Amazon ElastiCache Well-Architected Lens Reliability Pillar - Amazon ElastiCache
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Amazon ElastiCache Well-Architected Lens Reliability Pillar

REL 1: How are you supporting high availability (HA) architecture deployments?

Question-level introduction: Understanding the high availability architecture of Amazon ElastiCache will enable you to operate in a resilient state during availability events.

Question-level benefit: Architecting your ElastiCache clusters to be resilient to failures ensures higher availability for your ElastiCache deployments.

  • [Required] Determine the level of reliability you require for your ElastiCache cluster. Different workloads have different resiliency standards, from entirely ephemeral to mission critical workloads. Define needs for each type of environment you operate such as dev, test, and production.

    Caching engine: Memcached vs ElastiCache for Redis

    1. Memcached does not provide any replication mechanism and is used primarily for ephemeral workloads.

    2. ElastiCache for Redis offers HA features discussed below

  • [Best] For workloads that require HA, use ElastiCache for Redis in cluster mode with a minimum of two replicas per shard, even for small throughput requirement workloads that require only one shard.

    1. For cluster mode enabled, multi-AZ is enabled automatically.

      Multi-AZ minimizes downtime by performing automatic failovers from primary node to replicas, in case of any planned or unplanned maintenance as well as mitigating AZ failure.

    2. For sharded workloads, a minimum of three shards provides faster recovery during failover events as the Redis Cluster Protocol requires a majority of primary nodes be available to achieve quorum.

    3. Set up two or more replicas across Availability.

      Having two replicas provides improved read scalability and also read availability in scenarios where one replica is undergoing maintenance.

    4. Use Graviton2-based node types (default nodes in most regions).

      Amazon ElastiCache for Redis has added optimized performance on these nodes. As a result, you get better replication and synchronization performance, resulting in overall improved availability.

    5. Monitor and right-size to deal with anticipated traffic peaks: under heavy load, the ElastiCache for Redis engine may become unresponsive, which affects availability. BytesUsedForCache and DatabaseMemoryUsagePercentage are good indicators of your memory usage, whereas ReplicationLag is an indicator of your replication health based on your write rate. You can use these metrics to trigger cluster scaling.

    6. Ensure client-side resiliency by testing with the Failover API prior to a production failover event.


REL 2: How are you meeting your Recovery Point Objectives (RPOs) with ElastiCache?

Question-level introduction: Understand workload RPO to inform decisions on ElastiCache backup and recovery strategies.

Question-level benefit: Having an in-place RPO strategy can improve business continuity in the event of a disaster recovery scenarios. Designing your backup and restore policies can help you meet your Recovery Point Objectives (RPO) for your ElastiCache data. ElastiCache for Redis offers snapshot capabilities which are stored in Amazon S3, along with a configurable retention policy. These snapshots are taken during a defined backup window, and handled by the service automatically. If your workload requires additional backup granularity, you have the option to create up to 20 manual backups per day. Manually created backups do not have a service retention policy and can be kept indefinitely.

  • [Required] Understand and document the RPO of your ElastiCache deployments.

    • Be aware that Memcached does not offer any backup processes.

    • Review the capabilities of ElastiCache Backup and Restore features.

  • [Best] Have a well-communicated process in place for backing up your cluster.

    • Initiate manual backups on an as-needed basis.

    • Review retention policies for automatic backups.

    • Note that manual backups will be retained indefinitely.

    • Schedule your automatic backups during periods of low usage.

    • Perform backup operations against read-replicas to ensure you minimize the impact on cluster performance.

  • [Good] Leverage the scheduled backup feature of ElastiCache to regularly back up your data during a defined window.

    • Periodically test restores from your backups.

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REL 3: How do you support disaster recovery (DR) requirements?

Question-level introduction: Disaster recovery is an important aspect of any workload planning. ElastiCache for Redis offers several options to implement disaster recovery based on workload resilience requirements. With Amazon ElastiCache for Redis Global Datastore, you can write to your ElastiCache for Redis cluster in one region and have the data available to be read from two other cross-region replica clusters, thereby enabling low-latency reads and disaster recovery across regions.

Question-level benefit: Understanding and planning for a variety of disaster scenarios can ensure business continuity. DR strategies must be balanced against cost, performance impact, and data loss potential.

  • [Required] Develop and document DR strategies for all your ElastiCache components based upon workload requirements. ElastiCache is unique in that some use cases are entirely ephemeral and don’t require any DR strategy, whereas others are on the opposite end of the spectrum and require an extremely robust DR strategy. All options must be weighed against Cost Optimization – greater resiliency requires larger amounts of instrastructure.

    Understand the DR options available on a regional and multi-region level.

    • Multi-AZ Deployments are recommended to guard against AZ failure. Be sure to deploy with Cluster-Mode enabled in Multi-AZ architectures, with a minimum of 3 AZs available.

    • Global Datastore is recommended to guard against regional failures.

  • [Best] Enable Global Datastore for workloads that require region level resiliency.

    • Have a plan to failover to secondary region in case of primary degradation.

    • Test multi-region failover process prior to a failover over in production.

    • Monitor ReplicationLag metric to understand potential impact of data loss during failover events.

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REL 4: How do you effectively plan for failovers?

Question-level introduction: Enabling multi-AZ with automatic failovers is an ElastiCache best practice. In certain cases, ElastiCache for Redis replaces primary nodes as part of service operations. Examples include planned maintenance events and the unlikely case of a node failure or availability zone issue. Successful failovers rely on both ElastiCache and your client library configuration.

Question-level benefit: Following best practices for ElastiCache failovers in conjunction with your specific ElastiCache for Redis client library helps you minimize potential downtime during failover events.

  • [Required] For cluster mode disabled, use timeouts so your clients detect if it needs to disconnect from the old primary node and reconnect to the new primary node, using the updated primary endpoint IP address. For cluster mode enabled, the client library is responsible with detecting changes in the underlying cluster topology. This is accomplished most often by configuration settings in the ElastiCache for Redis client library, which also allow you to configure the frequency and the method of refresh. Each client library offers its own settings and more details are available in their corresponding documentation.


  • [Required] Successful failovers depend on a healthy replication environment between the primary and the replica nodes. Review and understand the asynchronous nature of Redis replication, as well as the available CloudWatch metrics to report on the replication lag between primary and replica nodes. For use cases that require greater data safety, leverage the Redis WAIT command to force replicas to acknowledge writes before responding to connected clients.


  • [Best] Regularly validate the responsiveness of your application during failover using the ElastiCache Test Failover API.


REL 5: Are your ElastiCache components designed to scale?

Question-level introduction: By understanding the scaling capabilities and available deployment topologies, your ElastiCache components can adjust over time to meet changing workload requirements. ElastiCache offers 4-way scaling: in/out (horizontal) as well as up/down (vertical).

Question-level benefit: Following best practices for ElastiCache deployments provides the greatest amount of scaling flexibility, as well as meeting the Well Architected principle of scaling horizontally to minimize the impact of failures.

  • [Required] Understand the difference between Cluster-mode Enabled and Cluster-mode Disabled topologies. In almost all cases it is recommended to deploy with Cluster-mode enabled as it allow for greater scalability over time. Cluster-mode disabled components are limited in their ability to horizontally scale by adding read replicas.

  • [Required] Understand when and how to scale.

    • For more READIOPS: add replicas

    • For more WRITEOPS: add shards (scale out)

    • For more network IO – use network optimized instances, scale up

  • [Best] Deploy your ElastiCache components with Cluster-mode enabled, with a bias toward more, smaller nodes rather than fewer, larger nodes. This effectively limits the blast radius of a node failure.

  • [Best] Include replicas in your clusters for enhanced responsiveness during scaling events

  • [Good] For cluster-mode disabled, leverage read replicas to increase overall read capacity. ElastiCache has support for up to 5 read replicas in cluster-mode disabled, as well as vertical scaling.

  • [Resources]: