Sign In

Communications of the ACM


Weathering the Unexpected

Weathering the Unexpected, illustration

Credit: Jon Proctor

back to top 

Whether it is a hurricane blowing down power lines, a volcanic-ash cloud grounding all flights for a continent, or a humble rodent gnawing through underground fibers—the unexpected happens. We cannot do much to prevent it, but there is a lot we can do to be prepared for it. To this end, Google runs an annual, companywide, multi-day Disaster Recovery Testing event—DiRT—the objective of which is to ensure that Google's services and internal business operations continue to run following a disaster. DiRT was developed to find vulnerabilities in critical systems and business processes by intentionally causing failures in them, and to fix them before such failures happen in an uncontrolled manner. DiRT tests both Google's technical robustness by breaking live systems, and our operational resilience by explicitly preventing critical personnel, area experts and leadership from participating. Where we are not resilient but should be, we try to fix it. (See the accompanying sidebar by Tom Limoncelli.)

For DiRT-style events to be successful, an organization first needs to accept system and process failures as a means of learning. Things will go wrong. When they do, the focus must be on fixing the error instead of reprimanding an individual or team for a failure of complex systems.

An organization also needs to believe that the value in learning from events like DiRT justifies the associated costs. These events are not cheap—they require a sizable engineering investment, are accompanied by considerable disruptions to productivity, and can cause user-facing issues or revenue loss. DiRT, for example, involves the work of hundreds of engineering and operations personnel over several days; and things do not always go according to plan. DiRT has caused accidental outages and in some cases revenue loss. Since DiRT is a companywide exercise, however, it has the benefit of having all the right people available at a moment's notice to contain such events should they arise.

However, to benefit the most from such recovery events, an organization also needs to invest in continuous testing of its services. DiRT-style, large, companywide events should be less about testing routine failure conditions such as a single-service failovers or on-call handoffs, and more about testing complex scenarios or less-tested interfaces between systems and teams. Complex failures are often merely a result of weaknesses in smaller parts of the system. As smaller components of the system get tested constantly, failures of larger components become less likely.

A simple example is testing an organization's ability to recover from the loss of a data center. Such a loss may be simulated by powering down the facility or by causing network links to fail. The response would theoretically involve a sequence of events, from redirecting traffic away from the lost data center to a series of single-service failovers in some specific order. All it would take to choke the recovery process, however, is the failure of a single instance of a core infrastructure service—such as DNS (Domain Name System) or LDAP (lightweight Directory Access Protocol)—to failover. Testing the failover of such a service can and should happen continuously and should not have to wait for a DiRT event.

Back to Top

Growing the Program

A good way to kick off such an exercise is to start small and let the exercise evolve. It is quite easy to make this a large and complex affair right from the start, which in turn will probably come with unexpected overhead and complications.

Starting small applies to not only the number of teams involved in the exercise, but also the complexity of the tests. A few easy-to-remember rules and a simple, repeatable exercise format go a long way toward engaging teams quickly. If not all teams buy in, then work with the few that do; and as the exercise proves itself useful, more teams will participate.

Google's experience serves as an example: DiRT in its original form focused only on testing critical user-facing services. The initial bar was that all major user-facing teams wrote tests and that the tests were safe and caused no disruption, although we did realize that some of the tests were not very useful. This got teams "playing." Over a few iterations, the exercise attracted many more teams and tolerated fewer low-quality/low-value tests.

The same can be said for test designs. While the quality of tests matters a lot and directly affects the value of the exercise, DiRT events do not have to begin with overly complicated tests or the perfect set of tests (they do not exist). DiRT started with individual groups testing failure scenarios specific to their service. The overarching "disaster" was merely theoretical. In a subsequent DiRT exercise, the first major outage tested was that of our primary source-control management servers, which exposed several non-replicated critical functions dependent on this system. As each piece was fixed, we progressed to a larger disaster involving a major "earthquake" in the Bay Area.

We simulated the earthquake by taking down a data center in the area that housed a number of our internal systems. While the outage uncovered several services that were singly homed, it also exposed other interesting dependencies. For example, to avoid being affected by the outage, some teams decided to failover services from the data center to their workstations. Since the "earthquake" occurred near Google headquarters in Mountain View, the testing team disconnected the Mountain View campus as well—which meant all these failovers had failed. Also, what many did not anticipate was that the data-center outage caused authentication systems to fail in unexpected ways, which in turn locked most teams out of their workstations.

When the engineers realized that the shortcuts had failed and that no one could get any work done, they all simultaneously decided it was a good time to get dinner, and we ended up DoS'ing our cafes. In keeping with the DiRT goals, several of these issues were fixed by the next test.

Today, production and internal systems, network and data-center operations, and several business units such as HR, finance, security, and facilities test during DiRT. In the most recent DiRT exercise, we brought down several data-center clusters, infrastructure hubs, and offices without notice. Most of the scenarios were resolved painlessly.

It is very important to mention that well before Google even considered the concept of DiRT, most operations teams were already continuously testing their systems and cross-training using formats of popular role-playing games. As issues were identified, fixes got folded into the design process. For many of these teams, DiRT merely provided a safe opportunity to test riskier failure conditions or less-tested interactions with other systems and teams.

Back to Top

What to Test

There are several angles to consider when designing tests for DiRT. The simplest case, as described earlier, is service-specific testing. This category tests that a service and its components are fault-tolerant. These tests are usually contained, needing only the immediate team to respond, and they uncover technical and operational issues including documentation gaps, stale configurations, or knowledge gaps in handling critical emergencies. Ideally, these tests become part of the service's continuous testing process.

More involved technical test cases create scenarios that cause multiple system failures in parallel. Examples include data-center outages, fiber cuts, or failures in core infrastructure that manifest in dependent services. Such tests have a lot more value if the team that designs them is cross-functional and incorporates technical leads and subject-matter experts from various areas in the company. These are the people who understand the intricacies of their services and are in excellent positions to enumerate dependencies and failure modes to design realistic and meaningful scenarios.

The goal of this category of tests is to identify weaknesses in the less-tested interfaces between services and teams. Such scenarios can be potentially risky and disruptive, and they may need the help of several teams to resolve the error condition. DiRT is an excellent platform for this category of testing since it is meant to be a companywide exercise and all teams necessary for issue resolution are available on demand.

An often-overlooked area of testing is business process and communications. Systems and processes are highly intertwined, and separating out testing of systems from testing of business processes isn't realistic: a failure of a business system will affect the business process, and conversely a working system is not very useful without the right personnel.

The previous "earthquake" scenario exposed several such examples, some of which are described here. The loss of the Bay Area disconnected both people and systems in Mountain View from the world. This meant that teams in geographically distributed offices needed to provide round-the-clock on-call coverage for critical operations. The configuration change that was needed to redirect alerts and pages to these offices, however, depended on a system that was affected by the outage. Even for these teams with fully global expertise, things did not go smoothly as a result of this process failure.

A more successful failover was an approvals-tracking system for internal business functions. The system on its own was useless, however, since all the critical approvers were in Mountain View and therefore unavailable. Unfortunately, they were the same people who had the ability to change the approval chain.

In the same scenario, we tested the use of a documented emergency communications plan. The first DiRT exercise revealed that exactly one person was able to find the plan and show up on the correct phone bridge at the time of the exercise. During the following drill, more than 100 people were able to find it. This is when we learned the bridge would not hold more than 40 callers. During another call, one of the callers put the bridge on hold. While the hold music was excellent for the soul, we quickly learned we needed ways to boot people from the bridge.

As another example, we simulated a long-term power outage at a data center. This test challenged the facility to run on backup generator power for an extended period, which in turn required the purchase of considerable amounts of diesel fuel without access to the usual chain of approvers at headquarters. We expected someone in the facility to invoke our documented emergency spend process, but since they didn't know where that was, the test takers creatively found an employee who offered to put the entire six-digit charge on his personal credit card. Copious documentation on how something should work doesn't mean anyone will use it, or that it will even work. The only way to make sure is through testing.

Of course, tests are of almost no value if no effort is put into fixing the issues that the tests surface. An organizational culture that embraces failure as a means of learning goes a long way toward getting teams both to find and to resolve issues in their systems routinely.

Back to Top

Risk Mitigation

DiRT tests can be disruptive, and failures should be expected to occur at any point. Several steps can be taken to minimize potential damage.

At minimum, all tests need to be thoroughly reviewed by a cross-functional technical team and accompanied by a plan to revert should things go wrong. If the test has never before been attempted, running it in a sandbox can help contain the effects. The flip side to sandboxing, though, is that sometimes these environments may have configurations that are significantly different from those in production, resulting in less realistic outcomes.

There are ways of testing without disrupting services: at Google, we "whitelist" services we already know won't be able to survive certain tests. In essence, they have already failed the test and there is no point in causing an outage for them when the failing condition is already well understood. While services can "prefail" and exempt themselves, there is no concept of "prepassing" the test—services have to make it through to "pass."

A centrally staffed command center that understands and monitors all tests going on at any given time makes DiRT a safer environment for testing. When the unforeseen happens, the team in the command center (comprised largely of technical experts in various areas) jumps in to revert the test or fix the offending issue.

Back to Top

The Team

At DiRT's core are two teams: a technical team and a coordination team.

The technical team is responsible for designing all major tests and evaluating all tests written by individual teams for quality and impact. The technical team is also responsible for actually causing the larger outages and monitoring them to make sure things do not go awry in the process. This is also the team that handles unforeseen side effects of tests.

The coordinators handle a large part of the planning, scheduling, and execution of tests. They work very closely with the technical team to make sure the tests do not conflict with each other and that preparation work (such as setting up sandboxes) for each of these tests is done ahead of DiRT.

Both teams populate the DiRT command center. At the helm is usually someone with a sufficiently large Rolodex. When not much is going on, the command center is filled with distractions; it houses very smart people with short attention spans who are low on sleep and high on caffeine. When things go wrong, however—and they do—they are alert, on target, and fully focused on firefighting and getting the error communicated, resolved, or rolled back—and, furthermore, filed for fixing.

Copious documentation on how something should work doesn't mean anyone will use it, or that it will even work. The only way to make sure is through testing.

The command center is also home to the person with one of the most fun 20% projects at Google: the storyteller who concocts and narrates the disaster, ranging from the attack of the zombies to a bizarre psychological thriller featuring an errant fortune-teller.

Back to Top


Whatever its flavor, disaster recovery testing events are an excellent vehicle to find issues in systems and processes in a controlled environment. The basic principle is to accept that failures happen and that organizations need to be prepared for them. Often, a solid executive sponsor and champion is instrumental in setting the right tone for the exercise. In Google's case, VP of operations Ben Treynor has championed both learning from continuous testing and preemptively fixing failures.

It is true that these exercises require a lot of work, but there is inestimable value in having the chance to identify and fix failures before they occur in an uncontrolled environment.

q stamp of ACM QueueRelated articles

Fault Injection in Production
John Allspaw

Thinking Clearly about Performance
Cary Millsap

Improving Performance on the Internet
Tom Leighton

Back to Top


Kripa Krishnan is a technical program manager at Google who has been running the company's disaster recovery program (DiRT) for six years. She also leads the Google Apps for Government effort. She is currently working on privacy and security infrastructure initiatives for Google Apps. Prior to Google, Kripa worked with the Telemedicine Program of Kosovo to set up telemedicine infrastructure and a virtual education network in the region.

Back to Top

©2012 ACM  0001-0782/11/01  $15.00

Permission to make digital or hard copies of part or all of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and full citation on the first page. Copyright for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, to republish, to post on servers, or to redistribute to lists, requires prior specific permission and/or fee. Request permission to publish from or fax (212) 869-0481.

The Digital Library is published by the Association for Computing Machinery. Copyright © 2012 ACM, Inc.


No entries found