Tuesday, April 29, 2014

JSON Web Tokens, OWIN, and AngularJS

I’m working on an exciting new project at the moment. The main UI element is a management console built with AngularJS that communicates with a HTTP/JSON API built with NancyFX and hosted using the Katana OWIN self host. I’m quite new to this software stack, having spent the last three years buried in SOA and messaging, but so far it’s all been a joy to work with. AngularJS makes building single page applications so easy, even for a newbie like me, that it almost feels unfair. I love the dependency injection, templating and model binding, and the speed with which you can get up and running. On the server side, NancyFx is perfect for building HTTP/JSON APIs. I really like the design philosophy behind it. The built-in dependency injection, component oriented design, and convention-over-configuration, for example, is exactly how I like build software. OWIN is a huge breakthrough for C# web applications. Decoupling the web server from the web framework is something that should have happened a long time ago, and it’s really nice to finally say goodbye to ASP.NET.

Rather than using cookie based authentication, I’ve decided to go with JSON Web Tokens (JWT). This is a relatively new authorization standard that uses a signed token, transmitted in a request header, rather than the traditional ASP.NET cookie based authorization.

There are quite a few advantages to JWT:

  • Cross Domain API calls. Because it’s just a header rather than a cookie, you don’t have any of the cross-domain browser problems that you get with cookies. It makes implementing single-sign-on much easier because the app that issues the token doesn’t need to be in any way connected with the app that consumes it. They merely need to have access to the same shared secret encryption key.
  • No server affinity. Because the token contains all the necessary user identification, there’s no for shared server state – a call to a database or shared session store.
  • Simple to implement clients. It’s easy to consume the API from other servers, or mobile apps.

So how does it work? The JWT token is a simple string of three ‘.’ separated base 64 encoded values:


Here’s an example:


The header and payload are simple JSON strings. In the example above the header looks like this:

{ "typ": "JWT", "alg": "HMACSHA256" }

This is defined in the JWT standard. The ‘typ’ is always ‘JWT’, and the ‘alg’ is the hash algorithm used to sign the token (more on this later).

The payload can be any valid JSON, although the standard does define some keys that client and server libraries should respect:

"user": "mike",
"exp": 123456789

Here, ‘user’ is a key that I’ve defined, ‘exp’ is defined by the standard and is the expiration time of the token given as a UNIX time value. Being able to pass around any values that are useful to your application is a great benefit, although you obviously don’t want the token to get too large.

The payload is not encrypted, so you shouldn’t put sensitive information it in. The standard does provide an option for encrypting the JWT inside an encrypted wrapper, but for most applications that’s not necessary. In my case, an attacker could get the user of a session and the expiration time, but they wouldn’t be able to generate new tokens without the server side shared-secret.

The token is signed by taking the header and payload, base  64 encoding them, concatenating with ‘.’ and then generating a hash value using the given algorithm. The resulting byte array is also base 64 encoded and concatenated to produce the complete token. Here’s some code (taken from John Sheehan’s JWT project on GitHub) that generates a token. As you can see, it’s not at all complicated:

/// <summary>
/// Creates a JWT given a payload, the signing key, and the algorithm to use.
/// </summary>
/// <param name="payload">An arbitrary payload (must be serializable to JSON via <see cref="System.Web.Script.Serialization.JavaScriptSerializer"/>).</param>
/// <param name="key">The key bytes used to sign the token.</param>
/// <param name="algorithm">The hash algorithm to use.</param>
/// <returns>The generated JWT.</returns>
public static string Encode(object payload, byte[] key, JwtHashAlgorithm algorithm)
var segments = new List<string>();
var header = new { typ = "JWT", alg = algorithm.ToString() };

byte[] headerBytes = Encoding.UTF8.GetBytes(jsonSerializer.Serialize(header));
byte[] payloadBytes = Encoding.UTF8.GetBytes(jsonSerializer.Serialize(payload));


var stringToSign = string.Join(".", segments.ToArray());

var bytesToSign = Encoding.UTF8.GetBytes(stringToSign);

byte[] signature = HashAlgorithms[algorithm](key, bytesToSign);

return string.Join(".", segments.ToArray());

Implementing JWT authentication and authorization in NancyFx and AngularJS

There are two parts to this: first we need a login API, that takes a username (email in my case) and a password and returns a token, and secondly we need a piece of OWIN middleware that intercepts each request and checks that it has a valid token.

The login Nancy module is pretty straightforward. I took John Sheehan’s code and pasted it straight into my project with a few tweaks, so it was just a question of taking the email and password from the request, validating them against my user store, generating a token and returning it as the response. If the email/password doesn’t validate, I just return 401:

using System;
using System.Collections.Generic;
using Nancy;
using Nancy.ModelBinding;
using MyApp.Api.Authorization;

namespace MyApp.Api
public class LoginModule : NancyModule
private readonly string secretKey;
private readonly IUserService userService;

public LoginModule (IUserService userService)
Preconditions.CheckNotNull (userService, "userService");
this.userService = userService;

Post ["/login/"] = _ => LoginHandler(this.Bind<LoginRequest>());

secretKey = System.Configuration.ConfigurationManager.AppSettings ["SecretKey"];

public dynamic LoginHandler(LoginRequest loginRequest)
if (userService.IsValidUser (loginRequest.email, loginRequest.password)) {

var payload = new Dictionary<string, object> {
{ "email", loginRequest.email },
{ "userId", 101 }

var token = JsonWebToken.Encode (payload, secretKey, JwtHashAlgorithm.HS256);

return new JwtToken { Token = token };
} else {
return HttpStatusCode.Unauthorized;

public class JwtToken
public string Token { get; set; }

public class LoginRequest
public string email { get; set; }
public string password { get; set; }

On the AngularJS side, I have a controller that calls the LoginModule API. If the request is successful, it stores the token in the browser’s sessionStorage, it also decodes and stores the payload information in sessionStorage. To update the rest of the application, and allow other components to change state to show a logged in user, it sends an event (via $rootScope.$emit) and then redirects to the application’s root path. If the login request fails, it simply shows a message to inform the user:

myAppControllers.controller('LoginController', function ($scope, $http, $window, $location, $rootScope) {
$scope.message = '';
$scope.user = { email: '', password: '' };
$scope.submit = function () {
.post('/api/login', $scope.user)
.success(function (data, status, headers, config) {
$window.sessionStorage.token = data.token;
var user = angular.fromJson($window.atob(data.token.split('.')[1]));
$window.sessionStorage.email = user.email;
$window.sessionStorage.userId = user.userId;
.error(function (data, status, headers, config) {
// Erase the token if the user fails to login
delete $window.sessionStorage.token;

$scope.message = 'Error: Invalid email or password';

Now that we have the JWT token stored in the browser’s sessionStorage, we can use it to ‘sign’ each outgoing API request. To do this we create an interceptor for Angular’s http module. This does two things: on the outbound request it adds an Authorization header ‘Bearer <token>’ if the token is present. This will be decoded by our OWIN middleware to authorize each request. The interceptor also checks the response. If there’s a 401 (unauthorized) response, it simply bumps the user back to the login screen.

myApp.factory('authInterceptor', function ($rootScope, $q, $window, $location) {
return {
request: function (config) {
config.headers = config.headers || {};
if($window.sessionStorage.token) {
config.headers.Authorization = 'Bearer ' + $window.sessionStorage.token;
return config;
responseError: function (response) {
if(response.status === 401) {
return $q.reject(response);

myApp.config(function ($httpProvider) {

The final piece is the OWIN middleware that intercepts each request to the API and validates the JWT token.

We want some parts of the API to be accessible without authorization, such as the login request and the API root, so we maintain a list of exceptions, currently this is just hard-coded, but it could be pulled from some configuration store. When the request comes in, we first check if the path matches any of the exception list items. If it doesn’t we check for the presence of an authorization token. If the token is not present, we cancel the request processing (by not calling the next AppFunc), and return a 401 status code. If we find a JWT token, we attempt to decode it. If the decode fails, we again cancel the request and return 401. If it succeeds, we add some OWIN keys for the ‘userId’ and ‘email’, so that they will be accessible to the rest of the application and allow processing to continue by running the next AppFunc.

using System;
using System.Collections.Generic;
using System.Threading.Tasks;

namespace MyApp.Api.Authorization
using AppFunc = Func<IDictionary<string, object>, Task>;

/// <summary>
/// OWIN add-in module for JWT authorization.
/// </summary>
public class JwtOwinAuth
private readonly AppFunc next;
private readonly string secretKey;
private readonly HashSet<string> exceptions = new HashSet<string>{

public JwtOwinAuth (AppFunc next)
this.next = next;
secretKey = System.Configuration.ConfigurationManager.AppSettings ["SecretKey"];

public Task Invoke(IDictionary<string, object> environment)
var path = environment ["owin.RequestPath"] as string;
if (path == null) {
throw new ApplicationException ("Invalid OWIN request. Expected owin.RequestPath, but not present.");
if (!exceptions.Contains(path)) {
var headers = environment ["owin.RequestHeaders"] as IDictionary<string, string[]>;
if (headers == null) {
throw new ApplicationException ("Invalid OWIN request. Expected owin.RequestHeaders to be an IDictionary<string, string[]>.");
if (headers.ContainsKey ("Authorization")) {
var token = GetTokenFromAuthorizationHeader (headers ["Authorization"]);
try {
var payload = JsonWebToken.DecodeToObject (token, secretKey) as Dictionary<string, object>;
environment.Add("myapp.userId", (int)payload["userId"]);
environment.Add("myapp.email", payload["email"].ToString());
} catch (SignatureVerificationException) {
return UnauthorizedResponse (environment);
} else {
return UnauthorizedResponse (environment);
return next (environment);

public string GetTokenFromAuthorizationHeader(string[] authorizationHeader)
if (authorizationHeader.Length == 0) {
throw new ApplicationException ("Invalid authorization header. It must have at least one element");
var token = authorizationHeader [0].Split (' ') [1];
return token;

public Task UnauthorizedResponse(IDictionary<string, object> environment)
environment ["owin.ResponseStatusCode"] = 401;
return Task.FromResult (0);

So far this is all working very nicely. There are some important missing pieces. I haven’t implemented an expiry key in the JWT token, or expiration checking in the OWIN middleware. When the token expires, it would be nice if there was some algorithm that decides whether to simply issue a new token, or whether to require the user to sign-in again. Security dictates that tokens should expire relatively frequently, but we don’t want to inconvenience the user by asking them to constantly sign in.

JWT is a really nice way of authenticating HTTP/JSON web APIs. It’s definitely worth looking at if you’re building single page applications, or any API-first software.

Wednesday, April 16, 2014

A Contractor’s Guide To Recruitment Agencies

I haven’t contracted through an agency for a long time, but I thought I’d write up my experiences from almost ten years of working as an IT contractor for anyone considering it as a career choice.

IT recruitment agencies provide a valuable service. Like any middle-man, their job is to bring buyers and sellers together. In this case the buyer is the end client, the company that needs a short-term resource to fill a current skills gap. The seller is you, the contactor offering the skill. The agency needs to do two things well: market intelligence - finding clients in need of resources and contractors looking to sell their skills; and negotiation – negotiating the highest price that the client will pay, and the lowest price that the contractor will work for. The agency’s income is a simple formula:

(client rate – contractor rate) * number of contractors placed.

Minimize the contractor rate, maximize the client rate, and place as many contractors as possible. That’s success.

Anyone with a phone can set themselves up as a recruitment agency. There are zero or low startup costs. The greatest difficultly most agencies face is finding clients. Finding contractors is a little easier, as I’ll explain presently. Having a good relationship with a large corporate or government client is a gold standard for any agency. Even better if that relationship is exclusive. Getting a foot in the door with one of these clients is very difficult, usually some long established, large agency has long ago stitched up a deal with someone high-up. But any company or organization in need of a contractor is a potential client, and agencies spend inordinate amounts of time in the search for names they can approach with potential candidates.

As I said before, finding contractors is somewhat easier. There are a number of well known websites, Jobserve is the most common one to use in the UK, so it’s merely a case of putting up a job description and waiting for the CVs to roll in. The agent will try to make the job sound as good as possible to maximize the chances of getting applications within the limits of the client’s job spec.

An ideal contractor for an agency is someone who the client wants to hire, and who is willing to work for the lowest possible rate, and who will keep the client happy by turning up every day and doing the work that the client expects. Since agencies take an on-going percentage of the daily rate, the longer the contract lasts the better. The agency will attempt to do some filtering to ‘add value’, but since few agencies have any real technology knowledge, this mainly consists of matching keywords and years-of-experience. Anyone with any experience of talking to agencies will know how frustrating it can be, “Do you know any ASPs?” “No, they don’t want .NET, they want C#.” I’m not making those quotes up. Ideally they will want to persuade the client that they have some kind of exclusive arrangement with ‘their’ contractors and that the client would not be able to hire them through anyone else. It can be very embarrassing for them if the client receives your CV through a competing agency as well as theirs.

The job hunt. How you should approach it.

Let’s say you’re a competent C# developer, how should you approach landing your dream contract role? The obvious first place to look are the popular jobsites. Do a search for C# contracts in your local area, or further afield if you’re willing to travel. Scan the job listings looking for anything that looks like it vaguely fits. Don’t be too fussy at this stage, you want to increase your chances by applying for as many jobs as possible. Once you’ve got a list of jobs it’s worth trying to see if you can work out who the company is. If you can make a direct contract with the client, so much the better. Don’t worry about feeling underhand, agencies do this to each other all the time, it’s part of the game.

Failing a direct contact, the next step is to email your CV to the agency. Remember they’ll be trying to match keywords, so it’s worth customizing your CV to the job advert. Make sure as many keywords as possible match those in the advert, remembering of course that you might have to back up your claims in an interview.

The next step is usually a short telephone conversation with the recruiter. This call is the beginning of the negotiations with the recruiter. Negotiating is their full time job, they are usually very good at it. Be very wary. Your attitude is that you are a highly qualified professional who is somewhat interested in the role, but it’s by no means the only possibility at this stage. Whatever you do, don’t appear desperate. Remember at this stage you are an unknown quantity. Most contractors a recruiter comes into contact with will be duds (there’s no barriers to entry in our profession either), and they will initially be suspicious of you. Confidently assert that you have all the experience you mention in your CV, and that, of course, you can do the job. There is no point in getting into any technical discussion with the recruiter, they simply won’t understand. Remember: match keywords and experience. At this stage, even if you’ve got doubts about the job, don’t express them, just appear keen and confident.

Sometimes there’s a rate mentioned on the advert, at other times it will just say ‘market rates’, which is meaningless. If the agent doesn’t bring up rates at this point, there’s no need to mention them. At this stage you are still an unknown quantity. Once the client has decided that they really want you, you are gold, and in a much stronger bargaining position. If there’s a rate conversation at the pre interview stage, try to stay non-committal. If there’s a range, say you’ll only work for the top number.

They may ask you for references. Your reply should be to politely say that you only give references after an interview. It’s a common trick to put an imaginary job on a jobsite then ask applicants for references. Remember, an agency’s main difficulty is finding clients and the references are used as leads. If you give them references you will never hear from them again, but your previous clients will be hounded with phone calls.

Another common trick is to ask you where else you are applying. They are looking for leads again. Be very non-committal. They may also ask you for names of people you worked for at previous jobs, this is just like asking for references, you don’t need to tell them. Sometimes it’s worth have a list of made up names to give out if they’re very persistent.

Next you will either hear back from the agent with an offer of an interview, or you won’t hear from them at all. No agency I’ve ever had contact with bothered to call me giving a reason why an interview hadn’t materialized. If you don’t hear from them, move on with applying for the next job. Constantly calling the agency smacks of desperation and won’t get you anywhere. There are multiple possible reasons that the interview didn’t materialize, the most common being that the job didn’t exist in the first place (see above).

At all times be polite and professional with the agent even if you’re convinced they’re being liberal with the truth.

If you get an interview, that’s good. This isn’t a post about interviewing, so let’s just assume that you were wonderful and the client really wants you. You’ll know this because you’ll get a phone call from the agent congratulating you on getting the role. You are now a totally different quantity in the agent’s eyes, a successful candidate, a valuable commodity, a guaranteed income stream for as long as the contract lasts. Their main job now is to get you to work for as little as possible while the client pays as much as possible. If you agreed a rate before the interview, now is their chance to try and lower it. You may well have a conversation like this: “I’m very sorry John, but the client is not able to offer the rate we agreed, I’m afraid it will have to be XXX instead.” Call their bluff. Your answer should be: “Oh that’s such a shame, I was really looking forward to working with them, but I my minimum rate is <whatever you initially agreed>. Never mind, it was nice doing business with you.” I guarantee they will call you back the next day telling you how hard they have been working on your behalf to persuade the client to increase your rate.

If you haven’t already agreed a rate, now is the time to have a good idea of the minimum you want to work for. Add 30% to it. That’s your opening rate with the agent. They will choke and tell you there’s no way that you’ll get that. Ask them for their maximum and choke in return. Haggle back and forth until you discover what their maximum is. If it’s lower than your minimum, walk away. You may have to walk away and wait for them to phone you. Of course you’ve got to be somewhere in the ballpark of the market rate or you won’t get the role. Knowing the market rate is tricky, but a few conversations with your contractor mates should give you some idea.

Once the rate has been agreed and you start work your interests are aligned with the agent. You both want the contract to last and and you both want to maintain a good relationship with the client. The agency should pay you promptly. Don’t put up with late or missing payments, just leave. Usually a threat to walk off site can work wonders with outstanding invoices. Beware, at the worst some agents can be downright nasty and bullying. I’ve been told that would never work in IT again by at least two different characters. It’s nice to see how that turned out. Just ignore bullies, except to make a note that you will never work for their agency again.

Agencies are a necessary evil until you have built up a good enough network and reputation that you don’t need to use them any more. Some are professional and honest, many aren’t, but if you understand their motivations and treat anything they say with a pinch of salt, you should be fine.

Thursday, April 03, 2014

A Docker ‘Hello World' With Mono

Docker is a lightweight virtualization technology for Linux that promises to revolutionize the deployment and management of distributed applications. Rather than requiring a complete operating system, like a traditional virtual machine, Docker is built on top of Linux containers, a feature of the Linux kernel, that allows light-weight Docker containers to share a common kernel while isolating applications and their dependencies.

There’s a very good Docker SlideShare presentation here that explains the philosophy behind Docker using the analogy of standardized shipping containers. Interesting that the standard shipping container has done more to create our global economy than all the free-trade treaties and international agreements put together.

A Docker image is built from a script, called a ‘Dockerfile’. Each Dockerfile starts by declaring a parent image. This is very cool, because it means that you can build up your infrastructure from a layer of images, starting with general, platform images and then layering successively more application specific images on top. I’m going to demonstrate this by first building an image that provides a Mono development environment, and then creating a simple ‘Hello World’ console application image that runs on top of it.

Because the Dockerfiles are simple text files, you can keep them under source control and version your environment and dependencies alongside the actual source code of your software. This is a game changer for the deployment and management of distributed systems. Imagine developing an upgrade to your software that includes new versions of its dependencies, including pieces that we’ve traditionally considered the realm of the environment, and not something that you would normally put in your source repository, like the Mono version that the software runs on for example. You can script all these changes in your Dockerfile, test the new container on your local machine, then simply move the image to test and then production. The possibilities for vastly simplified deployment workflows are obvious.

Docker brings concerns that were previously the responsibility of an organization’s operations department and makes them a first class part of the software development lifecycle. Now your infrastructure can be maintained as source code, built as part of your CI cycle and continuously deployed, just like the software that runs inside it.

Docker also provides docker index, an online repository of docker images.  Anyone can create an image and add it to the index and there are already images for almost any piece of infrastructure you can imagine. Say you want to use RabbitMQ, all you have to do is grab a handy RabbitMQ images such as https://index.docker.io/u/tutum/rabbitmq/ and run it like this:

docker run -d -p 5672:5672 -p 55672:55672 tutum/rabbitmq

The –p flag maps ports between the image and the host.

Let’s look at an example. I’m going to show you how to create a docker image for the Mono development environment and have it built and hosted on the docker index. Then I’m going to build a local docker image for a simple ‘hello world’ console application that I can run on my Ubuntu box.

First we need to create a Docker file for our Mono environment. I’m going to use the Mono debian packages from directhex. These are maintained by the official Debian/Ubuntu Mono team and are the recommended way of installing the latest Mono versions on Ubuntu.

Here’s the Dockerfile:

# monoxide mono-devel package on Ubuntu 13.10

FROM ubuntu:13.10
MAINTAINER Mike Hadlow <mike@suteki.co.uk>

RUN sudo DEBIAN_FRONTEND=noninteractive apt-get install -y -q software-properties-common
RUN sudo add-apt-repository ppa:directhex/monoxide -y
RUN sudo apt-get update
RUN sudo DEBIAN_FRONTEND=noninteractive apt-get install -y -q mono-devel

Notice the first line (after the comments) that reads, ‘FROM  ubuntu:13.10’. This specifies the parent image for this Dockerfile. This is the official docker Ubuntu image from the index. When I build this Dockerfile, that image will be automatically downloaded and used as the starting point for my image.

But I don’t want to build this image locally. Docker provide a build server linked to the docker index. All you have to do is create a public GitHub repository containing your dockerfile, then link the repository to your profile on docker index. You can read the documentation for the details.

The GitHub repository for my Mono image is at https://github.com/mikehadlow/ubuntu-monoxide-mono-devel. Notice how the Docker file is in the root of the repository. That’s the default location, but you can have multiple files in sub-directories if you want to support many images from a single repository.

Now any time I push a change of my Dockerfile to GitHub, the docker build system will automatically build the image and update the docker index. You can see image listed here: https://index.docker.io/u/mikehadlow/ubuntu-monoxide-mono-devel/

I can now grab my image and run it interactively like this:

$ sudo docker pull mikehadlow/ubuntu-monoxide-mono-devel
Pulling repository mikehadlow/ubuntu-monoxide-mono-devel
f259e029fcdd: Download complete
511136ea3c5a: Download complete
1c7f181e78b9: Download complete
9f676bd305a4: Download complete
ce647670fde1: Download complete
d6c54574173f: Download complete
6bcad8583de3: Download complete
e82d34a742ff: Download complete

$ sudo docker run -i mikehadlow/ubuntu-monoxide-mono-devel /bin/bash
mono --version
Mono JIT compiler version 3.2.8 (Debian 3.2.8+dfsg-1~pre1)
Copyright (C) 2002-2014 Novell, Inc, Xamarin Inc and Contributors. www.mono-project.com
TLS: __thread
SIGSEGV: altstack
Notifications: epoll
Architecture: amd64
Disabled: none
Misc: softdebug
LLVM: supported, not enabled.
GC: sgen

Next let’s create a new local Dockerfile that compiles a simple ‘hello world’ program, and then runs it when we run the image. You can follow along with these steps. All you need is a Ubuntu machine with Docker installed.

First here’s our ‘hello world’, save this code in a file named hello.cs:

using System;

namespace Mike.MonoTest
public class Program
public static void Main()
Console.WriteLine("Hello World");

Next we’ll create our Dockerfile. Copy this code into a file called ‘Dockerfile’:


FROM mikehadlow/ubuntu-monoxide-mono-devel

ADD . /src

RUN mcs /src/hello.cs
CMD ["mono", "/src/hello.exe"]

Once again, notice the ‘FROM’ line. This time we’re telling Docker to start with our mono image. The next line ‘ADD . /src’, tells Docker to copy the contents of the current directory (the one containing our Dockerfile) into a root directory named ‘src’ in the container. Now our hello.cs file is at /src/hello.cs in the container, so we can compile it with the mono C# compiler, mcs, which is the line ‘RUN mcs /src/hello.cs’. Now we will have the executable, hello.exe, in the src directory. The line ‘CMD [“mono”, “/src/hello.exe”]’ tells Docker what we want to happen when the container is run: just execute our hello.exe program.

As an aside, this exercise highlights some questions around what best practice should be with Docker. We could have done this in several different ways. Should we build our software independently of the Docker build in some CI environment, or does it make sense to do it this way, with the Docker build as a step in our CI process? Do we want to rebuild our container for every commit to our software, or do we want the running container to pull the latest from our build output? Initially I’m quite attracted to the idea of building the image as part of the CI but I expect that we’ll have to wait a while for best practice to evolve.

Anyway, for now let’s manually build our image:

$ sudo docker build -t hello .
Uploading context 1.684 MB
Uploading context
Step 0 : FROM mikehadlow/ubuntu-monoxide-mono-devel
---> f259e029fcdd
Step 1 : ADD . /src
---> 6075dee41003
Step 2 : RUN mcs /src/hello.cs
---> Running in 60a3582ab6a3
---> 0e102c1e4f26
Step 3 : CMD ["mono", "/src/hello.exe"]
---> Running in 3f75e540219a
---> 1150949428b2
Successfully built 1150949428b2
Removing intermediate container 88d2d28f12ab
Removing intermediate container 60a3582ab6a3
Removing intermediate container 3f75e540219a

You can see Docker executing each build step in turn and storing the intermediate result until the final image is created. Because we used the tag (-t) option and named our image ‘hello’, we can see it when we list all the docker images:

$ sudo docker images
hello latest 1150949428b2 10 seconds ago 396.4 MB
mikehadlow/ubuntu-monoxide-mono-devel latest f259e029fcdd 24 hours ago 394.7 MB
ubuntu 13.10 9f676bd305a4 8 weeks ago 178 MB
ubuntu saucy 9f676bd305a4 8 weeks ago 178 MB

Now let’s run our image. The first time we do this Docker will create a container and run it. Each subsequent run will reuse that container:

$ sudo docker run hello
Hello World

And that’s it.

Imagine that instead of our little hello.exe, this image contained our web application, or maybe a service in some distributed software. In order to deploy it, we’d simply ask Docker to run it on any server we like; development, test, production, or on many servers in a web farm. This is an incredibly powerful way of doing consistent repeatable deployments.

To reiterate, I think Docker is a game changer for large server side software. It’s one of the most exciting developments to have emerged this year and definitely worth your time to check out.

Tuesday, April 01, 2014

Docker: Bulk Remove Images and Containers

I’ve just started looking at Docker. It’s a cool new technology that has the potential to make the management and deployment of distributed applications a great deal easier. I’d very much recommend checking it out. I’m especially interested in using it to deploy Mono applications because it promises to remove the hassle of deploying and maintaining the mono runtime on a multitude of Linux servers.

I’ve been playing around creating new images and containers and debugging my Dockerfile, and I’ve wound up with lots of temporary containers and images. It’s really tedious repeatedly running ‘docker rm’ and ‘docker rmi’, so I’ve knocked up a couple of bash commands to bulk delete images and containers.

Delete all containers:

sudo docker ps -a -q | xargs -n 1 -I {} sudo docker rm {}

Delete all un-tagged (or intermediate) images:

sudo docker rmi $( sudo docker images | grep '<none>' | tr -s ' ' | cut -d ' ' -f 3)