AWS RDS Replication – Multi-AZ & Read Replica – Certification

RDS Multi-AZ & Read Replica Overview

  • DB instances replicas can be created in two ways Multi-AZ & Read Replica
  • Multi-AZ deployment
    • Multi-AZ deployment provides high availability and failover support
    • RDS automatically provisions and manages a synchronous standby replica in a different AZ (independent infrastructure in a physically separate location)
    • RDS automatically fails over to the standby so that database operations can resume quickly without administrative intervention in case of
      • Planned database maintenance
      • Software patching
      • Rebooting of the Primary instance
      • Primary DB instance connectivity or host failure, or an
      • Availability Zone failure
  • Read Replica
    • RDS uses the PostgreSQL, MySQL, and MariaDB DB engines’ built-in replication functionality to create a special type of DB instance called a Read Replica from a source DB instance.
    • Load on the source DB instance can be reduced by routing read queries from applications to the Read Replica.
    • Read Replicas allow elastic scaling beyond the capacity constraints of a single DB instance for read-heavy database workloads

Multi-AZ deployment

  • Multi-AZ deployments provides high availability and automatic failover support for DB instances
  • Multi-AZ helps improve the durability and availability of a critical system, enhancing availability during planned system maintenance, DB instance failure and Availability Zone disruption.
  • Multi-AZ is a High Availability feature is not a scaling solution for read-only scenarios; standby replica can’t be used to serve read traffic. To service read-only traffic, use a Read Replica.
  • Multi-AZ deployments for Oracle, PostgreSQL, MySQL, and MariaDB DB instances use Amazon technology, while SQL Server DB instances use SQL Server Mirroring.
  • In a Multi-AZ deployment,
    • RDS automatically provisions and maintains a synchronous standby replica in a different Availability Zone.
    • Copies of data are stored in different Availability Zones for greater levels of data durability.
    • Primary DB instance is synchronously replicated across Availability Zones to a standby replica to provide
      • data redundancy,
      • eliminate I/O freezes during snapshots and backups
      • and minimize latency spikes during system backups.
    • DB instances may have increased write and commit latency compared to a Single AZ deployment, due to the synchronous data replication
    • Transaction success is returned only if the commit is successful both on the primary and the standby DB
    • There might be a change in latency if the deployment fails over to the standby replica, although AWS is engineered with low-latency network connectivity between Availability Zones.
  • When using the BYOL licensing model, a license for both the primary instance and the standby replica is required
  • For production workloads, it is recommended to use Multi-AZ deployment with Provisioned IOPS and DB instance classes (m1.large and larger), optimized for Provisioned IOPS for fast, consistent performance.
  • When Single-AZ deployment is modified to a Multi-AZ deployment (for engines other than SQL Server or Amazon Aurora)
    • RDS takes a snapshot of the primary DB instance from the deployment and restores the snapshot into another Availability Zone.
    • RDS then sets up synchronous replication between the primary DB instance and the new instance.
    • This avoids downtime when conversion from Single AZ to Multi-AZ

RDS Multi-AZ Failover Process

  • In the event of a planned or unplanned outage of the DB instance,
    • RDS automatically switches to a standby replica in another AZ, if enabled for Multi-AZ.
    • Time it takes for the failover to complete depends on the database activity and other conditions at the time the primary DB instance became unavailable.
    • Failover times are typically 60-120 secs. However, large transactions or a lengthy recovery process can increase failover time.
    • Failover mechanism automatically changes the DNS record of the DB instance to point to the standby DB instance.
    • Multi-AZ switch is seamless to the applications as there is no change in the endpoint URLs but just needs to re-establish any existing connections to the DB instance.
  • RDS handles failover automatically so that database operations can be resumed as quickly as possible without administrative intervention.
  • Primary DB instance switches over automatically to the standby replica if any of the following conditions occur:
    • An Availability Zone outage
    • Primary DB instance fails
    • DB instance’s server type is changed
    • Operating system of the DB instance is undergoing software patching
    • A manual failover of the DB instance was initiated using Reboot with failover (also referred to as Forced Failover)
  • If the Multi-AZ DB instance has failed over, can be determined by
    • DB event subscriptions can be setup to notify you via email or SMS that a failover has been initiated.
    • DB events can be viewed via the Amazon RDS console or APIs.
    • Current state of your Multi-AZ deployment can be viewed via the RDS console and APIs.

Read Replica

  • Amazon RDS uses the MySQL, MariaDB, and PostgreSQL (version 9.3.5 and later) DB engines’ built-in replication functionality to create a Read Replica from a source DB instance.
  • Updates made to the source DB instance are asynchronously copied to the Read Replica.
  • Load on the source DB instance can be reduced by routing read queries from the applications to the Read Replica.
  • Using Read Replicas allow DB to elastically scale out beyond the capacity constraints of a single DB instance for read-heavy database workloads.
  • Read Replica operates as a DB instance that allows read-only connections; applications can connect to a Read Replica the same way they would to any DB instance.

Read Replica creation

  • Up to five Read Replicas can be created from one source DB instance.
  • Creation process
    • Automatic backups must be enabled on the source DB instance by setting the backup retention period to a value other than 0
    • Existing DB instance needs to be specified as the source.
    • RDS takes a snapshot of the source instance and creates a read-only instance from the snapshot.
    • RDS then uses the asynchronous replication method for the DB engine to update the Read Replica for any changes to the source DB instance.
  • RDS replicates all databases in the source DB instance.
  • RDS sets up a secure communications channel between the source DB instance and the Read Replica, if that Read Replica is in a different AWS region from the DB instance.
  • RDS establishes any AWS security configurations, such as adding security group entries, needed to enable the secure channel.
  • During the Read Replica creation, a brief I/O suspension on the source DB instance can be experienced as the DB snapshot occurs.
  • I/O suspension typically lasts about one minute and can be avoided if the source DB instance is a Multi-AZ deployment (in the case of Multi-AZ deployments, DB snapshots are taken from the standby).
  • Read Replica creation time can be slow if any long-running transactions are being executed and should wait for completion
  • For multiple Read Replicas created in parallel from the same source DB instance, only one snapshot is taken at the start of the first create action.
  • A Read Replica can be promoted to a new independent source DB, in which case the replication link is broken between the Read Replica and the source DB.  However, the replication continues for other replicas using the original source DB as the replication source

Read Replica Deletion & DB Failover

  • Read Replicas must be explicitly deleted, using the same mechanisms for deleting a DB instance.
  • If the source DB instance is deleted without deleting the replicas, each replica is promoted to a stand-alone, single-AZ DB instance.
  • If the source instance of a Multi-AZ deployment fails over to the standby, any associated Read Replicas are switched to use the secondary as their replication source.

Read Replica Storage & Compute requirements

  • A Read Replica, by default, is created with the same storage type as the source DB instance.
  • For replication to operate effectively, each Read Replica should have the same amount of compute & storage resources as the source DB instance.
  • Source DB instance, if scaled, Read Replicas should be scaled accordingly

Read Replica Features & Limitations

  • RDS does not support circular replication.
  • DB instance cannot be configured to serve as a replication source for an existing DB instance; a new Read Replica can be created only from an existing DB instance for e.g., if MyDBInstance replicates to ReadReplica1, ReadReplica1 can’t be configured to replicate back to MyDBInstance.  From ReadReplica1, only a new Read Replica can be created, such as ReadRep2.
  • Cross-Region Replication
    • MySQL, PostgresSQL (update from June 2016) or MariaDB Read Replica can be created in a different region than the source DB instance which helps to improve
      • disaster recovery capabilities (reduces RTO and RPO),
      • scale read operations into a region closer to end users,
      • migration from a data center in one region to another region
  • Read Replica can be created from other Read replicas as well. However, the replica lag is higher for these instances and there cannot be more than four instances involved in a replication chain.

Read Replica ComparisionRead Replica Use cases

  • Read Replicas can be used in variety of use cases, including:
    • Scaling beyond the compute or I/O capacity of a single DB instance for read-heavy database workloads, directing excess read traffic to Read Replica(s)
    • Serving read traffic while the source DB instance is unavailable for e.g. If the source DB instance cannot take I/O requests due to backups I/O suspension or scheduled maintenance, the read traffic can be directed to the Read Replica(s). However, the data might be stale.
    • Business reporting or data warehousing scenarios where business reporting queries can be executed against a Read Replica, rather than the primary, production DB instance.

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.
  1. You are running a successful multi-tier web application on AWS and your marketing department has asked you to add a reporting tier to the application. The reporting tier will aggregate and publish status reports every 30 minutes from user-generated information that is being stored in your web applications database. You are currently running a Multi-AZ RDS MySQL instance for the database tier. You also have implemented ElastiCache as a database caching layer between the application tier and database tier. Please select the answer that will allow you to successfully implement the reporting tier with as little impact as possible to your database.
    1. Continually send transaction logs from your master database to an S3 bucket and generate the reports off the S3 bucket using S3 byte range requests.
    2. Generate the reports by querying the synchronously replicated standby RDS MySQL instance maintained through Multi-AZ (Standby instance cannot be used as a scaling solution)
    3. Launch a RDS Read Replica connected to your Multi-AZ master database and generate reports by querying the Read Replica.
    4. Generate the reports by querying the ElastiCache database caching tier. (ElasticCache does not maintain full data and is simply a caching solution)
  2. A company is deploying a new two-tier web application in AWS. The company has limited staff and requires high availability, and the application requires complex queries and table joins. Which configuration provides the solution for the company’s requirements?
    1. MySQL Installed on two Amazon EC2 Instances in a single Availability Zone (does not provide High Availaility out of the box)
    2. Amazon RDS for MySQL with Multi-AZ
    3. Amazon ElastiCache (Just a caching solution)
    4. Amazon DynamoDB (Not suitable for complex queries and joins)
  3. Your 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)
    1. Deploy ElastiCache in-memory cache running in each availability zone
    2. Implement sharding to distribute load to multiple RDS MySQL instances (this is only a read contention, the writes work fine)
    3. Increase the RDS MySQL Instance size and Implement provisioned IOPS (not scalablethis is only a read contention, the writes work fine)
    4. Add an RDS MySQL read replica in each availability zone
  4. Your company has HQ in Tokyo and branch offices all over the world and is using logistics software with a multi-regional deployment on AWS in Japan, Europe and US. The logistic software has a 3-tier architecture and currently uses MySQL 5.6 for data persistence. Each region has deployed its own database. In the HQ region you run an hourly batch process reading data from every region to compute cross-regional reports that are sent by email to all offices this batch process must be completed as fast as possible to quickly optimize logistics. How do you build the database architecture in order to meet the requirements?
    1. For each regional deployment, use RDS MySQL with a master in the region and a read replica in the HQ region
    2. For each regional deployment, use MySQL on EC2 with a master in the region and send hourly EBS snapshots to the HQ region
    3. For each regional deployment, use RDS MySQL with a master in the region and send hourly RDS snapshots to the HQ region
    4. For each regional deployment, use MySQL on EC2 with a master in the region and use S3 to copy data files hourly to the HQ region
    5. Use Direct Connect to connect all regional MySQL deployments to the HQ region and reduce network latency for the batch process
  5. What would happen to an RDS (Relational Database Service) multi-Availability Zone deployment if the primary DB instance fails?
    1. IP of the primary DB Instance is switched to the standby DB Instance.
    2. A new DB instance is created in the standby availability zone.
    3. The canonical name record (CNAME) is changed from primary to standby.
    4. The RDS (Relational Database Service) DB instance reboots.
  6. Your business is building a new application that will store its entire customer database on a RDS MySQL database, and will have various applications and users that will query that data for different purposes. Large analytics jobs on the database are likely to cause other applications to not be able to get the query results they need to, before time out. Also, as your data grows, these analytics jobs will start to take more time, increasing the negative effect on the other applications. How do you solve the contention issues between these different workloads on the same data?
    1. Enable Multi-AZ mode on the RDS instance
    2. Use ElastiCache to offload the analytics job data
    3. Create RDS Read-Replicas for the analytics work
    4. Run the RDS instance on the largest size possible
  7. Will my standby RDS instance be in the same Availability Zone as my primary?
    1. Only for Oracle RDS types
    2. Yes
    3. Only if configured at launch
    4. No
  8. Is creating a Read Replica of another Read Replica supported?
    1. Only in certain regions
    2. Only with MySQL based RDS
    3. Only for Oracle RDS types
    4. No
  9. A user is planning to set up the Multi-AZ feature of RDS. Which of the below mentioned conditions won’t take advantage of the Multi-AZ feature?
    1. Availability zone outage
    2. A manual failover of the DB instance using Reboot with failover option
    3. Region outage
    4. When the user changes the DB instance’s server type
  10. When you run a DB Instance as a Multi-AZ deployment, the “_____” serves database writes and reads
    1. secondary
    2. backup
    3. stand by
    4. primary
  11. When running my DB Instance as a Multi-AZ deployment, can I use the standby for read or write operations?
    1. Yes
    2. Only with MSSQL based RDS
    3. Only for Oracle RDS instances
    4. No
  12. Read Replicas require a transactional storage engine and are only supported for the _________ storage engine
    1. OracleISAM
    2. MSSQLDB
    3. InnoDB
    4. MyISAM
  13. A user is configuring the Multi-AZ feature of an RDS DB. The user came to know that this RDS DB does not use the AWS technology, but uses server mirroring to achieve replication. Which DB is the user using right now?
    1. MySQL
    2. Oracle
    3. MS SQL
    4. PostgreSQL
  14. If I have multiple Read Replicas for my master DB Instance and I promote one of them, what happens to the rest of the Read Replicas?
    1. The remaining Read Replicas will still replicate from the older master DB Instance
    2. The remaining Read Replicas will be deleted
    3. The remaining Read Replicas will be combined to one read replica
  15. If you have chosen Multi-AZ deployment, in the event of a planned or unplanned outage of your primary DB Instance, Amazon RDS automatically switches to the standby replica. The automatic failover mechanism simply changes the ______ record of the main DB Instance to point to the standby DB Instance.
    1. DNAME
    2. CNAME
    3. TXT
    4. MX
  16. When automatic failover occurs, Amazon RDS will emit a DB Instance event to inform you that automatic failover occurred. You can use the _____ to return information about events related to your DB Instance
    1. FetchFailure
    2. DescriveFailure
    3. DescribeEvents
    4. FetchEvents
  17. The new DB Instance that is created when you promote a Read Replica retains the backup window period.
    1. TRUE
    2. FALSE
  18. Will I be alerted when automatic failover occurs?
    1. Only if SNS configured
    2. No
    3. Yes
    4. Only if Cloudwatch configured
  19. Can I initiate a “forced failover” for my MySQL Multi-AZ DB Instance deployment?
    1. Only in certain regions
    2. Only in VPC
    3. Yes
    4. No
  20. A user is accessing RDS from an application. The user has enabled the Multi-AZ feature with the MS SQL RDS DB. During a planned outage how will AWS ensure that a switch from DB to a standby replica will not affect access to the application?
    1. RDS will have an internal IP which will redirect all requests to the new DB
    2. RDS uses DNS to switch over to standby replica for seamless transition
    3. The switch over changes Hardware so RDS does not need to worry about access
    4. RDS will have both the DBs running independently and the user has to manually switch over
  21. Which of the following is part of the failover process for a Multi-AZ Amazon Relational Database Service (RDS) instance?
    1. The failed RDS DB instance reboots.
    2. The IP of the primary DB instance is switched to the standby DB instance.
    3. The DNS record for the RDS endpoint is changed from primary to standby.
    4. A new DB instance is created in the standby availability zone.
  22. Which of these is not a reason a Multi-AZ RDS instance will failover?
    1. An Availability Zone outage
    2. A manual failover of the DB instance was initiated using Reboot with failover
    3. To autoscale to a higher instance class (Refer link)
    4. The primary DB instance fails
  23. You need to scale an RDS deployment. You are operating at 10% writes and 90% reads, based on your logging. How best can you scale this in a simple way?
    1. Create a second master RDS instance and peer the RDS groups.
    2. Cache all the database responses on the read side with CloudFront.
    3. Create read replicas for RDS since the load is mostly reads.
    4. Create a Multi-AZ RDS installs and route read traffic to standby.
  24. How does Amazon RDS multi Availability Zone model work?
    1. A second, standby database is deployed and maintained in a different availability zone from master, using synchronous replication. (Refer link)
    2. A second, standby database is deployed and maintained in a different availability zone from master using asynchronous replication.
    3. A second, standby database is deployed and maintained in a different region from master using asynchronous replication.
    4. A second, standby database is deployed and maintained in a different region from master using synchronous replication.
  25. A customer is running an application in US-West (Northern California) region and wants to setup disaster recovery failover to the Asian Pacific (Singapore) region. The customer is interested in achieving a low Recovery Point Objective (RPO) for an Amazon RDS multi-AZ MySQL database instance. Which approach is best suited to this need?
    1. Synchronous replication
    2. Asynchronous replication
    3. Route53 health checks
    4. Copying of RDS incremental snapshots
  26. A user is using a small MySQL RDS DB. The user is experiencing high latency due to the Multi AZ feature. Which of the below mentioned options may not help the user in this situation?
    1. Schedule the automated back up in non-working hours
    2. Use a large or higher size instance
    3. Use PIOPS
    4. Take a snapshot from standby Replica
  27. Are Reserved Instances available for Multi-AZ Deployments?
    1. Only for Cluster Compute instances
    2. Yes for all instance types
    3. Only for M3 instance types
  28. My Read Replica appears “stuck” after a Multi-AZ failover and is unable to obtain or apply updates from the source DB Instance. What do I do?
    1. You will need to delete the Read Replica and create a new one to replace it.
    2. You will need to disassociate the DB Engine and re associate it.
    3. The instance should be deployed to Single AZ and then moved to Multi- AZ once again
    4. You will need to delete the DB Instance and create a new one to replace it.
  29. What is the charge for the data transfer incurred in replicating data between your primary and standby?
    1. No charge. It is free.
    2. Double the standard data transfer charge
    3. Same as the standard data transfer charge
    4. Half of the standard data transfer charge
  30. A user has enabled the Multi AZ feature with the MS SQL RDS database server. Which of the below mentioned statements will help the user understand the Multi AZ feature better?
    1. In a Multi AZ, AWS runs two DBs in parallel and copies the data asynchronously to the replica copy
    2. In a Multi AZ, AWS runs two DBs in parallel and copies the data synchronously to the replica copy
    3. In a Multi AZ, AWS runs just one DB but copies the data synchronously to the standby replica
    4. AWS MS SQL does not support the Multi AZ feature
  31. A company is running a batch analysis every hour on their main transactional DB running on an RDS MySQL instance to populate their central Data Warehouse running on Redshift. During the execution of the batch their transactional applications are very slow. When the batch completes they need to update the top management dashboard with the new data. The dashboard is produced by another system running on-premises that is currently started when a manually-sent email notifies that an update is required The on-premises system cannot be modified because is managed by another team. How would you optimize this scenario to solve performance issues and automate the process as much as possible?
    1. Replace RDS with Redshift for the batch analysis and SNS to notify the on-premises system to update the dashboard
    2. Replace RDS with Redshift for the batch analysis and SQS to send a message to the on-premises system to update the dashboard
    3. Create an RDS Read Replica for the batch analysis and SNS to notify me on-premises system to update the dashboard
    4. Create an RDS Read Replica for the batch analysis and SQS to send a message to the on-premises system to update the dashboard.

18 thoughts on “AWS RDS Replication – Multi-AZ & Read Replica – Certification

  1. Question (2)
    i am not sure about the answer cause Multi-AZ only without specifying that it will be a read replica will not help, i thought (c. Amazon ElastiCache) would be better answer , what do you think ?

    1. Key point in the questions is
      1. limited staff and requires high availability,
      2. application requires complex queries and table joins

      #A is ruled out cause of 1, as it would need manual failover
      #C is rules out cause its not a DB but a caching solution
      #D is rules out cause of 2
      #B is the correct option as Multi AZ provides complex queries and table joins with ability to failover without manual intervention

  2. Excellent article with real-life scenarios and explainations right to the point. It sums up more than a whole semester in our colleges these days (Germany). Thank you for this high-quality information, I will use it in the next few weeks when I’ll be building a data-warehouse & a report-section for one of my bigger clients. It was a pleasure to read – thank you very much!

    1. Elastic cache is for mere caching of the query results for improving performance by not hitting the database.
      Whereas the Analytics job would require actual data to perform the analysis on but mostly in read only mode

  3. FROM : https://aws.amazon.com/rds/faqs/

    Q: Will I be alerted when automatic failover occurs?

    Yes, Amazon RDS will emit a DB Instance event to inform you that automatic failover occurred. You can use the DescribeEvents to return information about events related to your DB Instance, or click the “DB Events” section of the AWS Management Console.

  4. Hey..

    I have a postgres in an ec2 on AWS . Now we are planing to move on RDS . & we do not want any downtime . We were planning on postgres replication between the post gres on ec2 & RDS postgres . Is this possible ? If yes can you give me pointers. ?

    1. I would prefer to create a database backup and import it directly to the RDS database. Is the database a production database ? Whats the size ? as the operation can be performed with minimal downtime given both the resources are within AWS.

      1. Yes its a prod database . The size is around 500gb . for that i will have to take incremental backups..which i don’t want to.

        AWS migration tools is there vut the source version must be 9.4 or above we r on 9.3

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