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Instrumentation v. log aggregation?

12 October 2016

On the Go kit mailing list, Chris Ford recently asked

I came across the instrumentation part of Go kit and I am wondering if there is any advantage over aggregating log events? e.g. the instrumentation examples are about response time, request count and these kind of metrics. You could simply log every request and count the request events in the log afterwards. Am I correct about this assumption or am I missing something? Because I do not see a use case that can’t be realized with log aggregation. Is guess the performance and realtimeness is better with instrumentation, but besides that?

Great question, and one I see a lot. In theory Chris is correct: if you logged the complete details of every request to a system like ELK, in a way that was queryable, you could construct the same information (99th quantile latencies, error rates, etc.) post-hoc using a sufficiently sophisticated query language. The reason to use (and, spoiler alert, almost always prefer) instrumentation is ultimately about complexity, maintainability, and fitness to purpose.

First of all, you want to consider plain data volume: a human- and machine-readable log line for each request with enough information is going to generate a lot of traffic. I can speak from firsthand experience to say that this can quickly outpace actual production traffic and completely saturate all kinds of resources: your network, your log shippers, the CPUs on the VMs themselves, the machines/cluster responsible for storing your logs and making them queryable, etc. I’m happy to acknowledge that this seems counterintuitive, and like pointless worrying when you’re starting out; trust me that it becomes problematic a lot sooner than you think.

Then there’s complexity in terms of what you actually want to accomplish by tracking this stuff. The whole point is observability: getting some view into the inner workings of the distributed system you’ve built. That means processing latencies, quantiles, error rates, top-K resource consumers, etc. as first-order concepts in some sense, rather than the text output of a map/reduce process over some log data. And, ideally, you want those things on a close-to-realtime view of your infra, with granularity on the order of seconds. Again, all of this is in theory computable from text or structured application logs, but not in a way that is nearly as useful, and not in a way that allows that information to serve well as the foundation for more sophisticated feedback systems, reports, and as a tool for system operators.

Finally, there’s costs within the service itself. This loops back to my first point: spitting out a line of text to stdout or rsyslog or whatever for every trackable thing is a lot more expensive (CPU) than how the good instrumentation systems do things: Prometheus instrumentation, for example, involves just updating a register in memory, and at thousands of QPS that absolutely adds up. There’s also a point about software engineering lurking in here: recording and making available good instrumentation about how your service is doing is part of the contract of that service to its runtime environment. Keeping your 99th quantile latencies below 10ms (for example) is part of your service’s SLA, and recording/emitting data to validate that SLA is as much a necessary thing to test for as anything else in your business logic. All this is to say you want reliable, purpose-built primitives for this kind of thing: it’s very difficult to define a contract for your log lines, or unit test them properly.

For more on the latest point, including many many good reasons for investing in proper instrumentation, see the Site Reliability Engineering book from O’Reilly. And for more on logging v. instrumentation generally, shout out to me & my eponymous blog post Logging v instrumentation.

Welcome to the new Go kit website!

01 July 2016

Go kit started its life as just a talk at a small conference, and spent its first 18 months as a GitHub repo with lots of breaking changes. (Sorry about that.) Many design discussions took place, initially in RFCs, and then in GitHub issues, on the mailing list, and especially in the Slack channel. While the code can capture the rationale and outcome of those discussions, it can’t so easily capture the context: the conversations themselves, the alternatives, the pros and cons of all the decisions, the intended usage, the caveats. And it’s often these things that are the most valuable, especially to newcomers.

So, welcome to the new Go kit website! The primary purpose will be to capture and document the context behind the decisions that shaped Go kit. I’ll record and keep supplementary information to help you understand and effectively apply Go kit in your organization.

Initially, I’ll focus on building up an extensive FAQ based on common questions and conversation in Slack, the mailing list, etc. But no resource has proved nearly as useful as the examples, so I’ve also ported the examples and tutorials from the repo and will be adding to them as time goes on.

Happy programming 🏌