Tutorial - Validating data submitted by a client

What you’ll learn

  1. How to define rules to automatically validate incoming data
  2. How to customise the error messages that are returned to your web service clients


  1. Follow the Granitic installation instructions
  2. Read the before you start tutorial
  3. Either have completed tutorial 4 or clone the tutorial repo and navigate to json/005/recordstore in your terminal.


Making sure that data submitted to a web service is safe and correct before processing it is a vital step. Even if your web services are for internal use only, you should never trust callers to always provide valid data.

Performing validation in code can be a tedious process, generating lots of boilerplate code that can be difficult to read. Granitic tries to make your code cleaner and your validation logic easier to understand by defining validation as rules in your configuration files.

Defining rules

We will be adding validation to the /artist POST endpoint we built in the previous tutorial

Open your config/base.json file and set its contents to:

  "Facilities": {
    "HTTPServer": true,
    "JSONWs": true,
    "RuntimeCtl": true,
    "ServiceErrorManager": true

    "GlobalLogLevel": "INFO",
    "ComponentLogLevels": {
      "artistHandlerLogic": "TRACE"

  "submitArtistRules": [
    ["Name",  "STR",  "REQ"]

Rules in Granitic are expressed as a JSON array of string arrays. Each entry in the top level array maps to the name of a field that we want to validate on the target struct that our web service has parsed HTTP request data into. In this case our target is the artist.Submission struct which currently looks like:

type Submission struct {
  Name              string
  FirstYearActive   *types.NilableInt64

We have defined a single rule:

  ["Name",  "STR",  "REQ"]

The first element in the array is the field we’re checking on the target object. The second is the expected data type. The rest of the elements are the validation operations that should be applied to the field.

In this cause we are stating that a single operation should be applied. REQ means the field is required and validation will fail if it is not provided in the JSON body (this is different to the field being present but empty)

We will cover many of the available operations throughout this tutorial, but if you’d like to skip ahead, the validation GoDoc provides a complete reference.

Service Error Manager

You might have noticed that we’ve enabled a new Granitic facility:

   "ServiceErrorManager": true

this facility manages error messages for web service calls. We’ll start using this shortly, but if you’d like to find out more, refer to the GoDoc

Enabling validation

Validation is invoked by the handler component associated with your endpoint. In this case, it’s the submitArtistHandler defined in your comp-def/base.json file. We’ll need to modify this component’s configuration and also declare a new component:

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

"submitArtistValidator": {
  "type": "validate.RuleValidator",
  "DefaultErrorCode": "INVALID_ARTIST",
  "Rules": "$submitArtistRules"

We’ve created a new component submitArtistValidator of type validate.RuleValidator. This is the code that will actually apply the validation rules. We’ve given it a reference to the validation rules we’ve defined in our config:

  "Rules": "$submitArtistRules"

The DefaultErrorCode is used to lookup an error message to return if there isn’t a specific error message defined for a particular validation operation.

validate.RuleValidator is in a package that we haven’t referenced before in our components.json file, so you’ll have to amend the packages array at the top of the file to include:


Component references

Our component definition file is instructing Granitic to inject a reference to submitArtistValidator into the submitArtistHandler component’s AutoValidator field. This relationship is defined by the line

  "AutoValidator": "+submitArtistValidator"

The + symbol tells Granitic that this isn’t an ordinary string, but a reference to a another component.

Setting a default error message

Earlier we enabled the Service Error Manager facility. This facility will refuse to start if we haven’t defined error messages for each error code in use by our code. We’ve used the code INVALID_ARTIST as the default message to use if a more specific one isn’t available, so open your config/base.json and add the following:

"serviceErrors": [
  ["C", "INVALID_ARTIST", "Cannot create an artist with the information provided."]

The “C” indicates that this type of error is a client error where the problem was caused by a mistake made by the calling client. The types of error found during a web service call affect the HTTP status code returned by the web service call. More information can be found in the GoDoc

Service Errors config path

serviceErrors is the config path where the Service Error Manager facility expects to find your error message definitions. If you want to use a different config path, you need to override the config value ServiceErrorManager.ErrorDefinitions

Testing the first validation rule

If you haven’t already done so, please look at the Testing POST services section in the previous tutorial which explains how to use a browser plugin to test web service POST endpoints.

Start your service by navigating to your tutorial project in a terminal and run:

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

And POST the following JSON to http://localhost:8080/artist

  "FirstYearActive": 2010

You should see a response similar to:

       "Message":"Cannot create an artist with the information provided."

and the HTTP status code will be 400 Bad Request You can see that Granitic shows you the names of the fields that errors were found on and a list of each error found on the field. If this is too verbose for your application, you can control how error responses are formatted by providing your own ws.ErrorFormatter

Debugging validation

If you set ApplicationLogger.GlobalLogLevel or your submitArtistValidator component’s log level to DEBUG the console will log each operation run on a field. Refer to the logging tutorial to learn how to do this.

Refining validation

At the moment, you could submit a zero-length Name and the service would accept it, which is probably not what we want. Change your validation rule to look like:

  ["Name",  "STR",  "REQ", "LEN:5-50"]

This will reject the POST if the artist’s name is fewer than five characters or greater than 50. The following JSON will be accepted:

  "Name": "Some Artist",
  "FirstYearActive": 2010

Whitespace around strings

The above rule would accept a name made up of five space characters. Try submitting:

  "Name": "     ",
  "FirstYearActive": 2010

Granitic validation provides an operation TRIM which means that a string will have leading and trailing whitespace removed before any other checks take place. Change your rule to:

  ["Name",  "STR",  "REQ", "TRIM", "LEN:5-50"]

and try again (the Name should be rejected).


This TRIM operation does not permanently alter the data that is submitted to your service. If you submit a Name of “ABCD” it will be validated as “ABCD” but will be passed to the rest of your code as “ABCD”. String validation supports an alternative operation HARDTRIM that permanently trims the supplied data.

Error codes and messages

At the moment our code just returns the same generic error code and message regardless of the problem. You can configure Granitic to return a specific code and message for each problem.

Change your rule to:

   ["Name",  "STR",  "REQ:NAME_MISSING", "TRIM", "LEN:5-50:NAME_BAD_LENGTH"]

and your serviceErrors to

 "serviceErrors": [
   ["C", "INVALID_ARTIST", "Cannot create an artist with the information provided."],
   ["C", "NAME_MISSING", "You must supply the Name field on your submission."],
   ["C", "NAME_BAD_LENGTH", "Names must be 5-50 characters in length."]

Now each check on Name has a specific error message, as well as a catch-all message for any other errors found on the submitted data.

More detailed rules and halting validation

Fields are checked in the order they are specified in your JSON configuration. Let’s add a second rule that checks that the optionally supplied FirstYearActive is in a reasonable range and change the first rule so that only letters and spaces are allowed in names:

    ["FirstYearActive",   "INT",  "RANGE:1700|2100:FIRST_ACTIVE_INVALID"]

We’ve added a new check on Name to make sure it matches the regular expression ^[A-Z]| +$ (letters or spaces only).

We’ve added a second rule to make sure that FirstYearActive is in the range 1700 to 2100 (note the pipe separator).

We’ve also added two new error codes, so add the following messages to your serviceErrors:

  ["C", "NAME_BAD_CONTENT", "Names can only contain letters and spaces."],
  ["C", "FIRST_ACTIVE_INVALID", "FirstYearActive must be in the range 1700-2100"]

Breaking mid-rule

If you restart your service and send the following JSON:

  "Name": "",
  "FirstYearActive": -1

You’ll see that Name fails two tests - the length check and the regex check. It doesn’t make sense to execute the REG operation if the LEN operation has already failed. Change the rule to:

  ["FirstYearActive",   "INT",  "RANGE:1700|2100:FIRST_ACTIVE_INVALID"]

and you’ll see that only the LEN operation applies. BREAK stops executing the current rule if the previous operation failed and moves on to the next rule.

Abandoning all following checks and rules

Sometimes a check is so fundamental that there is no point processing any further rules. Change the Name rule to:


and notice that the FirstYearActive checks are no longer executed if any problems are found with Name because of the STOPALL operation

Advanced validation

This tutorial covers some of the most commons use cases for automatic validation, but Granitic offers much greater depth including invoking components to validate fields, checking for mutual exclusivity between fields and defining callbacks to be invoked at certain points during validation. Refer to the Further reading section below for links to the GoDoc where these features are explained.


  • Granitic can validate received data using rules defined in JSON configuration.
  • Error messages can be as specific as you choose - a message for each check, for each field or just one generic message.
  • Granitic’s default JSON structure for error responses can be overridden.
  • Each validation rule is associated with a field on the web service’s ‘unmarshalling target’.
  • Rules are built from a series of validation operations.
  • Individual rules or the entire set of rules can be configured to halt when an error is found.

Further reading


The next tutorial covers the reading of data from an RDBMS