An example CloudFormation template for AWS AppSync

AppSync-Example.yaml

---
Description: AWSAppSync DynamoDB Example
Resources:
  GraphQLApi:
    Type: "AWS::AppSync::GraphQLApi"
    Properties:
      Name: AWSAppSync DynamoDB Example
      AuthenticationType: AWS_IAM

  PostDynamoDBTableDataSource:
    Type: "AWS::AppSync::DataSource"
    Properties:
      ApiId: !GetAtt GraphQLApi.ApiId
      Name: PostDynamoDBTable
      Description: The Post DynamoDB table in us-west-2
      Type: AMAZON_DYNAMODB
      ServiceRoleArn: "arn:aws:iam::YOUR_ACCOUNT_ID:role/AppSyncTutorialAmazonDynamoDBRole"
      DynamoDBConfig:
        AwsRegion: "us-west-2"
        TableName: "AppSyncTutorial-Post"

  QueryGetPostResolver:
    Type: "AWS::AppSync::Resolver"
    DependsOn: Schema
    Properties:
      ApiId: !GetAtt GraphQLApi.ApiId
      TypeName: Query
      FieldName: getPost
      DataSourceName: !GetAtt PostDynamoDBTableDataSource.Name
      RequestMappingTemplate: |
        {
            "version" : "2017-02-28",
            "operation" : "GetItem",
            "key" : {
                "id" : $util.dynamodb.toDynamoDBJson($ctx.args.id)
            }
        }
      ResponseMappingTemplate: "$utils.toJson($ctx.result)"

  QueryAllPostResolver:
    Type: "AWS::AppSync::Resolver"
    DependsOn: Schema
    Properties:
      ApiId: !GetAtt GraphQLApi.ApiId
      TypeName: Query
      FieldName: allPost
      DataSourceName: !GetAtt PostDynamoDBTableDataSource.Name
      RequestMappingTemplate: |
        {
            "version" : "2017-02-28",
            "operation" : "Scan",
            #if( $ctx.args.count )
                "limit": $ctx.args.count,
            #end
            #if( ${ctx.args.nextToken} )
                "nextToken": "${ctx.args.nextToken}"
            #end
        }
      ResponseMappingTemplate: |
        {
            "posts": $utils.toJson($ctx.result.items),
            #if( ${ctx.result.nextToken} )
                "nextToken": "${ctx.result.nextToken}",
            #end
        }

  QueryAllPostsByAuthorResolver:
    Type: "AWS::AppSync::Resolver"
    DependsOn: Schema
    Properties:
      ApiId: !GetAtt GraphQLApi.ApiId
      TypeName: Query
      FieldName: allPostsByAuthor
      DataSourceName: !GetAtt PostDynamoDBTableDataSource.Name
      RequestMappingTemplate: |
        {
            "version" : "2017-02-28",
            "operation" : "Query",
            "index" : "author-index",
            "query" : {
              "expression": "author = :author",
                "expressionValues" : {
                  ":author" : $util.dynamodb.toDynamoDBJson($ctx.args.author)
                }
            },
            #if( $ctx.args.count )
                "limit": $ctx.args.count,
            #end
            #if( ${ctx.args.nextToken} )
                "nextToken": "${ctx.args.nextToken}",
            #end
        }
      ResponseMappingTemplate: |
        {
            "posts": $utils.toJson($ctx.result.items),
            #if( ${ctx.result.nextToken} )
                "nextToken": "${ctx.result.nextToken}",
            #end
        }

  QueryAllPostsByTagResolver:
    Type: "AWS::AppSync::Resolver"
    DependsOn: Schema
    Properties:
      ApiId: !GetAtt GraphQLApi.ApiId
      TypeName: Query
      FieldName: allPostsByTag
      DataSourceName: !GetAtt PostDynamoDBTableDataSource.Name
      RequestMappingTemplate: |
        {
            "version" : "2017-02-28",
            "operation" : "Scan",
            "filter": {
              "expression": "contains (tags, :tag)",
                "expressionValues": {
                  ":tag": $util.dynamodb.toStringJson($ctx.args.tag)
                }
            },
            #if( $ctx.args.count )
                "limit": $ctx.args.count,
            #end
            #if( ${ctx.args.nextToken} )
                "nextToken": "${ctx.args.nextToken}"
            #end
        }
      ResponseMappingTemplate: |
        {
            "posts": $utils.toJson($ctx.result.items),
            #if( ${ctx.result.nextToken} )
                "nextToken": "${ctx.result.nextToken}",
            #end
        }

  MutationaddPostResolver:
    Type: "AWS::AppSync::Resolver"
    DependsOn: Schema
    Properties:
      ApiId: !GetAtt GraphQLApi.ApiId
      TypeName: Mutation
      FieldName: addPost
      DataSourceName: !GetAtt PostDynamoDBTableDataSource.Name
      RequestMappingTemplate: |
        #set( $d = $util.dynamodb )
        #set( $values = $d.toMapValues($ctx.args))
        $!{values.put("ups", $d.toNumber(1))}
        $!{values.put("downs", $d.toNumber(0))}
        $!{values.put("version", $d.toNumber(1))}
        $!{values.put("created", $d.toDynamoDB($util.time.nowISO8601()))}
        $!{values.put("lastUpdated", $values.get("created"))}
        {
            "version" : "2017-02-28",
            "operation" : "PutItem",
            "key" : {
                "id" : $d.toStringJson($utils.autoId())
            },
            "attributeValues" : $util.toJson($values),
        }
      ResponseMappingTemplate: "$util.toJson($ctx.result)"

  MutationAddCommentResolver:
    Type: "AWS::AppSync::Resolver"
    DependsOn: Schema
    Properties:
      ApiId: !GetAtt GraphQLApi.ApiId
      TypeName: Mutation
      FieldName: addComment
      DataSourceName: !GetAtt PostDynamoDBTableDataSource.Name
      RequestMappingTemplate: |
        {
            "version" : "2017-02-28",
            "operation" : "UpdateItem",
            "key" : {
                "id" : $util.dynamodb.toStringJson($ctx.args.id)
            },
            "update" : {
                "expression" : "SET comments = list_append(if_not_exists(comments, :emptyList), :newComment), lastUpdated = :lastUpdated ADD version :plusOne",
                "expressionValues" : {
                    ":emptyList": $util.dynamodb.toListJson([]),
                    ":newComment" : $util.dynamodb.toListJson([$util.map.copyAndRetainAllKeys($ctx.args, ["author","comment"])]),
                    ":plusOne" : $util.dynamodb.toNumberJson(1),
                    ":lastUpdated" : $util.dynamodb.toDynamoDBJson($util.time.nowISO8601())
                }
            },
            "condition" : {
                "expression" : "attribute_exists(id)"
            },
        }
      ResponseMappingTemplate: "$util.toJson($ctx.result)"

  MutationAddTagResolver:
    Type: "AWS::AppSync::Resolver"
    DependsOn: Schema
    Properties:
      ApiId: !GetAtt GraphQLApi.ApiId
      TypeName: Mutation
      FieldName: addTag
      DataSourceName: !GetAtt PostDynamoDBTableDataSource.Name
      RequestMappingTemplate: |
        {
            "version" : "2017-02-28",
            "operation" : "UpdateItem",
            "key" : {
                "id" : $util.dynamodb.toStringJson($ctx.args.id)
            },
            "update" : {
                "expression" : "ADD tags :tags, version :plusOne SET lastUpdated = :lastUpdated",
                "expressionValues" : {
                    ":tags" : $util.dynamodb.toStringSetJson([$ctx.args.tag]),
                    ":plusOne" : $util.dynamodb.toNumberJson(1),
                    ":lastUpdated" : $util.dynamodb.toDynamoDBJson($util.time.nowISO8601()),
                }
            },
            "condition" : {
                "expression" : "attribute_exists(id)"
            },
        }
      ResponseMappingTemplate: "$util.toJson($ctx.result)"

  MutationRemoveTagResolver:
    Type: "AWS::AppSync::Resolver"
    DependsOn: Schema
    Properties:
      ApiId: !GetAtt GraphQLApi.ApiId
      TypeName: Mutation
      FieldName: removeTag
      DataSourceName: !GetAtt PostDynamoDBTableDataSource.Name
      RequestMappingTemplate: |
        {
            "version" : "2017-02-28",
            "operation" : "UpdateItem",
            "key" : {
                "id" : $util.dynamodb.toStringJson($ctx.args.id)
            },
            "update" : {
                "expression" : "DELETE tags :tags ADD version :plusOne SET lastUpdated = :lastUpdated",
                "expressionValues" : {
                    ":tags" : $util.dynamodb.toStringSetJson([$ctx.args.tag]),
                    ":plusOne" : $util.dynamodb.toNumberJson(1),
                    ":lastUpdated" : $util.dynamodb.toDynamoDBJson($util.time.nowISO8601()),
                }
            },
            "condition" : {
                "expression" : "attribute_exists(id)"
            },
        }
      ResponseMappingTemplate: "$util.toJson($ctx.result)"

  MutationDeletePostResolver:
    Type: "AWS::AppSync::Resolver"
    DependsOn: Schema
    Properties:
      ApiId: !GetAtt GraphQLApi.ApiId
      TypeName: Mutation
      FieldName: deletePost
      DataSourceName: !GetAtt PostDynamoDBTableDataSource.Name
      RequestMappingTemplate: |
        {
            "version" : "2017-02-28",
            "operation" : "DeleteItem",
            "key": {
                "id": $util.dynamodb.toStringJson($ctx.args.id)
            },
            #if( $ctx.args.containsKey("expectedVersion") )
                "condition" : {
                    "expression"       : "attribute_not_exists(id) OR version = :expectedVersion",
                    "expressionValues" : {
                        ":expectedVersion" : $util.dynamodb.toNumberJson($ctx.args.expectedVersion)
                    }
                },
            #end
        }
      ResponseMappingTemplate: "$util.toJson($ctx.result)"

  MutationUpvotePostResolver:
    Type: "AWS::AppSync::Resolver"
    DependsOn: Schema
    Properties:
      ApiId: !GetAtt GraphQLApi.ApiId
      TypeName: Mutation
      FieldName: upvotePost
      DataSourceName: !GetAtt PostDynamoDBTableDataSource.Name
      RequestMappingTemplate: |
        {
            "version" : "2017-02-28",
            "operation" : "UpdateItem",
            "key" : {
                "id" : $util.dynamodb.toStringJson($ctx.args.id)
            },
            "update" : {
                "expression" : "ADD ups :plusOne, version :plusOne SET lastUpdated = :lastUpdated",
                "expressionValues" : {
                    ":plusOne" : $util.dynamodb.toNumberJson(1),
                    ":lastUpdated" : $util.dynamodb.toDynamoDBJson($util.time.nowISO8601())
                }
            },
            "condition" : {
                "expression" : "attribute_exists(id)"
            },
        }
      ResponseMappingTemplate: "$util.toJson($ctx.result)"

  MutationDownvotePostResolver:
    Type: "AWS::AppSync::Resolver"
    DependsOn: Schema
    Properties:
      ApiId: !GetAtt GraphQLApi.ApiId
      TypeName: Mutation
      FieldName: downvotePost
      DataSourceName: !GetAtt PostDynamoDBTableDataSource.Name
      RequestMappingTemplate: |
        {
            "version" : "2017-02-28",
            "operation" : "UpdateItem",
            "key" : {
                "id" : $util.dynamodb.toStringJson($ctx.args.id)
            },
            "update" : {
                "expression" : "ADD downs :plusOne, version :plusOne SET lastUpdated = :lastUpdated",
                "expressionValues" : {
                    ":plusOne" : $util.dynamodb.toNumberJson(1),
                    ":lastUpdated" : $util.dynamodb.toDynamoDBJson($util.time.nowISO8601()),
                }
            },
            "condition" : {
                "expression" : "attribute_exists(id)"
            },
        }
      ResponseMappingTemplate: "$util.toJson($ctx.result)"

  MutationUpdatePostResolver:
    Type: "AWS::AppSync::Resolver"
    DependsOn: Schema
    Properties:
      ApiId: !GetAtt GraphQLApi.ApiId
      TypeName: Mutation
      FieldName: updatePost
      DataSourceName: !GetAtt PostDynamoDBTableDataSource.Name
      RequestMappingTemplate: |
        #set( $ddb = $util.dynamodb )
        {
            "version" : "2017-02-28",
            "operation" : "UpdateItem",
            "key" : {
                "id" : $ddb.toDynamoDBJson($ctx.args.id)
            },
            ## Set up some space to keep track of things we're updating **
            #set( $expNames  = {} )
            #set( $expValues = {} )
            #set( $expSet = {} )
            #set( $expAdd = {} )
            #set( $expRemove = [] )
            ## Increment "version" by 1 **
            $!{expAdd.put("version", ":one")}
            $!{expValues.put(":one", $ddb.toDynamoDB(1))}
            ## Set the "lastUpdated" timestamp **
            $!{expSet.put("lastUpdated", ":lastUpdated")}
            $!{expValues.put(":lastUpdated", $ddb.toDynamoDB($util.time.nowISO8601()))}
            ## Iterate through each argument, skipping "id" and "expectedVersion" **
            #foreach( $entry in $util.map.copyAndRemoveAllKeys($ctx.args, ["id","expectedVersion"]).entrySet() )
                #if( $util.isNull($entry.value) )
                    ## If the argument is set to "null", then remove that attribute from the item in DynamoDB **
                    #set( $discard = ${expRemove.add("#${entry.key}")} )
                    $!{expNames.put("#${entry.key}", "${entry.key}")}
                #else
                    ## Otherwise set (or update) the attribute on the item in DynamoDB **
                    $!{expSet.put("#${entry.key}", ":${entry.key}")}
                    $!{expNames.put("#${entry.key}", "${entry.key}")}
                    $!{expValues.put(":${entry.key}", $ddb.toDynamoDB($entry.value))}
                #end
            #end
            ## Start building the update expression, starting with attributes we're going to SET **
            #set( $expression = "" )
            #if( !${expSet.isEmpty()} )
                #set( $expression = "SET" )
                foreach( $entry in $expSet.entrySet() )
                    set( $expression = "${expression} ${entry.key} = ${entry.value}" )
                    if ( $foreach.hasNext )
                        set( $expression = "${expression}," )
                    end
                end
            end
            ## Continue building the update expression, adding attributes we're going to ADD **
            #if( !${expAdd.isEmpty()} )
                #set( $expression = "${expression} ADD" )
                #foreach( $entry in $expAdd.entrySet() )
                    #set( $expression = "${expression} ${entry.key} ${entry.value}" )
                    #if ( $foreach.hasNext )
                        #set( $expression = "${expression}," )
                    #end
                #end
            #end
            ## Continue building the update expression, adding attributes we're going to REMOVE **
            #if( !${expRemove.isEmpty()} )
                #set( $expression = "${expression} REMOVE" )
                #foreach( $entry in $expRemove )
                    #set( $expression = "${expression} ${entry}" )
                    #if ( $foreach.hasNext )
                        #set( $expression = "${expression}," )
                    #end
                #end
            #end
            ## Finally, write the update expression into the document, along with any expressionNames and expressionValues **
            "update" : {
                "expression" : "${expression}",
                #if( !${expNames.isEmpty()} )
                    "expressionNames" : $utils.toJson($expNames),
                #end
                #if( !${expValues.isEmpty()} )
                    "expressionValues" : $utils.toJson($expValues),
                #end
            },
            "condition" : {
                "expression"       : "attribute_exists(id) and version = :expectedVersion",
                "expressionValues" : {
                    ":expectedVersion" : $ddb.toDynamoDBJson($context.arguments.expectedVersion)
                }
            }
        }
      ResponseMappingTemplate: "$util.toJson($ctx.result)"

  Schema:
    Type: "AWS::AppSync::GraphQLSchema"
    Properties:
      ApiId: !GetAtt GraphQLApi.ApiId
      Definition: |
        type Comment {
            author: String!
            comment: String!
        }
        type Mutation {
            addComment(id: ID!, author: String!, comment: String!): Post
            addTag(id: ID!, tag: String!): Post
            removeTag(id: ID!, tag: String!): Post
            deletePost(id: ID!, expectedVersion: Int): Post
            upvotePost(id: ID!): Post
            downvotePost(id: ID!): Post
            updatePost(
                id: ID!,
                author: String,
                title: String,
                content: String,
                url: String,
                expectedVersion: Int!
            ): Post
            addPost(
                author: String!,
                title: String!,
                content: String!,
                url: String!
            ): Post!
        }
        type PaginatedPosts {
            posts: [Post!]!
            nextToken: String
        }
        type Post {
            id: ID!
            author: String
            title: String
            content: String
            url: String
            ups: Int!
            downs: Int!
            version: Int!
            tags: [String!]
            comments: [Comment!]
            created: String
            lastUpdated: String
        }
        type Query {
            allPostsByTag(tag: String!, count: Int, nextToken: String): PaginatedPosts!
            allPostsByAuthor(author: String!, count: Int, nextToken: String): PaginatedPosts!
            allPost(count: Int, nextToken: String): PaginatedPosts!
            getPost(id: ID!): Post
        }
        schema {
            query: Query
            mutation: Mutation
        }

Outputs:
  GraphQLApiARN:
    Description: The App ID of the GraphQL endpoint.
    Value: !Ref GraphQLApi
  GraphQLApiId:
    Description: The App ID of the GraphQL endpoint.
    Value: !GetAtt GraphQLApi.ApiId
  GraphQLApiEndpoint:
    Description: The URL for the GraphQL endpoint.
    Value: !GetAtt GraphQLApi.GraphQLUrl
  PostDynamoDBTableDataSourceARN:
    Description: The ARN for the Post DynamoDB table DataSource.
    Value: !Ref PostDynamoDBTableDataSource
  PostDynamoDBTableDataSourceName:
    Description: The ARN for the Post DynamoDB table DataSource.
    Value: !GetAtt PostDynamoDBTableDataSource.Name

when I try, addPost just works. but updatePost throws exception , not sure what's wrong.

{
"data": {
"updatePost": null
},
"errors": [
{
"path": [
"updatePost"
],
"data": null,
"errorType": "MappingTemplate",
"errorInfo": null,
"locations": [
{
"line": 50,
"column": 3,
"sourceName": null
}
],
"message": "Encountered "" at velocity[line 80, column 2]\nWas expecting one of:\n "(" ...\n ...\n <ESCAPE_DIRECTIVE> ...\n <SET_DIRECTIVE> ...\n "##" ...\n "\\\\" ...\n "\\" ...\n ...\n "#" ...\n "#" ...\n "]]#" ...\n <STRING_LITERAL> ...\n ...\n <IF_DIRECTIVE> ...\n <ELSEIF_DIRECTIVE> ...\n <ELSE_DIRECTIVE> ...\n <INTEGER_LITERAL> ...\n <FLOATING_POINT_LITERAL> ...\n ...\n <BRACKETED_WORD> ...\n ...\n ...\n "{" ...\n "}" ...\n <EMPTY_INDEX> ...\n "
}
]
}

I used this type of update resolver as well in my application but didn't work too.

Support DeltaSync?

I suspect the template has changed. I no longer work on AppSync, so I cannot assist further. I suggest reaching out to @dabit3 if you need to discuss updates.