Deploying a monorepo Serverless app with interdependent services
It’s common for your monorepo Serverless app to have services that are dependent on each other. In this post we’ll look at how the dependencies affect your deployments.
The short version is that:
- When you introduce a new dependency in your app you cannot deploy the services concurrently.
- However, if these services have been deployed before, you can deploy them concurrently.
Let’s look at why this happens. Assume our monorepo Serverless Framework app has the following structure.
/ package.json libs/ services/ serviceA/ serverless.yml handler.js serviceB/ serverless.yml handler.js serviceC/ serverless.yml handler.js
There are three services called
serviceC. Let’s say
serviceA creates an SNS topic and exports the topic’s ARN in its
service: serviceA ... resources: - Outputs: ATopicArn: Value: Ref: ATopic Export: Name: ATopicArn
serviceC use the SNS topic as their Lambda triggers:
service: serviceB ... functions: handler: handler: handler.main events: - sns: 'Fn::ImportValue': ATopicArn
If you are deploying the above app for the first time, you have to deploy
serviceA first, such that the export value
ATopicArn exists. Then deploy
serviceC. If you were to deploy all three services concurrently,
serviceC will fail with CloudFormation throwing the following error:
serviceB - No export named ATopicArn found.
It’s basically saying that the ARN referenced in its
serverless.yml does not exist. That makes sense because we haven’t created it yet!
Once the three services have been successfully deployed, you can deploy them concurrently. This is because the referenced ARN is created after the first deployment.
Adding new dependencies
Say you add a new SNS topic in
serviceC once again subscribe to the topic. The first deployment after the change, will again fail if all the services are deployed concurrently. You need to deploy
serviceA first, and then deploy
And that’s all there is to it! The interdependencies are created by services referencing resources that are created in another service. When you first deploy the app, make sure the service that is creating the resource is deployed first. With Seed, we handle this using the concept of Deploy Phases and to keep your builds fast, we check if a service has been updated.
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