In Part 1 of this blog series, we looked at how to use AWS compute, networking, and security services to create a foundation for a multi-Region application.
Data is at the center of many applications. In this post, Part 2, we will look at AWS data services that offer native features to help get your data where it needs to be.
In Part 3, we’ll look at AWS application management and monitoring services to help you build, monitor, and maintain a multi-Region application.
Considerations with replicating data
Data replication across the AWS network can happen quickly, but we are still limited by the speed of light. For this reason, data consistency must be considered when building a multi-Region application. Generally speaking, the longer a physical distance is, the longer it will take the data to get there.
When building a distributed system, consider the consistency, availability, partition tolerance (CAP) theorem. This theorem states that an application can only pick 2 out of the 3, and tradeoffs should be considered.
- Consistency – all clients always have the same view of data
- Availability – all clients can always read and write data
- Partition Tolerance – the system will continue to work despite physical partitions
Achieving consistency and availability is common for single-Region applications. For example, when an application connects to a single in-Region database. However, this becomes more difficult with multi-Region applications due to the latency added by transferring data over long distances. For this reason, highly distributed systems will typically follow an eventual consistency approach, favoring availability and partition tolerance.
Replicating objects and files
To ensure objects are in multiple Regions, Amazon Simple Storage Service (Amazon S3) can be set up to replicate objects across AWS Regions automatically with one-way or two-way replication. A subset of objects in an S3 bucket can be replicated with S3 replication rules. If low replication lag is critical, S3 Replication Time Control can help meet requirements by replicating 99.99% of objects within 15 minutes, and most within seconds. To monitor the replication status of objects, Amazon S3 events and metrics will track replication and can send an alert if there’s an issue.
Traditionally, each S3 bucket has its own single, Regional endpoint. To simplify connecting to and managing multiple endpoints, S3 Multi-Region Access Points create a single global endpoint spanning multiple S3 buckets in different Regions. When applications connect to this endpoint, it will route over the AWS network using AWS Global Accelerator to the bucket with the lowest latency. Failover routing is also automatically handled if the connectivity or availability to a bucket changes.
For files stored outside of Amazon S3, AWS DataSync simplifies, automates, and accelerates moving file data across Regions and accounts. It supports homogeneous and heterogeneous file migrations across Elastic File System (Amazon EFS), Amazon FSx, AWS Snowcone, and Amazon S3. It can even be used to sync on-premises files stored on NFS, SMB, HDFS, and self-managed object storage to AWS for hybrid architectures.
File and object replication should be expected to be eventually consistent. The rate at which a given dataset can transfer is a function of the amount of data, I/O bandwidth, network bandwidth, and network conditions.
Scheduled backups can be set up with AWS Backup, which automates backups of your data to meet business requirements. Backup plans can automate copying backups to one or more AWS Regions or accounts. A growing number of services are supported, and this can be especially useful for services that don’t offer real-time replication to another Region such as Amazon Elastic Block Store (Amazon EBS) and Amazon Neptune.
Figure 1 shows how these data transfer services can be combined for each resource.
Figure 1. Storage replication services
Spanning non-relational databases across Regions
Amazon DynamoDB global tables provide multi-Region and multi-writer features to help you build global applications at scale. A DynamoDB global table is the only AWS managed offering that allows for multiple active writers in a multi-Region topology (active-active and multi-Region). This allows for applications to read and write in the Region closest to them, with changes automatically replicated to other Regions.
Global reads and fast recovery for Amazon DocumentDB (with MongoDB compatibility) can be achieved with global clusters. These clusters have a primary Region that handles write operations. Dedicated storage-based replication infrastructure enables low-latency global reads with a lag of typically less than one second.
Keeping in-memory caches warm with the same data across Regions can be critical to maintain application performance. Amazon ElastiCache for Redis offers global datastore to create a fully managed, fast, reliable, and secure cross-Region replica for Redis caches and databases. With global datastore, writes occurring in one Region can be read from up to two other cross-Region replica clusters – eliminating the need to write to multiple caches to keep them warm.
Spanning relational databases across Regions
For applications that require a relational data model, Amazon Aurora global database provides for scaling of database reads across Regions in Aurora PostgreSQL-compatible and MySQL-compatible editions. Dedicated replication infrastructure utilizes physical replication to achieve consistently low replication lag that outperforms the built-in logical replication database engines offer, as shown in Figure 2.
Figure 2. SysBench OLTP (write-only) stepped every 600 seconds on R4.16xlarge
With Aurora global database, one primary Region is designated as the writer, and secondary Regions are dedicated to reads. Aurora MySQL supports write forwarding, which forwards write requests from a secondary Region to the primary Region to simplify logic in application code. Failover testing can happen by utilizing managed planned failover, which will change the active write cluster to another Region while keeping the replication topology intact. All databases discussed in this post employ eventual consistency when used across Regions, but Aurora PostgreSQL has an option to set the maximum a replica lag allowed with managed recovery point objective (managed RPO).
Logical replication, which utilizes a database engine’s built-in replication technology, can be set up for Amazon Relational Database Service (Amazon RDS) for MariaDB, MySQL, Oracle, PostgreSQL, and Aurora databases. A cross-Region read replica will receive these changes from the writer in the primary Region. For applications built on RDS for Microsoft SQL Server, cross-Region replication can be achieved by utilizing the AWS Database Migration Service. Cross-Region replicas allow for quicker local reads and can reduce data loss and recovery times in the case of a disaster by being promoted to a standalone instance.
For situations where a longer RPO and recovery time objective (RTO) are acceptable, backups can be copied across Regions. This is true for all of the relational and non-relational databases mentioned in this post, except for ElastiCache for Redis. Amazon Redshift can also automatically do this for your data warehouse. Backup copy times will vary depending on size and change rates.
A purpose-built database strategy offers many benefits, Figure 3 forms a purpose-built global database architecture.
Figure 3. Purpose-built global database architecture
Data is at the center of almost every application. In this post, we reviewed AWS services that offer cross-Region data replication to get your data where it needs to be quickly. Whether you need faster local reads, an active-active database, or simply need your data durably stored in a second Region, we have a solution for you. In the 3rd and final post of this series, we’ll cover application management and monitoring features.