Explore GraphQL using introspection queries

GraphQL services support introspection queries. Unfortunatelly, on production environments introspections usually blocked for sequrity reasons. But I found Deuche Bahn one if you don't have your own to play with https://bahnql.herokuapp.com/graphql

Insomnia template for below examples

Check the list of possible queries and mutations.

{
  __schema {
    queryType {
      name
      fields{
        name
      }
    }

    mutationType{
      name
      fields{
        name
      }
    }
  }
}

Or simplified version where common code moved into fragment.

{
  __schema {
    queryType {
      ...TypeFields
    }

    mutationType {                                           
      ...TypeFields
    }
  }
}

fragment TypeFields on __Type
{
  name
  fields {
    name
  }   
}

For Deuche Bahn service I got response below. I would not present all the responses - you can download insomnia app and try by yourself.

{
	"data": {
		"__schema": {
			"queryType": {
				"name": "Query",
				"fields": [
					{
						"name": "routing"
					},
					{
						"name": "stationWithEvaId"
					},
					{
						"name": "stationWithStationNumber"
					},
					{
						"name": "stationWithRill100"
					},
					{
						"name": "search"
					},
					{
						"name": "nearby"
					},
					{
						"name": "parkingSpace"
					}
				]
			},
			"mutationType": null
		}
	}
}

We can see there some queries but no mutations (mutationType: null).
Keep in mind, some nasty services would throw error for such query (when there are no mutations). Then just remove `mutationType` piece.

Good services have also documentation. Just add description field into previous fragment.

fragment TypeFields on __Type
{
  name
  fields {
    name
    description
  }   
}

Keep in mind, depreacted functions are not shown by default. But we can access them and check the reason it is deprecated.

fragment TypeFields on __Type
{
  name
  fields {
    name
    isDeprecated
    deprecationReason
  }   
}

Check the list of possible input parameters.

{
  __schema {
    queryType {
      ...TypeFields
    }

    mutationType {                                           
      ...TypeFields
    }
  }
}

fragment TypeFields on __Type
{
  name
  fields {
    name
    args {
      name
      type {
        name
      }
    defaultValue
    }
  }
}

Check the list of possible output parameters.

{
  __schema {
    queryType {
      ...TypeFields
    }

    mutationType {                                           
      ...TypeFields
    }
  }
}

fragment TypeFields on __Type
{
  name
  fields {
    name
    type {
      ofType {
        name
      }
    }
  }   
}

Check complex types.

Finally, explore the types. The argument is not always a simple string on number. It is often some complex data structures.

{
  __schema {
    queryType {
      ...TypeFields
    }

    mutationType {                                           
      ...TypeFields
    }
  }
}

fragment TypeFields on __Type
{
  name
  fields {
    name
    type {
      ofType {
        name
      }
    }
  }   
}

It was just overview of basic GraphQL cases. In real life you can have more interesting cases with inheritance, unions, enums.
Use insomnia code compleat functionality to explore them all.