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CRE life lessons: The practicalities of dark launching

August 11, 2017
Adrian Hilton

Customer Reliability Engineer, SRE

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Editor's note: This is the second of a two-part series on dark launches. You can read the first post here.

In the first part of this series, we introduced you to the concept of dark launches. In a dark launch, you take a copy of your incoming traffic and send it to the new service, then throw away the result. Dark launches are useful when you want to launch a new version of an existing service, but don’t want nasty surprises when you turn it on.

This isn’t always straightforward as it sounds, however. In this blog post, we’ll look at some of the circumstances that can make things difficult for you, and teach you how to work around them.

Finding a traffic source

Do you actually have existing traffic for your service? If you’re launching a new web service which is not more-or-less-directly replacing an existing service, you may not.

As an example, say you’re an online catalog company that lets users browse items from your physical store’s inventory. The system is working well, but now you want to give users the ability to purchase one of those items. How would you do a dark launch of this feature? How can you approximate real usage when no user is even seeing the option to purchase an item?

One approach is to fire off a dark-launch query to your new component for every user query to the original component. In our example, we might send a background “purchase” request for an item whenever the user sends a “view” request for that item. Realistically, not every user who views an item will go on to purchase it, so we might randomize the dark launch by only sending a “purchase” request for one in every five views.

This will hopefully give you an approximation of live traffic in terms of volume and pattern. Note that this can’t be expected to be totally accurate when it comes to to live traffic when the service is launched. But, it’s better than nothing.

Dark launching mutating services

Generally, a read-only service is fairly easy to dark-launch. A service with queries that mutate backend storage is far less easy. There are still strong reasons for doing the dark launch in this situation, because it gives you some degree of testing that you can’t reasonably get elsewhere, but you’ll need to invest significant effort to get the most from dark-launching.

Unless you’re doing a storage migration, you’ll need to make significant effort/payoff tradeoffs doing dark launches for mutating queries. The easiest option is to disable the mutates for the dark-launch traffic, returning a dummy response after the mutate is prepared but before it’s sent. This is safe, but it does mean that you’re not getting a full measurement of the dark launched service — what if it has a bug that causes 10% of the mutate requests to be incorrectly specified?

Alternatively, you might choose to send the mutation to a temporary duplicate of your existing storage. This is much better for the fidelity of your test, but great care will be needed to avoid sending real users the response from your temporary duplicate. It would also be very unfortunate for everyone if, at the end of your dark launch, you end up making the new service live when it’s still sending mutations to the temporary duplicate storage.

Storage migration

If you’re doing a storage migration — moving an existing system’s stored data from one storage system to another (for instance, MySQL to MongoDB because you’ve decided that you don’t really need SQL after all) — you’ll find that dark launches will be crucial in this migration, but you’ll have to be particularly careful about how you handle mutation-inducing queries. Eventually you’ll need mutations to take effect in both your old and new storage systems, and then you’ll need to make the new storage system the canonical storage for all user queries.

A good principle is that, during this migration, you should always make sure that you can revert to the old storage system if something goes wrong with the new one. You should know which of your systems (old and new) is the master for a given set of queries, and hence holds the canonical state. The mastership generally needs to be easily mutable and able to revert responsibility to the original storage system without losing data.

The universal requirement for a storage migration is a detailed written plan reviewed by not just your system stakeholders but also by your technical experts from the involved systems. Inevitably, your plan will miss things and will have to adapt as you move through the migration. Moving between storage systems can be an awfully big adventure — expect us to address this in a future blog post.

Duplicate traffic costs

The great thing about a well-implemented dark launch is that it exercises the full service in processing a query, for both the original and new service. The problem this brings is that each query costs twice as much to process. That means you should do the following:

  • Make sure your backends are appropriately provisioned for 2x the current traffic. If you have quota in other teams’ backends, make sure it’s temporarily increased to cover the dark launch as well.
  • If you’re connection-sensitive, ensure that your frontends have sufficient slack to accommodate a 2x connection count.
  • You should already be monitoring latency from your existing frontends, but keep a close eye on this monitoring stat and consider tightening your existing alerting thresholds. As service latency increases, service memory likely also increases, so you’ll want to be alert for either of these stats breaching established limits.

In some cases, the service traffic is so large that a 100% dark launch is not practical. In these instances, we suggest that you determine the largest percentage launch that is practical and plan accordingly, aiming to get the most representative selection of traffic in the dark launch. Within Google, we tend to launch a new service to Googlers first before making the service public. However, experience has taught us that Googlers are often not representative of the rest of the world in how they use a service.

An important consideration if your service makes substantial use of caching is that a sub-50% dark launch is unlikely to see material benefits from caching and hence will probably significantly overstate estimated load at 100%.

You may also choose to test-load your new service at over 100% of current traffic by duplicating some traffic — say, firing off two queries to the new service for every original query. This is fine, but you should scale your quota increases accordingly. If your service is cache-sensitive, then this approach will probably not be useful as your cache hit rate will be artificially high.

Because of the load impact of duplicate traffic, you should carefully consider how to use load shedding in this experiment. In particular, all dark launch traffic should be marked “sheddable” and hence be the first requests to be dropped by your system when under load.

In any case, if your service on-call sees an unexpected increase in CPU/memory/latency, they should drop the dark launch to 0% and see if that helps.


If you’re thinking about a dark launch for a new service, consider writing a dark launch plan. In that plan, make sure you answer the following questions:

  • Do you have existing traffic which you can fork and send to your new service?
  • Where will you fork the traffic: the application frontend, or somewhere else?
  • Will you fire off the message to the new backend asynchronously, or will you wait for it and impose a timeout?
  • What will you do with requests that generate mutations?
  • How and where will you log the responses from the original and new services, and how will you compare them?
    • Are you logging the following things: response code, backend latency, and response message size?
    • Will you be diffing responses? Are there fields that cannot meaningfully be diffed which you should skip in your comparison?
  • Have you made sure that your backends can handle 2x the current peak traffic, and have you given them temporary quota for it?
    • If not, at what percentage traffic will you stop the dark launch?
  • How are you going to select traffic for participation in the dark launch percentage: randomly, or by hashing on a key such as user ID?
  • Which teams need to know that this dark launch is happening? Do they know how to escalate concerns?
  • What’s your rollback plan after you make your new service live?

It may be that you don’t have enough surprises or excitement in your life; in that case, you don’t need to worry about dark launches. But if you feel that your service gives you enough adrenaline rushes already, dark launching is a great technique to make service launches really, really boring.

The complete series

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