djspiewak

Thread Pools

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

1. CPU-bound
2. Blocking IO
3. Non-blocking IO polling

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

odersky

Principled Meta Programming for Scala

This note outlines a principled way to meta-programming in Scala. It tries to combine the best ideas from LMS and Scala macros in a minimalistic design.

• LMS: Types matter. Inputs, outputs and transformations should all be statically typed.

• Macros: Quotations are ultimately more easy to deal with than implicit-based type-lifting

• LMS: Some of the most interesting and powerful applications of meta-programming

djspiewak

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

gvolpe

Shared State in pure Functional Programming

Newcomers to Functional Programming are often very confused about the proper way to share state without breaking purity and end up having a mix of pure and impure code that defeats the purpose of having pure FP code in the first place.

Reason why I decided to write up a beginner friendly guide :)

Use Case

We have a program that runs three computations at the same time and updates the internal state to keep track of the

gvolpe

Dependency Injection in Functional Programming

There exist several DI frameworks / libraries in the Scala ecosystem. But the more functional code you write the more you'll realize there's no need to use any of them.

A few of the most claimed benefits are the following:

• Dependency Injection.
• Life cycle management.
• Dependency graph rewriting.

Daenyth

// Based on https://github.com/http4s/contrib/blob/master/aws/src/main/scala/org/http4s/contrib/aws/AwsSigner.scala

import java.util.Date

import cats.data.Kleisli
import cats.effect.{Effect, Sync}
import cats.implicits._
import fs2.Stream
import org.http4s.client.Client
import org.http4s.{Header, Request}

Daenyth

package teikametrics

import scala.collection.SortedMap

/**
  * Immutable implementation of an LRU cache.
  *
  * @author Twitter
  *
  *  Copy pasted from the version previously in twitter-util v19.1.0 at

Daenyth

package teikametrics
import java.nio.charset.{Charset, StandardCharsets}

import akka.util.ByteString
import fs2.{Chunk, Pipe, RaiseThrowable, Stream}
import teikametrics.ByteEncoding._

/** Streamed bytes claiming to be encoded under `BE`
  *
  * @param chunkSize A chunk size to use for streaming operations, in order to use a consistent value and minimize array copying

Daenyth

Concurrent resource caching for cats

Motivation

cats-effect Resource is extremely handy for managing the lifecycle of stateful resources, for example database or queue connections. It gives a main interface of:

trait Resource[F[_], A] {
  /** - Acquire resource
    * - Run f
 * - guarantee that if acquire ran, release will run, even if `use` is cancelled or `f` fails

nafg

import java.sql.Types

import scala.annotation.tailrec
import scala.concurrent.Await
import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.duration.Duration
import scala.meta._

import slick.dbio.DBIO
import slick.jdbc.meta.{MColumn, MQName, MTable}