September 21, 2023


Technology Room

Service Supply Index: A Driver for Reliability

9 min read

Buyer-first: Shifting from Hero Engineering to Reliability Engineering

From the start, Slack has all the time had a powerful give attention to the client expertise, and customer love is one among our core values. Slack has grown from a small workforce to hundreds of workers over time and this buyer love has all the time included a give attention to service reliability.

In a small startup, it’s manageable to have a reactive reliability focus. For instance, one engineer can troubleshoot and resolve a systemic challenge — we all know them as Hero Engineers. You may additionally realize it as an operations workforce, or a small workforce of Website Reliability Engineers which might be all the time on-call. As the corporate grows, these tried and practiced measures fail to scale, and also you’re left with pockets of tribal data riddled with burnout because the system turns into too advanced to be managed by just a few people.

With any quickly rising advanced product, it’s arduous to maneuver away from a reactionary give attention to user-impacting points.  Reliability practitioners at Slack have developed efficient methods to reply, mitigate, and study from these points by means of Incident Management and Response processes and fostering Service Possession — these contribute to a tradition of reliability first as an entire. One of many key elements of each the Incident Administration program and the Service Possession program is the Service Supply Index.

When you’re driving a reliability tradition in a service-oriented firm, you should have a measurement of your service reliability earlier than all else, and this metric is quintessential in driving decision-making processes and setting buyer expectations. It permits groups to talk the identical language of reliability when you could have one frequent understanding.

Introducing the Service Supply Index

A dashboard displaying the SDI-R of 5.15 today, 90,273 500 errors, 18,224 502 errors, 32,323 503 errors, and 1,955 504 errors since 00:00 UTC, and a chart with the SDI-R and a 30-day weighted average

The Service Supply Index – Reliability (SDI-R for brief) is a composite metric of the success of jobs-to-be-done by Slack’s customers and Slack’s uptime as reported on our Slack System Status website. It’s a composite measure  of profitable API calls and content material supply (as measured on the edge), together with vital person workflows (e.g. sending a message, loading a channel, utilizing a huddle).

It is a company-wide metric with visibility as much as the chief stage, and in observe is applied fairly just by:

API Availability
availability api = profitable requests / complete requests

Total Availability
availability general = uptime standing website * availability api

Chances are you’ll be asking why uptime and availability are completely different; uptime is set by monitoring key workflows which might be important to Slack’s usability and if the provision of any of these important person interactions drops under a predetermined threshold, we rely the minutes that the service is under that threshold to find out downtime.

Since small adjustments in availability (~0.0001) can have a drastic impression on the client expertise, we convert availability to a 9s illustration, the place 99% availability is 2 9s, 99.9% availability is 3 9s, and 99.99% availability is 4 9s, and so forth.

We monitor day by day and hourly aggregates of availability, monitoring it over time in order that we will spot developments and establish regressions and enhancements.

We preserve company-wide targets on this metric when it comes to the variety of days in 1 / 4 that we meet availability targets.

The Reliability Engineering workforce is basically answerable for responding to and triaging regressions in availability that trigger or can probably trigger us to overlook these targets, however like all vital effort we’re removed from alone in assembly our targets:

  • Engineering Management: Resolve prioritization and unblock wanted options to regressions systemically and tactically
  • Service Homeowners: Debug, perceive, and mitigate the foundation reason behind regressions, enhancing the providers they personal over time
  • Reliability Engineering: Support service homeowners, develop tooling, and establish threats that have to be resolved to take care of availability

All events mix SDI-R regressions with incident and buyer impression knowledge to align on an important points and drive them to conclusion.

We’ve discovered that by treating SDI-R as a “canary within the coal mine” as a substitute of ready for points to turn out to be incidents, we’ve been capable of resolve reliability threats extra proactively. Points are:

  • Simpler to know and debug, for the reason that variety of issues breaking without delay is diminished
  • Recognized earlier, giving extra time to scope and implement any right options
  • Typically solved earlier than prospects even discover, stopping outages solely

Rising the Service Supply Index from an thought to a program: Adoption

The SDI got here to fruition from an idea by our Chief Architect Keith Adams by which he tried to quantify the standard of a service with 4 measurements: Safety, Efficiency, High quality, and Reliability.

  • Safety: How rapidly are we addressing safety vulnerabilities? Observe ticket shut price.
  • Efficiency: Is our service delivering responses to prospects well timed? Observe API latency or shopper efficiency.
  • High quality: How rapidly are we addressing open software program defects? Observe ticket shut price.
  • Reliability: Is our service reliably delivering requests to prospects? Observe error charges.

Over time, every of these 4 areas have developed into their very own separate packages and are tracked as key metrics firm broad. We’ll speak in regards to the Reliability program right here and the way we have been capable of set up a typical language that groups perceive and use to prioritize their work.

Slack—as a customer-first group—established a excessive bar of high quality and maintains a 99.99% availability SLA in buyer agreements. This requires a program that ensures the metric is being tracked and that there’s accountability.

The primary facet of this system is visibility — we should perceive and see the sign of how effectively we’re assembly the SLA.

As soon as now we have visibility, we deliver accountability. We publish this metric to a management group or firm broad group of stakeholders, and set up an goal of Reliability in planning. As soon as the target is printed, and the important thing result’s monitored, we will then set up a hyperlink between the SDI and groups. The SDI permits us to hyperlink regressions to providers, which will be mapped to a workforce. As soon as the connection is made, we will then prioritize fixes or tradeoffs to right the regression earlier than it turns into a SLA breach.

Scaling motion, studying, and prioritization

SDI-R is successfully an error price range that helps us resolve how a lot time the corporate and particular person groups ought to spend on launching new options, and once we should cease function work to give attention to availability. On this approach, it helps us stability prioritization of investments throughout the corporate by means of a typical view of person impression.

Due to our sturdy perception in Service Possession, we’ve invested in instruments and processes that assist scale understanding and determination of SDI-R impacting points.

We purpose to get the Proper Folks, in entrance of the Proper Downside, on the Proper Time

Monitoring, alerting, and observability instruments are vital to scale the engineering response to customer-impacting points. We noticed a number of frequent use instances that have been price automating to make it simpler for service homeowners to take care of service stage goals (SLOs) and reply to regressions. The primary of which, Webapp Possession Software, is answerable for automating the setup of alerts, SLOs, and dashboards for Slack API endpoints utilizing a typical set of metrics and infrastructure. Service homeowners can usually reply to and resolve an alert earlier than it turns into an SDI-R regression, using a typical set of logging, metrics, and tracing to feed again data of availability into the Software program Growth Lifecycle. The second of which is Omni, Slack’s Service Catalog answerable for being a system of document for possession and escalation. Omni consists of SDI-R knowledge alongside owned APIs and infrastructure elements, enabling the escalation of points in dependencies and for us to mechanically route regressions to the suitable workforce. These instruments are very efficient in making certain response and determination of acute points.

We purpose to do the issues that finest serve our prospects

Organizationally, it can be crucial that we set up the proper boards and instruments to know ongoing regressions and for efficient re-prioritization of investments to strike the proper stability between reliability and have work. The primary of those is the Engineering Monday Assembly, a daily discussion board for re-prioritization of investments and understanding by engineering management of ongoing buyer points and SDI-R regressions. Secondly, we report group and workforce stage aggregates of SDI-R that enable breakdown by organizational duty and monitoring of success over time. Each of those assist be sure that our organization-wide aim can scale and that each one groups are aligned in direction of the client expertise. Typically we’ve discovered that groups self-service make the most of these studies to seek out power points that slowly degrade the client expertise, however are in any other case not caught in incidents or alerting.

Not each system is ideal; there have been many classes

As we’ve labored with SDI-R over a few years, it has developed over time to be sure that it will probably deliver most worth to our prospects.

Not all API requests are the identical

One of many issues we realized is that not all API requests are the identical. We’d encounter points for particular customers that might be important for them however not transfer the general metric. This led to the institution of a breakdown of SDI-R for under our largest organizations and a weighting of various APIs by significance to correctly symbolize the client impression regressions in them might have. Typically we’d discover that regressions would have an effect on our largest prospects first as they pushed the boundaries of our merchandise and infrastructure, however that with this breakdown we’d be capable of resolve them proactively in the identical approach as the general SDI-R rating.

Lengthy-term maintainability

The delayed nature of SDI-R reporting generally led to a disconnect between the time that a difficulty occurred and when it impacted SDI-R. Nonetheless, we’ve discovered that as we’ve scaled SDI-R by means of service-specific alerting this has mattered much less, since by the point a difficulty was impacting SDI-R it could have already been captured by an alert.

It has turn out to be more and more beneficial to spend money on sustaining availability headroom by proactively fixing points earlier than our availability targets are prone to being violated. This proactive nature not solely reduces operational toil, however can also be common observe in debugging and different abilities essential to triage and perceive regressions.

SDI-R has been so profitable as an method we’ve adopted it to make sure the provision of latest Slack merchandise and infrastructure as we scale, specifically for our GovSlack atmosphere.

Our method should constantly evolve

Over time with new product launches, buyer wants, and adjustments to our infrastructure it can be crucial that we constantly iterate on our metrics and processes in order that we will maintain determining the easiest way to measure our personal success. No enterprise is static, and we should not be afraid to study from failures and iterate to enhance our reliability over time.


As organizations quickly develop, it’s usually tough to remain proactive whereas additionally prioritizing availability and product work collectively. By specializing in our prospects, we’ve discovered SDI-R helpful in placing this delicate stability. For each product and infrastructure, the client is an important factor and data-driven approaches mixed with the proper processes are important in direction of holding our prospects glad and productive.


We wished to provide a shout out to all of the those that have contributed to this journey: 

Adam Fuchs, Ajay Patel, John Suarez, Bipul Pramanick, Justin Jeon, Nandini Tata, Shivam Shukla and all of these at Slack who’ve put our prospects first.


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