Cell Developer Expertise at Slack

At Slack, the objective of the Cell Developer Expertise Group (DevXp) is to empower builders to ship code with confidence whereas having fun with a nice and productive engineering expertise. We use metrics and surveys to measure productiveness and developer expertise, reminiscent of developer sentiment, CI stability, time to merge (TTM), and check failure price.

We’ve gotten a number of worth out of our deal with cellular developer expertise, and we predict most firms under-invest on this space. On this put up we’ll focus on why having a DevXp crew improves effectivity and happiness, the price of not having a crew, and the way the crew recognized and resolved some widespread developer ache factors to optimize the developer expertise.

How it began

A couple of cellular engineers realized early on that engineers who have been employed to write down native cellular code may not essentially have experience within the technical areas round their developer expertise. They thought that if they may make the developer expertise for all cellular engineers higher, they may not solely assist engineers be extra productive, but in addition delight our prospects with quicker, higher-quality releases. They acquired collectively and fashioned an ad-hoc crew to deal with the most typical developer ache factors. The cellular developer expertise crew has grown from three individuals in 2017 to eight individuals at the moment. In our 5 years as a crew, now we have centered on these areas:

  • Native improvement expertise and IDE usability
  • Our rising codebase. Making certain visibility into problematic areas of the codebase that require consideration
  • Steady Integration usability and extensibility
  • Automation check infrastructure and automatic check flakiness
  • Holding the primary department inexperienced. Ensuring the most recent major is all the time buildable and shippable

The price of not investing in a cellular developer expertise crew

A cellular engineer normally begins a function by making a department on their native machine and committing their code to GitHub. When they’re prepared, they create a pull request and assign it to a reviewer. As soon as a pull request is opened or a subsequent commit has been added to the department, the next CI jobs get kicked off:

  • Jobs that construct artifacts
  • Jobs that run checks
  • Jobs that run static evaluation

As soon as the reviewer approves the pull request and all checks cross on CI, the engineer may merge the pull request in the primary department. Right here is the visualization of the developer movement and the movement interruptions related to every space.

Here’s a tough estimate of the price of some developer ache factors and the fee to the corporate for not addressing these ache factors because the crew grows:

Whereas builders can be taught to resolve a few of these points, the time spent and the fee incurred just isn’t justifiable because the crew grows. Having a devoted crew that may deal with these downside areas and figuring out methods to make the developer groups extra environment friendly will make sure that builders can preserve an intense product focus.


Our crew companions with the cellular engineering groups to prioritize which developer ache factors to deal with, utilizing the next strategy:

  • Take heed to prospects and work alongside them. We are going to associate with a cellular engineer as they’re engaged on a function and observe their challenges.
  • Survey the builders. We conduct a quarterly survey of our cellular engineers the place we observe common Web Promoter Rating (NPS) round cellular improvement.
  • Summarize developer ache factors. We distill the suggestions into working areas that we are able to cut up up as a crew and deal with.
  • Collect metrics. It is vital that we measure earlier than we begin addressing a ache level to make sure that an answer we deploy truly fixes the difficulty, and to know the precise influence our answer had on the issue space. We give you metrics to trace that correlate with the issue areas builders have and observe them on dashboards. This permits us to see the metrics change over time.
  • Spend money on experiments that enhance developer ache factors. We are going to consider options to the issues by both consulting with different firms that additionally develop at this scale, or by arising with a singular answer ourselves.
  • Think about using third-party instruments. We consider whether or not it makes extra sense to make use of current options or to construct out our personal options.
  • Repeat this course of. As soon as we launch an answer, we have a look at the metrics to make sure that it strikes the needle in the precise route; solely then can we transfer onto the following downside space.

Developer pains

Let’s dive into some developer ache factors so as of severity and look at how the cellular developer expertise crew addressed them. For every ache level, we’ll begin with some quotes from our builders after which define the steps we took.

CI check jobs that take a very long time to finish

When a developer has to attend a very long time for checks to run on their pull requests, they change to engaged on a special process and lose context on the unique pull request. When the check outcomes return, if there is a matter they should handle, they must re-orient themselves with the unique process they have been engaged on. This context switching takes a toll on developer productiveness. The next are two quotes from our quarterly cellular engineering survey in 2018.


Quicker CI time! I believe that is requested lots, however it could be wonderful to have this improved

Jenkins construct instances are fairly excessive and it could be nice if we are able to scale back these

From 1 to 10 builders, we had a few hundred checks and ran all of them serially utilizing Xcodebuild for iOS and Firebase Take a look at lab for Android.

Operating the checks serially labored for a few years, till the check job time began to take virtually an hour. One of many options we thought of was introducing parallelization to the check suites. As a substitute of working the entire checks serially, we may cut up them into shards and run them in parallel. Right here is how we solved this downside on the iOS and Android platforms.


We thought of writing our personal device to attain this, however then found a device known as Bluepill that was open sourced by Linkedin. It makes use of Xcodebuild underneath the hood, however added the flexibility to shard and execute checks in parallel. Integrating Bluepill decreased our complete check execution time to about 20 minutes.

Utilizing Bluepill labored for just a few extra years till our unit check job began to as soon as once more take virtually 50 minutes. Slack iOS engineers have been including extra check suites to run, and we may not merely rely solely on parallelization to decrease TTM.

How transferring to a contemporary construct system helped drive down CI job instances

Our subsequent technique was to implement a caching layer for our check suites. The objective was to solely run the checks that wanted to be run on a particular pull request, and return the remaining check outcomes from cache. The issue was that Xcodebuild doesn’t assist caching. To implement check caching we wanted to maneuver to a special construct system:s Bazel. We utilized Bazel’s disk cache on CI machines so builds from completely different pull requests can reuse construct outputs from one other consumer’s construct slightly than constructing every new output domestically.

Along with the Bazel disk cache, we use the bazel-diff device that enables us to find out the precise affected set of impacted targets between two Git revisions. The 2 revisions we examine are the tip of the primary department, and the final commit on the builders department. As soon as now we have the listing of targets that have been impacted, we solely check these targets.

With the Bazel construct system and bazel-diff, we have been in a position to lower TTM to a mean of 9 minutes, with a minimal TTM  of 4.5 minutes. This implies builders can get the suggestions they want on their pull request quicker, and extra rapidly get again to collaborating with others and dealing on their options.


Within the early days, TTM was round 50 minutes, and Firebase Take a look at Lab (FTL) didn’t have check sharding.  We constructed an in-house check sharder on high of FTL known as Gasoline to interrupt checks into a number of shards and name FTL APIs to run every check shard in parallel. This introduced TTM from 50+ minutes to underneath 20 minutes. Right here is the excessive degree overview:

We continued utilizing Gasoline for 2 and a half years, after which moved to an open supply check sharder known as Flank. We proceed to make use of Flank at the moment to run Android purposeful and end-to-end UI checks.

Take a look at-related failures 

When a verify fails on a pull request due to flaky or unrelated check failures, it has the potential to take the developer out of movement, and presumably influence different builders as properly. Let’s check out just a few culprits inflicting non-related pull request failures and the way now we have addressed them.

Fragile automation frameworks

From 2015 to early 2017, we used the Calabash testing framework that interacted with the UI and wrapped that logic in Cucumber to make the steps human readable. Calabash is a “blackbox” check automation framework and wishes a devoted automation crew to write down and handle checks. We noticed that the extra checks that have been added, the extra fragile the check suites grew to become. When a check failed on a pull request, the developer would attain out to an Automation Engineer to grasp the failure, try to repair it, then rerun it once more and hope that one other fragile check doesn’t fail their construct. This resulted in an extended suggestions loop and elevated TTM.

Because the crew grew we determined to maneuver away from Calabash and switched to Espresso as a result of Espresso was tightly coupled with the Android OS and can be written within the native language (Java or Kotlin). Espresso is highly effective as a result of it’s conscious of the interior workings of the Android OS and will interface with it simply. This additionally meant that Android builders may simply write and modify checks as a result of they have been written within the language they have been most comfy with. A couple of advantages to focus on with migrations:

  • This helped to shift testing duty from our devoted automation crew to builders, to allow them to write checks as wanted to check the logic within the code
  • Testing time went from ~350 minutes to ~60 minutes once we moved from Calabash to Espresso and FTL

Flaky checks

In early 2018 the developer sentiment in the direction of testing was poor and brought on a number of developer ache. Listed here are couple of quotes from our developer survey:


Flimsy checks are nonetheless a bottleneck generally. We must always have a greater approach monitoring them and ping the proprietor to repair earlier than it causes an excessive amount of friction

Flaky checks gradual me right down to a halt – there needs to be a extra streamlined course of in place for continuing with PR’s as soon as flaky checks are discovered (as an alternative of blocking a merge because it occurs now)

At one level, 57% of the check failures in our major department have been as a consequence of flaky checks and the share was even greater on developer pull requests. We spent a while studying about flaky checks and managed to get them underneath management lately by constructing a system to auto-detect and suppress flaky checks to make sure developer expertise and movement is uninterrupted. Here’s a detailed article outlining our strategy and the way we diminished check failures price from 57% to 4% 

CI-related failures

For a few years we used Jenkins to energy the cellular CI infrastructure, utilizing Groovy-based .jenkinsfiles. Whereas it labored, it was additionally the supply of a number of frustration for builders. These issues have been probably the most impactful:

  • Frequent downtime
  • Decreased efficiency of the system
  • Failure to choose up Git webhooks, and subsequently not beginning pull request CI jobs
  • Failure to replace the pull request when a job fails
  • Issue in debugging failures as a consequence of poor UX

After flaky checks, CI downtime was the largest bottleneck negatively impacting the cellular crew’s productiveness. Listed here are some quotes from our builders relating to Jenkins:


Want extra dependable hooks between the jenkins CI and GitHub. When issues do go unsuitable, there are generally no hyperlinks in GH to go to the precise place. Additionally, generally CI passes however would not report again to GH so PR is caught in limbo till I manually rebuild stuff

Jenkins is a ache. Take away the Blue Ocean jenkins UI that’s complicated and everybody hates

Jenkins is a multitude to me. There are too many hyperlinks and I solely care about what broke and what button/hyperlink do I must click on on to retry. The whole lot else is noise

After utilizing Jenkins for greater than six years, we migrated away from it to BuildKite, which has had 99.96% uptime to date. Webhook-related points have fully disappeared, and the UX is easy sufficient for builders to navigate without having our crew’s assist. This has not solely improved developer expertise but in addition decreased the triage load for our crew.

The instant influence of the migration was an 8% enhance in CI stability from ~87% to 95%  and diminished Time to Merge by 41% from ~34 minutes to ~20 minutes

Merge conflicts

Battle whereas including new modules or recordsdata to the Xcode venture for iOS 

Because the variety of iOS engineers at Slack grew previous 20, one space of fixed frustration was the checked in Xcode venture file. The Xcode venture file is an XML file that defines the entire Xcode venture’s targets, construct configurations, preprocessor macros, schemes, and way more. As a small crew, it’s straightforward to make modifications to this file and commit them to the primary department with out inflicting any points, however because the variety of engineers will increase, the possibilities of inflicting a battle by making a change on this file additionally will increase.


“I believe the priority is extra so the xcode venture file, resolving conflicts on that factor is painful and error inclined. I’m undecided what the most effective strategy is to assuaging this doable ache level, particularly if they’ve added new code recordsdata.”

“I had a dozen or so conflicts within the venture file that I needed to manually resolve. Not an enormous subject in itself however once you’re anticipating to merge a PR it may be a shock”

The answer we applied was to make use of a device known as Xcodegen. Xcodegen allowed us to delete the checked in .xcodeproj file and create an Xcode venture dynamically utilizing a YAML file that contained definitions of all of our Xcode targets. We linked this device to a command line interface in order that iOS engineers may create an Xcode venture from the command line. One other profit was that the entire venture and goal degree settings are outlined in code, not within the Xcode GUI, which made the settings simpler to search out and edit.

After adopting Bazel we took it a step additional and created the YAML file dynamically from our Bazel construct descriptions.

A number of concurrent merges to major have the potential to interrupt major

To date now we have talked about completely different points that builders can expertise when writing code domestically and opening a pull request. However what occurs when a number of builders try to land their pull requests to the primary department concurrently? With a big crew, a number of merges to major occur all through the day which might make a developer’s pull requests stale rapidly. The longer a developer waits to merge, the bigger the prospect of a merge battle.

An rising variety of merge conflicts began inflicting the primary department to fail as a consequence of concurrent merges and began to negatively have an effect on developer productiveness. Till the merge battle is resolved, the primary department would stay damaged and pause all productiveness. At one level merge conflicts have been breaking the primary department a number of instances a day. Extra builders began requesting a merge queue.


We maintain breaking the primary department. We’d like a merge queue.

We brainstormed completely different options and in the end landed on utilizing a 3rd celebration answer known as Aviator, and mixed it with our in-house device Mergebot. We felt that constructing and sustaining a merge queue could be an excessive amount of work for us and that the most effective answer was to depend on an organization that was spending all of their time engaged on this downside. With Aviator, builders add their pull request to a queue as an alternative of straight merging to the primary department, and as soon as within the queue, Aviator will merge major into the developer branches and run the entire required checks. If a pull request was discovered to interrupt major, then the merge queue rejects it and the developer is notified through Slack. This method helps keep away from any merge conflicts.


Means higher now with Aviator. Solely ache level is I am unable to merge my pull requests and must depend on Aviator. Aviator takes hours to merge my PR to grasp. Which makes me anxious.

Being an early adopter means you get some advantages but in addition some ache. We labored carefully with the Aviator crew to establish and handle developer pains reminiscent of elevated time to merge a pull request in the primary department and failure reporting on a pull request when it’s dropped out of queue as a consequence of a battle.

Checking pull request progress/standing

This can be a request we acquired in 2017 in one in all our developer surveys:


Would actually love well timed alerts for PR assignments, feedback, approvals and many others. Additionally could be good if we may get a DM if our builds cross (slightly than solely the alert for once they fail) with the choice to merge it proper there from slack if now we have all of the wanted approvals.

Later within the 12 months we created a service which screens Git occasions and sends Slack notifications to the pull request creator and pull request reviewer accordingly. The bot is called “Mergebot” and can notify the pull request creator when a remark is added to their pull request or its standing modifications. It’ll additionally notify the pull request reviewer when a pull request is assigned to them. Mergebot has helped shorten the pull request evaluate course of and maintain builders in movement. That is yet one more instance of how saving simply 5 minutes of developer time saved ~$240,000 for a 100-developer crew in a 12 months.

Not too long ago github rolled out an identical function known as “github scheduled reminder” which, as soon as opted into, notifies a developer of any PR replace by means of Slack notification. Whereas it covers the essential reminder half, Mergebot remains to be our developer’s most well-liked bot because it doesn’t require specific opt-in and in addition permits pull requests to be merged by means of a click on of the button by means of Slack.


We wish Slack to be the most effective place on the earth to make software program, and a method that we’re doing that’s by investing within the cellular developer expertise. Our crew’s mission is to maintain builders within the movement and make their working lives simpler, extra nice, and extra productive.  Listed here are some direct quotes from our cellular builders:


Dev XP is nice. Thanks for all the time taking suggestions from the cellular improvement groups! I do know you care 💪

We’re utilizing fashionable practices. Bazel is nice. I really feel extremely supported by DevXP and their arduous work.

The instruments work properly. The code is modularized properly. Devxp is responsive and useful and continues to iterate and enhance.

Are a lot of these developer expertise challenges attention-grabbing to you? If that’s the case, join us!