Tips for Scaling Web Apps

by alex_gaynor

This blog post is a short list of things you can do, on basically any web project, to improve performance, scalability, and cost. In 2015, a medium sized server (8 cores, 24GB of RAM) is capable of serving hundreds-to-thousands of HTTP requests per second. This post is a guide to making sure you aren't wasting your resources on things that are already solved problems.

All of these assume you have monitoring to track your availability, and metrics to track various aspects of your site's performance. You want to be looking at 99th percentile numbers, not 50th percentile. A 50th percentile page load time means half of your users are experiencing something slower, so this can be very misleading.

Many of these are things you can do right from the start, not lessons you need to learn the hard way.

Static Assets

Put all your static assets (CSS, Javascript, Images) behind a CDN that reverse proxies to your servers and caches stuff at their PoPs. This will get you good performance for users and effectively infinite scalability (for static assets).

Assets should all have unique names for content. That means when you change the contents of a Javascript file your asset pipeline should give the final file your webservers serve a unique name. This lets you set HTTP caching headers for the far future, which lets web browsers actually cache the data on users' computers, saving you entire HTTP requests. A simple trick to get a unique name is to make the filename that your asset pipeline emits simply be the sha256 of the file's contents.


If you can get away with it, storing your session as a signed (and optionally encrypted) cookie is great, this saves you from needing to do any database lookups on your server. You should be using TLS for all connections, and setting the secure flag on the cookie. And for the love of god, do not implement the crypto signing/encrypting/verifying/decrypting bits yourself. Use an existing library.

If signed cookies are insufficient for some reason (e.g. large payload size), your best bet is to use a simple K/V store. For most applications, best-effort persistence is sufficient, so something dirt simple like memcached works great.

Load balancing

Your web nodes should be stateless. Put them behind a load balancer from the beginning, and practice (read: develop automation for) adding and removing them. It should be completely trivial and safe for you to add web nodes whenever you need to, and delete old nodes for any reason.

If your load balancer lets you pick a distribution strategy, minimum number of connections is a safe bet.

Set up your load balancer to perform health checks on the backends every couple of seconds.

Object storage

For things like user uploads, or other blob storage (any static media that isn't a part of the site itself), use an object storage API (Amazon's S3 is the gold standard). Avoid local disk: it's hard to scale and not worth the effort, hosted solutions are cheap and easy.


For most applications, a simple relational database is fine. Further, most applications become bottlenecked on reads well before they become bottlenecked on writes.

It should be easy for you to set up additional read replicas as needed, and to get application servers performing queries against them. Monitor the replication lag from the primary to the replicas.

Develop automation for failing over your primary to a secondary.

For any analytics workloads you have (reports, dashboards, or unusual administrative pages) have an additional read replica that is specific to them. Analytics workloads tend to "look" different from regular transactional queries, and mixing them on a single server can degrade performance due to excessive trashing of caches.

Don't rely on database migrations being fast. Your application code should be able to handle the database in the "pre-migration", "post-migration", and "during-migration" states. For example, to add a new column you'd have the following steps:

  1. Add a new nullable column to your database.
  2. Deploy application code to write to that column.
  3. Run a script in the background to backfill the data for older rows.
  4. Verify the integrity of all the data.
  5. Finally, deploy application code to actually read from the column.

Deployment and Automation

Any maintenance task you perform should be encoded in software.

Actually practice your automation. Delete and replace servers all the time, this is the only way to make it easy and safe.

Dark reads can be a powerful technique for testing and load testing new features or infrastructure. Dark reads are where you have visitors to your site make requests to an endpoint in the background (e.g. with an AJAX request), but don't actually do anything with the responses. This lets you measure the performance of a new feature based on more realistic load patterns.

In a similar vein, rather than meticulous branch maintenance and testing environments, use feature flags to deploy new changes instantly, but only make them available to certain subsets of users.

Dark reads and feature flags work particularly nicely together. Deploy dark reads for a new feature behind a flag, gradually increase the amount of traffic that goes through that path, watching your metrics and optimizing as necessary. When you're confident that it will all work, move from dark reads to actually launching the feature.

Hi, I'm Alex. I'm a software engineer at Mozilla, working on Firefox security. Before that I was a software engineer with the U.S. Digital Service. I'm an avid open source contributor and live in Washington, DC.