I have just published v1.1.2 of lie.js

The library now uses more fluent names and matches the default used for timeouts in Javascript.

This is now being built and published to npm using CircleCI. The minimal test dependencies are kept upto date using dependabot.

Thanks to @ll782 for suggesting improvements.

The point of the library is to introduce timeouts into javascript Promises. It’s only by making timeouts explicit can you build a reliable application. Currently it allows rejects and success with a default value.

I plan to add typescript declarations to the project.

I have just published my first open source npm package, lie.js.

yarn add lie.js

It provides two useful functions creating a Promise that times out with a given error and another that will succeed with a default after a timeout.

The practical use of these is that you can use them with Promise.race to create functions with a bounded response time. It makes it easier to keep within a time budget if you can say go get a value or return a default if too slow.

A developers toolbox should include:

1. A Statically typed language
2. A Dynamically typed language
3. A scripting language

This can be less that 3 different languages.

You also need to know when to use each of them.

Currently my go-tos are:

1. Groovy

2. Elixir

3. Bash or Powershell

Groovy is a halfway house. You can use @CompileStatic to enforce the parts that you want. It has enough types to allow Intellij to perform essential refactorings.

Elixir is a concurrency-oriented language that is dynamically typed and functional.

Bash is great for glueing together small unix utilities. You can get a long way using sed, awk, grep, jq and curl.

For interactive investigation of systems there is always the powershell REPL.

Hub is a cli for github.

I mostly use it for creating pull-requests or to navigate to the github page for the current repository.

It can be used for more, including calling github api’s

Here is an example of fetching a file from a repo (which would work based upon the credentials attached to hub).

hub api -X GET /repos/chriseyre2000/sample-readme/contents/README.md -H "Accept: application/vnd.github.VERSION.raw"

This fetches the content of the file.

Intend to use this to analyse the package.json of a range of projects.

These will be passed into https://github.com/chriseyre2000/package_compare so that I can load a neo4j database. This will allow me to analyse the package dependencies.

Microservices can result in a lot of packages ….

This is a script that can be used to load a node package.json from github into a local neo4j database:

#/bin/bash
owner=$1
project=$2

hub api -X GET /repos/$owner/$project/contents/package.json -H "Accept: application/vnd.github.VERSION.raw" > $project.json
package_compare $project.json localhost neo4j magicbeans

This book will cover all of the functions in the Elixir library that are provided out of the box.

This is currently at an early stage, but hopefully having all the modules and global functions in one module will be of use.

This is a work through an discussion of Metaprogramming Elixir.

The entry point of metaprogramming in Elixir is the quite macro.

quote do: 1 + 2
{:+, [context: Elixir, import: Kernel], [1, 2]}

This allows the transformation of a code block into the abstract syntax tree (AST). This can then be manipulated like any other data structure and return to the generated code. The practical upshot of this is that half of the Elixir language is actually macros. You can if you need to write your own macros to perform complex operations simply.

There are recommendations to limit the use of macros to things that you can’t solve with normal functions. It’s possible with macros to create a powerful equivalent of inheritance, but must be used sparingly to avoid magical code.

Most of this work takes place at compile time rather than at runtime.

You use require to import macros.

General warning: in languages that have implicit returns adding logging at the end of a function masks the result. Capture the result, log then return result.

unquote is a technique where a macro can read variables from the hosts context. This is how you pass parameters into a macro.

This is how to wrap a callback api in promises:

// Sample function with callback
function clientFactory() {
  return {
    request: (name, callback) => {
      callback(name)
    }
  }
}

let client = clientFactory()

client.request('a', console.log)

// Wrapping a callback into a promise

function returnCallbackResult(input) {
  let promise = new Promise(
    resolver => {
      let client = clientFactory()
      client.request(input, (a) => {
        resolver(a)
      })
  })
  return promise
}

returnCallbackResult('b').then( console.log )

// How to wrap a promise

function chainingPromise(input) {
  let promise = new Promise(
    resolver => {
      input.then( (a) => resolver(`wrapping ${a}`))
    }
  )
  return promise
}

chainingPromise( returnCallbackResult( 'c' ) ).then(console.log)

I have finally finished working through the example in the book.

https://github.com/chriseyre2000/mastery ( 94529b1)

This is a great introduction to OTP covering some details that I have not seen elsewhere. In particular the ability to import an OTP project and not have it auto start will be useful elsewhere.

The last few chapters do seem rushed, and the example code does not quite match what you get when it is typed in. However it is a great example of building a domain model in Elixir and then adding on the external links later. This would count as Hexagonal Architecture.

This is more of a training course that a book. You would have trouble simply reading through without working on the exercise. This does take a significant amount of time.

We are now onto writing the outside in tests.

I have got as far as the first test. This is failing with authentication errors.

[error] Postgrex.Protocol (#PID<0.251.0>) failed to connect: ** (Postgrex.Error) FATAL 28P01 (invalid_password) password authentication failed for user "chris*****"

iex -S mix came to the rescue here:

h Ecto.Adapters.SQL.Sandbox.checkout

Checks a connection out for the given repo.

The process calling checkout/2 will own the connection until it calls checkin/2
or until it crashes when then the connection will be automatically reclaimed by
the pool. 

## Options

  • :sandbox - when true the connection is wrapped in a transaction.
    Defaults to true.
  • :isolation - set the query to the given isolation level.
  • :ownership_timeout - limits how long the connection can be owned.
    Defaults to the value in your repo config in config/config.exs (or
    preferably in config/test.exs), or 60000 ms if not set. The timeout exists
    for sanity checking purposes, to ensure there is no connection leakage, and
    can be bumped whenever necessary.

This was the important clue that I had not got my test database configured correctly.

This is the new version of config/test.exs

use Mix.Config

config :mastery_persistence, MasteryPersistence.Repo, 
                             database: "mastery_test", 
                             hostname: "localhost", 
                             port: 54320, 
                             pool: Ecto.Adapters.SQL.Sandbox,
                             username: "postgres",
                             password: "postgres"

config :mastery, :persistence_fn, &Mastery.Persistence.record_response/2

I had “forgotten” the username here.

I now have the persistence tests implemented: https://github.com:chriseyre2000/mastery (dc6c5bf)

I am now onto chapters 7 – 9 which deal with OTP. This is the really interesting part of the book. I am unsure that my sample code is accurate as there are very few tests and the supplied sample code has diverged from that which you enter if you are following along. I can’t tell if I have made a mistake or if the supplied code is wrong (my suspicion is a little of both). This has taught me a lot about debugging Elixir applications which will be useful. What looked like a cryptic error message was actually precise and detailed where the genserver received a Map when it was expecting an atom. Adding guard clauses is a great way to flush out these problems.

The book is written as a long exercise where you add code snippets to your application to update. Given that I am using an epub format I can’t simply cut and paste the code, so I am typing it in. This is interesting since epub adds a trailing - when wrapping lines which takes a while to get used to. This has a certain error rate so I frequently use `iex -S mix` to check for typos. This works well for sections where the required pieces are defined first. These later chapters were written outside-in (a technique that I like, but normally back with tests) so this does not help.

One of the downsides of these long exercises is that it can be hard to find a mistake in a snippet that has been updated several times.

The database additions have slowed me down as I don’t currently have a local working postgres version. The samples for the database completely miss out the credentials required, but this information is available elsewhere.

Here is the docker-compose.yml that I got to work (note that the port is marked host:container):

version: "3"
services:
  db:
    image: "postgres:11"
    environment:
      - POSTGRES_PASSWORD=postgres
    container_name: "my_postgres"
    ports:
      - "54320:5432"
    volumes:
      - my_dbdata:/var/lib/postgresql/data
volumes:
  my_dbdata: {}

In practice we would use a better username and password. I can have a portable database setup by using: docker-compose up -d db

Here is the config that I used to get the database setup:

use Mix.Config

config :mastery_persistence, MasteryPersistence.Repo,
  database: "mastery_dev",
  hostname: "localhost",
  port: 54320,
  username: "postgres",
  password: "postgres"

The use of OTP fails to mention the default 5 second timeout behaviour built into OTP requests. It does cover using timeouts to handle late responses and has a good explination of the use of via.

Poncho projects are an interesting concept. You just create a full mix application in a subdirectory (or a parallel directory). This allows it to be moved into a distinct repository later on should it be needed, but is less overhead than an umbrella project.

Now on to integrating the mastery_persistence into mastery. This part looks a bit rushed. They are asking us to change files that have not yet been created (config/config.exs).

I have worked through the examples and pushed the finished details to:

github.com:chriseyre2000/mastery

I am currently at: 6963329

All that remains is the testing chapter and going back and making sure everything works.