Request & Lambda Context

Lambda Web Adapter forwards API Gateway request context and Lambda invocation context to your web application via HTTP headers.

Request Context

API Gateway sends metadata (requestId, requestTime, apiId, identity, authorizer) for each request. This is forwarded in the x-amzn-request-context header as a JSON string.

The identity and authorizer fields are particularly useful for client authorization.

// Express.js example
app.get('/', (req, res) => {
    const requestContext = JSON.parse(req.headers['x-amzn-request-context']);
    const sourceIp = requestContext.identity?.sourceIp;
    const userId = requestContext.authorizer?.claims?.sub;
});

See the API Gateway docs for the full request context schema.

Lambda Context

The Lambda invocation context (function name, memory, timeout, request ID, etc.) is forwarded in the x-amzn-lambda-context header as a JSON string.

# Flask example
@app.route('/')
def index():
    lambda_context = json.loads(request.headers.get('x-amzn-lambda-context', '{}'))
    function_name = lambda_context.get('function_name')
    remaining_time = lambda_context.get('deadline_ms')

See the Lambda Context docs for the full list of available properties.