Nikolaos Christidis chriniko13

Quick Tips for Fast Code on the JVM

I was talking to a coworker recently about general techniques that almost always form the core of any effort to write very fast, down-to-the-metal hot path code on the JVM, and they pointed out that there really isn't a particularly good place to go for this information. It occurred to me that, really, I had more or less picked up all of it by word of mouth and experience, and there just aren't any good reference sources on the topic. So… here's my word of mouth.

This is by no means a comprehensive gist. It's also important to understand that the techniques that I outline in here are not 100% absolute either. Performance on the JVM is an incredibly complicated subject, and while there are rules that almost always hold true, the "almost" remains very salient. Also, for many or even most applications, there will be other techniques that I'm not mentioning which will have a greater impact. JMH, Java Flight Recorder, and a good profiler are your very best friend! Mea

Thread Pools

Thread pools on the JVM should usually be divided into the following three categories:

CPU-bound
Blocking IO
Non-blocking IO polling

Each of these categories has a different optimal configuration and usage pattern.

Kafka 0.11.0.0 (Confluent 3.3.0) added support to manipulate offsets for a consumer group via cli kafka-consumer-groups command.

List the topics to which the group is subscribed

kafka-consumer-groups --bootstrap-server <kafkahost:port> --group <group_id> --describe

Note the values under "CURRENT-OFFSET" and "LOG-END-OFFSET". "CURRENT-OFFSET" is the offset where this consumer group is currently at in each of the partitions.

Reset the consumer offset for a topic (preview)

Removing the last commit

To remove the last commit from git, you can simply run git reset --hard HEAD^ If you are removing multiple commits from the top, you can run git reset --hard HEAD~2 to remove the last two commits. You can increase the number to remove even more commits.

If you want to "uncommit" the commits, but keep the changes around for reworking, remove the "--hard": git reset HEAD^ which will evict the commits from the branch and from the index, but leave the working tree around.

If you want to save the commits on a new branch name, then run git branch newbranchname before doing the git reset.

Scala: Type Families, Sealed Traits, and Exhaustive Pattern Matching

By: Matt Barackman

What is a Type Family?

A collection of objects or case classes that share a sealed trait.

In the example below, the type family would be a collection of traffic light colors with Red, Yellow, and Green as member objects.

	Below are the Big O performance of common functions of different Java Collections.


	List \| Add \| Remove \| Get \| Contains \| Next \| Data Structure
	---------------------\|------\|--------\|------\|----------\|------\|---------------
	ArrayList \| O(1) \| O(n) \| O(1) \| O(n) \| O(1) \| Array
	LinkedList \| O(1) \| O(1) \| O(n) \| O(n) \| O(1) \| Linked List
	CopyOnWriteArrayList \| O(n) \| O(n) \| O(1) \| O(n) \| O(1) \| Array

	'use strict';

	var Promise = require('bluebird');
	var knex = require('../../src/db').knex;

	var tables = [
	'organizations',
	'campaigns',
	'donors',
	'pledges',

	package io.vertx.lang.scala.http

	import io.vertx.core.file.OpenOptions
	import io.vertx.scala.core.Vertx
	import io.vertx.scala.core.streams.Pump
	import org.junit.runner.RunWith
	import org.scalatest.concurrent.AsyncAssertions
	import org.scalatest.junit.JUnitRunner
	import org.scalatest.{FlatSpec, Matchers}