Pre-deploy Script

This is a very advanced operation and requires attention. Otherwise, it can break the deployment for your app.

This script will run right before your container (i.e. app) gets updated due to a configuration change or app deploy. In this script, you can modify the Docker service object, invoke an HTTP call, and literally do anything. The template for this script is:

var preDeployFunction = function (captainAppObj, dockerUpdateObject) {
    return Promise.resolve()
        .then(function(){

            // Do something in a Promise form

            // In the end, return the "possibly-modified" dockerUpdateObject
            return dockerUpdateObject;
        });
};

Note that captainAppObj, is the app object as saved in /captain/config.conf file, and dockerUpdateObject is the service update object that is being passed to Docker to update the service (environmental vars, image version and etc). This object is as per Docker docs.

Since this script will be executed in CaptainDuckDuck process, you'll get access to all node dependecies that CaptainDuckDuck has, see CaptainDuckDuck/app-backend/package.json. For example, the following script injects a UUID mapped to the deployed version to service label with every update:

var uuid = require('uuid/v4');

var preDeployFunction = function (captainAppObj, dockerUpdateObject) {
    return Promise.resolve()
        .then(function(){

            dockerUpdateObject.TaskTemplate.ContainerSpec.Labels[uuid()] =
                                                         captainAppObj.deployedVersion+ '';
            return dockerUpdateObject;
        });
};

Note that this pre-deploy script, particularly Docker service update object, is complicated. Hence, it is strongly recommended to use this pre-deploy method if you are an expert user. For example, note that how an empty string is being added to the deployed version in this line:

dockerUpdateObject.TaskTemplate.ContainerSpec.Labels[uuid()] = captainAppObj.deployedVersion+ '';

Removing this causes an error. To see logs, you need to run docker service logs captain-captain --follow. Even the error from Docker is not very clear. All in all, this is an advanced feature and is not recommended for beginners and intermiate users.