David Liman dvliman

A curated list of AWS resources to prepare for the AWS Certifications

A curated list of awesome AWS resources you need to prepare for the all 5 AWS Certifications. This gist will include: open source repos, blogs & blogposts, ebooks, PDF, whitepapers, video courses, free lecture, slides, sample test and many other resources.

Index:

A composable pattern for pure state machines with effects

State machines are everywhere in interactive systems, but they're rarely defined clearly and explicitly. Given some big blob of code including implicit state machines, which transitions are possible and under what conditions? What effects take place on what transitions?

There are existing design patterns for state machines, but all the patterns I've seen complect side effects with the structure of the state machine itself. Instances of these patterns are difficult to test without mocking, and they end up with more dependencies. Worse, the classic patterns compose poorly: hierarchical state machines are typically not straightforward extensions. The functional programming world has solutions, but they don't transpose neatly enough to be broadly usable in mainstream languages.

Here I present a composable pattern for pure state machiness with effects,

Persistent "pipes" in Linux

In a project I'm working on I ran into the requirement of having some sort of persistent FIFO buffer or pipe in Linux, i.e. something file-like that could accept writes from a process and persist it to disk until a second process reads (and acknowledges) it. The persistence should be both across process restarts as well as OS restarts.

AFAICT unfortunately in the Linux world such a primitive does not exist (named pipes/FIFOs do not persist

Journey

I was using Boot2Docker 1.2 (OSX) and wanted to use volume for MongoDB. First nothing was happening because 1.2 has no Guest Additions and volumes don't work. There is a workaround by making a boot2docker.iso from master which has Guest Additions.

But then Mongo didn't like putting data on VirtualBox's shared folders:

[initandlisten] 	WARNING: This file system is not supported. For further information see:
[initandlisten] http://dochub.mongodb.org/core/unsupported-filesystems

	# LVDB - LLOOGG Memory DB
	# Copyriht (C) 2009 Salvatore Sanfilippo <[email protected]>
	# All Rights Reserved

	# TODO
	# - cron with cleanup of timedout clients, automatic dump
	# - the dump should use array startsearch to write it line by line
	# and may just use gets to read element by element and load the whole state.
	# - 'help','stopserver','saveandstopserver','save','load','reset','keys' commands.
	# - ttl with milliseconds resolution 'ttl a 1000'. Check ttl in dump!

	lists:sort(fun({_, X}, {_, Y}) -> X > Y end,
	dict:to_list(lists:foldl(
	fun(Pid, Dict) ->
	InitialCall = case erlang:process_info(Pid, initial_call) of
	{initial_call,{proc_lib,init_p,A}} ->
	case erlang:process_info(Pid, dictionary) of
	{dictionary, D} -> proplists:get_value('$initial_call', D, undefined);
	_ -> {proc_lib,init_p,A}
	end;
	{initial_call,{erlang,apply,A}} ->

	lists:sort(fun({_, X}, {_, Y}) -> X > Y end,
	dict:to_list(lists:foldl(
	fun(Pid, Dict) ->
	InitialCall = case erlang:process_info(Pid, initial_call) of
	{initial_call,{proc_lib,init_p,A}} ->
	case erlang:process_info(Pid, dictionary) of
	{dictionary, D} -> proplists:get_value('$initial_call', D, undefined);
	_ -> {proc_lib,init_p,A}
	end;
	{initial_call,{erlang,apply,A}} ->

	#cribbed from http://vimeo.com/52569901 (Twilio carrier call origination moderation)
	# The idea is that many fan-in queues can enqueue at any rate, but
	# dequeue needs to happen in a rate-controlled manner without allowing
	# any individual input queue to starve other queues.
	# http://en.wikipedia.org/wiki/Leaky_bucket (second sense, "This version is referred to here as the leaky bucket as a queue.")
	#
	# requires:
	# redis 2.6+
	# redis-py>=2.7.0
	# anyjson

	% Post/Query string parameter validation.
	%
	% Mark Allen, [email protected]
	%
	% Specs [{{name, "param_name"}, {required, true}, {type, function}}, ... ]
	% Input [{"name", "value}, ...] just you'd get from post or query string parameters

	-module(t).
	-export([test/0, test1/0, test2/0]).

	package main

	import (
	"encoding/json"
	"log"
	"net/http"
	"reflect"
	"time"

	"github.com/gorilla/context"