无服务器框架中的最佳实践

Question

I am new beginner in serverless framwork. When study Best Practices in Serverless. here

I have a question about "Initialize external services outside of your Lambda code". How to implement it? For example: Below code in handler.js

const getOneUser = (event, callback) => {
  let response = null;
  // validate parameters
  if (event.accountid && process.env.SERVERLESS_SURVEYTABLE) {
    let docClient = new aws.DynamoDB.DocumentClient();
    let params = {
      TableName: process.env.SERVERLESS_USERTABLE,
      Key: {
        accountid: event.accountid,
      }
    };
    docClient.get(params, function(err, data) {
      if (err) {
        // console.error("Unable to get an item with the request: ", JSON.stringify(params), " along with error: ", JSON.stringify(err));
        return callback(getDynamoDBError(err), null);
      } else {
        if (data.Item) { // got response
          // compose response
          response = {
            accountid: data.Item.accountid,
            username: data.Item.username,
            email: data.Item.email,
            role: data.Item.role,
          };
          return callback(null, response);
        } else {
          // console.error("Unable to get an item with the request: ", JSON.stringify(params));
          return callback(new Error("404 Not Found: Unable to get an item with the request: " + JSON.stringify(params)), null);
        }
      }
    });
  }
  // incomplete parameters
  else {
    return callback(new Error("400 Bad Request: Missing parameters: " + JSON.stringify(event)), null);
  }
};

The question is that how to initial DynamoDB outside of my Lambda code.

Update 2:

Is below code optimized?

Handler.js

let survey = require('./survey');
module.exports.handler = (event, context, callback) => {
    return survey.getOneSurvey({
      accountid: event.accountid,
      surveyid: event.surveyid
    }, callback);
};

survey.js

let docClient = new aws.DynamoDB.DocumentClient();
module.exports = (() => {
  const getOneSurvey = (event, callback) {....
      docClient.get(params, function(err, data)...
      ....
  };

  return{
     getOneSurvey : getOneSurvey,
  }
})();

Answer 1

Here's the quote in question:

Initialize external services outside of your Lambda code

When using services (like DynamoDB) make sure to initialize outside of your lambda code. Ex: module initializer (for Node), or to a static constructor (for Java). If you initiate a connection to DDB inside the Lambda function, that code will run on every invoke.

In other words, in the same file, but outside of -- before -- the actual handler code.

let docClient = new aws.DynamoDB...
...
const getOneUser = (event, callback) => {
....
  docClient.get(params, ...

When the container starts, the code outside the handler runs. When subsequent function invocations reuse the same container, you save resources and time by not instantiating the external services again. Containers are often reused, but each container only handles one concurrent request at a time, and how often they are reused and for how long is outside your control... Unless you update the function, in which case any existing containers will no longer be reused, because they'd have the old version of the function.

Your code will work as written, but isn't optimized.

The caveat with this approach that arises in current generation Node.js Lambda functions (Node 4.x/6.x) is that some objects -- notably, those that create literal persistent connections to external services -- will prevent the event loop from becoming empty (a common example is a mysql database connection, which is holding a live TCP connection to the server; by contrast, a DynamoDB "connection" is actually connectionless, since it's transport protocol is HTTPS). In this case you need to either take a different approach or allow lambda to not wait for an empty event loop before freezing the container, by setting context.callbackWaitsForEmptyEventLoop to false before calling the callback... but only do this if needed and only if you fully understand what it means. Setting it by default because some guy on the Internet said it was a good idea will potentially bring you mysterious bugs, later.