Facebook Engineering Statement Around Outage

Image credit: Facebook Engineering

Read our timeline from yesterday here

Just now, Facebook has come out with a first source statement confirming that the error happened because of an error in maintenance. We will snip out a few paragraphs with the most important information here.

This outage was triggered by the system that manages our global backbone network capacity. The backbone is the network Facebook has built to connect all our computing facilities together, which consists of tens of thousands of miles of fiber-optic cables crossing the globe and linking all our data centers.

The data traffic between all these computing facilities is managed by routers, which figure out where to send all the incoming and outgoing data. And in the extensive day-to-day work of maintaining this infrastructure, our engineers often need to take part of the backbone offline for maintenance — perhaps repairing a fiber line, adding more capacity, or updating the software on the router itself.

This was the source of yesterday’s outage. During one of these routine maintenance jobs, a command was issued with the intention to assess the availability of global backbone capacity, which unintentionally took down all the connections in our backbone network, effectively disconnecting Facebook data centers globally. Our systems are designed to audit commands like these to prevent mistakes like this, but a bug in that audit tool didn’t properly stop the command.

This change caused a complete disconnection of our server connections between our data centers and the internet. And that total loss of connection caused a second issue that made things worse.

One of the jobs performed by our smaller facilities is to respond to DNS queries. DNS is the address book of the internet, enabling the simple web names we type into browsers to be translated into specific server IP addresses. Those translation queries are answered by our authoritative name servers that occupy well known IP addresses themselves, which in turn are advertised to the rest of the internet via another protocol called the border gateway protocol (BGP).

To ensure reliable operation, our DNS servers disable those BGP advertisements if they themselves can not speak to our data centers, since this is an indication of an unhealthy network connection. In the recent outage the entire backbone was removed from operation,  making these locations declare themselves unhealthy and withdraw those BGP advertisements. The end result was that our DNS servers became unreachable even though they were still operational. This made it impossible for the rest of the internet to find our servers.

All of this happened very fast. And as our engineers worked to figure out what was happening and why, they faced two large obstacles: first, it was not possible to access our data centers through our normal means because their networks were down, and second, the total loss of DNS broke many of the internal tools we’d normally use to investigate and resolve outages like this.

Our primary and out-of-band network access was down, so we sent engineers onsite to the data centers to have them debug the issue and restart the systems. But this took time, because these facilities are designed with high levels of physical and system security in mind. They’re hard to get into, and once you’re inside, the hardware and routers are designed to be difficult to modify even when you have physical access to them. So it took extra time to activate the secure access protocols needed to get people onsite and able to work on the servers. Only then could we confirm the issue and bring our backbone back online.

Once our backbone network connectivity was restored across our data center regions, everything came back up with it. But the problem was not over — we knew that flipping our services back on all at once could potentially cause a new round of crashes due to a surge in traffic. Individual data centers were reporting dips in power usage in the range of tens of megawatts, and suddenly reversing such a dip in power consumption could put everything from electrical systems to caches at risk.

Helpfully, this is an event we’re well prepared for thanks to the “storm” drills we’ve been running for a long time now. In a storm exercise, we simulate a major system failure by taking a service, data center, or entire region offline, stress testing all the infrastructure and software involved. Experience from these drills gave us the confidence and experience to bring things back online and carefully manage the increasing loads. In the end, our services came back up relatively quickly without any further systemwide failures. And while we’ve never previously run a storm that simulated our global backbone being taken offline, we’ll certainly be looking for ways to simulate events like this moving forward.


The most important parts we got out of this:

  • The issue was in the backbone of Facebook, not individual datacenters
  • A command that should’ve never been executed took down the backbone’s connections
  • This should’ve been prevented, but due to a bug in the audit tool this was still executed
  • The DNS servers have disabled BGP when they couldn’t connect to their datacenters
  • The DNS server itself was still up and running during this
  • Engineers were sent on-site
  • Physical and system security prevented a quick fix
  • Load was distributed well after, as this could’ve caused another outage as systems get overwhelmed
  • Cloudflare’s article was fully correct with what they reported, but this gave a few more details

You can find the full article here

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