Connecting AWS ECS Services with the CDK

Question

I want to deploy two containers into an ECS cluster, one gets called with HTTP from outside, and then it calls the other container, also via HTTP.

const cluster = new ecs.Cluster(this, "mycluster", {});
cluster.addDefaultCloudMapNamespace({ name: "local" });

new ecsPatterns.ApplicationLoadBalancedFargateService(this, "abc", {
  cluster,
  taskImageOptions: {
    containerPort: 8000,
    image: ecs.ContainerImage.fromRegistry("my/abc-image:latest"),
  },
});

const xyztask = new ecs.FargateTaskDefinition(this, "xyztask");
const xyz = xyztask.addContainer("xyzcontainer", {
  image: ecs.ContainerImage.fromRegistry("my/xyz-image:latest"),
});
xyz.addPortMappings({ containerPort: 8000 });
new ecs.FargateService(this, "xyz", {
  cluster,
  taskDefinition: xyztask,
  cloudMapOptions: { name: "xyz" },
});

The abc service looks like this:

const axios = require("axios");
const bodyParser = require("body-parser");
const express = require("express");

const app = express();
app.use(bodyParser.json());

app.post("/", async ({ body: { x } }, response) => {
  response.end(JSON.stringify({ x }));
  await axios.post(
    `http://xyz.local:8000/`,
    { x },
    { timeout: 3000 }
  );
});

app.listen(8000);

The xyz service looks like this:

const axios = require("axios");
const bodyParser = require("body-parser");
const express = require("express");

const app = express();
app.use(bodyParser.json());

app.post("/", async ({ body: { x } }, response) => {
  response.end();
});

app.listen(8000);

The abc service is available from the outside, but somehow the request to xyz always fails.

Do you definitely want to use CloudMap here or was this an implementation attempt? I was going to try spin something similar up but I hesitated when I saw you were using CloudMap and wanted to check. — Alex Bailey
It was an attempt. But the CM domain resolved to an IP, so I had the impression that part was working and I assumed CM was the cleanest way here. — K..

Martin Löper Martin Löper · Accepted Answer · 2020-12-23T22:01:05

I try to make an educated guess as I see two starting points here:

a) The issue is related to DNS resolution.
You can check if this is the case by starting an EC2 instance inside the same subnet as the abc service and running: nslookup xyz.local. If the dns resolution yields the IP of the xyz service container(s), then DNS resolution is not an issue.

b) The security group of the xyz service blocks incoming calls from the abc service. Outgoing connections are allowed by default, but incoming ones are usually not (except for the ApplicationLoadBalancedFargateService construct, which opens ALB's security group for connections from everywhere by default [1]). I guess the FargateService construct on the contrary does not open any incoming container ports by default, so you have to connect both services manually:

// initialize both services as before
const abc = new ecsPatterns.ApplicationLoadBalancedFargateService(...)
const xyz = new ecs.FargateService(...);

// add the ingress rule to xyz service for abc service's traffic via TCP port 8000
xyz.connections.allowFrom(abc, Port.tcp(8000));

I did not try this out yet, but these are two possible causes of your issue that come into my mind.

References

[1] https://github.com/aws/aws-cdk/blob/53f092f8a658e7b72adc996a0d06b18f7ca7ab4d/packages/%40aws-cdk/aws-ecs-patterns/lib/base/application-load-balanced-service-base.ts#L69

Connecting AWS ECS Services with the CDK

1 Answers

References