Why there are tags in Go?

A small tutorial with code examples about tags in Golang programming language
10 August 2017   410

Hype.Codes team continues to answer popular technical questions about programming languages. We made a research and found out what tags "do" in Go.

A tag for a field allows you to attach meta-information to the field which can be acquired using reflection. Usually it is used to provide transformation info on how a struct field is encoded to or decoded from another format (or stored/retrieved from a database), but you can use it to store whatever meta-info you want to, either intended for another package or for your own use.

As mentioned in the documentation of reflect.StructTag, by convention the value of a tag string is a space-separated key:"value" pairs, for example:

type User struct {
    Name string `json:"name" xml:"name"`
}

The key usually denotes the package that the subsequent "value" is for, for example jsonkeys are processed/used by the encoding/json package.

If multiple information is to be passed in the "value", usually it is specified by separating it with a comma (','), e.g.

Name string `json:"name,omitempty" xml:"name"`

Usually a dash value ('-') for the "value" means to exclude the field from the process (e.g. in case of json it means not to marshal or unmarshal that field).

Example of accessing your custom tags using reflection

We can use reflection (reflect package) to access the tag values of struct fields. Basically we need to acquire the Type of our struct, and then we can query fields e.g. with Type.Field(i int) or Type.FieldByName(name string). These methods return a value of StructFieldwhich describe / represent a struct field; and StructField.Tag is a value of type StructTagwhich describes / represents a tag value.

Previously we talked about "convention". This convention means that if you follow it, you may use the StructTag.Get(key string) method which parses the value of a tag and returns you the "value" of the key you specify. The convention is implemented / built into this Get() method. If you don't follow the convention, Get() will not be able to parse key:"value" pairs and find what you're looking for. That's also not a problem, but then you need to implement your own parsing logic.

Also there is StructTag.Lookup() (was added in Go 1.7) which is "like Get() but distinguishes the tag not containing the given key from the tag associating an empty string with the given key".

So let's see a simple example:

type User struct {
    Name  string `mytag:"MyName"`
    Email string `mytag:"MyEmail"`
}

u := User{"Bob", "bob@mycompany.com"}
t := reflect.TypeOf(u)

for _, fieldName := range []string{"Name", "Email"} {
    field, found := t.FieldByName(fieldName)
    if !found {
        continue
    }
    fmt.Printf("\nField: User.%s\n", fieldName)
    fmt.Printf("\tWhole tag value : %q\n", field.Tag)
    fmt.Printf("\tValue of 'mytag': %q\n", field.Tag.Get("mytag"))
}

Output :

Field: User.Name
    Whole tag value : "mytag:\"MyName\""
    Value of 'mytag': "MyName"

Field: User.Email
    Whole tag value : "mytag:\"MyEmail\""
    Value of 'mytag': "MyEmail"

Brigade launched

Event-driven scripting tool for Kubernetes released by Microsoft
31 October 2017   447

Microsoft has unveiled its new Open Source development for the needs of DevOps, a Brigade utility designed to run scripts that are executed on a Kubernetes cluster on an event.

The Brigade utility was created by former employees of Deis, a company that Microsoft bought earlier this year. Deis was working on Kubernetes, Helm and Draft. The purpose of Brigade is to "script simple and complex workflows using JavaScript." The solution allows to associate containers by running them sequentially or in parallel and invoking scripts based on time, events in GitHub (also supported by "DockerHub and other popular web services"), push operations in Docker or other triggers. Readme of the project describes it as "a tool for creating pipelines for Kubernetes".

Brigade architecture
Brigade architecture 

Brigade is written in Go and TypeScript / JavaScript, it functions as a service inside Kubernetes. The job (task) in Brigade is a JavaScript script that is interpreted by the product service, which leads to the creation of the necessary resources in Kubernetes. Next, Brigade expects events and performs the corresponding task trigger. It is assumed that the new solution is well suited for the tasks of continuous integration and delivery of applications (CI / CD), tk. simplifies automated testing, assembly of artifacts and releases, management of software deployment.

Check GitHub for more information.