Retries Can Kill You

In a large-scale distributed system, it’s inevitable that some requests will fail. Even if your collaborating systems work perfectly, sooner or later you will experience temporary network issues and other intermittent errors. That’s why a lot of people try to paint over this issue by implementing retries in their applications. Unfortunately, if it’s not done properly, this can cause serious stability problems.

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Prometheus and Spring Boot Health Checks

When trying to set up alerting for Spring Boot services with Prometheus, I discovered the synthetic “up” time series which is great for checking whether the monitoring system can reach my service instances. While this is a great thing, I also wanted to alert on the health status of my instances, as reported by /actuator/health. Unfortunately, there is nothing in Spring Boot’s /actuator/prometheus endpoint that I could use.

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DNS Over HTTPS

With much of the web switching to HTTPS and at least some people becoming more concerned about privacy, DNS has recently come into the spotlight because it provides an ISP with the ability to monitor which websites a user visits. Multiple mitigations have been proposed – Android for example is going to support DNS Over TLS, which tunnels good old DNS payload over TLS. And now there is a working group at the IETF developing DNS Over HTTPS (DoH), which layers DNS on top of HTTP/2.

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A Quick Introduction to sed(1)

The sed(1) stream editor is one of the most powerful tools from the classic Unix tool box. It is a close cousin to the ed(1) command line editor and a descendant of the ex(1) editor, the command line mode of vi(1). In this article I’ll show a few idioms that I frequently use in practice.

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Spring Boot: Logging Failed Logins

In many applications it’s important to react to failed logins and other security-critical events, for example to log the failed login attempt to a file or to display a captcha after repeated failures. Spring-based applications come with a simple mechanism to access this kind of information using Application Events.

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The Curse of Convenience Methods

In the old days, many Java APIs were fairly low level and pretty generic. You often had to explicitly select a concrete implementation, provide lots of parameters, and generally needed to know how things worked. This has changed in recent years – modern APIs provide lots of convenience functionality that raises the level of abstraction and increases productivity. I like this as much as anybody else, but unfortunately it can also lead to subtle bugs.

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Empty InputStream with Spring MVC

The other day, I was trying to build a simple batch upload interface for a Java web application running Spring Boot with Spring MVC on Apache Tomcat. But when I tried reading the InputStream in my controller, it was always empty. Fortunately, this turned out to be quite easy to fix.

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