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Golang - What is go:embed Directive in Go?


Golang - What is go:embed Directive in Go?

Introduction


In the previous article we delved into the fascinating world of Go Generics and learnt how to incorporate it in our go programs. In this article we will explore yet another cool feature in Go, the go embed directive.


Go has been one of the fastest growing programming languages in the recent years. It is known for its simplicity, robustness and performance. When it comes to features, the Go language is rapidly expanding with new features being added in each new release. The go:embed directive was one such feature which was introduced in go 1.16.


This simple yet powerful feature lets you access and manage static assets such as configuration files, certificates and much more like never before! This article will help you with everything you need to know about the go:embed directive in order to leverage its benefits in your day to day work.


What is go:embed directive?


The first thing we need to know is that, the go:embed is a compiler directive. What this means is, all of the processing that happens when we use the go:embed directive in go, happens at compile time. It is a special directive that is used to include/embed files or directories directly into the compiled binary of our go program.


How to use go:embed directive?


Using the go embed feature is simple. To use this feature in our go program we simply need to add a comment line `// go:embed`, followed by a space and the path to the file or folder which needs to be embeded. This comment must be added directly above the variable into which we want to our embed files or folders.


Both files and folders can be embeded. We can also use patterns or wildcard characters (such as *) along with the file/folder paths. The syntax for using go embed directive is as shown below:


Syntax

//go:embed <file/folder path or pattern>
var variableName embed.FS

Patterns will be useful in cases where we have to embed multiple files or folders. For example to embed all the .txt files whose name starts with "config", you need to specify the following:

//go:embed config*.txt
var variableName embed.FS

When the go compiler sees the `// go:embed` comment anywhere in the go code, it interprets it as a special comment. The compiler processes this comment and embeds the files mentioned in the directive directly into the variable that we have defined. This is particularly useful in scenarios where we have to bundle static assets, configuration files, templates, or any other type of data directly into our program binary. Remember all of this happens at compile time. So, you need to have these files in the mentioned path at compile time.


Note: Go makes use of the embed package for processing the go embed directive.


Example


Lets understand this with the help of an example. Lets create a sample go project named sample-project. Create a text file example.txt in the current working directory where the main.go file is located. Your project structure should now look something like this:


sample go project structure

Now lets add some text to our example.txt file.


sample-project/example.txt






And then add following code to main.go.


sample-project/main.go



















In the above example, the //go:embed example.txt comment indicates that the file example.txt in the projects' root directory should be embedded into the content variable. content is a variable of type embed.FS from embed package, which is a go core library. Now that example.txt is embeded into the content variable, we can access it using the ReadFile() method (from embed package) as shown in line 12 in the above code. Remember, the embed package needs to be imported (line 4) in order to use this feature. The embed directive and package are available from Go 1.16. So, you will only be able to use this feature if you are using Go 1.16 or later versions.


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Reading files using os or ioutil package vs using go:embed


Using Go os or ioutil Libraries


When we use the go os or ioutil library to read files (method 1, method2), we are accessing those files at runtime. The file must be present on the file system when the program is executed. This method is suitable for use cases where our program needs to read files that are dynamic in nature (files whose content keeps changing) or to read files that are not available at the time of compilation (Only available at runtime).


Using go:embed Directive


The go:embed directive as mentioned earlier is used to embed files or folders at the time of compilation, directly into the compiled binary of the go program. This is the recommended approach for reading files with static content.


Summary


In summary, the decision on whether to choose approach 1 or approach 2 depends on our use case and we can choose either of them by considering the following main factors:

  1. Whether we want to perform read operation at compile time or runtime.

  2. Is it a file with frequently changing content or file with static content.


Limitations


The go:embed directive offers significant advantages, but it is also important for us to understand its limitations:


  1. Large Binaries: Since go:embed is a compile time directive it will have an impact on the size of our compiled binary (built using go build command), especially if the embeded files/folders are large. So, it is important that we take this into consideration and embed only those files/folders that our application really needs.

  2. Security: Since go:embed is usually used to store static files such as certificates, configurations and these files are directly stored within the binary, it is our responsibility to secure any sensitive information contained within these files.

  3. Not for Dynamic Content: As discussed earlier, go:embed directive is used for static files. If your application involves dynamically changing files, go:embed might not be the right choice, in such cases it is better considering the go os or io packages for reading files.

  4. Empty folders: Currently embedding empty folders is not supported by this feature.

  5. The go embed feature can only be used to embed files in the current working directory or subdirectories under it. It cannot be used to embed files outside the current directory.


Conclusion


The go:embed directive is a powerful tool that redefines the way developers manage static assets and resources. It simplifies deployment, reduces external dependencies and enables self contained applications. By understanding its benefits, knowing when to use it and recognizing its limitations, we as developers can harness the full potential of the go embed feature. We hope this article has helped you in this aspect and you will be able to make an informed choice the next time you have to use this feature in your go project.


That's all we had to cover in this article, thank you for your time. If you have any questions or doubts, please let us know in the comments section below, we will be happy to answer you.


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Sep 25, 2023

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