package main
import "fmt"
func main() {
//variable lenght arrays
slice := []int{1, 2, 3, 4}
slice = append(slice, 4)
fmt.Println(slice)
//fixed lenght arrays
fixed_slice := [4]int{1, 2, 3, 4}
//cannot append another element - compiler error!
//fixed_slice = append(slice, 4)
fmt.Println(fixed_slice)
//another way to define a slice
another_slice := make([]int, 10)
//initially filled with zeros
fmt.Println(another_slice)
//add a different number in the first position
another_slice[0] = 45
fmt.Println(another_slice)
}
Golang also uses pointers, but it doesn’ need any memory allocation code or anything.
It’s very straightforward.
The ampersand & creates a pointer, and the asteryx * allows you to get the value referenced by the pointer.
Simply:
package main
import (
"fmt"
)
func main() {
s := "this is a string"
string_pointer := &s
fmt.Println(s)
//reference the pointer with asteryx
fmt.Println("reference by pointer: ", *string_pointer)
//reference the pointer with asteryx
fmt.Println("address: ", string_pointer)
}
import (
"fmt"
"os"
"strconv"
)
func main() {
var sum int
//exclude for argument from Args
for _, a := range os.Args[1:] {
i, err := strconv.Atoi(a)
if err != nil {
panic(fmt.Sprintf("Invalid value : %v", err))
}
sum += i
}
fmt.Printf("Sum = %v\n", sum)
}
As you can see, if you run the code with string parameters you get an error:
go run error_handling.go 1 a c
Then you get:
panic: Invalid value : strconv.Atoi: parsing "a": invalid syntax
goroutine 1 [running]:
main.main()
C:/Users/lliparul/Desktop/go/3_ERROR_HANDLING/error_handling.go:19 +0x159
exit status 2
Inspired by the course “Getting started with Go Programming Language” by Matthew Spaulding (Packt Publishing)
Go was developed by some developers at Google and meant to be a better system programming language than C and C++.
It’s compiled and not interpreted.
It’s fast and suitable for concurrency.
Go uses vendoring for depedepency management. It means that the cofe of the dependencies is included in the project, once the dependency is downloaded.
Hello world program
A simple Hello world program (a file called “hello_world.go”) looks like this:
package main
import "fmt"
func main() {
fmt.Println("Hello, World!")
}
As you can see the main function is the entry point to run golang applications.
Once golang is installed on your machine, you can simply run:
go run hello_world.go
The package fmt is a fundamental library to deal with strings, etc.
Assignments, data types, control structures
To assign values, columns and equal simbols are used. You can use data types like int, but also omit them.
a := 5
var b int = 4
Strings always use double quotes.
fmt.Println("a is negative!")
Of course you have conditional clauses like, if and switch:
if a < 0 {
fmt.Println("a is negative!")
} else if a > b {
fmt.Println("a is bigger than b")
}
switch a {
case 10:
fmt.Println("It's 10")
case 5:
fmt.Print("It's five")
}
And three types of for loops.
Simple loop:
for i := 0; i < 4; i++ {
for i := 0; i < 4; i++ {
fmt.Println(i)
}
Range loop:
for _, i := range []int{1, 2, 3, 4} {
fmt.Println(i)
}
Using Handlers, we can create tasks in Ansible that can be executed only if a change occurs on the machine.
Let’s get back to the nginx playbook used in the previous article and modify the installtion task, by adding the handler for the update, to be executed if there is a new nginx version, when apt-get update is run on the linux machine:
To run one of the Docker official image, you can simply use “docker run”:
$ docker run centos
The first time you run it, the image can´t be found and must be downloaded:
Once downloaded, the image is saved in the docker cache and can be reused if necessary.
If it´s an official image, you would only need to enter the image name only.
But it´s taken from a user´s repo the format is: <username>/<image>.
The 100% Free software linux image Trisquel, for example, is taken from the repository of a user called “kpengboy“.
In this case, even if you are logged in, you can´t run “docker run”. You need to pull it first:
$docker pull kpengboy/trisquelbash
To see the list of the images donwload so far, run:
$ docker images
If you also want to start a bash in the container, use “-it” for the interactive mode:
$ docker run -it kpengboy/trisquel
Then you can work with the bash shell:
As default, docker will download the latest version available, but you can additionally specify a version (called “tag”) you want to work with:
$ docker run redis:4.0
If you want to access a webapp or, for example, a database, you generally get an assigned url with a port number. But you can change the ports, by adding them as parameters.
For example, if you want to run mysql and access it at a different port than 3306, use the “-p” option to give <host>:<container-port>:
$ docker run -p 52000:3307 mysql
To persist your data, even when the container is killed, you can mount a persistent volume with “-v”:
$ docker run -v /opt/data/mysql mysql
You can pass an environment variable to you container, which might help you to avoid modifying the image and achieve some strategic solution:
$ docker run -e BACKGROUND_COLOR simple-web-app
To see which containers are running:
$ docker ps
To see the stopped containers to
$ docker ps -a
If you want to get the details of a specific container:
$ docker inspect vibrant_chatelet
You can check the log by running:
$ docker logs vibrant_chatelet
You can start a container in background mode with the “-d” option:
$ docker -d kpengboy/trisquel
To remove a container (even if it´s running)
$ docker rm vibrant_chatelet
Or by containerID
$ docker rm 0d6d64f9053c
To remove the image you need to make sure that no container is running it first. You migh use “docker stop” or just remove it directly.
NodePort and Load balancer are used to expose the service outside the cluster In addition to creating the ClusterIp, this will allocate a port in the range 30000-32767 on every node of the cluster, to route to the clusterIP. For example:
If you don´t specify the target port, a a random port will be used.
Exposed services automatically register with the Kubernetes internal DNS, which make it easier to access them by names rather than IPs. You can get the url by using nslookup, like:
$ kubectl exec -it busybox2 --nslookup nginx
HEADLESS SERVICE
If you put in the spec “clusterIp” as “none”, you get a headless service. It is possible to create a service grouping that does not allocate an IP address or forward traffic, if there is a reason that you want to definitively control what specific pods you connect and communicate with. This kind of service is called a headless service. You can request this setup by explicitly setting ClusterIP to None within the service definition:
For these kind of services, DNS entries will be created that point to the Pods backing the service, and that DNS will be automatically updated as Pods matching the selector come online (or disappear).
ENDPOINT
You can expose a remote system as a service internally by creating an endpoint for it.
For example, if you had a remote TCP service running on the internet at port 1976 at the IP address 1.2.3.4, you could define a Service and Endpoint to reference that external-to-kubernetes system:
Now we can use that name to ask kubectl to set up a proxy that will forward all traffic from a local port we specify to a port associated with the Pod we determine.
$ kubectl port-forward flask-1599974757-b68pw 5000:5000
Forwarding from 127.0.0.1:5000 -> 5000
Forwarding from [::1]:5000 -> 5000
This is forwarding any and all traffic that gets created on your local machine at TCP port 5000 to TCP port 5000 on the Pod flask-1599974757-b68pw.
If we inspect the service account, we can find the name of the secret we are using:
Then we can inpect the token by running:
$ kubectl get secret my-sa-token-m4mmz -o yaml
In the token section, we can see the secret encoded in base64. To read it in clear text simply:
$ echo <tokenstring> | base64 -d
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