is negatively impacted when any "in-sync" replica is running slower than other replicas in the In-Sync Replica (ISR) set.

Monitor the health of your cluster with the rpk cluster health command, which tells you if any brokers are down, and if you have any leaderless partitions.

Rack awareness

Rack awareness is one of the most important features for HA. It lets Redpanda spread partition replicas across available brokers in different failure zones. Rack awareness ensures that no more than a minority of replicas are placed on a single rack, even during cluster balancing.

Make sure you assign separate rack IDs that actually correspond to a physical separation of brokers.

See also: Enable Rack Awareness

Partition leadership

Raft uses a heartbeat mechanism to maintain leadership authority and to trigger leader elections. The partition leader sends a periodic heartbeat to all followers to assert its leadership. If a follower does not receive a heartbeat over a period of time, then it triggers an election to choose a new partition leader.

See also: Partition leadership elections

Producer acknowledgment

Producer acknowledgment defines how producer clients and broker leaders communicate their status while transferring data. The acks value determines producer and broker behavior when writing data to the event bus.

See also: Producer Acknowledgement Settings

Partition rebalancing

By default, Redpanda rebalances partition distribution when brokers are added or decommissioned. Continuous Data Balancing additionally rebalances partitions when brokers become unavailable or when disk space usage exceeds a threshold.

See also: Cluster Balancing

Tiered Storage and disaster recovery

In a disaster, your secondary cluster may still be available, but you need to quickly restore the original level of redundancy by bringing up a new primary cluster. In a containerized environment such as Kubernetes, all state is lost from pods that use only local storage. HA deployments with Tiered Storage address both these problems, since it offers long-term data retention and topic recovery.

See also: Tiered Storage

Single-AZ deployments

When deploying a cluster for high availability into a single AZ or data center, you need to ensure that, within the AZ, single points of failure are minimized and that Redpanda is configured to be aware of any discrete failure domains within the AZ. This is achieved with Redpanda’s rack awareness, which deploys n Redpanda brokers across three or more racks (or failure domains) within the AZ.

Single-AZ deployments in the cloud have less network costs than multi-AZ deployments, and you can leverage resilient power supplies and networking infrastructure within the AZ to mitigate against all but total-AZ failure scenarios. You can balance the benefits of increased availability and fault tolerance against any increase in cost, performance, and complexity:

  • Cost: Redpanda operates the same Raft consensus algorithm whether it’s in HA mode or not. There may be infrastructure costs when deploying across multiple racks, but these are normally amortized across a wider datacenter operations program.

  • Performance: Spreading Redpanda replicas across racks and switches increases the number of network hops between Redpanda brokers; however, normal intra-data center network latency should be measured in microseconds rather than milliseconds. Ensure that there’s sufficient bandwidth between brokers to handle replication traffic.

  • Complexity: A benefit of Redpanda is the simplicity of deployment. Because Redpanda is deployed as a single binary with no external dependencies, it doesn’t need any infrastructure for ZooKeeper or for a Schema Registry. Redpanda also includes cluster balancing, so there’s no need to run Cruise Control.

Single-AZ infrastructure

In a single-AZ deployment, ensure that brokers are spread across at least three failure domains. This generally means separate racks, under separate switches, ideally powered by separate electrical feeds or circuits. Also, ensure that there’s sufficient network bandwidth between brokers, particularly considering shared uplinks, which could be subject to high throughput intra-cluster replication traffic. In an on-premises network, this HA configuration refers to separate racks or data halls within a data center.

Cloud providers support various HA configurations:

  • AWS partition placement groups allow spreading hosts across multiple partitions (or failure domains) within an AZ. The default number of partitions is three, with a maximum of seven. This can be combined with Redpanda’s replication factor setting, so each topic partition replica is guaranteed to be isolated from the impact of hardware failure.

  • Microsoft Azure flexible scale sets let you assign VMs to specific fault domains. Each scale set can have up to five fault domains, depending on your region. Not all VM types support flexible orchestration; for example, Lsv2-series only supports uniform scale sets.

  • Google Cloud instance placement policies let you specify how many availability domains you can have (up to eight) when using the Spread Instance Placement Policy.

    Google Cloud doesn’t divulge which availability domain an instance has been placed into, so you must have an availability domain for each Redpanda broker. Essentially, this isn’t enabled with rack awareness, but it’s the only possibility for clusters with more than three brokers.

You can automate this using Terraform or a similar infrastructure-as-code (IaC) tool. See AWS, Azure, and GCP.

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