DEV Community: Jayapriya Pai

Rapid Web App Development in Ruby: Introduction

Jayapriya Pai — Sun, 28 Jun 2020 07:16:15 +0000

Photo by Divyadarshi Acharya on Unsplash

In this blog series, I will be covering how to create a web app using ruby with minimal effort. Assuming you have ruby installed let's get started.

I am going to use sinatra framework here, so first step is to install sinatra gem.

gem install sinatra

Now create a new folder to store your app files. Since I am going to create a simple search app which has contact information, naming it as contacts. Open this folder in your editor now. I am using visual studio code.

Create a file named app.rb with the following contents

# app.rb

require 'sinatra'

get '/' do
  'Welcome to contacts app!'
end

Now from your terminal, run ruby app.rb, you will see a similar message on your screen now.

slashpai@pai  ~/contacts   ruby app.rb

[2020-06-28 12:05:09] INFO  WEBrick 1.4.2
[2020-06-28 12:05:09] INFO  ruby 2.6.6 (2020-03-31) [x86_64-linux]
== Sinatra (v2.0.8.1) has taken the stage on 4567 for development with backup from WEBrick
[2020-06-28 12:05:09] INFO  WEBrick::HTTPServer#start: pid=21875 port=4567

From your browser, go to localhost:4567, you should see the content on web page as Welcome to contacts app!. That was easy right?

Now let's understand how it worked. When you typed localhost:4567 in your browser, it checked in app.rb file if route for this url is defined, since this is root of website, it will look for route with /. And that's what we have defined with get method in app.rb file.

In Sinatra, a route is an HTTP method paired with a URL pattern. Each route is associated with a block. Routes will be evaluated in the same order as they are defined. The first route that matched will be invoked.

Now let's add more routes in app.rb file and restart app using ruby app.rb

# app.rb

require 'sinatra'

get '/' do
  'Welcome to contacts app!'
end

get '/home' do
    'Home page'
end

get '/contacts' do
    'This page is under construction'
end

Now visit home and contacts pages. You should see same messages as defined in routes.

You might have noticed, you need to restart app.rb everytime you make a change. How we can simplify this?

We will use the gem 'sinatra-contrib' for this. Go ahead and install this gem now

gem install sinatra-contrib

Now in your app.rb file, require the following library which comes as part of sinatra-contrib gem

require 'sinatra/reloader'

Run your app now ruby app.rb. While the app is running make any changes to the route messages to verify auto reload is working. The site will display updated content without needing the app restart.

The complete app.rb file

# app.rb

require 'sinatra'
require 'sinatra/reloader'

get '/' do
  'Welcome to contacts app!'
end

get '/home' do
    'Home page'
end

get '/contacts' do
    'This page is under construction'
end

Hope that was helpful, we will see more on Sinatra web development in next post!

Quickly creating kubernetes manifest files using kubectl

Jayapriya Pai — Mon, 13 Apr 2020 16:52:32 +0000

When someone is just starting with kubernetes, writing manifest files can be a bit difficult. In this post I will try to give some tips in writing manifest files easily.

Pods

Use the generator to create pods. Verify if the given entries are syntactically correct by doing a dry run.

kubectl run --generator=run-pod/v1 <pod name> --image <image to use> --dry-run

Save the pod definition to a yaml file for future changes as well as reference

kubectl run --generator=run-pod/v1 <pod name> --image <image to use> --dry-run -o yaml > pod.yaml

Example:

slashpai@pai  ~  (⎈ |kind-pai:default) kubectl run --generator=run-pod/v1 nginx-pod --image nginx --dry-run        
pod/nginx-pod created (dry run)
slashpai@pai  ~  (⎈ |kind-pai:default) kubectl run --generator=run-pod/v1 nginx-pod --image nginx --dry-run -o yaml > pod.yaml
slashpai@pai  ~  (⎈ |kind-pai:default) cat pod.yaml 
apiVersion: v1
kind: Pod
metadata:
  creationTimestamp: null
  labels:
    run: nginx-pod
  name: nginx-pod
spec:
  containers:
  - image: nginx
    name: nginx-pod
    resources: {}
  dnsPolicy: ClusterFirst
  restartPolicy: Always
status: {}
slashpai@pai  ~  (⎈ |kind-pai:default)

Deployment

Generator for deployment is deprecated so following way helps. Verify if given entries are syntactically correct by doing a dry run

kubectl create deployment <deployment name> --image <image to use> --dry-run

Save the deployment definition to a yaml file for future changes as well as reference

kubectl create deployment <deployment name> --image <image to use> --dry-run -o yaml > deployment.yaml

Example:

slashpai@pai  ~  (⎈ |kind-pai:default) kubectl create deployment nginx-deployment --image nginx --dry-run
deployment.apps/nginx-deployment created (dry run)
slashpai@pai  ~  (⎈ |kind-pai:default) kubectl create deployment nginx-deployment --image nginx --dry-run -o yaml > deployment.yaml
slashpai@pai  ~  (⎈ |kind-pai:default) cat deployment.yaml 
apiVersion: apps/v1
kind: Deployment
metadata:
  creationTimestamp: null
  labels:
    app: nginx-deployment
  name: nginx-deployment
spec:
  replicas: 1
  selector:
    matchLabels:
      app: nginx-deployment
  strategy: {}
  template:
    metadata:
      creationTimestamp: null
      labels:
        app: nginx-deployment
    spec:
      containers:
      - image: nginx
        name: nginx
        resources: {}
status: {}
slashpai@pai  ~  (⎈ |kind-pai:default)

ReplicaSet

There is no direct way I could find to create this from kubectl. I found this trick though till I could find a better way. Use the same way to create deployment and modify kind to ReplicaSet in the yaml file and remove fields specific to deployment. I found this with trial and error, so this is the diff file.

slashpai@pai  ~  (⎈ |kind-pai:default) diff deployment.yaml replicaset.yaml 
2c2
< kind: Deployment
---
> kind: ReplicaSet
13d12
<   strategy: {}
24c23
< status: {}
---
>

I removed the fields strategy as well as status from replicaset.yaml. While creating ReplicaSet file this way, give the deployment name suffix like replicaset instead of deployment. I haven't done that in this example since I wanted to show what fields I exactly need to change.

ConfigMap

Even if there are more than one entry to be added in a config map, just add one and then add rest in manifest file directly so you can save time in adding one by one in command line. Verify first syntax wise correct using dry-run.

kubectl create configmap <config map name> --from-literal key=value --dry-run

Save the deployment definition to a yaml file for future changes as well as reference

kubectl create configmap <config map name> --from-literal key=value --dry-run -o yaml > configmap.yaml

Example:

slashpai@pai  ~  (⎈ |kind-pai:default) kubectl create configmap mongo-config --from-literal user_id=mongo --dry-run
configmap/mongo created (dry run)
slashpai@pai  ~  (⎈ |kind-pai:default) kubectl create configmap mongo-config --from-literal user_id=mongo --dry-run -o yaml > configmap.yaml
slashpai@pai  ~  (⎈ |kind-pai:default) cat configmap.yaml 
apiVersion: v1
data:
  user_id: mongo
kind: ConfigMap
metadata:
  creationTimestamp: null
  name: mongo

You may want to move the data section below metadata since that's where we place this normally.

Adding more entries in manifest file

Now you must have got an idea how to create manifest files for most used kubernetes objects. You can tweak like this for other objects not mentioned. The fields in manifest file created this way is not enough, if you have specific needs or need to add more fields.

Let's see how we can add more fields in a pod definition file. For example, you want to inject an environment variable into a container from a ConfigMap. Since we know container related info is inside spec section of the pod, we can use kubectl explain pod.spec.containers to get the exact field. See how this is done below.

slashpai@pai  ~  (⎈ |kind-pai:default) kubectl explain pod.spec.containers|grep env
     container's environment. If a variable cannot be resolved, the reference in
     are expanded using the container's environment. If a variable cannot be
   env  <[]Object>
     List of environment variables to set in the container. Cannot be updated.
   envFrom  <[]Object>
     List of sources to populate environment variables in the container. The

I grepped for env, since I know field starts with env. If you don't know the field, you would need to scroll down the explain output and figure this out.

From this I got to know it's the envFrom field that we need to use configmap and it's a list type (important to note the type). We can check the fields that comes under envFrom similar way like above. But if we are only interested to know the fields without it's description, just add the recursive flag

slashpai@pai  ~  (⎈ |kind-pai:default) kubectl explain pod.spec.containers.envFrom --recursive
KIND:     Pod
VERSION:  v1

RESOURCE: envFrom <[]Object>

DESCRIPTION:
     List of sources to populate environment variables in the container. The
     keys defined within a source must be a C_IDENTIFIER. All invalid keys will
     be reported as an event when the container is starting. When a key exists
     in multiple sources, the value associated with the last source will take
     precedence. Values defined by an Env with a duplicate key will take
     precedence. Cannot be updated.

     EnvFromSource represents the source of a set of ConfigMaps

FIELDS:
   configMapRef <Object>
      name  <string>
      optional  <boolean>
   prefix   <string>
   secretRef    <Object>
      name  <string>
      optional  <boolean>
slashpai@pai  ~  (⎈ |kind-pai:default)

That drilled down the config to use as given below in the spec section of pod. Note the type for envFrom, which we had figured out to be of list type.

spec:
  containers:
  - name: mongo
    image: mongo
    envFrom:
    - configMapRef:
        name: mongo-config

Surely you can go to kubernetes documentation and get this info. But I found this is more easier since I don't like leaving my terminal so often.

I hope this helps you create the manifest files with ease now. Thank you for reading this post.