Translating an enterprise Spring webapp to Clojure
March 21, 2019
How to translate the concepts and patterns we know from enterprise Java applications to Clojure? It is not just a different syntax, the whole philosophy of the language is different. The thing is, many concepts and patterns do not translate - you just do things differently. We will look shortly at how we can solve common enterprise concerns in Clojure, compared to Java.
This post is intended for an experienced Java developer curious about how his object-oriented, enterprise know-how would translate into the world of functional programming.
Clojure vs Java: Troubleshooting an application in production
March 7, 2019
I have just gone through the painful experience of troubleshooting a remote Java webapp in a production-like environment and longed for Clojure’s explore-and-edit-running-app REPL. I want to demonstrate and contrast the tools the two languages offer for this case.
Clojure vs Java: The benefit of Few Data Structures, Many Functions over Many Unique Classes
March 6, 2019
In Clojure we use again and again the same data structures and have many functions operating on them. Java programmers, on the other hand, create a unique class for every grouping of data, with its own “API” (getters, setters, return types, …) for accessing and manipulating the data. Having been forced to translate between two such “class APIs” I want to share my experience and thus demonstrate in practical terms the truth in the maxim
Solution design in Java/OOP vs. Clojure/FP - I/O anywhere or at the boundaries? - experience
March 1, 2019
As a Clojure developer thrown into an “enterprise” Java/Spring/Groovy application, I have a unique opportunity to experience and think about the differences between functional (FP) and object-oriented programming (OOP) and approach to design. Today I want to compare how the solution would differ for a small subsystem responsible for checking for and progressing the process of fixing data discrepancies. The main question we will explore will be where do we deal with external effects, i.e. I/O.
(We are going to explore here an application of the Functional Core, Imperative Shell architecture. You can learn more about it in the Related resources section.)
It will only take one hour… (On why programmers suck at estimating and the perils of software development)
February 13, 2019
“It will only take about an hour,” I said to her. Two days later, a pull request awaits review. Where has all that time gone? What are the sources of delay in software development and how can we make it faster?
We have a Java/Groovy Spring Boot webapp, mainly running a bunch of batch jobs fetching, transforming and combining data. It is challenging to troubleshoot production issues because some production APIs are only accessible from the production servers and it is difficult and possibly dangerous to run the application in full production setup locally. Fortunately, we can now connect a REPL to the running application, get hold of its Spring beans, and interact with it (invoking remote calls, checking the returned data, …), which is a real life-saver and something I want to demonstrate and describe here.
How to use Clojure 1.10 pREPL to connect to a remote server (WIP)
January 31, 2019
Clojure 1.10 includes a new, program-friendly REPL or prepl (pronounced as “preppy,” not p-repl). However there is still very little documentation about it, though it is reportedly in making (it is alpha, after all). Here I want to demonstrate how to start it and how to connect to it in a primitive way (I hope to improve the user experience of the client eventually).
AWS RDS: How to find out login credentials to the database
December 21, 2018
To log in to your AWS RDS database (Oracle in my case) you need login credentials, but what are these for a newly created DB? The password is the master user password you entered during DB creation and which you can change via the Console.
Migrating from Wordpress.com to a static site generated by GatsbyJS
December 13, 2018
I am moving my blog over from Wordpress.com to a statically generated site using Gatsby. Wordpress has served me well in many years but it isn’t really fit for writing (about) code and the latest updates have made it even more difficult for me. With Gatsby I get a quick site and full control over everything (using JavaScript, React, and any of the tons of plugins for Gatsby).
The content from my old blog is coming soon, it is work in progress :-)
Java: Simulating various connection problems with Toxiproxy
November 26, 2018
Java: Simulating various connection problems with Toxiproxy
Simulate various connection issues with Toxiproxy and Java's HttpURLConnection
to see what kind of errors get produced: connect timed out vs. read timed out vs. connection refused ... .
Results:
System: openjdk 11.0.1 2018-10-16
(.setConnectTimeout 1) => java.net.SocketTimeoutException: connect timed out
(.setReadTimeout 1) => javax.net.ssl.SSLProtocolException: Read timed out on HTTPS, java.net.SocketTimeoutException: Read timed out on HTTP
(or Toxiproxy with 5s latency or timeout )
Nothing listening at the port => java.net.ConnectException: Connection refused
Toxiproxy with no upstream configured (i.e. the port is open, but nothing happesn with the connection)
=> javax.net.ssl.SSLHandshakeException: Remote host terminated the handshake on HTTPS, java.net.SocketTimeoutException: Read timed out on HTTP
(What I haven't been able to simulate (yet) is "connection interrupted/broken", i.e. java.net.SocketException Connection reset (perhaps you closed it and try to write to it anyway?) and java.net.SocketException Connection reset by peer (perhaps when dropped by a firewall/the server/...?).)
The setup
Prerequisities
To /etc/hosts add:
127.0.0.1 proxied.google.com
The toxiproxy setup
Start toxiproxy:
docker pull shopify/toxiproxy
# BEFORE we `run` it: case #3
docker run --rm -p 5555:5555 -p 6666:6666 -p 8474:8474 --name toxiproxy -it shopify/toxiproxy
Configure it (we could just POST to :8474 but using the CLI is easier):
$ docker exec -it toxiproxy /bin/sh
/ # cd /go/bin/
# ./toxiproxy-cli create google -l 0.0.0.0:6666 -u www.google.com:443 # BEFORE this is run: case #4
# ./toxiproxy-cli toxic add google -t latency -a latency=5000 # case #2
Added downstream latency toxic 'latency_downstream' on proxy 'google
# ./toxiproxy-cli toxic remove google -n latency_downstream
Removed toxic 'latency_downstream' on proxy 'google'
# ./toxiproxy-cli toxic add google -t timeout -a timeout=2000 # case #2
Added downstream timeout toxic 'timeout_downstream' on proxy 'google'
# ./toxiproxy-cli toxic remove google -n timeout_downstream
Removed toxic 'timeout_downstream' on proxy 'google'
# ./toxiproxy-cli toxic add google -t limit_data -a bytes=5000 # case #5
Added downstream limit_data toxic 'limit_data_downstream' on proxy 'google'
The test code
(import '[java.net URL HttpURLConnection])
(->
(doto ^HttpURLConnection (.openConnection (URL."https://proxied.google.com:6666/"))
;; BEWARE: JVM *must* be started with `-Dsun.net.http.allowRestrictedHeaders=true` to allow setting the Host:
(.setRequestProperty"Host""www.google.com")
(.setConnectTimeout1000)
(.setReadTimeout1000))
(.getInputStream)
slurp)
How I got fired and learned the importance of communication and play time
November 4, 2018
When I came to the office one late autumn morning in 2005, I have been shocked to find out that - without any warning signs whatsoever - I hd been fired. That day I have learned the importance of communication. Their criticism was justified but the thing is, nobody bothered to tell me anything during my 11 months in the company. I received exactly 0 feedback about my behaviour or work. The company ended up at court with its client - which both explains why they were stressed and was also caused by bad communication. So communication - even, or especially under stress - is really important. It must be open, transparent, and broad.
The funny thing is that I still do the things they fired me for.
Clojure - comparison of gnuplot, Incanter, oz/vega-lite for plotting usage data
November 4, 2018
What is the best way to plot memory and CPU usage data (mainly) in Clojure? I will compare gnuplot, Incanter with JFreeChart, and vega-lite (via Oz). (Spoiler: I like Oz/vega-lite most but still use Incanter to prepare the data.)
You know how it goes - suddenly people complain your app does not work, your are getting plenty of timeouts or other errors in your error tracking tool, you find the backend app that is misbehaving and finally "fix" the problem by restarting it. Phew!
Monitoring process memory/CPU usage with top and plotting it with gnuplot
October 17, 2018
If you want to monitor the memory and CPU usage of a particular Linux process for a few minutes, perhaps during a performance test, you can capture the data with top and plot them with gnuplot. Here is how:
