Write resolvers

We need to create two resolvers, one for the getDateTime query and another for the pong mutation

Preparing the files

Both resolvers are simple unit resolvers. To setup a unit resolver, two files are required:

• A request mapping template, defaults to request.vtl, and
• A response mapping template, defaults to response.vtl

To proceed, create the appropriate directories and files.

mkdir vtl/resolvers/Query/getDateTime vtl/resolvers/Mutation/pong
touch vtl/resolvers/Query/getDateTime/{request,response}.vtl vtl/resolvers/Mutation/pong/{request,response}.vtl

This would resuslt in the following directory tree:

vtl
├── functions
└── resolvers
    ├── Mutation
    │   └── pong
    │       ├── request.vtl
    │       └── response.vtl
    └── Query
        └── getDateTime
            ├── request.vtl
            └── response.vtl

Query.getDateTime

To return the current date time, a data source (e.g. a DynamoDB table) is not necessary. Therefore, we can set this resolver's data source to None.

vtl/resolvers/Query/getDateTime/request.vtl

##@butler.dataSource('none')

{
    "version": "2018-05-29"
}

The none data source key is a reserved key used internally by AppSync Butler to refer to a None data source.

vtl/resolvers/Query/getDateTime/response.vtl

$util.toJson($util.time.nowFormatted("yyyy-MM-dd HH:mm:ssZ"))

Mutation.pong

Similarly, this a None (local) data source resolver.

vtl/resolvers/Mutation/pong/request.vtl

##@butler.dataSource('none')

{
    "version": "2018-05-29",
    "payload": {
        "word": "$ctx.args.word"
    }
}

vtl/resolvers/Mutation/pong/response.vtl

#set($word = $ctx.result.word)
#set($reversed = "")
#set($end = $word.length() - 1)

#foreach ($i in [$end..0])
    #set($reversed = "${reversed}$word.charAt($i)")
#end

$util.toJson($reversed)

tip

The above resolver revereses the character sequence. When given the input word is "hello", it outputs "olleh". Based on this resolver, a fun exercise would be to check whether the input word is a palindrome or not.