March 25, 2023

At Slack, the objective of the Cell Developer Expertise Crew (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 check failure price.

We’ve got gotten loads of worth out of our deal with cell developer expertise, and we predict most corporations under-invest on this space. On this publish we are going to talk about why having a DevXp staff improves effectivity and happiness, the price of not having a staff, and the way the staff recognized and resolved some widespread developer ache factors to optimize the developer expertise.

How it began

Just a few cell engineers realized early on that engineers who had been employed to put in writing native cell 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 cell engineers higher, they may not solely assist engineers be extra productive, but in addition delight our prospects with sooner, higher-quality releases. They acquired collectively and shaped an ad-hoc staff to handle the most typical developer ache factors. The cell developer expertise staff has grown from three individuals in 2017 to eight individuals immediately. In our 5 years as a staff, we’ve got targeted on these areas:

  • Native improvement expertise and IDE usability
  • Our rising codebase. Guaranteeing visibility into problematic areas of the codebase that require consideration
  • Steady Integration usability and extensibility
  • Automation check infrastructure and automatic check flakiness
  • Conserving the principle department inexperienced. Ensuring the newest fundamental is at all times buildable and shippable

The price of not investing in a cell developer expertise staff

A cell engineer normally begins a characteristic 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 might merge the pull request in the principle 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 staff grows:

Whereas builders can be taught to resolve a few of these points, the time spent and the associated fee incurred will not be justifiable because the staff grows. Having a devoted staff that may deal with these drawback areas and figuring out methods to make the developer groups extra environment friendly will make sure that builders can keep an intense product focus.


Our staff companions with the cell engineering groups to prioritize which developer ache factors to deal with, utilizing the next method:

  • Take heed to prospects and work alongside them. We are going to accomplice with a cell engineer as they’re engaged on a characteristic and observe their challenges.
  • Survey the builders. We conduct a quarterly survey of our cell engineers the place we monitor normal Web Promoter Rating (NPS) round cell improvement.
  • Summarize developer ache factors. We distill the suggestions into working areas that we are able to break up up as a staff and sort out.
  • Collect metrics. It will be important that we measure earlier than we begin addressing a ache level to make sure that an answer we deploy really fixes the difficulty, and to know the precise affect our resolution had on the issue space. We provide you with metrics to trace that correlate with the issue areas builders have and monitor 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 corporations that additionally develop at this scale, or by developing with a novel resolution 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 take a look at the metrics to make sure that it strikes the needle in the best course; solely then can 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 cell developer expertise staff 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 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 job 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 job they had been engaged on. This context switching takes a toll on developer productiveness. The next are two quotes from our quarterly cell engineering survey in 2018.


Quicker CI time! I feel that is requested rather a lot, however it might be wonderful to have this improved

Jenkins construct instances are fairly excessive and it might be nice if we are able to 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 Check 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-about was introducing parallelization to the check suites. As an alternative of operating all the checks serially, we might break 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 software to realize this, however then found a software referred to as Bluepill that was open sourced by Linkedin. It makes use of Xcodebuild beneath the hood, however added the flexibility to shard and execute checks in parallel. Integrating Bluepill decreased our whole check execution time to about 20 minutes.

Utilizing Bluepill labored for a number of extra years till our unit check job began to as soon as once more take virtually 50 minutes. Slack iOS engineers had been including extra check suites to run, and we might 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 selected 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 somewhat than constructing every new output domestically.

Along with the Bazel disk cache, we use the bazel-diff software that permits us to find out the precise affected set of impacted targets between two Git revisions. The 2 revisions we evaluate are the tip of the principle department, and the final commit on the builders department. As soon as we’ve got the listing of targets that had been impacted, we solely check these targets.

With the Bazel construct system and bazel-diff, we had been capable of 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 sooner, 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 Check Lab (FTL) didn’t have check sharding.  We constructed an in-house check sharder on high of FTL referred to as Gas 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 beneath 20 minutes. Right here is the excessive degree overview:

We continued utilizing Gas for 2 and a half years, after which moved to an open supply check sharder referred to as Flank. We proceed to make use of Flank immediately to run Android useful and end-to-end UI checks.

Check-related failures 

When a examine 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 affect different builders as effectively. Let’s check out a number of culprits inflicting non-related pull request failures and the way we’ve got 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 staff to put in writing and handle checks. We noticed that the extra checks that had been added, the extra fragile the check suites turned. 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 staff 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 inside workings of the Android OS and will interface with it simply. This additionally meant that Android builders might simply write and modify checks as a result of they had been written within the language they had been most snug with. Just a few advantages to spotlight with migrations:

  • This helped to shift testing duty from our devoted automation staff 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 direction of testing was poor and prompted loads 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 manner monitoring them and ping the proprietor to repair earlier than it causes an excessive amount of friction

Flaky checks gradual me all the way 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 a substitute of blocking a merge because it occurs now)

At one level, 57% of the check failures in our fundamental department had been on account of flaky checks and the proportion was even greater on developer pull requests. We spent a while studying about flaky checks and managed to get them beneath management in recent times 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 method 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 cell CI infrastructure, utilizing Groovy-based .jenkinsfiles. Whereas it labored, it was additionally the supply of loads of frustration for builders. These issues had been essentially the most impactful:

  • Frequent downtime
  • Decreased efficiency of the system
  • Failure to choose up Git webhooks, and due to this fact not beginning pull request CI jobs
  • Failure to replace the pull request when a job fails
  • Problem in debugging failures on account of poor UX

After flaky checks, CI downtime was the largest bottleneck negatively impacting the cell staff’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 mistaken, there are generally no hyperlinks in GH to go to the best 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 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 with no need our staff’s assist. This has not solely improved developer expertise but in addition decreased the triage load for our staff.

The instant affect of the migration was an 8% improve 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 information to the Xcode challenge for iOS 

Because the variety of iOS engineers at Slack grew previous 20, one space of fixed frustration was the checked in Xcode challenge file. The Xcode challenge file is an XML file that defines all the Xcode challenge’s targets, construct configurations, preprocessor macros, schemes, and way more. As a small staff, it’s simple to make adjustments to this file and commit them to the principle 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 challenge file, resolving conflicts on that factor is painful and error susceptible. I’m unsure what one of the best method is to assuaging this doable ache level, particularly if they’ve added new code information.”

“I had a dozen or so conflicts within the challenge file that I needed to manually resolve. Not an enormous problem 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 software referred to as Xcodegen. Xcodegen allowed us to delete the checked in .xcodeproj file and create an Xcode challenge dynamically utilizing a YAML file that contained definitions of all of our Xcode targets. We linked this software to a command line interface in order that iOS engineers might create an Xcode challenge from the command line. One other profit was that all the challenge 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 fundamental have the potential to interrupt fundamental

To this point we’ve got 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 are attempting to land their pull requests to the principle department concurrently? With a big staff, 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 principle department to fail on account of concurrent merges and began to negatively have an effect on developer productiveness. Till the merge battle is resolved, the principle department would stay damaged and pause all productiveness. At one level merge conflicts had been breaking the principle department a number of instances a day. Extra builders began requesting a merge queue.


We hold breaking the principle department. We want a merge queue.

We brainstormed completely different options and in the end landed on utilizing a 3rd celebration resolution referred to as Aviator, and mixed it with our in-house software Mergebot. We felt that constructing and sustaining a merge queue could 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 instantly merging to the principle department, and as soon as within the queue, Aviator will merge fundamental into the developer branches and run all the required checks. If a pull request was discovered to interrupt fundamental, then the merge queue rejects it and the developer is notified through Slack. This technique helps keep away from any merge conflicts.


Manner 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 intently with the Aviator staff to determine and handle developer pains similar to elevated time to merge a pull request in the principle department and failure reporting on a pull request when it’s dropped out of queue on account of a battle.

Checking pull request progress/standing

This can be a request we obtained in 2017 in considered one of our developer surveys:


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

Later within the yr we created a service which displays 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 adjustments. It should additionally notify the pull request reviewer when a pull request is assigned to them. Mergebot has helped shorten the pull request assessment course of and hold 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 staff in a yr.

Not too long ago github rolled out the same characteristic referred to as “github scheduled reminder” which, as soon as opted into, notifies a developer of any PR replace by Slack notification. Whereas it covers the fundamental reminder half, Mergebot continues to be our developer’s most popular bot because it doesn’t require express opt-in and likewise permits pull requests to be merged by a click on of the button by Slack.


We would like Slack to be one of the best place on this planet to make software program, and a method that we’re doing that’s by investing within the cell developer expertise. Our staff’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 cell builders:


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

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

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

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