We were asked to prepare a generic document for a LAMP-based startup, running one on machine in the corner of an office, looking to cope with massive scale, disaster recovery, and self-healing resilience, all within three months. Here’s what we came up with given only the above in the context of Amazon Web Services offerings. Most of their services are just shortcuts for doing the same thing yourself.
This document outlines the ways in which AWS can address the concerns of rapid growth from a proof-of-concept to a hard-hit Internet website. It will address options comprehensively, with an eye toward the future. However, as is the case with rapid growth of early stage startups, the question will often come down to priorities.
The first step toward scalability is to remove state from components, starting with the PHP. Once this is done, non-sticky load balancing via Elastic Load Balancers (ELBs) can spread the load over an arbitrary number of machines. State can be maintained in a noSQL database such as SimpleDB, but code will have to be altered. Alternatively, to get started quickly, there exists a single PHP configuration directive to maintain sessions in memcached. Such a cache is necessary in any case to widen the most often-encountered bottleneck, the relational database. ElastiCache may be leveraged quickly here, for which modest developer time will be needed to wrap all database calls with simple cacheing calls. In general, it is a good idea to abstract out all database calls one step further so that when the time comes to scale the DB via sharding or moving some data to a noSQL store, the developer time required won’t overwhelm. Also, it makes it easier to split reads and writes so that a number of slaves, read replicas in the case of RDS, can offload read traffic from the master. The final piece of the puzzle, necessary for AutoScaling, is automation. Each machine needs to be able to come up with everything it needs to start serving immediately. Here, CloudFormation, coupled with server role management software like Chef, can be used to fully automate the process.
Self-healing Fault Tolerance
Once state is removed from machines, AutoScaling can maintain a minimum number of healthy instances to keep serving in the event of failure. However, if the app itself is having trouble, only a decoupled architecture can mitigate the effect of such failure. If the app functions can be split into two classes, one that presents data and one that processes it, one can fail while the other keeps going. Typically, presentation is much less likely to fail, and if there is are queues in place for processing, dev/ops can get to work on the failure while the queue grows with backed-up tasks. If the processing layer is stateless as well, and if its AutoScaling is keyed to the queue size, it can catch back up very quickly after the issue is resolved. The presentation of data, albeit possibly a bit stale, continues the whole time. If appropriate, GSLB can distribute load across multiple regions. We recently worked with UltraDNS to make their service available for use with ELBs, and Dyn is reported to offer it as well.
DR can be split up into two aspects: continuity of service and data loss. With the above, the stack can be configured quickly to spread itself out across multiple physical data centers located in each region’s Availability Zones (AZs), including the MySQL DB with the multi-AZ feature of RDS. However, in the event that Virginia slides into the ocean, continuity of service can be managed, but data loss proves much more difficult. CloudFormation stacks, for example, can be configured to fully launch the stack in any AWS region with one button as long as the software is region-agnostic. MySQL data loss, however, is more tricky. Periodic off-site backups help, but MySQL replication across the Internet is not reliable. Data loss can be minimized by spreading the data across multiple regions and perhaps using GSLB to distribute traffic among them. NoSQL DB replication like MongoDB’s is often much more forgiving. It is for this reason, along with the bottleneck issues, that using a NoSQL store whenever possible is recommended.
Removing state for scalability requires that local filesystems be used only for ephemeral processing. This may take the form of storing things like images on S3 and pointing all clients there.
Latency can be reduced in several ways. First and most effective is serving static content via a Content Delivery Network (CDN) such as CloudFront. The origin can be a host or S3, and if the content is suitable for cacheing CloudFront minimizes latency by providing local copies around the world. The aforementioned GSLB can help minimize latency of traffic to the EC2 instances as well.
Security of data at rest and in transit
First, encryption can be used everywhere. ELBs can be configured to use https, the machines to use ssh for administrative access of course. In addition, traffic to and from the various AWS services can all be encrypted as well. Security Groups should tightly restrict access to data at rest. For example, a database should only be available to the application layer. Virtual Private Cloud (VPC) can be used to allow full access to dev/ops while further restricting access, even if it is already encrypted. Add to this multi-factor authentication and user-based access control with IAM for a very locked-down environment. Assuming the application is in a three-tier architecture, the public would only have access to front-end machines and S3/CloudFront, the middle tier only to the DB and everything only to rigorously screened users. Furthermore, IAM is granular enough to restrict role accounts for write access to services like S3. If inter-region tunnels are required, for, say, international replication, VPC does not yet offer cross-region tunnels, but its gateways can be used in conjunction with tunneling software.
I’ve offered an outline of an architecture in a perfect world. To scale within one to three months, not all of this can be reasonably implemented by a small startup. We recommend starting with abstracting out all database calls and the introduction of cacheing using ElastiCache. During the abstraction, keep a keen on at least being able to separate DB reads from writes quickly. See if a move to RDS is easy. Also, remove state from the application layer and stick it behind and ELB. This should be achievable within one month. Automation should then be put in-place, AutoScaling enabled, and wrapped up in a region- independent CloudFormation stack. At this point, read replicas can be used with the read/ write split in the database abstraction layer to alleviate any MySQL bottlenecks. The move to a non-filesystem data store like S3 should be implemented if it wasn’t already as part of state removal. Enable CloudFront for the static content and the business should be able to handle foreseeable growth in the next three months.