Horizontal Scalability

Learn about scaling web applications and architecture of every Pantheon environment.


Pantheon's distributed infrastructure facilitates horizontal scalability through the automated process of provisioning additional lightweight containers. This allows us to take sites from hundreds of pages served to hundreds of millions without downtime.

Vertical vs. Horizontal Scalability

Vertical Scalability: Reconfigure the existing architecture of a single machine to increase available resources (CPUs, memory, etc.) to scale up

Horizontal Scalability: Provision additional containers within a cluster of distributed machines to scale out

Vertical scalability is often used as a starting point for sites running on traditional hosts. Resources are scaled up on a single machine until a cluster-style architecture can be implemented to achieve horizontal scalability. For a site to handle copious amounts of traffic and activity, it must transcend a single server.

In addition to extremely high overhead costs, common pitfalls include:

  • Surprise architectural migrations
  • Problems with shared instances
  • Downtime while someone resizes a server
  • One-off science projects to build out your complex snowflake cluster
  • Last-minute requests for additional resources

Pantheon eliminates these risks entirely by running sites on a web-scale infrastructure from the start. Provisioning more containers to handle viral traffic happens at the speed of software through an automated process.

Container Architecture

Pantheon's infrastructure is based on a grid model. Each application container is created with an optimized PHP stack and isolated NGINX, APC cache, and PHP worker agents. Containers automatically bind your site's codebase with a dedicated MySQL container, networked filesystem, and any enabled addon services such as Redis and/or Apache Solr.

For more information on containers, see All About Application Containers.

Add and Remove Application Containers

Add containers by upgrading the site's plan within the Site Dashboard to a Performance Medium plan or higher. If the additional container(s) are no longer needed, simply downgrade the plan within the Site Dashboard to remove.

The platform does not assume when additional containers are added or removed in response to traffic and/or resource consumption.

New Relic Pro

Consider enabling New Relic Pro for your site. You'll get access to a wide array of metrics that provide a nearly real-time look into the performance of a web application. Making it easy for you to monitor to your performance, with the added benefit of speeding up the support process by helping our support team visualize corresponding performance and symptoms.

For more information, see New Relic Pro.

Handle Traffic Spikes

You'll need to decide how to distribute traffic across the available PHP app servers. Open-source tools like Nginx, HAProxy, and Pound can fill this role, but you can also solve this with hardware (e.g. an F5 appliance) or with a cloud-based load balancer (e.g. Amazon’s ELBs).

Managing Temporary Files

The /tmp directory is not shared across application containers, making temporary files created by your site's framework inaccessible for requests served by another container. A plan for managing these files should be implemented prior to scaling the site out. For more details, see Temporary File Management with Multiple Application Containers.