event_handler.md

title	description
Event Handler	Utility

Event handler decorators for common Lambda events

AppSync Resolver Decorator

New in 1.14.0

AppSync resolver decorator is a concise way to create lambda functions to handle AppSync resolvers for multiple typeName and fieldName declarations. This decorator builds on top of the AppSync Resolver data class and therefore works with Amplify GraphQL Transform Library{target="_blank"} (@function), and AppSync Direct Lambda Resolvers{target="_blank"}

Key Features

• Works with any of the existing Powertools utilities by allow you to create your own lambda_handler function
• Supports an implicit handler where in app = AppSyncResolver() can be invoked directly as app(event, context)
• resolver decorator has flexible or strict matching against fieldName
• Arguments are automatically passed into your function
• AppSyncResolver includes current_event and lambda_cotext fields can be used to pass in the original AppSyncResolver or LambdaContext objects

Amplify GraphQL Example

Create a new GraphQL api via amplify add api and add the following to the new schema.graphql

=== "schema.graphql"

```typescript hl_lines="7-10 17-18 22-25"
@model
type Merchant
{
    id: String!
    name: String!
    description: String
    # Resolves to `get_extra_info`
    extraInfo: ExtraInfo @function(name: "merchantInfo-${env}")
    # Resolves to `common_field`
    commonField: String  @function(name: "merchantInfo-${env}")
}

type Location {
    id: ID!
    name: String!
    address: Address
    # Resolves to `common_field`
    commonField: String  @function(name: "merchantInfo-${env}")
}

type Query {
  # List of locations resolves to `list_locations`
  listLocations(page: Int, size: Int): [Location] @function(name: "merchantInfo-${env}")
  # List of locations resolves to `list_locations`
  findMerchant(search: str): [Merchant] @function(name: "searchMerchant-${env}")
}
```

Create two new simple Python functions via amplify add function and run pipenv install aws-lambda-powertools to add Powertools as a dependency. Add the following example lambda implementation

=== "merchantInfo/src/app.py"

```python hl_lines="1-2 6 8-9 13-14 18-19 24 26"
from aws_lambda_powertools.event_handler import AppSyncResolver
from aws_lambda_powertools.logging import Logger, Tracer, correlation_paths

tracer = Tracer()
logger = Logger()
app = AppSyncResolver()

@app.resolver(type_name="Query", field_name="listLocations")
def list_locations(page: int = 0, size: int = 10):
    # Your logic to fetch locations
    ...

@app.resolver(type_name="Merchant", field_name="extraInfo")
def get_extra_info():
    # Can use `app.current_event.source["id"]` to filter within the Merchant context
    ...

@app.resolver(field_name="commonField")
def common_field():
    # Would match all fieldNames matching 'commonField'
    ...

@tracer.capture_lambda_handler
@logger.inject_lambda_context(correlation_id_path=correlation_paths.APPSYNC_RESOLVER)
def lambda_handler(event, context):
    app.resolve(event, context)
```

=== "searchMerchant/src/app.py"

```python hl_lines="1 3 5-6"
from aws_lambda_powertools.event_handler import AppSyncResolver

app = AppSyncResolver()

@app.resolver(type_name="Query", field_name="findMerchant")
def find_merchant(search: str):
    # Your special search function
    ...
```

Example AppSync resolver events

=== "Query.listLocations event"

```json hl_lines="2-7"
{
  "typeName": "Query",
  "fieldName": "listLocations",
  "arguments": {
    "page": 2,
    "size": 1
  },
  "identity": {
    "claims": {
      "iat": 1615366261
      ...
    },
    "username": "mike",
    ...
  },
  "request": {
    "headers": {
      "x-amzn-trace-id": "Root=1-60488877-0b0c4e6727ab2a1c545babd0",
      "x-forwarded-for": "127.0.0.1"
      ...
    }
  },
  ...
}
```

=== "Merchant.extraInfo event"

```json hl_lines="2-5 14-17"
{
  "typeName": "Merchant",
  "fieldName": "extraInfo",
  "arguments": {
  },
  "identity": {
    "claims": {
      "iat": 1615366261
      ...
    },
    "username": "mike",
    ...
  },
  "source": {
    "id": "12345",
    "name: "Pizza Parlor"
  },
  "request": {
    "headers": {
      "x-amzn-trace-id": "Root=1-60488877-0b0c4e6727ab2a1c545babd0",
      "x-forwarded-for": "127.0.0.1"
      ...
    }
  },
  ...
}
```

=== "*.commonField event"

```json hl_lines="2 3"
{
  "typeName": "Merchant",
  "fieldName": "commonField",
  "arguments": {
  },
  "identity": {
    "claims": {
      "iat": 1615366261
      ...
    },
    "username": "mike",
    ...
  },
  "request": {
    "headers": {
      "x-amzn-trace-id": "Root=1-60488877-0b0c4e6727ab2a1c545babd0",
      "x-forwarded-for": "127.0.0.1"
      ...
    }
  },
  ...
}
```

=== "Query.findMerchant event"

```json hl_lines="2-6"
{
  "typeName": "Query",
  "fieldName": "findMerchant",
  "arguments": {
    "search": "Brewers Coffee"
  },
  "identity": {
    "claims": {
      "iat": 1615366261
      ...
    },
    "username": "mike",
    ...
  },
  "request": {
    "headers": {
      "x-amzn-trace-id": "Root=1-60488877-0b0c4e6727ab2a1c545babd0",
      "x-forwarded-for": "127.0.0.1"
      ...
    }
  },
  ...
}
```