Anthony Steele


The Bug That Took A Year To Fix

Here is another war story.

One of the issues that was raised to the team that I was on, was an order that could not be processed. Just one order at a time, once in a while, for no clear reason. Everything looked good, and yet it failed.

The cause was eventually tracked down, and the fix applied, but that took the best part of a year of intermittent effort.

The Bug

The team that I was on, if I remember it right, had a service called OrderPartnerWorker, that received messages over SNS and SQS on AWS. The affected message was called something like OrderAccepted, and it contained an order id for the new order. We processed it, eventually forwarding it to the correct third party.

Sometimes, around once per week, this failed. The team sending the message insisted that all was well at their end. We didn’t have much to go on, so simply had to add logging around the OrderPartnerWorker code and wait for it to recur. Logging was improving across the whole system at this time, from an initial low standard. And when it recurred, if it was noticed and someone had time, it would get looked at again.

This phase lasted for months.

The Friction

Losing orders is about the most serious failure that could happen in this business: the company doesn’t get to make money on the transaction, and the customer is annoyed. If there is an outage during which no orders are processed at all, that is was taken very seriously; but this wasn’t an outage, just a recurring blip on individual orders.

But were was the issue? The two systems disagreed. There were two teams involved, so there is a political, human dimension. The message exchanged literally formed the point of contact between their systems. JIRA tickets formed the point of contact between the people.

I’m sorry to say that in this point the teams, who were seldom in the same room, had different priorities, and did not see eye-to-eye. It reached an impasse. The downstream team knew that there was a real issue. The upstream team claimed that “if there is an issue, it’s not ours” and had more urgent things to do than investigate someone elses “one order a week” issues. The downstream team wanted correctness, not ongoing late-night call-outs; and we were worried that if it happened twice this week and once the week before; there was nothing to stop it happening hundreds of times next week, in which case the CTO would be breathing down our neck.

Loud conversations were had in meetings, embarrassing bystanders. Fingers were pointed. There are still memories.

This phase lasted for more months.

The Find

We eliminated many possible causes: it wasn’t some resource exhaustion or concurrency thing, correlated with high load - it was only around twice as likely to occur when there were twice as many orders. SQS retries were working normally.

After a while the logging was good enough to see what was failing, and the investigation turned to timing issues. This was finally the right track.

What was happening in this case is that the OrderAccepted message was received, the OrderPartnerWorker immediately went to the order API and tried to get details of the accepted order over HTTP. The order API seemed to reply “What accepted order? 404 It doesn’t exist!”. More confounding, the order API was not erroring or running slow according to logs, and it has the “missing” order details.

But the order API was not the OrderAccepted message sender. The order API also listened to other (further upstream) messages about orders and put data in in its store, to respond to later GET requests.

It was more a design issue than anything else. Both the upstream and downstream systems were actually working as designed all the time, the fault lay in the pipes between them, and in a third system. There was a design fix to the design issue, which took a while to roll through the whole system.

The Reasons

SNS and SQS are extremely reliable and scalable systems, I would happily use them regardless of if I am handling 1 message per week, or if I am handling millions of messages per day. The consumers would look different: a lambda in the first case vs. multiple instances of a full app in the second, but that’s a another story.

Almost all messages are delivered over SNS once, very quickly; but there are edge cases, outliers and failures, and at scale they happen. Given e.g. a million messages per day, a “one in a million” event will happen daily. SNS and SQS tend to fail in the direction of delivering a message late, or delivering a message multiple times rather than dropping the message. It has “at least once” delivery.

Once in a long while, on the outer edges of possibility, there was tens of seconds of delay in the message flow to order API, so it created the order data much later than expected. In this case OrderPartnerWorker queried order API before it knew about that order yet.

This didn’t correlate with load on our system, but it might correlate with load on AWS’s SQS infrastructure across all users of the AWS platform in that region.

Put it this way: Source sends the message, OrderApi receives it, stores it and answers api requests. So Source -> OrderApi is the fast path, and the slow path is Source -> A -> B -> OrderPartnerWorker -> OrderApi. Every once in a long while, the “slow” path was actually faster.

The Learnings

It was an “eventual consistency” problem, a race condition. In a distributed system, if your service is told that “order 123456 has just been accepted”, you can’t always assume that other services have order 123456 in that same “accepted” state right then. They might not have got there yet, or they might have moved on to another state. And the sequence in which it’s designed to happen is not always the sequence in which it does happen.

The design fix was that the OrderAccepted message was revised to contain not just an order id, but to be a “fat message” containing a JSON representation of the order, so no further data needed to be queried. The call to the order API was eliminated, and the problem went away.

The consequences were that, in general, calling back for details about the message just received started to be rightly regarded as bad idea.

Even when things go well it’s not the best design: As part of the push for reliability, some services were designated “mission critical”, i.e. we wanted them to be running always, more so than other services. In the original design, OrderPartnerWorker was mission critical, therefor so was the order API because the order details query coupled them: OrderPartnerWorker could not function without Order Api and it’s uptime could never be higher. The “fat message” decoupled them; it eliminated that dependency and improved reliability.

And when things don’t go well, as in this bug, then the OrderPartnerWorker should be able to process the message not by relying on the current state of other services, but from data that is either in the message itself, or that it already has stored when listening to other messages.

A while later we learned that the common technical term for this pattern is (unsurprisingly) not “fat message”, It is Event-Carried State Transfer.

The other thing is that distributed systems are hard, the edge cases will surprise you. We knew some things about SQS’s edge cases, and so we checked that our message handlers were idempotent (they were) and retried (they were), But still the issue was not anticipated. You can’t plan for everything that happens at scale so good logging and monitoring are important. As is learning from others best practices.

Another thing is the hard parts can be at the seams, which can be both technical and political seams. When teams disagree on how severe the issue is; when one team feels the consequences (in the form of on-call issues), but another team owns the fix, this is a recipe for friction.