Architecting an Highly Available and Scalable WordPress Site in AWS

Why this presentation ?

• WordPress is used by over 14.7% of Alexa Internet's "top 1 million" websites
• As of August 2011 WordPress powers 22% of all new websites
• As of December 2011, version 3.0 had been downloaded over 65 million times
• Provide some architectural insights to build Highly Scalable and Available WordPress sites in AWS
• Help startups and companies who are new to AWS- WordPress with some blueprints

Architecting an Highly Available and Scalable WordPress Site in AWS

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FAQ for this architecture can be found here

WordPress Architecture Requirements in AWS

• Ability to Serve millions of Pages per day
• Ability to scale out/down dynamically depending upon traffic
• Ability to perform well even with scaling demands 
• Should be Highly available without Single Points of Failure
• Automated Operations – Backup and Monitoring
• Cost effective 

Why AWS is better for WordPress Scaling?

• AWS provides flexibility to Scale up and Scale out
• AWS provides Building block services which is inherently robust and fault tolerant
• AWS provides full range of options from CDN , OS flavors , Storage Pool , Database , Load Balancers , Monitoring etc
• Pay as you go – Save cost by leveraging Auto Scaled WordPress EC2 instances
• We can start small and grow big in AWS

AWS building blocks used

• Amazon Route53 for managing and answering DNS queries
• Amazon Elastic Load Balancer for load balancing HTTP/S requests to WordPress Instances
• RDS MySQL Database Server for storing the WordPress content
• Simple Storage Service (S3) for storing the WordPress application assets , snapshots and log files
• Amazon CloudFront CDN for delivering the application static assets from nearest edge locations
• Amazon CloudWatch for monitoring and sending SNS alerts

Highly Scalable Architecture for WordPress application

Load Balancing Tier

• Amazon Elastic Load Balancing will Load balance the http requests to WordPress EC2 instances
• ELB will direct the requests to WordPress EC2 across Multiple Availability Zones ( for HA)
• Health check , LB algorithms and SSL termination will be done by the Load Balancer
• Amazon Elastic Load Balancing can keep expanding its capacity automatically depending upon the traffic 
• Amazon ELB works with Route 53 and Auto Scaling seamlessly
• Amazon ELB is priced @“ Pay for use” model
• Amazon ELB is a managed service , so No Maintenance headaches , SW/HW upgrades ,Capacity planning  and manual intervention for expansion

How scalable is this architecture ?

• Scalability and Elasticity is built in most of the layers in this architecture
• WordPress EC2 instances can be scaled out and down depending upon the traffic 
• We can expand the number of WordPress EC2 instances from 1 to 100+ automatically during load peaks
• We can reduce the number of WordPress EC2 instances from 100+ to 1 automatically during valleys
• Can serve millions of pages with ease 
• Pay only for the EC2 instance hours used
• Read Scalability is built in the Database layer with RDS Read Replicas
• New RDS Read Replicas can be added / removed with ease
• Tips:  During Campaigns alone we can run more RDS Read Replicas , Not more than max 5 RDS Read replicas are recommended , Keep Read Replica’s and RDS Master same EC2 size for better performance.

• Clustered + Distributed File System of the Storage pool layer can be manually scaled in case needed
• Minimum 2 EC2 Large instances should be used for Storage Pool Layer ( for HA and better IO)
• Design the Storage Pool Layer with HA (very critical)
• AWS building blocks like S3 , CloudFront , CloudWatch , SNS used in this architecture  are inherently designed for scalability

How High Availability and Fault tolerance is built in this Architecture ?

• HA @ WordPress Layer
• Multiple WordPress EC2 instances avoid single point of failure
• WordPress EC2 instances are launched across multiple –AZ’s inside a region for High Availability
• HA @ DB Layer
• RDS MySQL Master and Standby are launched in 2 different availability zones for High Availability
• RDS Read Replicas are created in Multiple –AZ’s
• HA @ Storage Pool Layer
• GlusterFS is used for the Storage Pool Layer
• Two EC2 instances used for Storage Pool Layer
• Storage Pool is setup in replicated mode for High Availability 
• AWS building blocks like S3 , Cloud Front , CloudWatch , ELB , SNS ,EBS used in this architecture are inherently designed for Fault tolerance and HA

How the Database Tier is Architected ?

• RDS Master and Standby provide High availability
• RDS Read Replicas provide Read performance
• HyperDB plugin is configured to use multiple endpoints like RDS master and RDS read replicas in this architecture
• DB security groups will allow DB access only to WordPress EC2 instances
• Periodic Dumps , Snapshots and Point in time recovery is possible in this architecture

What performance aspects are taken care in this Architecture ?

• APC (or) Xcache plugin can be used for PHP opscode caching
• W3TotalCache+CloudFront (or) BatCache+Memcached can be used for Page caching
• Multiple RDS Read Replicas for Read performance
• RDS Master and Read Replicas are separated to get independent write and read performance
• Amazon ELB + Auto Scaling improves the overall Site performance when the load increases

Content Delivery Network

• Amazon CloudFront will be the Content Delivery Network (CDN) 
• W3TotalCache plugin will be configured to use Amazon CloudFront for enhanced performance and reduced latency
• Static assets , templates , themes , images etc will delivered from the nearest edge locations of the CDN

Deployment

• Distributed File Storage Pool is configured between WordPress Management and Content instances
• WordPress Management and Content instances share the common storage pool for files and plugins
• Deployment of files and plugins will happen through the WordPress management node
• Files and plugins will be immediately available for use in the auto scaled WordPress EC2 instances

Monitoring and Alerts

• Amazon CloudWatch will monitor the CPU and Network utilization of the entire setup
• Amazon CloudWatch alarms configured with Amazon SNS provides Email/SMS alerts to System Administrators

Backups

• Custom ops scripts will backup the necessary files from Storage pool to S3 periodically
• S3 can be configured to remove old backups automatically
• RDS MySQL is configured to take periodic data dumps and DB snapshots
• RDS layer can be recovered point in time from the backups

Security

• Suitably hardened OS for WordPress Mgmt and WordPress instances
• Firewall (or) AWS security groups configured between all the layers in the architecture
• SFTP/FTP access only to the WordPress management node
• AWS IAM policies to manage user account access
• Install WordPress Security plugins 

Advantages

• Highly Available and Scalable architecture
• Can elastically scale out to serve millions of hits in a day
• Can grow with load demands in future
• Usage of inherently fault tolerant AWS building blocks adds Stability
• High performance using CDN and suitable cache plugins
• Monitoring , Backup and Recovery is built in
• Pay for use 

Disadvantages

• Complex to setup and maintain
• Will not be ideal for smaller sites that do not need scale
• Will not be cost efficient for sites that have less traffic

Key Points to Remember 

• Log files generated in WordPress EC2 instances have to rotated 
• Use Amazon EBS for Storage Pool and WordPress EC2 instances 
• Start with EC2 m1.Large Instances for the storage pool and WordPress instances
• More Memory – Better Performance for DB
• Start with RDS Large for Master 
• Keep RDS Master and RDS Read Replica’s in same size to improve read performance
• RDS MySQL supports only Innodb engine
• Separate the WordPress Management and WordPress Content instances for scalability
• Do not Scale out/down rapidly in a hour, it will cost more in AWS
• Combine AWS On-Demand and Reserved Instance pricing to get more savings per month
• Amazon Availability Zones(AZ’s) are distinct physical locations with Independent power , cooling ,network and security having Low latency network connectivity between them inside the same region
• Leverage them in WordPress , Storage Pool and DB layers for HA as mentioned in the architecture 

Related Articles:


FAQ for this architecture can be found here (Must read)
Architecting WordPress in AWS : Part 1 : Single Server Setup
Architecting WordPress in AWS : Part 2 : Multi tiered setup
Scaling WordPress in AWS to serve million page hits