Go library to make HTTP API clients and servers.

Package api2 provides types and functions used to define interfaces of client-server API and facilitate creation of server and client for it. You define a common structure (GetRoutes, see below) in Go and api2 makes both HTTP client and HTTP server for you. You do not have to do JSON encoding-decoding yourself and to duplicate schema information (data types and path) in client.

Organize your code in services. Each service provides some domain specific functionality. It is a Go type whose methods correspond to exposed RPC's of the API. Each method has the following signature:

func(ctx, *Request) (*Response, error)

Let's define a service Foo with method Bar.

type Foo struct {
	...
}

type BarRequest struct {
	// These fields are stored in JSON format in body.
	Name string `json:"name"`

	// These fields are GET parameters.
	UserID int `query:"user_id"`

	// These fields are headers.
	FileHash string `header:"file_hash"`

	// These fields are cookies.
	Foo string `cookie:"foo"`

	// URL parameters present in URL template like "/path/:product".
	Product string `url:"product"`

	// These fields are skipped.
	SkippedField int `json:"-"`
}

type BarResponse struct {
	// These fields are stored in JSON format in body.
	FileSize int `json:"file_size"`

	// These fields are headers.
	FileHash string `header:"file_hash"`

	// These fields are skipped.
	SkippedField int `json:"-"`
}

func (s *Foo) Bar(ctx context.Context, req *BarRequest) (*BarResponse, error) {
	...
}

A field must not have more than one of tags: json, query, header, cookie. Fields in query, header and cookie parts are encoded and decoded with fmt.Sprintf and fmt.Sscanf. Strings are not decoded with fmt.Sscanf, but passed as is. Types implementing encoding.TextMarshaler and encoding.TextUnmarshaler are encoded and decoded using it. Cookie in Response part must be of type http.Cookie. If no field is no JSON field in the struct, then HTTP body is skipped.

You can also set HTTP status code of response by adding a field of type int with tag use_as_status:"true" to Response. 0 is interpreted as 200. If Response has status field, no HTTP statuses are considered errors.

If you need the top-level type matching body JSON to be not a struct, but of some other kind (e.g. slice or map), you should provide a field in your struct with tag use_as_body:"true":

type FooRequest struct {
	// Body of the request is JSON array of strings: ["abc", "eee", ...].
	Body []string `use_as_body:"true"`

	// You can add 'header', 'query' and 'cookie' fields here, but not 'json'.
}

If you use use_as_body:"true", you can also set is_protobuf:"true" and put a protobuf type (convertible to proto.Message) in that field. It will be sent over wire as protobuf binary form.

You can add use_as_body:"true" is_raw:"true" to a []byte field, then it will keep the whole HTTP body.

Streaming. If you use use_as_body:"true", you can also set is_stream:"true". In this case the field must be of type io.ReadCloser. On the client side put any object implementing io.ReadCloser to such a field in Request. It will be read and closed by the library and used as HTTP request body. On the server side your handler should read from the reader passed in that field of Request. (You don't have to read the entire body and to close it.) For Response, on the server side, the handler must put any object implementing io.ReadCloser to such a field of Response. The library will use it to generate HTTP response's body and close it. On the client side your code must read from that reader the entire response and then close it. If a streaming field is left nil, it is interpreted as empty body.

Now let's write the function that generates the table of routes:

func GetRoutes(s *Foo) []api2.Route {
	return []api2.Route{
		{
			Method:    http.MethodPost,
			Path:      "/v1/foo/bar/:product",
			Handler:   s.Bar,
			Transport: &api2.JsonTransport{},
		},
	}
}

You can add multiple routes with the same path, but in this case their HTTP methods must be different so that they can be distinguished.

If Transport is not set, DefaultTransport is used which is defined as &api2.JsonTransport{}.

Error handling. A handler can return any Go error. JsonTransport by default returns JSON. Error() value is put into "error" field of that JSON. If the error has HttpCode() int method, it is called and the result is used as HTTP return code. You can pass error details (any struct). For that the error must be of a custom type. You should register the error type in JsonTransport.Errors map. The key used for that error is put into "code" key of JSON and the object of the registered type - into "detail" field. The error can be wrapped using fmt.Errorf("%w" ...). See custom_error_test.go for an example.

In the server you need a real instance of service Foo to pass to GetRoutes. Then just bind the routes to http.ServeMux and run the server:

// Server.
foo := NewFoo(...)
routes := GetRoutes(foo)
api2.BindRoutes(http.DefaultServeMux, routes)
log.Fatal(http.ListenAndServe(":8080", nil))

The server is running. It serves foo.Bar function on path /v1/foo/bar with HTTP method Post.

Now let's create the client:

// Client.
routes := GetRoutes(nil)
client := api2.NewClient(routes, "http://127.0.0.1:8080")
barRes := &BarResponse{}
err := client.Call(context.Background(), barRes, &BarRequest{
	Product: "product1",
	...
})
if err != nil {
	panic(err)
}
// Server's response is in variable barRes.

The client sent request to path "/v1/foo/bar/product1", from which the server understood that product=product1.

Note that you don't have to pass a real service object to GetRoutes on client side. You can pass nil, it is sufficient to pass all needed information about request and response types in the routes table, that is used by client to find a proper route.

You can make GetRoutes accepting an interface instead of a concrete Service type. In this case you can not get method handlers by s.Bar, because this code panics if s is nil interface. As a workaround api2 provides function Method(service pointer, methodName) which you can use:

type Service interface {
	Bar(ctx context.Context, req *BarRequest) (*BarResponse, error)
}

func GetRoutes(s Service) []api2.Route {
	return []api2.Route{
		{Method: http.MethodPost, Path: "/v1/foo/bar/:product", Handler: api2.Method(&s, "Bar"), Transport: &api2.JsonTransport{}},
	}
}

If you have function GetRoutes in package foo as above you can generate static client for it in file client.go located near the file in which GetRoutes is defined:

api2.GenerateClient(foo.GetRoutes)

GenerateClient can accept multiple GetRoutes functions, but they must be located in the same package.

You can find an example in directory example. To build and run it:

$ cd api2/
$ go install ./example/...
$ app &
$ client
test
87672h0m0s
ABC XYZ
302 https://example.com/user?id=user123
secret

Code generation code is located in directory example/gen. To regenerate file client.go run:

$ go generate github.com/starius/api2/example