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Amazon ElastiCache

  • ElastiCache is a distributed in-memory cache environment in the AWS Cloud.
  • ElastiCache works with both the Redis and Memcached engines.

AWS Training Amazon ElastiCache

Components

  • ElastiCache Nodes
    • A node is a fixed-size chunk of secure, network-attached RAM. A node can exist in isolation from or in some relationship to other nodes.
    • Every node within a cluster is the same instance type and runs the same cache engine. Each cache node has its own Domain Name Service (DNS) name and port.
  • If a maintenance event is scheduled for a given week, it will be initiated and completed at some point during the 60 minute maintenance window you specify.
  • Elasticache can be used for storing session state.
  • ElastiCache Redis
    • Existing applications that use Redis can use ElastiCache with almost no modification.
    •  Features
      • Automatic detection and recovery from cache node failures.
      • Multi-AZ with automatic failover of a failed primary cluster to a read replica in Redis clusters that support replication.
      • Redis (cluster mode enabled) supports partitioning your data across up to 250 shards.
      • The node or shard limit can be increased to a maximum of 500 per cluster if the Redis engine version is 5.0.6 or higher.
      • Redis supports in-transit and at-rest encryption with authentication so you can build HIPAA-compliant applications.
      • Flexible Availability Zone placement of nodes and clusters for increased fault tolerance.
      • Data is persistent.
      • Can be used as a datastore.
      • Not multi-threaded.
      • Amazon ElastiCache for Redis supports self-service updates, which allows you to apply service updates at the time of your choosing and track the progress in real-time.
    • Cache data if:
      • It is slow or expensive to acquire when compared to cache retrieval.
      • It is accessed with sufficient frequency.
      • It is relatively static, or if rapidly changing, staleness is not a significant issue.
    • Redis sorted sets guarantee both uniqueness and element ordering. Each time a new element is added to the sorted set it’s reranked in real time. It’s then added to the set in its appropriate numeric position.
    • In the Redis publish/subscribe paradigm, you send a message to a specific channel not knowing who, if anyone, receives it.  Recipients of the message are those who are subscribed to the channel.
    • Redis hashes are hashes that map string names to string values.
    • Components
      • Redis Shard – a grouping of one to six related nodes. A Redis (cluster mode disabled) cluster always has one shard. A Redis (cluster mode enabled) cluster can have 1–90 shards.
        • A multiple node shard implements replication by have one read/write primary node and 1–5 replica nodes.
        • If there is more than one node in a shard, the shard supports replication with one node being the read/write primary node and the others read-only replica nodes.
      • Redis Cluster – a logical grouping of one or more ElastiCache for Redis Shards. Data is partitioned across the shards in a Redis (cluster mode enabled) cluster.
    • For improved fault tolerance, have at least two nodes in a Redis cluster and enabling Multi-AZ with automatic failover.
    • Replica nodes use asynchronous replication mechanisms to keep synchronized with the primary node.
    • If any primary has no replicas and the primary fails, you lose all that primary’s data.
    • You can use backup and restore to migrate to Redis (cluster mode enabled) and resize your Redis (cluster mode enabled).
    • Redis (cluster mode disabled) vs Redis (cluster mode enabled)

AWS Training Amazon ElastiCache

    • You can vertically scale up or scale down your sharded Redis Cluster on demand. Amazon ElastiCache resizes your cluster by changing the node type, while the cluster continues to stay online and serve incoming requests.
    • You can set up automatic snapshots or initiate manual backups, and then seed new ElastiCache for Redis clusters. You can also export your snapshots to an S3 bucket of your choice for disaster recovery, analysis or cross-region backup and restore.
    • Endpoints
      • Single Node Redis (cluster mode disabled) Endpoints – used to connect to the cluster for both reads and writes.
      • Multi-Node Redis (cluster mode disabled) Endpoints – use the primary endpoint for all writes to the cluster. The read endpoint points to your read replicas.
      • Redis (cluster mode enabled) Endpoints – has a single configuration endpoint. By connecting to the configuration endpoint, your application is able to discover the primary and read endpoints for each shard in the cluster.
    • Parameter Groups
      • Cache parameter group is a named collection of engine-specific parameters that you can apply to a cluster.
      • Parameters are used to control memory usage, eviction policies, item sizes, and more.
    • Redis Security
      • ElastiCache for Redis node access is restricted to applications running on whitelisted EC2 instances. You can control access of your cluster by using subnet groups or security groups. By default, network access to your clusters is turned off.
      • By default, all new ElastiCache for Redis clusters are launched in a VPC environment. Use subnet groups to grant cluster access from Amazon EC2 instances running on specific subnets.
      • ElastiCache for Redis supports TLS and in-place encryption for nodes running specified versions of the ElastiCache for Redis engine.
      • You can use your own customer managed customer master keys (CMKs) in AWS Key Management Service to encrypt data at rest in ElastiCache for Redis.
    • Redis Backups
      • A point-in-time copy of a Redis cluster.
      • Backups consist of all the data in a cluster plus some metadata.
    • Global Datastore
      • A new feature that provides fully managed, secure cross-region replication. You can now write to your ElastiCache for Redis cluster in one region and have the data available for reading in two other cross-region replica clusters.
      • In the unlikely event of regional degradation, one of the healthy cross-region replica clusters can be promoted to become the primary cluster with full read/write capabilities.

ElastiCache Memcached

  • Features

    • Automatic detection and recovery from cache node failures.
    • Automatic discovery of nodes within a cluster enabled for automatic discovery, so that no changes need to be made to your application when you add or remove nodes.
    • Flexible Availability Zone placement of nodes and clusters.
    • ElastiCache Auto Discovery feature for Memcached lets your applications identify all of the nodes in a cache cluster and connect to them.
    • ElastiCache node access is restricted to applications running on whitelisted EC2 instances. You can control the instances that can access your cluster by using subnet groups or security groups.
    • It is not persistent.
    • Supports large nodes with multiple cores or threads.
    • Does not support multi-AZ failover or replication
    • Does not support snapshots
  • Components

    • Memcached cluster – a logical grouping of one or more ElastiCache Nodes. Data is partitioned across the nodes in a Memcached cluster.
      • Memcached supports up to 100 nodes per customer for each Region with each cluster having 1–20 nodes.
      • When you partition your data, use consistent hashing.
    • Endpoint – the unique address your application uses to connect to an ElastiCache node or cluster.
      • Each node in a Memcached cluster has its own endpoint.
      • The cluster also has an endpoint called the configuration endpoint.
    • ElastiCache parameter group – a named collection of engine-specific parameters that you can apply to a cluster. Parameters are used to control memory usage, eviction policies, item sizes, and more.
    • ElastiCache allows you to control access to your clusters using security groups. By default, network access to your clusters is turned off.
    • A subnet group is a collection of subnets that you can designate for your clusters running in a VPC environment. If you create a cluster in a VPC, then you must specify a cache subnet group. ElastiCache uses that cache subnet group to choose a subnet and IP addresses within that subnet to associate with your cache nodes.
  • Mitigating Failures

    • Node Failures
      • Spread your cached data over more nodes. Because Memcached does not support replication, a node failure will always result in some data loss from your cluster.
    • Availability Zone Failure
      • Locate your nodes in as many Availability Zones as possible. In the unlikely event of an AZ failure, you will lose the data cached in that AZ, not the data cached in the other AZs.
  • ElastiCache uses DNS entries to allow client applications to locate servers (nodes). The DNS name for a node remains constant, but the IP address of a node can change over time.

Caching Strategies

  • Lazy Loading – a caching strategy that loads data into the cache only when necessary.
    • Only requested data is cached.
    • Node failures are not fatal.
    • There is a cache miss penalty.
    • Stale data.
  • Write Through – adds data or updates data in the cache whenever data is written to the database.
    • Data in the cache is never stale.
    • Write penalty vs. Read penalty. Every write involves two trips: A write to the cache and a write to the database.
    • Missing data.
    • Cache churn.
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  • By adding a time to live (TTL) value to each write, we are able to enjoy the advantages of each strategy and largely avoid cluttering up the cache with superfluous data.

Scaling ElastiCache for Memcached Clusters

  • Scaling Memcached Horizontally
    • The Memcached engine supports partitioning your data across multiple nodes. Because of this, Memcached clusters scale horizontally easily. A Memcached cluster can have 1 to 20 nodes. To horizontally scale your Memcached cluster, just add or remove nodes.
  • Scaling Memcached Vertically
    • When you scale your Memcached cluster up or down, you must create a new cluster. Memcached clusters always start out empty unless your application populates it.

Monitoring

  • The service continuously monitors the health of your instances. In case a node experiences failure or a prolonged degradation in performance, ElastiCache will automatically restart the node and associated processes.
  • ElastiCache provides both host-level metrics and metrics that are specific to the cache engine software. These metrics are measured and published for each Cache node in 60-second intervals.
  • Monitor events with ElastiCache Events. When significant events happen on a cache cluster, including failure to add a node, success in adding a node, the modification of a security group, and others, ElastiCache sends a notification to a specific SNS topic.
  • Monitor costs with tags.

Redis VS Memcached

  • Memcached is designed for simplicity while Redis offers a rich set of features that make it effective for a wide range of use cases.

Redis cluster mode enabled vs disabled vs Memcached

Pricing

  • With on-demand nodes you pay only for the resources you consume by the hour without any long-term commitments.
  • With Reserved Nodes, you can make a low, one-time, up-front payment for each node you wish to reserve for a 1 or 3 year term. In return, you receive a significant discount off the ongoing hourly usage rate for the Node(s) you reserve.
  • ElastiCache provides storage space for one snapshot free of charge for each active ElastiCache for Redis cluster. Additional backup storage is charged.
  • EC2 Regional Data Transfer charges apply when transferring data between an EC2 instance and an ElastiCache Node in different Availability Zones of the same Region.

Free Amazon ElastiCache Tutorials on YouTube:
https://www.youtube.com/user/AmazonWebServices/search?query=elasticache

Amazon ElastiCache-related Cheat Sheets:

Note: If you are studying for the AWS Certified Database Specialty exam, we highly recommend that you take our AWS Certified Database – Specialty Practice Exams and read our Database Specialty exam study guide.

AWS Certified Database Specialty Practice Exams

Validate Your Knowledge

Question 1

A startup is developing a distributed cache for session management using Amazon ElastiCache for Redis with cluster mode enabled. The cache layer will be used by various applications running in AWS. The cluster shall use the default port for Redis connection.

Which combination of steps should the Database Specialist do to protect the ElastiCache cluster from unauthorized access? (Select TWO.)

  1. Enable Transparent Data Encryption (TDE) in the ElastiCache cluster.
  2. Set the associated security group to allow inbound traffic on TCP port 6379 from trusted clients only.
  3. Enable encryption in-transit and encryption at-rest on the ElastiCache cluster including Redis AUTH. Configure the clients to use the auth-token parameter when connecting to the Redis cluster.
  4. Ensure that the associated security group allows inbound traffic on TCP port 11211 from trusted clients only.
  5. Integrate AWS Web Application Firewall (WAF) with the ElastiCache Redis cluster.

Correct Answers: 2,3 

To help keep your data secure, Amazon ElastiCache and Amazon EC2 provide mechanisms to guard against unauthorized access of your data on the server.

Amazon ElastiCache for Redis also provides optional encryption features for data on clusters running Redis versions 3.2.6, 4.0.10 or later:

– In-transit encryption encrypts your data whenever it is moving from one place to another, such as between nodes in your cluster or between your cluster and your application.

– At-rest encryption encrypts your on-disk data during sync and backup operations.

If you want to enable in-transit or at-rest encryption, you must meet the following conditions:

– Your cluster or replication group must be running Redis 3.2.6, 4.0.10 or later.

– Your cluster or replication group must be created in a VPC based on Amazon VPC.

– Optionally, you can also use AUTH and the AUTH token (password) needed to perform operations on this cluster or replication group.

Amazon ElastiCache in-transit encryption is an optional feature that allows you to increase the security of your data at its most vulnerable points—when it is in transit from one location to another. Because there is some processing needed to encrypt and decrypt the data at the endpoints, enabling in-transit encryption can have some performance impact. You should benchmark your data with and without in-transit encryption to determine the performance impact for your use cases.

At-rest encryption can be enabled on a replication group only when it is created. Because there is some processing needed to encrypt and decrypt the data, enabling at-rest encryption can have a performance impact during these operations. You should benchmark your data with and without at-rest encryption to determine the performance impact for your use cases.

Redis authentication tokens enable Redis to require a token (password) before allowing clients to run commands, thereby improving data security. When you use Redis AUTH with your ElastiCache for Redis cluster, you should follow a strict token policy to enhance the security of your ElastiCache cluster. To connect to your cluster, you have to include the parameter --auth-token (API: AuthToken) with the correct token when you create your replication group or cluster. You must also include the token in all subsequent commands to the replication group or cluster.

The default TCP port for Redis is 6379. This port must be allowed in the associated security group of the cluster. For Memcached, the default port is 11211.

Hence, the correct answers are:

  • Set the associated security group to allow inbound traffic on TCP port 6379 from trusted clients only.
  • Enable encryption in-transit and encryption at-rest on the ElastiCache cluster including Redit AUTH. Configure the clients to use the auth-token parameter when connecting to the Redis cluster.

The option that says: Enable Transparent Data Encryption (TDE) in the ElastiCache cluster is incorrect because TDE is not an available encryption feature in ElastiCache. This is only applicable to the Microsoft SQL Server.

Tutorials Dojo Study Guide and Cheatsheet

The option that says: Ensure that the associated security group allows inbound traffic on TCP port 11211 from trusted clients only is incorrect because this the default port for Memcached, and not for Redis.

The option that says: Integrate AWS Web Application Firewall (WAF) with the ElastiCache Redis cluster is incorrect because you can’t directly integrate AWS WAF with ElastiCache. It is more appropriate to integrate AWS WAF to your CloudFront web distribution, Application Load Balancers, or Amazon API Gateway for your APIs.

References:
https://docs.aws.amazon.com/AmazonElastiCache/latest/red-ug/in-transit-encryption.html
https://docs.aws.amazon.com/AmazonElastiCache/latest/red-ug/at-rest-encryption.html
https://docs.aws.amazon.com/AmazonElastiCache/latest/red-ug/auth.html

Note: This question was extracted from our AWS Certified Database Specialty Practice Exams.

Question 2

A developer is moving a legacy web application from their on-premises data center to AWS. The application is concurrently used by thousands of people and it stores the user session state in memory. Its on-premises server usually reaches 100% CPU Utilization every time there is a surge on the number of people accessing the application.

Which of the following is the best way to re-factor the performance and availability of the application’s session management once it is migrated to AWS?

  1. Use an ElastiCache for Redis cluster to store the user session state of the application.
  2. Store the user session state of the application using CloudFront.
  3. Use an ElastiCache for Memcached cluster to store the user session state of the application.
  4. Use Sticky Sessions with Local Session Caching.

Correct Answer: 1

Amazon ElastiCache for Redis is a blazing fast in-memory data store that provides sub-millisecond latency to power internet-scale real-time applications. Built on open-source Redis and compatible with the Redis APIs, ElastiCache for Redis works with your Redis clients and uses the open Redis data format to store your data. Your self-managed Redis applications can work seamlessly with ElastiCache for Redis without any code changes. ElastiCache for Redis combines the speed, simplicity, and versatility of open-source Redis with manageability, security, and scalability from Amazon to power the most demanding real-time applications in Gaming, Ad-Tech, E-Commerce, Healthcare, Financial Services, and IoT.

In order to address scalability and provide a shared data storage for sessions that can be accessible from any individual web server, you can abstract the HTTP sessions from the web servers themselves. A common solution to for this is to leverage an In-Memory Key/Value store such as Redis and Memcached. While Key/Value data stores are known to be extremely fast and provide sub-millisecond latency, the added network latency and added cost are the drawbacks. An added benefit of leveraging Key/Value stores is that they can also be utilized to cache any data, not just HTTP sessions, which can help boost the overall performance of your applications.

With Redis, you can keep your data on disk with a point in time snapshot which can be used for archiving or recovery. Redis also lets you create multiple replicas of a Redis primary. This allows you to scale database reads and to have highly available clusters. Hence, the correct answer for this scenario is to use an ElastiCache for Redis cluster to store the user session state of the application.

The option that says: Store the user session state of the application using CloudFrontis incorrect because CloudFront is not suitable for storing user session data. It is primarily used as a content delivery network.

The option that says: Use an ElastiCache for Memcached cluster to store the user session state of the applicationis incorrect. Although using ElastiCache is a viable answer, Memcached is not as highly available as Redis.

The option that says: Use Sticky Sessions with Local Session Caching is incorrect. Although this is also a viable solution, it doesn’t offer durability and high availability compared to a distributed session management solution. The best solution for this scenario is to use an ElastiCache for Redis cluster.

References:
https://aws.amazon.com/caching/session-management
https://aws.amazon.com/elasticache/redis-vs-memcached/
https://aws.amazon.com/elasticache/redis/

Note: This question was extracted from our AWS Certified Developer Associate Practice Exams.

For more AWS practice exam questions with detailed explanations, visit the Tutorials Dojo Portal:

Tutorials Dojo AWS Practice Tests

References: 
https://docs.aws.amazon.com/AmazonElastiCache/latest/red-ug/
https://aws.amazon.com/elasticache/redis-details/
https://docs.aws.amazon.com/AmazonElastiCache/latest/mem-ug/
https://aws.amazon.com/elasticache/redis-vs-memcached/
https://aws.amazon.com/elasticache/features/
https://aws.amazon.com/elasticache/pricing/

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