In our practice, we hear a variety of misconceptions and misinterpretations in relation to the benefits of moving workloads ‘into the cloud’. You should be very wary if someone wants to make you believe that the pure migration of a traditional application to a cloud services vendor will make it any more scalable or reliable. Of course, you can scale vertically in increasing the size of your compute nodes. However, this still restricts you to the maximum size of the instances available. Scaling horizontally on the other hand, in distributing your workload over multiple instances, requires special considerations with regard to optimizing the application design.
As part of your migration strategy, you should also critically review your existing application components and consider if any of the high-level capabilities provided by your cloud vendor can deliver the same functionality at reduced costs, increased reliability, higher flexibility, et cetera. The story on how this startup ditched AWS from marketing software provider Moz also serves as a timely reminder that your mileage may vary depending on the type of your workload and non-functional requirements. Therefore we can only provide a general guide as each option needs to be considered against its advantages and disadvantages against the overall solution.
Please note that we tend to angle our posts along the service catalog of Amazon Web Services (AWS). However, most of the strategies and patterns we describe will also apply to other cloud vendors that provide capabilities with similar characteristics. With that in mind, let’s review our two-tier example application from the first post on Architecting on AWS: the best services to build a two-tier application from this series. To support the elasticity and reliability of our solution, we should consider a few concerns from the application perspective.
Optimizing the application design: session management
In the design, we are using the Elastic Load Balancer (ELB) for the distribution of load between instances. By default, the ELB routes each request to the instance with the smallest load. This is also referred to as stateless load balancing and needs to be considered in the design of your application. Unless you are having strong reasons to store your session state in memory on the web server, you should migrate to an external session provider. A common approach for this is the use of a session table within a relational database system. However, this doesn’t come without risks as the database system may be unnecessarily flooded with session requests. This is potentially impacting on the overall performance of the application and causing scalability issues in the database tier.
On AWS you are better off to use DynamoDB. Amazon DynamoDB is a key-value data store (with recently added support for document data model) which delivers configurable, predictable performance. For that reason, DynamoDB is an ideal candidate to consider for the management of session state. As a fully managed service, you won’t even have to worry about any operational or administrative cost.
A word on RDS and scaling
If you are an observant reader you may have already spotted a snag. If you really want your overall solution to be truly scalable, you need to ensure that this is applied to each tier of the application. This is posing a couple of issues in the traditional relational database space. While you have the ability to ‘crank-up’ the amount of provisioned IOPS for your RDS instances, there is no ability to autoscale database instances in the same fashion as EC2.
A common design pattern is the separation of write operations from read operations. By the general nature of storage systems read operations tend to be proportionally higher than database write operations. For that reason, you can offload read requests to a dedicated (set of) read replicas to provide some level of scalability. Based on the underlying design limitations within the storage engines though, this cannot be provided fully automated. Amongst other limitations, we also need to highlight that the support of read replicas in RDS is limited to MySQL, PostgreSQL and the new Aurora storage engines.
Alternative database technologies
All this is obviously not going to be an issue for you if you are dealing with very steady and predictable workloads. If you do have the need to deliver a scalable solution though, you will eventually get to the stage where you need to consider alternative mechanisms to reduce the load on your relational database environment. An obvious choice would be the consideration of alternative database technologies like Amazon DynamoDB or Amazon SimpleDB for the data that doesn’t require a relational structure.
You can also reduce the strain on your application and database services in employing caching services like Amazon CloudFront. As described in a previous post, CloudFront provides a large number of edge locations across the globe that act like a massive cache for web and streaming content. The cache behaviour settings allow you to optimise the cache behaviour for the unique needs of your application. As an added bonus this will also improve the overall user experience for your customers.
Finally, we briefly want to touch on the Amazon S3 storage service. Again, many traditional application designs make us of relational or file system resources for the storage of BLOB objects. While you can certainly continue on that path, we recommend to rethink that approach for a number of reasons. For one, you obviously need to continue to provide and manage your own file system or relational database environment. You also have to ensure that the systems are always up-to-date, ensure that you have got enough available disk space available and the systems are actually available to meet your service level agreements.
If those operational reasons haven’t put you off yet, you may want to consider the actual costs for storage. Based on the figures for my ‘home’ region Sydney, the cost of storing 100 GByte of data on S3 is approximately 30% of the cost of Elastic Block Storage or RDS. And the pure cost of storage doesn’t even include the cost for utility compute to power the relational database or file system environments. So unless there is a specific need for a direct attached, high performing local disk e.g. for the hosting of a COTS solution like SAP, we strongly recommend to consider the use of the S3 object store where applicable.
With those initial teasers in mind, you should start exploring the AWS service catalogue and our rich training content on Cloud Academy to consider what other services you could utilise to address the unique concerns of your solution.