Tutorial - Capturing data from service calls

What you’ll learn

  1. How Granitic captures data from HTTP web service calls and binds it to your application’s data structures


  1. Follow the Granitic installation instructions
  2. Read the before you start tutorial
  3. Either have completed tutorial 3 or clone the tutorial repo and navigate to json/004/recordstore in your terminal.

Data capture

Granitic’s core purpose is to streamline the process of building web and micro services in Go. One of the most time consuming aspects of buliding web services is writing code to read inbound data from an HTTP request and binding that data to your application’s data structures. This tutorial explains how Granitic makes this easier (the next tutorial will cover the validation of the data you capture).

Web services that are provided as HTTP endpoints generally allow callers to supply data in three different parts of the HTTP request:

  1. The HTTP request’s body
  2. The path component of the URL
  3. The query component of the URL

or some combination of the above. Granitic automates the process of extracting these three types of data from the request and mapping the data to fields on your application’s data structures.

Web service design patterns

Granitic is agnostic as to which web service design pattern (REST, RPC etc) your application follows. The majority of the examples in these tutorials are ‘REST-like’, but that is not to suggest that Granitic favours REST over other patterns.


Callers of HTTP web services may also supply meta-data about the request as request headers. Granitic gives you application access to these headers but does not currently offer a way of automatically binding those headers to your data structures.

Path and query binding

Our tutorial application already supports a single GET endpoint to allow us to recover the details of a recording artist. Start your tutorial application by opening a terminal, navigating to the folder containing the tutorial application and running:

grnc-bind && go build && ./recordstore

and visit http://localhost:8080/artist to see what happens when you execute a GET request.

We want to allow the caller of this endpoint to specify which artist they’d like details for. Open artist/get.go and add the following struct to the end of the file.

type ArtistQuery struct {
  ID int
  NormaliseName *types.NilableBool

Make sure you add github.com/graniticio/granitic/v2/types to that file’s list of imports if your IDE hasn’t already done it for you.

You should also change the signature for the method:

  func (gl *GetLogic) Process(ctx context.Context, req *ws.Request, res *ws.Response)


  func (gl *GetLogic) ProcessPayload(ctx context.Context, req *ws.Request, res *ws.Response, q *ArtistQuery)

As we now want Granitic to use some of the logical contents of the HTTP request (the payload) as data to populate the new ArtistQuery type.

Nilable types

A side-effect of Go’s system of zero values for variables is that it can make recovering data from web service calls ambiguous. For example, if you accept a boolean via a query parameter and the value of the boolean is false, how does your code know if it’s false because:

  • The caller explicitly set the value to false or
  • The caller didn’t supply that parameter at all, so the variable just defaulted to false.

Granitic’s solution this problem is to provide a set of ‘nilable’ struct versions of primitive types (see the Godoc) that provide additional methods to indicate whether the value was explicitly set by the caller or was an automatic zero value.

Configuring path binding

A common REST-like technique is to allow a caller to specify the ID of the required resource (in this case a recording artist) into the path of the request. E.g. /artist/1234. We will configure Granitic to extract that ID and inject it into the ID field of the ArtistRequest struct you defined above - this process is known as path binding in Granitic.

If you open the file comp-def/common.json you will see:

"artistHandler": {
  "type": "handler.WsHandler",
  "HTTPMethod": "GET",
  "Logic": {
    "type": "artist.GetLogic",
    "EnvLabel": "$environment.label(DEV)"
  "PathPattern": "^/artist"

The component artistHandler is an instance of handler.WsHandler It provides all of the automatic web-service processing features supported by Granitic.

We can define how path binding will work by configuring this component. Change the definition of your artistHandler component so it looks like:

"artistHandler": {
  "type": "handler.WsHandler",
  "HTTPMethod": "GET",
  "Logic": {
    "type": "artist.GetLogic",
    "EnvLabel": "$environment.label(DEV)"
  "PathPattern": "^/artist/([\\d]+)[/]?$",
  "BindPathParams": ["ID"]

Parsing data into a target struct

The new method signature:

  func (gl *GetLogic) ProcessPayload(ctx context.Context, req *ws.Request, res *ws.Response, q *ArtistQuery)

on your logic component tells Granitic that incoming data (in the HTTP request body, path and parameters) should be parsed into a new instance of the struct artist.ArtistQuery. In Granitic terminology, this struct is called the target.

Change your GetLogic struct to look like:

type ArtistLogic struct {
  EnvLabel string
  Log      logging.Logger

func (al *ArtistLogic) ProcessPayload(ctx context.Context, req *ws.WsRequest, res *ws.WsResponse, ar *ArtistRequest) {

	gl.Log.LogTracef("Request for artist with ID %d", q.ID)

	name := "Some Artist"

	res.Body = Info{
		Name: name,


Capturing data in request paths

We’ve altered the regular expression that this endpoint expects to


so that in order to match an incoming request that request’s path:

  1. Must start with the string `/artist/’
  2. Must then have a sequence of one or more digits (the ID of the artist)
  3. May optionally have a trailing / character (this is a convenience for the caller)

We’ve defined a regular expression group (round brackets) around the part of the path that will be considered as representing the requested ID.

We’ve also added a new field BindPathParams and set it to an array of strings. The number of strings in this array should match the number of groups in the PathPattern regex. Here we are saying that the value of the first regex group should be injected into a field called ‘ID’ on the target for data extraction, which in this case is artist.ArtistQuery

Testing your changes

Stop, rebuild and restart your application:

grnc-bind && go build && ./recordstore

Visiting http://localhost:8080/artist will now result in a 404 Not Found error, but visiting http://localhost:8080/artist/1234 should result in a response and a log line similar to:

14/Mar/2019:08:19:19 Z TRACE [artistHandlerLogic] Request for artist with ID 1234

Binding query parameters

The technique for binding query parameters (key-value pairs in the request URL after the ? symbol) to your ‘target’ object is very similar to that used for binding path parameters.

We will demonstrate this by adding support for a boolean parameter to normalise the name of the artist in the response.

Edit your comp-def/common.json file and add the following to the definition of your artistHandler component:

   "FieldQueryParam": {
      "NormaliseName": "normalise"

This is instructing Granitic to populate a field (NormaliseName) on your target with the value of a particular query parameter (normalise).

Modify your ArtistLogic.ProcessPayload method to look like

func (al *ArtistLogic) ProcessPayload(ctx context.Context, req *ws.WsRequest, res *ws.WsResponse, ar *ArtistRequest) {

	gl.Log.LogTracef("Request for artist with ID %d", q.ID)

	name := "Some Artist"

	if q.NormaliseName.Bool() {
		name = strings.ToUpper(name)

	res.Body = Info{
		Name: name,

Rebuild and restart your application. Visiting http://localhost:8080/artist/1234?normalise=true will now cause the returned artist’s name to be capitalised.

Extracting data from the request body

Path parameters and query parameters are only useful for submitting limited amounts of semi-structured data to a web service. More common is to use a POST or PUT request to include more complex data in the body of an HTTP request. Granitic has built-in support for accepting data in an HTTP request body as JSON or XML. The following examples all use JSON, refer to the facility/ws and ws/xml GoDoc to discover how to use XML instead.

We will create a new endpoint to accept details of a new artist as POSTed JSON.

Create a new file artist/post.go with the following contents:

package artist

import (

type PostLogic struct {
  Log      logging.Logger

func (pl *PostLogic) ProcessPayload(ctx context.Context, req *ws.Request, res *ws.Response, s *Submission) {
  pl.Log.LogInfof("New artist: '%s'", s.Name)

  res.Body = CreatedResource{0}

type Submission struct {
  Name *types.NilableString
  FirstYearActive *types.NilableInt64

type CreatedResource struct{
  ID int64

This defines new endpoint logic that will expect a populated Submission to be supplied as the ws.WsRequest.RequestBody.

In order to have this code invoked, we will need to add the following to the components section of our comp-def/common.json file:

"submitArtistHandler": {
  "type": "handler.WsHandler",
  "HTTPMethod": "POST",
  "PathPattern": "^/artist[/]?$",
  "Logic": {
    "type": "artist.PostLogic"

Check your config/common.json file and make sure the GlobalLogLevel is set to INFO

grnc-bind && go build && ./recordstore

Testing POST services

Testing POST and PUT services is more complex than GET services as browsers don’t generally have built-in mechanisms for setting the body of a request. There are several browser extensions available that facilitate this sort of testing. The following instructions are based on Advanced Rest Client (ARC) for Chrome

POST a new artist

  1. Open ARC
  2. Set ‘Request URL’ to http://localhost:8080/artist
  3. Change the method picklist to POST
  4. Click on Body and set Body content type to application/json
  5. Enter the ‘test JSON’ below into the large grey text area
  6. Press SEND
  7. You should receive a JSON formatted response with an ID of 0 and see a log line similar to: 14/Mar/2019:09:24:06 Z INFO [submitArtistHandlerLogic] New artist: 'Another Artist'


  "Name": "Another Artist",
  "FirstYearActive": 2010


  • Granitic can extract data from the path, query and body of an HTTP request and bind it to your custom Go structs.
  • All this behaviour is configurable by changing the configuration of your handler components
  • Handler components are instances of handler.WsHandler

Further reading


The next tutorial covers the validation of data submitted to web services