February 9, 2023


Technology Room

Cell Developer Expertise at Slack

15 min read

At Slack, the aim of the Cell Developer Expertise Workforce (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, similar to developer sentiment, CI stability, time to merge (TTM), and take a look at failure fee.

We have now gotten quite a lot of worth out of our give attention to cellular developer expertise, and we predict most firms under-invest on this space. On this put up we are going to talk about why having a DevXp workforce improves effectivity and happiness, the price of not having a workforce, and the way the workforce recognized and resolved some frequent developer ache factors to optimize the developer expertise.

How it began

A number of cellular engineers realized early on that engineers who had 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 might make the developer expertise for all cellular engineers higher, they might not solely assist engineers be extra productive, but in addition delight our clients with quicker, higher-quality releases. They received collectively and fashioned an ad-hoc workforce to deal with the commonest developer ache factors. The cellular developer expertise workforce has grown from three individuals in 2017 to eight individuals at this time. In our 5 years as a workforce, we now have centered on these areas:

  • Native growth 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 take a look at infrastructure and automatic take a look at flakiness
  • Holding the primary department inexperienced. Ensuring the most recent fundamental is at all times buildable and shippable

The price of not investing in a cellular developer expertise workforce

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 go 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 associated fee to the corporate for not addressing these ache factors because the workforce grows:

Whereas builders can be taught to resolve a few of these points, the time spent and the associated fee incurred is just not justifiable because the workforce grows. Having a devoted workforce that may give attention to these drawback areas and figuring out methods to make the developer groups extra environment friendly will be certain that builders can preserve an intense product focus.


Our workforce companions with the cellular engineering groups to prioritize which developer ache factors to give attention to, utilizing the next strategy:

  • Hearken to clients 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 normal Web Promoter Rating (NPS) round cellular growth.
  • Summarize developer ache factors. We distill the suggestions into working areas that we will cut up up as a workforce and sort out.
  • Collect metrics. It is vital that we measure earlier than we begin addressing a ache level to make sure that an answer we deploy really fixes the problem, and to know the precise impression our resolution had on the issue space. We provide you with metrics to trace that correlate with the issue areas builders have and observe them on dashboards. This enables 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 developing with a singular resolution ourselves.
  • Think about using third-party instruments. We consider whether or not it makes extra sense to make use of present options or to construct out our personal options.
  • Repeat this course of. As soon as we launch an answer, we take a look at the metrics to make sure that it strikes the needle in the precise course; solely then will we transfer onto the following drawback space.

Developer pains

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

CI take a look at 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 swap to engaged on a special process and lose context on the unique pull request. When the take a look at outcomes return, if there is a matter they should deal with, they need to re-orient themselves with the unique process they had 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.


Sooner CI time! I feel that is requested so much, however it might be superb to have this improved

Jenkins construct occasions are fairly excessive and it might be nice if we will cut 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 take a look at job time began to take virtually an hour. One of many options we thought-about was introducing parallelization to the take a look at 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 drawback on the iOS and Android platforms.


We thought-about writing our personal device to realize this, however then found a device known as Bluepill that was open sourced by Linkedin. It makes use of Xcodebuild beneath the hood, however added the power to shard and execute checks in parallel. Integrating Bluepill decreased our whole take a look at execution time to about 20 minutes.

Utilizing Bluepill labored for just a few extra years till our unit take a look at job began to as soon as once more take virtually 50 minutes. Slack iOS engineers had been including extra take a look at 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 occasions

Our subsequent technique was to implement a caching layer for our take a look at suites. The aim was to solely run the checks that wanted to be run on a selected pull request, and return the remaining take a look at outcomes from cache. The issue was that Xcodebuild doesn’t help caching. To implement take a look at 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 fairly than constructing every new output regionally.

Along with the Bazel disk cache, we use the bazel-diff device that permits 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 we now have the record of targets that had been impacted, we solely take a look at these targets.

With the Bazel construct system and bazel-diff, we had been in a position to lower TTM to a median 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 take a look at sharding.  We constructed an in-house take a look at sharder on prime of FTL known as Gasoline to interrupt checks into a number of shards and name FTL APIs to run every take a look at shard in parallel. This introduced TTM from 50+ minutes to beneath 20 minutes. Right here is the excessive stage overview:

We continued utilizing Gasoline for 2 and a half years, after which moved to an open supply take a look at sharder known as Flank. We proceed to make use of Flank at this time to run Android practical and end-to-end UI checks.

Take a look at-related failures 

When a test fails on a pull request due to flaky or unrelated take a look at failures, it has the potential to take the developer out of movement, and probably impression different builders as properly. Let’s check out just a few culprits inflicting non-related pull request failures and the way we now 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” take a look at automation framework and desires a devoted automation workforce to write down and handle checks. We noticed that the extra checks that had been added, the extra fragile the take a look at suites turned. When a take a look at 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 take a look at doesn’t fail their construct. This resulted in an extended suggestions loop and elevated TTM.

Because the workforce 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 also be written within the native language (Java or Kotlin). Espresso is highly effective as a result of it’s conscious of the inside 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 had been written within the language they had been most comfy with. A number of advantages to spotlight with migrations:

  • This helped to shift testing duty from our devoted automation workforce 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 after we moved from Calabash to Espresso and FTL

Flaky checks

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


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

Flaky checks sluggish me all the way down to a halt – there must be a extra streamlined course of in place for continuing with PR’s as soon as flaky checks are discovered (as a substitute of blocking a merge because it occurs now)

At one level, 57{101417768b20d6f92316fd7d46662a6fe9bfb6b86ff5a782d317a3e9c879583b} of the take a look at failures in our fundamental department had been because of flaky checks and the proportion was even larger on developer pull requests. We spent a while studying about flaky checks and managed to get them beneath 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 decreased take a look at failures fee from 57{101417768b20d6f92316fd7d46662a6fe9bfb6b86ff5a782d317a3e9c879583b} to 4{101417768b20d6f92316fd7d46662a6fe9bfb6b86ff5a782d317a3e9c879583b} 

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 quite a lot of frustration for builders. These issues had 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
  • Problem in debugging failures because of poor UX

After flaky checks, CI downtime was the most important bottleneck negatively impacting the cellular workforce’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 fallacious, there are generally no hyperlinks in GH to go to the precise place. Additionally, generally CI passes however does 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 have to 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{101417768b20d6f92316fd7d46662a6fe9bfb6b86ff5a782d317a3e9c879583b} uptime thus far. Webhook-related points have utterly disappeared, and the UX is easy sufficient for builders to navigate with no need our workforce’s assist. This has not solely improved developer expertise but in addition decreased the triage load for our workforce.

The fast impression of the migration was an 8{101417768b20d6f92316fd7d46662a6fe9bfb6b86ff5a782d317a3e9c879583b} improve in CI stability from ~87{101417768b20d6f92316fd7d46662a6fe9bfb6b86ff5a782d317a3e9c879583b} to 95{101417768b20d6f92316fd7d46662a6fe9bfb6b86ff5a782d317a3e9c879583b}  and decreased Time to Merge by 41{101417768b20d6f92316fd7d46662a6fe9bfb6b86ff5a782d317a3e9c879583b} from ~34 minutes to ~20 minutes

Merge conflicts

Battle whereas including new modules or information 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 far more. As a small workforce, 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 feel the priority is extra so the xcode venture file, resolving conflicts on that factor is painful and error susceptible. I’m unsure what one of the best strategy is to assuaging this doable ache level, particularly if they’ve added new code information.”

“I had a dozen or so conflicts within the venture file that I needed to manually resolve. Not an enormous challenge 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 stage 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 fundamental have the potential to interrupt fundamental

Thus far we now have talked about completely different points that builders can expertise when writing code regionally and opening a pull request. However what occurs when a number of builders are attempting to land their pull requests to the primary department concurrently? With a big workforce, a number of merges to fundamental 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 growing variety of merge conflicts began inflicting the primary department to fail because 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 had been breaking the primary department a number of occasions a day. Extra builders began requesting a merge queue.


We maintain breaking the primary department. We want a merge queue.

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


Manner higher now with Aviator. Solely ache level is I can not merge my pull requests and need to 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 intently with the Aviator workforce to establish and deal with developer pains similar to 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 because of a battle.

Checking pull request progress/standing

It is a request we obtained in 2017 in certainly one of our developer surveys:


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

Later within the yr we created a service which screens Git occasions and sends Slack notifications to the pull request writer and pull request reviewer accordingly. The bot is known as “Mergebot” and can notify the pull request writer when a remark is added to their pull request or its standing modifications. It would additionally notify the pull request reviewer when a pull request is assigned to them. Mergebot has helped shorten the pull request evaluation 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 workforce in a yr.

Lately 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 fundamental reminder half, Mergebot continues to be our developer’s most well-liked bot because it doesn’t require express opt-in and likewise permits pull requests to be merged by means of a click on of the button by means of Slack.


We would like Slack to be one of the best place on the planet to make software program, and a method that we’re doing that’s by investing within the cellular developer expertise. Our workforce’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 at all times taking suggestions from the cellular growth groups! I do know you care 💪

We’re utilizing fashionable practices. Bazel is nice. I really feel extremely supported by DevXP and their onerous 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 fascinating to you? If that’s the case, join us!

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