AWS ElastiCache is a managed web service that lets you easily deploy and run Memcached or Redis protocol-compliant cache clusters in the cloud
ElastiCache is available in two flavours: Memcached and Redis
simplify and offload the management, monitoring, and operation of in-memory cache environments, enabling the engineering resources to focus on developing applications
automate common administrative tasks required to operate a distributed cache environment.
improves the performance of web applications by allowing retrieval of information from a fast, managed, in-memory caching system, instead of relying entirely on slower disk-based databases.
not only to improve load & response times to user actions and queries, but also reduce the cost associated with scaling web applications
helps automatically detect and replace failed cache nodes, providing a resilient system that mitigates the risk of overloaded databases, which can slow website and application load times
provides enhanced visibility into key performance metrics associated with the cache nodes through integration with CloudWatch
code, applications, and popular tools already using Memcached or Redis environments work seamlessly, with being protocol- compliant with Memcached and Redis environments
ElastiCache provides in-memory caching which can
significantly improve latency and throughput for many
read-heavy application workloads for e.g. social networking, gaming, media sharing and Q&A portals or
compute-intensive workloads such as a recommendation engine
improve application performance by storing critical pieces of data in memory for low-latency access.
be used to cache results of I/O-intensive database queries or the results of computationally-intensive calculations.
ElastiCache currently allows access only from the EC2 network and cannot be accessed from outside networks like on-premises servers
Redis is an open source, BSD licensed, advanced key-value cache & store
ElastiCache enables the management, monitoring and operation of a Redis node; creation, deletion and modification of the node
ElastiCache for Redis can be used as a primary in-memory key-value data store, providing fast, sub millisecond data performance, high availability and scalability up to 16 nodes plus up to 5 read replicas, each of up to 3.55 TiB of in-memory data
ElastiCache for Redis supports
Redis Master/Slave replication.
Multi-AZ operation by creating read replicas in another AZ
Backup and Restore feature for persistence by snapshotting
ElastiCache for Redis can be vertically scaled upwards by selecting a larger node type, however it cannot be scaled down
Parameter group can be specified for Redis during installation, which acts as a “container” for Redis configuration values that can be applied to one or more Redis primary clusters
Append Only File (AOF)
provides persistence and can be enabled for recovery scenarios
if a node restarts or service crash, Redis will replay the updates from an AOF file, thereby recovering the data lost due to the restart or crash
cannot protect against all failure scenarios, cause if the underlying hardware fails, a new server would be provisioned and the AOF file will no longer be available to recover the data
Enabling Redis Multi-AZ as a Better Approach to Fault Tolerance, as failing over to a read replica is much faster than rebuilding the primary from an AOF file
Redis Read Replica
Read Replicas help provide Read scaling and handling failures
Read Replicas are kept in sync with the Primary node using Redis’s asynchronous replication technology
Redis Read Replicas can help
Horizontal scaling beyond the compute or I/O capacity of a single primary node for read-heavy workloads.
Serving read traffic while the primary is unavailable either being down due to failure or maintenance
Data protection scenarios to promote a Read Replica as primary node, in case the primary node or the AZ of the primary node fails
ElastiCache supports initiated or forced failover where it flips the DNS record for the primary node to point at the read replica, which is in turn promoted to become the new primary
Read replica cannot span across regions and may only be provisioned in the same or different AZ of the same Region as the cache node primary
ElastiCache for Redis shard consists of a primary and up to 5 read replicas
Redis asynchronously replicates the data from the primary node to the read replicas
ElastiCache for Redis Multi-AZ mode
provides enhanced availability and smaller need for administration as the node failover is automatic
impact on the ability to read/write to the primary is limited to the time it takes for automatic failover to complete
no longer needs monitoring of Redis nodes and manually initiating a recovery in the event of a primary node disruption
During certain types of planned maintenance, or in the unlikely event of ElastiCache node failure or AZ failure,
it automatically detects the failure,
selects a replica, depending upon the read replica with the smallest asynchronous replication lag to the primary, and promotes it to become the new primary node
it will also propagate the DNS changes so that the the primary endpoint remains the same
If Multi-AZ is not enabled,
ElastiCache monitors the primary node
in case the node becomes unavailable or unresponsive, it will repair the node by acquiring new service resources
it propagates the DNS endpoint changes to redirect the node’s existing DNS name to point to the new service resources.
If the primary node cannot be healed and you will have the choice to promote one of the read replicas to be the new primary
Redis Backup & Restore
Backup and Restore allows users to create snapshots of the Redis clusters
Snapshots can be used for recovery, restoration, archiving purpose or warm start an ElastiCache for Redis cluster with preloaded data
Snapshots can created on a cluster basis and uses Redis’ native mechanism to create and store an RDB file as the snapshot
Increased latencies for a brief period at the node might be encountered while taking a snapshot, and is recommended to be taken from a Read Replica minimizing performance impact
Snapshots can be created either automatically (if configured) or manually
ElastiCache for Redis cluster when deleted removes the automatic snapshots. However, manual snapshots are retained
Memcached is an in-memory key-value store for small chunks or arbitrary data
ElastiCache for Memcached can be used to cache a variety of objects
from the content in persistent data stores such as RDS, DynamoDB, or self-managed databases hosted on EC2) to
dynamically generated web pages (with Nginx for example), or
transient session data that may not require a persistent backing store
ElastiCache for Memcached
can be scaled Vertically by increasing the node type size
can be scaled Horizontally by adding and removing nodes
does not support persistence of data
ElastiCache for Memcached cluster can have
nodes can span across multiple AZs within the same region
maximum of 20 nodes per cluster with a maximum of 100 nodes per region (soft limit and can be extended)
ElasticCache for Memcached supports auto discovery, which enables automatic discovery of cache nodes by clients when they are added to or removed from an ElastiCache cluster
ElastiCache Mitigating Failures
ElastiCache should be designed to plan so that failures have a minimal impact upon your application and data
Mitigating Failures when Running Memcached
Mitigating Node Failures
spread the cached data over more nodes
as Memcached does not support replication, a node failure will always result in some data loss from the cluster
having more nodes will reduce the proportion of cache data lost
Mitigating Availability Zone Failures
locate the nodes in as many availability zones as possible, only the data cached in that AZ is lost, not the data cached in the other AZs
Mitigating Failures when Running Redis
Mitigating Cluster Failures
Redis Append Only Files (AOF)
enable AOF so whenever data is written to the Redis cluster, a corresponding transaction record is written to a Redis AOF
when Redis process restarts, ElastiCache creates a replacement cluster and provisions it and repopulating it with data from AOF
It is time consuming
AOF can get big
Using AOF cannot protect you from all failure scenarios
Redis Replication Groups
A Redis replication group is comprised of a single primary cluster which your application can both read from and write to, and from 1 to 5 read-only replica clusters.
Data written to the primary cluster is also asynchronously updated on the read replica clusters
When a Read Replica fails, ElastiCache detects the failure, replaces the instance in the same AZ and synchronizes with the Primary Cluster
Redis Multi-AZ with Automatic Failover, ElastiCache detects Primary cluster failure, promotes a read replica with least replication lag to primary
Multi-AZ with Auto Failover is disabled, ElastiCache detects Primary cluster failure, creates a new one and syncs the new Primary with one of the existing replicas
Mitigating Availability Zone Failures
locate the clusters in as many availability zones as possible
AWS Certification Exam Practice Questions
Questions are collected from Internet and the answers are marked as per my knowledge and understanding (which might differ with yours).
AWS services are updated everyday and both the answers and questions might be outdated soon, so research accordingly.
AWS exam questions are not updated to keep up the pace with AWS updates, so even if the underlying feature has changed the question might not be updated
Open to further feedback, discussion and correction.
What does Amazon ElastiCache provide?
A service by this name doesn’t exist. Perhaps you mean Amazon CloudCache.
An Amazon EC2 instance with the Memcached software already pre-installed.
You are developing a highly available web application using stateless web servers. Which services are suitable for storing session state data? Choose 3 answers.
Elastic Load Balancing
Amazon Relational Database Service (RDS)
AWS Storage Gateway
Which statement best describes ElastiCache?
Reduces the latency by splitting the workload across multiple AZs
A simple web services interface to create and store multiple data sets, query your data easily, and return the results
Offload the read traffic from your database in order to reduce latency caused by read-heavy workload
Managed service that makes it easy to set up, operate and scale a relational database in the cloud
Our company is getting ready to do a major public announcement of a social media site on AWS. The website is running on EC2 instances deployed across multiple Availability Zones with a Multi-AZ RDS MySQL Extra Large DB Instance. The site performs a high number of small reads and writes per second and relies on an eventual consistency model. After comprehensive tests you discover that there is read contention on RDS MySQL. Which are the best approaches to meet these requirements? (Choose 2 answers)
Deploy ElastiCache in-memory cache running in each availability zone
Implement sharding to distribute load to multiple RDS MySQL instances
Increase the RDS MySQL Instance size and Implement provisioned IOPS
Add an RDS MySQL read replica in each availability zone
You are using ElastiCache Memcached to store session state and cache database queries in your infrastructure. You notice in CloudWatch that Evictions and Get Misses are both very high. What two actions could you take to rectify this? Choose 2 answers
Increase the number of nodes in your cluster
Tweak the max_item_size parameter
Shrink the number of nodes in your cluster
Increase the size of the nodes in the cluster
You have been tasked with moving an ecommerce web application from a customer’s datacenter into a VPC. The application must be fault tolerant and well as highly scalable. Moreover, the customer is adamant that service interruptions not affect the user experience. As you near launch, you discover that the application currently uses multicast to share session state between web servers, In order to handle session state within the VPC, you choose to:
Store session state in Amazon ElastiCache for Redis (scalable and makes the web applications stateless)
Create a mesh VPN between instances and allow multicast on it
Store session state in Amazon Relational Database Service (RDS solution not highly scalable)
Enable session stickiness via Elastic Load Balancing (affects user experience if the instance goes down)
When you are designing to support a 24-hour flash sale, which one of the following methods best describes a strategy to lower the latency while keeping up with unusually heavy traffic?
Launch enhanced networking instances in a placement group to support the heavy traffic (only improves internal communication)
Apply Service Oriented Architecture (SOA) principles instead of a 3-tier architecture (just simplifies architecture)
Use Elastic Beanstalk to enable blue-green deployment (only minimizes download for applications and ease of rollback)
Use ElastiCache as in-memory storage on top of DynamoDB to store user sessions (scalable, faster read/writes and in memory storage)
You are configuring your company’s application to use Auto Scaling and need to move user state information. Which of the following AWS services provides a shared data store with durability and low latency?
AWS ElastiCache Memcached (does not provide durability as if the node is gone the data is gone)
Amazon Simple Storage Service
Amazon EC2 instance storage
Your application is using an ELB in front of an Auto Scaling group of web/application servers deployed across two AZs and a Multi-AZ RDS Instance for data persistence. The database CPU is often above 80% usage and 90% of I/O operations on the database are reads. To improve performance you recently added a single-node Memcached ElastiCache Cluster to cache frequent DB query results. In the next weeks the overall workload is expected to grow by 30%. Do you need to change anything in the architecture to maintain the high availability for the application with the anticipated additional load and Why?
You should deploy two Memcached ElastiCache Clusters in different AZs because the RDS Instance will not be able to handle the load if the cache node fails.
If the cache node fails the automated ElastiCache node recovery feature will prevent any availability impact. (does not provide high availability, as data is lost if the node is lost)
Yes you should deploy the Memcached ElastiCache Cluster with two nodes in the same AZ as the RDS DB master instance to handle the load if one cache node fails. (Single AZ affects availability as DB is Multi AZ and would be overloaded is the AZ goes down)
No if the cache node fails you can always get the same data from the DB without having any availability impact. (Will overload the database affecting availability)
A read only news reporting site with a combined web and application tier and a database tier that receives large and unpredictable traffic demands must be able to respond to these traffic fluctuations automatically. What AWS services should be used meet these requirements?
Stateless instances for the web and application tier synchronized using ElastiCache Memcached in an autoscaling group monitored with CloudWatch and RDS with read replicas.
Stateful instances for the web and application tier in an autoscaling group monitored with CloudWatch and RDS with read replicas (Stateful instances will not allow for scaling)
Stateful instances for the web and application tier in an autoscaling group monitored with CloudWatch and multi-AZ RDS (Stateful instances will allow not for scaling &multi-AZ is for high availability and not scaling)
Stateless instances for the web and application tier synchronized using ElastiCache Memcached in an autoscaling group monitored with CloudWatch and multi-AZ RDS (multi-AZ is for high availability and not scaling)
You have written an application that uses the Elastic Load Balancing service to spread traffic to several web servers. Your users complain that they are sometimes forced to login again in the middle of using your application, after they have already logged in. This is not behavior you have designed. What is a possible solution to prevent this happening?