Last Monday evening we received this tweet:
@freeagent your servers are running really slow tonight, making data input a real drag— Warwicka (@Warwicka) January 13, 2014
Naturally we take anything like this seriously so we started digging. First stop was New Relic which shows us average application and browser response times as well as a whole host of other useful metrics. Everthing looked normal. OK, time to hit the logs in search of any long running requests.
While searching for all long running requests, we noticed a few patterns. Firstly that the long running requests were only long running on the load balancers and not the application servers. Secondly that most of these long running requests were from BT-owned IP addresses.
Then as suddenly as this started, it stopped. This isn’t the first time we’ve had issues with BT. Guess they had some incident which they resolved.
Along comes Tuesday evening and again:
@freeagent sorry, but again tonight, site is so slow. It's taking over a minute to load a page. I'm on a very fast connection here as well— Warwicka (@Warwicka) January 14, 2014
Two disruptions in two nights, each lasting only a couple of hours. This was odd. I decided that if this was going to happen again, I wanted to be ready for it
Lets rewind a few months. A friend of mine told me about the RIPE Atlas project and kindly gave me some credits to try it out. RIPE are trying to build the largest internet measurement network by giving probes away to people to host on their networks. In return for hosting a probe you gain credits which can be used to query other probes.
The great thing about these probes is most of them are hosted on residential internet connections and not in data centres which usually have lots of bandwidth and redundant links.
I applied for a probe and a few weeks later it arrived.
With users reporting problems which we believed to be network related it was time to test this out.
I set up 2 measurements:
- Find 10 UK probes and ping our app every 5min.
- Find 10 BT probes and ping our app every 5min.
The results were interesting and confirmed exactly what we were seeing. The results from the UK wide probes showed that only BT was having.
Looking at the BT probes we see that nearly all of them have packet loss or increased ping times at the same time.
It’s important to remember here that these are hosted on home connections so we do expect them to have fluctuations every now and again.
As it happens the problem turned out not to be with BT at all but with a company our hosting provider has a BGP peering with. BT customers were favouring this route which was flapping causing the packet loss and slowness our customers were seeing.
Thankfully this issue is now resolved and we’ve been carefully monitoring it over the last week or so.