Category: Golang

In Go or Golang, declaring an interface is pretty simple and easy.

We just defined an interface named Printer that required an implementer to have a method named Print which takes a string parameter and returns nothing. Interfaces are implemented implicitly in Go. Any type that has the Print(string) method implements the interface. There is no need to use any implements keyword or anything of that sort.

In the above example, the Terminal type implements the Printer interface because it implements the methods required by the interface. Here’s a runnable, full code example:

We declared our printer variable to be of type Printer which is the interface. Since the Terminal type implements the Printer interface, we can pass Terminal{} to the printer variable and later call the Print method on it.

Interface and the Method sets

As you can understand, a method set is a set of methods on a type. The method set of an interface type (for example Printer here) is it’s interface, that is the Print method in this example. The method set of a type T (for example Terminal) contains the methods which can take a T type receiver. In our above code, the Print method takes the type Terminal so it’s included in Terminal‘s method set. The corresponding pointer type, *T has a method set that includes all methods with a receiver type of *T as well as the methods defined on the receiver type T. So *Terminal type contains any method that either takes Terminal or Terminal as a receiver type. So the Print method is also in the method set for *Terminal .

Method Set of T includes all methods receiving just T.
Method Set of *T includes all methods receiving either T or *T.

So the method set of *T includes the method set of T anyway. But by now, you might be wondering why this is so important. It is very important to understand the method set of a type because whether it implements an interface or not depends on the method set. To understand things further, let’s take a quick look at the following example:

If you try to run this code on the go playground or try to run/compile it on your machine, you shall get an error message like this:

Can you guess what’s happening? Well, our Print method has a receiver type of *Terminal however we are trying to assign the type Terminal to printer. The Print method falls in the method set of *Terminal and not Terminal. So in this particular example, *Terminal type actually implements the interface, not the base Terminal type. We can just assign &Terminal to printer and it will work fine. Try the codes here – https://play.golang.org/p/MvyD0Ls8xb 🙂

Another interesting thing, since *Terminal also includes the method set defined on Terminal, this could would be valid just fine – https://play.golang.org/p/xDmNGBcwsM. This is why understanding the method set of a type is important to understand which interfaces it implements.

The Curious Case of Method Calls

We have seen how the method set of *T includes methods receiving both T and *T but the method set of T is confined to methods that only take T and not *T. Now you might be thinking – I have seen codes like the following snippet:

Here, the Print method receives a *Terminal type but how are we calling it on Terminal type? From what we have seen before, Terminal should not have the method set defined to take a *Terminal receiver, how is this call being made?

Well, the code x.m() works fine if the m method takes the x type as receiver. That is fine with us. But if the method m is to take the type *x and we try to call x.m() – that shouldn’t work, right? The proper call should be (&x).m() – no? Yes, correct. But Go provides us a shortcut here. If the method m is defined to take a *x type as receiver and base type x is addressable, x.m() works as a shortcut for (&x).m(). Go provides us with that shortcut to keep things simpler. So whether you have a pointer or a value, it doesn’t matter, as long as the type is addressable, you can call the method set of *x on x using the very same syntax. However, please remember that this shortcut is not available while working with interfaces.

The Empty Interface

The type interface{} has zero methods defined on it. And every type in Go implements zero or more methods. So their method set actually satisfies the emtpy interface aka interface{}. So if a variable is of type interface{}, we can pass any type to it.

We want to store different types in the same slice? Map values can be of different types? Just use interface{}.

So, when we’re not sure of a type, or we need the type to flexible / dynamic, we can use interface{} to store them.

Type Assertion

While we can store any type in a interface{} type, not all types are the same. For example, you can not use the string functions on an integer type. Go would not accept if you blindly want to pass interface{} in an operation where a very specific type is expected. Take a look:

Even though we have a string value stored against the name key, Go actually stores it as a type interface{} and thus it won’t allow us to use it like a string. Luckily, interface values do store the underlying value and type. So we can use type assertion to assert that the underlying value can behave like a certain type.

This works:

The unKnownMap["name"].(string) part – we’re doing the type assertion here. If the type assertion succeeds, we can use the value as a string. If it does not succeed, we will get a panic.

Getting Type of an interface{}

If you have an interface{} and want to know what it holds underneath, you can use the %T format in Printf family of calls.

Type Switch

You can also use a switch statement with an interface{} to deal with different possible types.

The i.(type) gets you the type of the variable. Please remember it only works with a switch statement.

Go aka Golang is a very promising programming language with a lot of potential. It’s very performant, easy to grasp and maintain, productive and backed by Google. In our earlier posts, we have tried to provide guidelines to learn Go and later we saw how to work with JSON in Go. In this blog post, we’re going to see how we can make http requests using Go. We shall make use of the net/http package in Go which provides all the stuff we need to make http requests or create new http servers. That is, this package would help you do all things “http”. To check / verify that we made correct requests, we would be using httpbin which is a nice service to test our http client requests.

A Simple HTTP Request

Let’s make a very simple GET request and see how we can read the response. We would be sending a simple HTTP GET request to https://httpbin.org/get and read the response. For that we can just import the net/http package and use the http.Get function call. Let’s see an example:

We have created a separate MakeRequest function and called it from our main function. So going ahead, we will just see the changes inside this function and won’t need to think about the entire program. Inside this function, we have passed the url to http.Get and received two values – the response object and any errors that might have happened during the operation. We did a check to see if there were any errors. If there weren’t any errors, err would be nil. Please note that this err would be reported only if there was an issue connecting to the server and getting a response back. However, it would not be concerned about what http status code the server sent. For example, if the server sends a http 500 (which is internal server error), you will get that status code and error message on the resp object, not on err.

Next, we read the resp.Body which implements the io.ReadCloser interface and we can use ioutil.ReadAll to fully read the response. This function also returns two values – a byte slice ([]byte) and err. Again, we check for any potential errors in reading the response body. If there were no errors, we print out the body. Please note the string(body) part. Here, we’re converting the byte slice to a string. If we don’t do it, log.Println would print out representation of the byte slice, a list of all the bytes in that slice, individually. But we want a string representation. So we go ahead and make the conversion.

We would see the printed output is a JSON string. You will notice the httpbin service outputs JSON messages. So in the next example, we would see how we can send and read JSON messages.

JSON Requests and Responses

Now let’s send a JSON message. How do we do that? If you’re coming from Python / Node / Ruby, you might be used to passing a dictionary like structure to your favorite requests library and just mention it should be sent as JSON. Your library does the conversion for you and sends the request with required headers. In Go, however, things are more explicit, which is a good thing in fact. You will know what you’re doing, how you’re doing it. If the JSON related functionality is new to you, please do check our blog post – Golang: Working with JSON.

In Go, we would first convert our data structure to a byte slice containing the JSON representation of the data. Then we pass it to the request with the proper content type. Let’s see a code example:

We first created message which is a map containing a string value, an integer value and another embedded map. Then we json.Marshal it to get the []byte out of it. We also check for any errors that might happen during the marshalling. Next, we make a POST request using the http.Post function. We pass the url, our content type, which is JSON and then we create and pass a new bytes.Buffer object from the bytes representation. Why do we need to create a buffer here? The http.Post function expects an implementation of io.Reader – which is a brilliant design, anything that implements an io.Reader can be passed here. So we could even read this part from disk or network or any custom readers we want to implement. In our case, we can just create a bytes buffer which implements the io.Reader interface. We send the request and check for errors.

Next we declare another result variable (which is also a map type) to store the results returned from the request. We could read the full body first (like previous example) and then do json.Unmarshal on it. However, since the resp.Body is an io.Reader, we can just pass it to json.NewDecoder and then call Decode on it. Remember, we have to pass a pointer to our map, so we passed &result instead of just result.  The Decode function returns an error too. But we assumed it would not matter and didn’t check. But best practice would have been to handle it as well. We logged the result and result["data"]. The httpbin service sends different information about the request as the response. You can see those in the result map. If you want to see the data you sent, they will be in the data key of the result map.

Posting Form

In our last example, we have submitted JSON payload. What if we wanted to submit form values? We have the handy http.PostForm function for that. This function takes the url and url.Values from net/url package. The url.Values is a custom type which is actually map[string][]string internally. That is – it’s a map which contains string keys and against each key, there can be multiple string values ([]string). In a form request, you can actually submit multiple values against one field name. That’s the reason it’s a slice of string, instead of just a key to value mapping.

Here’s an example code snippet:

We would be reading the form key from the result map to retrieve our form values. We have seen how we can easily send form values using the net/http package. Next we would like to send a file along with typical form fields. For that we would also need to learn how to customize http requests on our own.

Custom Clients / Requests

The http.Get, http.Post or http.PostForm calls we have seen so far uses a default client already created for us. But now we are going to see how we can initialize our own Client instances and use them to make our own Requests. Let’s first see how we can create our own clients and requests to do the same requests we have made before. A quick example follows:

As you can see, we just take a new instance of http.Client and then create a new request by calling http.NewRequest function. It takes the http method, url and the request body. In our case, it’s a plain GET request, so we pass nil for the body. We then call the Do method on the client  and parse the response body. So that’s it – create a client, create a request and then let the client Do the request. Interestingly the client also has convenient methods like Get, Post, PostForm – so we can directly use them. That’s what http.Get, http.Post, http.PostForm and other root level functions actually do. They call these methods on the DefaultClient which is already created beforehand. In effect, we could just do:

And it would work similarly. Now you might be wondering – why not just use the DefaultClient, why create our own? What is the benefit?

Customizing the Client

If we look at the definition of the http.Client structure, it has these fields:

If we want, we can set our own transport implementation, we can control how the redirection is handled, pass a cookie jar to save cookies and pass them to the next request or simply set a timeout. The timeout part is often very significant in making http requests. The DefaultClient does not set a timeout by default. So if a malicious service wants, it can start blocking your requests (and your goroutines) indefinitely, causing havoc in your application. Customizing the client gives us more control over how the requests are sent.

File Upload

For uploading files while sending a http request, we need to use the mime/multipart package with the net/http package. We will first see the code example and then walk through it to understand what we’re doing. The code might seem a lot (it includes a lot of error handling) and complex. But please bear with me, once you go through the code and understand what’s happening, it will seem so simpler 🙂

So what are we doing here?

• First we are opening the file we want to upload. In our case, I have created a file named “name.txt” that just contains my name.
• We create a bytes.Buffer to hold the request body we will be passing with our http.Request later on.
• We create a multipart.Writer object and pass a pointer to our bytes.Buffer object so the multipart writer can write necessary bytes to it.
• The multipart writer has convenient methods to create a form file or a form field. It gives us back a writer to which we can write our file content or the field values. We create a file field and copy our file contents to it. Then we create a normal field and write “Value” to it.
• Once we have written our file and normal form field, we call the Close method on the multipart writer object. Closing it writes the final, ending boundary to the underlying bytes.Buffer object we passed to it. This is necessary, otherwise the request body may remain incomplete.
• We create a new post request like we saw before. We passed the bytes.Buffer we created as the request body. The body now contains the multi part form data written with the help of the mime/multipart package.
• We send the request as before. But we set the content type by calling multiPartWriter.FormDataContentType() – which ensures the correct content type and boundary being set.
• We decode the response from httpbin and check the output.

If everything goes well, we will see the form field and the file name in the response we received from httpbin. The concept here is simple. We are sending a http request with a custom body. We could construct the request body ourselves but we just took the help of the mime/multipart package to construct it in a relatively easier fashion.

Always Close The Response Body

Here’s a lesson I learned the hard way. When we make a http request, we get a response and an error back. We may feel lazy and decide not to check for errors or close the response body (just like in the examples above). And from the laziness comes disaster. If we do not close the response body, the connection may remain open and cause resource leak. But if the error is not nil that is in case of an error, the response can be nil. So we can’t just do a defer resp.Body.Close() in this case. We have to properly check error and then close the response body.

Always Use a Timeout

Try to use your own http client and set a timeout. Not setting a timeout can block the connection and the goroutine and thus cause havoc. So do something like this:

 

We previously wrote an introductory blog post on Golang describing why and how the language is gaining traction quickly. Go aka Golang is being used by many large scale systems for various purposes. One thing common in most complex systems is that we have to communicate with other systems / services. JSON is a very popular data interchange format for this kind of scenarios. In fact, it is so popular that you may even go ahead and call it the de-facto data interchange format on the internet.  If you have followed any of our REST API tutorials (Django / Flask), you might have also noticed they all output JSON. In this blog post, we will see the different ways we can work with JSON in Golang.

Creating JSON

We can use the encoding/json package to easily create JSON from our Go data structures. There’s a few things to consider while creating JSON in Go. One of them is the choice of data structure.

From Structs

Structs are very convenient to use and often come as cheaper compared to map or other complex structures. We can pass any instance of a struct to the json.Marshal function and get back the JSON as a slice of bytes ([]byte).

Output should be:

Things to note here:

• The field name has to start with a capital letter – this is important and will be discussed soon
• You can nest other structures, in this example our Emails field contain a list of strings
• The json.Marshal function returns the JSON and error, don’t forget to handle the error. You may not often get errors from the Marshal function but in some cases where some types can not be converted into JSON, you will get errors. So look out for it.
• The returned JSON is in the form of bytes list. So if you want to use it as string, you will need to convert it.

Choosing Field Names

If you have seen the output, the keys/fields in the created JSON structure all start with a capital letter (our struct fields were similar, so this is no surprise). If you have a curious and adventurous mind, you might have tried to go ahead and convert the struct fields into lower caps. But that doesn’t work, does it? Check it out – https://play.golang.org/p/93eDoFSjnW – because if a field / member name does not start with a capital letter, it is not exported. External code can not access it. This is why our name and age fields are not part of the generated JSON structure.

But don’t worry, there’s a simple way of “tagging” our struct fields so we can describe how to marshal our structs. Let’s modify our codes to look like this:

Next to each field, we have provided tags to describe how this field should be marshalled or unmarshalled. Now we can see you expected results here: https://play.golang.org/p/xlcjU1_VSE 🙂

If you don’t understand the tags, don’t worry, try reading this answer on StackOverflow – https://stackoverflow.com/questions/10858787/what-are-the-uses-for-tags-in-go. We will understand the concepts more when we use structs for various purposes (for example mapping to database tables, but for now, let’s not worry).

Omitting Empty Fields

What if some of the fields are empty? Let’s try that.

The output would be:

If a field is empty, we might want to omit that from our JSON. We can do so by using the omitempty flag in our json field tags.

Now if we check the output again:

Nice, no?

Skipping Fields

Let’s say in our struct, we need to keep the Age field. But we don’t want it to be a part of the produced JSON. We can use the - flag in the json tag for that particular field.

The output would be:

Even though the struct had the Age field set, it wasn’t included in the output. This comes very handy in some cases, for example when a User struct has a Password field that we don’t want to serialize into JSON.

Using Maps and Slices

So far we have used structs. We can also use maps and slices instead. Here’s a quick code example:

And using slices:

They both work as expected. But in most codebases I have come across, structs are more widely used.

 

Parsing JSON

We have so far seen how we can generate JSON from our go data. Now we will see the opposite. We will be parsing JSON into Go data structures.

Into Structs

We will first see how we can parse JSON data into structs. It’s quite similar to what we did earlier. We will be using the Unmarshal function which takes bytes and pointer to any interface{} type. It reads through the JSON and stores the data in the struct we pass as the second parameter. Let’s see an example:

Here json_bytes hold the JSON we want to process. We already have a Person type with tagged fields. We just need to pass this json_bytes and a pointer to an instance of Person to the Unmarshal function. Please note the pointer is important. We have to pass a pointer otherwise the parser would not be able to write to the struct.

If the struct doesn’t have some fields which are present in the JSON, those will be silently ignored. In the same way, if the struct has fields which are not available in the JSON, they will be ignored too.

In the above example, the struct has a field named Address which the JSON doesn’t provide. On the other hand, the JSON has the Score key which the struct knows nothing about. In this case, masnun.Address will be empty string.

Into Maps / Slices

We have previously mentioned how structs are cheaper and more widely used than maps. But there’s these use cases where we can not be certain about the structure of the JSON data we want to parse. In such cases, maps can be very useful. Let’s see:

See? We have passed map[string]interface{} and received all the data in JSON. But please remember, the values to each key in the map will be of type interface{}. If we want to extract part of the data, for example, one of the emails and then use it as a string, I will have to manually convert it to a string.

For example this code will fail:

We will get an error:

That’s what I was trying to explain just above. The Emails key has a value of interface{} type. Let’s cast it to a list of interface{} first. Then we can take an element (which will be again interface{} type). We further cast it to a string.

You may be wondering why couldn’t we just get Emails as []string? Well, Go doesn’t know the types of the values in our JSON. So it uses interface{} . That is when it stores Emails, it stores it as a list of unknown types or a list of interface{}. That is why we first need to get it as a list and then we can take individual items and further convert them to the type we want.

Now it works fine 🙂

Streaming JSON Encoding and Decoding

The json package offers NewEncoder and NewDecoder functions which would get us Encoder and Decoder types. These types can work with other objects that support io.Reader and io.Writer interfaces to offer streaming support.

Streaming JSON from a File

We can open a JSON file using the os.Open function and stream it using the json.NewDecoder function. Here’s a quick example:

We opened a file which implements the io.Reader interface. So we can use it with our Decoder type. We created a decoder with the file reader and then called the Decode method on it. That’s all we needed 🙂

Streaming JSON into a File

Writing JSON is also very similar. We need to open a file in write mode, grab the io.Writer and pass it to json.NewEncoder – then we can pass our data to the Encode method to stream the json into the file.

 

Custom Marshal / Unmarshal

If we want to change how our own types are marshalled or unmarshalled, we can implement the json.Marshaler and json.Unmarshaler interfaces. It’s actually simple. We need to define the MarshalJSON and UnmarshalJSON methods on our structs and we’re done. Here’s an example from the official documentation:

Pretty nice, no?