Why there are tags in Go?

A small tutorial with code examples about tags in Golang programming language
10 August 2017   713

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"

NGINX to Release Unit 1.3 Beta

Developers expanded the ability to run web applications in Python, PHP, Perl, Ruby and Go
16 July 2018   60

In open access, a beta version of the NGINX Unit 1.3 application server was released. Developers continued to expand the ability to run web applications in Python, PHP, Perl, Ruby and Go. The project code is written in C and is distributed under the Apache 2.0 license.

Features

Version 1.3 eliminates the problems with handling errors when installing HTTP connections.

Among other changes:

  • parameter max_body_size to limit the size of the body of the request;
  • new parameters for setting timeouts when setting up an HTTP connection:
         "settings": {
              "http": {
                  "header_read_timeout": 30,
                  "body_read_timeout": 30,
                  "send_timeout": 30,
                  "idle_timeout": 180,
                  "max_body_size": 8388608
              }
          },
  • automatic use of the Bundler where possible in the Ruby module;
  • http.Flusher interface in the module for the Go language;
  • The possibility of using characters in the UTF-8 encoding in the request headers.

The first version of the NGINX 1.1 application server was released in mid-April 2018. Under the control of NGINX Unit, several applications can be executed simultaneously in different programming languages, the startup parameters of which can be changed dynamically without the need to edit the configuration files and restart.