As GOV.UK gets bigger, we often need to revisit the ways that we originally solved some problems. One thing that's changed recently is how we prepare for disaster recovery.
The reality of working in technology is that software systems fail, more often than we'd like and usually in ways that are beyond our control. The process of thinking up high-level failure scenarios and solutions for them is called disaster recovery.
One extreme scenario for GOV.UK is that all our infrastructure disappears. All of of our applications, the servers they run on and even the infrastructure provider we're using. Gone.
Trying to solve this problem directly and up front is difficult. It's too generic and we don't know the root cause yet. But, we can give ourselves some time while we assess the situation and start to resume normal service for our users.
Creating a static copy of GOV.UK
One of our solutions at the time was to run a task overnight to visit every page on GOV.UK that wasn't a form and save it to disk. This process would take a couple of hours to complete. We'd then transfer the files to some backup machines that were ready to be switched over to should the worst occur.
We called the code that did this the GOV.UK Mirror.
Fast forward to present day
Various agencies and departments have transitioned to GOV.UK and we're now up to over 140,000 pages. We found that the GOV.UK Mirror was now taking close to 50 hours to complete. It meant that a given page could be up to two days out of date should we need to switch to our mirror. For pages like foreign travel advice that update more regularly than once a day this was a problem.
What problems are we trying to solve?
We knew that the full crawl was taking too long at 50 hours. We would want this to complete within a day. We also couldn't crawl certain pages that get updated often on an ad-hoc basis such as foreign travel advice. Finally, there was no way of pausing or stopping the mirror process mid-run. We couldn't continue easily from the last good state so we had to restart from the beginning each time.
We made a conscious decision to split the GOV.UK Mirror into two components. A producer to give us the initial set of URLs and a consumer to crawl them, and write the pages to disk. The two components would communicate using a message queue. This way, we'd remove our reliance on the nightly task to complete the work and could use the message queue for crawling ad-hoc pages. Using a message queue also meant we could continue where we left off.
The producer is now a simpler process that retrieves a list of URLs from our Content API and publishes them to an exchange. Most of the work is done in the consumer component, which is written in Go and the message queue broker we're using is RabbitMQ.
We wanted the ability to horizontally scale out crawling to improve the rate at which we completed the work we were given. We could achieve parallelism on the queue by increasing the number of consumers, but we wouldn't be able to keep track of URLs that had been crawled across the nodes. We needed to think beyond a single process running at one time.
We used Redis to share state across the workers. We use Redis to keep track of URLs that had been crawled before and check whether or not to crawl them as we pick up URLs from the message queue. Now we can have many message queue consumers to get through work faster based on our workload. The total time for a full crawl is now 4 hours.
What have we learnt?
We had been running the GOV.UK Mirror for long enough to know which areas we didn't like, from operating it through to functionality we knew to be missing. Not only that, but we also understood the problem better than we did at GOV.UK's release. There was no magical epiphany that occurred; this is the nature of writing software – you have to adapt and update as you know more.
After two years of running GOV.UK we're finding that we have to revisit many of the choices we had made. The site has grown a lot and it's time to take another look at many of the applications we built back in 2012.
You can find the code here: https://github.com/alphagov/govuk_crawler_worker/
If this sounds like a good place to work, take a look at Working for GDS - we're usually in search of talented people to come and join the team.