Tutorial - Shared validation rules

What you’ll learn

  1. How to write validation rules that can be shared between multiple endpoints
  2. How to delegate validation of a field to a Granitic component


  1. Follow the Granitic installation instructions
  2. Read the before you start tutorial
  3. Followed the setting up a test database section of tutorial 6
  4. Either have completed tutorial 7 or clone the tutorial repo and navigate to json/008/recordstore in your terminal.

Test database

If you didn’t follow tutorial 6, please work through the ‘Setting up a test database‘ section which explains how to run Docker and MySQL with a pre-built test database.

Shared rules

The validation rules we’ve expressed in config/base.json currently look like:

"submitArtistRules": [
  ["Name",             "STR",  "REQ:NAME_MISSING", "TRIM", "STOPALL", "LEN:5-50:NAME_BAD_LENGTH", "BREAK", "REG:^[A-Z]| +$:NAME_BAD_CONTENT"],
  ["FirstYearActive",  "INT",  "RANGE:1700|2100:FIRST_ACTIVE_INVALID"]

These rules are specific to submitting an artist, but some rules (like checking to see if an artist exists) are likely to be useful in a number of places. Granitic provides a mechanism for defining rules in a way in which they can be shared. Open comp-def/common.json and add this component:

"sharedRuleManager": {
  "type": "validate.UnparsedRuleManager",
  "Rules": "$sharedRules"

In the same file, modify the submitArtistValidator component so its definition looks like:

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

We now need to edit config/base.json to add some shared rules. Add the following:

"sharedRules": {
  "artistExistsRule": ["INT", "EXT:artistExistsChecker"]

EXT (short for external) is an operation that delegates validation of a field to another Granitic component, in this case a component named artistExistsChecker that will need to implement the validate.ExternalInt64Validator interface.

We need to alter the existing submitArtistRules in config/base.json so that they use the shared we’ve just defined when checking the set of ‘related artists’ that are provided when creating a new artist:

"submitArtistRules": [
  ["Name",             "STR",  "REQ:NAME_MISSING", "TRIM", "STOPALL", "LEN:5-50:NAME_BAD_LENGTH", "BREAK", "REG:^[A-Z]| +$:NAME_BAD_CONTENT"],
  ["FirstYearActive",  "INT",  "RANGE:1700|2100:FIRST_ACTIVE_INVALID"],
  ["RelatedArtists", "SLICE",  "ELEM:artistExistsRule:NO_SUCH_RELATED"]

ELEM is an operation that causes a shared rule to be applied to each element of a slice. We have introduced an new error code NO_SUCH_RELATED, so we’ll need to add that to our serviceErrors in config/base.json. Add the following:

  ["C", "NO_SUCH_RELATED", "Related artist does not exist"]

Optional exercise

You’ll notice that the configuration file config/base.json and the component definition file comp-def/common.json are getting quite complex now. Try refactoring these into multiple files (e.g config/base.json, config/validation.json, config/messages.json)

Validation component

We now need to build the component that actually performs the database check. Create a new file db/validate.go and set its contents to:

package db

import (

type ArtistExistsChecker struct{
  DbClientManager rdbms.ClientManager
  Log logging.Logger

func (aec *ArtistExistsChecker) ValidInt64(id int64) (bool, error) {

  dbc, _ := aec.DbClientManager.Client()

  var count int64

  // Execute a query that counts how many artists in the database share the ID we are checking
  // If the count is zero, that artist doesn't exist.
  if _, err := dbc.SelectBindSingleQIDParam("CHECK_ARTIST", "ID", id, &count); err != nil {
    return false, err
  } else {
    return count > 0, nil

An we’ll need to add a new query to resource/queries/artist:


    id = ${ID}

The last step is to register this new checker as a component by adding the following to your comp-def/common.json file:

"artistExistsChecker": {
  "type": "db.ArtistExistsChecker"

Binding database results to a basic type

Previous examples have shown how to bind database results into a struct or slice of structs. In the Go code above

  dbc.SelectBindSingleQIDParam("CHECK_ARTIST", "ID", id, &count)

we are binding the results of the database call to an int64. You may supply a basic type (string, int etc) instead of a struct when your query is guaranteed to return a single row with a single column.

Building and testing

Start your service by navigating to the folder where you have yout tutorial project and running:

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

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

  "Name": "Another Artist",
  "RelatedArtists": [-1, 1, 9999]

(see the data capture tutorial for instructions on using a browser plugin to do this) and you should get a result like:

        "Message":"Related artist does not exist."
        "Message":"Related artist does not exist."


  • Validation rules can be defined globally so they can be re-used by multiple endpoints
  • When validating slices, the validation of each element can be delegated to another validation rule
  • When validating ints, floats and strings your validation rule can delegate to another component, as long as it implements validate.ExternalInt64Validator, validate.ExternalFloat64Validator or validate.ExternalStringValidator
  • Database results can be bound to a basic type as long as your query returns one row with one column.