Defining probes for each container is a best practice as it allows to make sure a container is running and ready to receive traffic.
Acorn offers a way to specify probes using the probes property in the definition of a container. liveness, readiness and startup are the 3 types of probes available:
- a liveness probe makes sure the container is up and running (it restarts the container if the probe fails)
- a readiness probe makes sure the container is ready to receive traffic
- a startup probe is used to give a container additional time to start
In this step we will just add a readiness probe to the voteui but the same approach would apply to the others containers of the VotingApp.
Change the voteui container specifying a readiness probe as follows (this probe ensures voteui container is able to serve request on port 8080):
voteui: {
labels: {
component: "voteui"
}
if args.dev {
dirs: "/usr/share/nginx/html": "./vote-ui"
}
build: {
context: "./vote-ui"
}
ports: publish : "80/http"
scale: args.replicas
memory: 128Mi
probes: [
{
type: "readiness"
initialDelaySeconds: 10
periodSeconds: 5
http: {
url: "http://localhost:8080"
}
}
]
}
Run the application using this new version of the Acornfile:
acorn run -n vote .
You should see that voteui never becomes ready:
...
[containers: voteui is not ready]
This is a normal behavior as we specified a wrong port number in the readiness probe definition (we used port 8080 whereas voteui container actually listens on port 80). Let's fix that and update the probe definition of the voteui container so it looks like the following:
voteui: {
labels: {
component: "voteui"
}
if args.dev {
dirs: "/usr/share/nginx/html": "./vote-ui"
}
build: {
context: "./vote-ui"
}
ports: publish : "80/http"
scale: args.replicas
memory: 128Mi
probes: [
{
type: "readiness"
initialDelaySeconds: 10
periodSeconds: 5
http: {
url: "http://localhost"
}
}
]
}
Stop the application and restart it with this corrected version of the specification:
acorn rm app --force --all
acorn run -n vote .
This time you should see the application is up and running after a couple of seconds.
In this step we've only added a readiness probe to voteui container to illustrate the way probes are defined. Feel free to add such probes to the other containers of the VotingApp.
Acornfile you should have at the end of this step...
labels: {
application: "votingapp"
}
args: {
replicas: 3
}
profiles: {
dev: {
replicas: 1
}
test: {
replicas: 2
}
}
containers: {
voteui: {
labels: {
component: "voteui"
}
if args.dev {
dirs: {
"/usr/share/nginx/html": "./vote-ui"
}
}
build: {
context: "./vote-ui"
}
ports: publish : "80/http"
scale: args.replicas
memory: 128Mi
probes: [
{
type: "readiness"
initialDelaySeconds: 10
periodSeconds: 5
http: {
url: "http://localhost"
}
}
]
}
vote: {
labels: {
component: "vote"
}
build: {
target: std.ifelse(args.dev, "dev", "production")
context: "./vote"
}
if args.dev {
dirs: {
"/app": "./vote"
}
}
ports: "5000/http"
memory: 128Mi
}
redis: {
labels: {
component: "redis"
}
image: "redis:7.0.5-alpine3.16"
ports: "6379/tcp"
dirs: {
if !args.dev {
"/data": "volume://redis"
}
}
memory: 128Mi
}
worker: {
labels: {
component: "worker"
}
build: "./worker/go"
env: {
"POSTGRES_USER": "secret://db-creds/username"
"POSTGRES_PASSWORD": "secret://db-creds/password"
}
memory: 128Mi
}
db: {
labels: {
component: "db"
}
image: "postgres:15.0-alpine3.16"
ports: "5432/tcp"
env: {
"POSTGRES_USER": "secret://db-creds/username"
"POSTGRES_PASSWORD": "secret://db-creds/password"
"PGDATA": "/var/lib/postgresql/data/db"
}
dirs: {
if !args.dev {
"/var/lib/postgresql/data": "volume://db"
}
}
memory: 128Mi
}
result: {
labels: {
component: "result"
}
build: {
target: std.ifelse(args.dev, "dev", "production")
context: "./result"
}
if args.dev {
dirs: {
"/app": "./result"
}
}
ports: "5000/http"
env: {
"POSTGRES_USER": "secret://db-creds/username"
"POSTGRES_PASSWORD": "secret://db-creds/password"
}
memory: 128Mi
}
resultui: {
labels: {
component: "resultui"
}
build: {
target: std.ifelse(args.dev, "dev", "production")
context: "./result-ui"
}
if args.dev {
dirs: {
"/app": "./result-ui"
}
}
ports: publish : "80/http"
memory: std.ifelse(args.dev, 1Gi, 128Mi)
}
}
secrets: {
"db-creds": {
type: "basic"
params: {
usernameLength: 7
usernameCharacters: "a-z"
passwordLength: 10
passwordCharacters: "A-Za-z0-9"
}
data: {
username: ""
password: ""
}
}
}
volumes: {
if !args.dev {
"db": {
size: "100M"
}
"redis": {
size: "100M"
}
}
}
Note: you can find more information about probes in the Acorn documentation