Figure 1: Caddy goes in front; the service from Part 1 remains unchanged.

Between a running service and a usable application lie more steps than a single line of proxy configuration suggests.

Caddy becomes the only service visible from the outside. It accepts the requests, terminates TLS, obtains and renews the certificate itself, and passes everything else through to127.0.0.1:8080.

The application has to learn that it is behind a proxy. A request that comes in through Caddy appears, from Jetty’s point of view, to originate from127.0.0.1, is unencrypted, and carries a different hostname than the one the browser called. Without correction, Vaadin sets session cookies withoutSecure and builds redirects tohttp://. Chapter 4 deals with that.

Vaadin Push keeps a persistent connection open. It survives a reverse proxy only if the proxy explicitly passes it through.

After that, operations begin: configuration outside the archive, secrets and file permissions, a repeatable procedure for the next release, a controlled rollback — and the question of what happens to the running sessions during all of this.

The path from the outside

At the end of this part, the application is reachable under its own domain name over HTTPS, set up, and updatable. The service itself stays exactly where Part 1 parked it: bound to127.0.0.1, hardened, unchanged. What is added goes in front — not inside.

Installing Caddy

The role of the reverse proxy

Jetty listens on the loopback address and cannot be reached from the outside. Caddy sits exactly in between: it accepts the requests from the internet, terminates TLS, and forwards them unencrypted to127.0.0.1:8080.

This separates two jobs that would otherwise have to be handled by the same process. The application server takes care of the application; everything related to public reachability — certificates, redirects, compression, access logging — sits in front of it.

Installation

sudo apt-get install debian-keyring debian-archive-keyring apt-transport-https curlcurl -1sLf'https://dl.cloudsmith.io/public/caddy/stable/gpg.key'\| sudo gpg --dearmor -o /usr/share/keyrings/caddy-stable-archive-keyring.gpgcurl -1sLf'https://dl.cloudsmith.io/public/caddy/stable/debian.deb.txt'\| sudo tee /etc/apt/sources.list.d/caddy-stable.listsudo apt-get updatesudo apt-get install caddy

The difference from Part 1, Chapter 10

What is remarkable is what doesnot have to be done afterwards:

systemctl is-enabled caddy# enabledsystemctl is-active caddy# activegetent passwd caddy# caddy:/var/lib/caddy:/usr/sbin/nologin

The service is already running, with its own system account and a finished unit definition. For Jetty, Chapters 9 and 10 required the same work by hand.

That is no coincidence but the difference between a distribution and an application runtime. Caddy is a finished program with a clear operating model; Jetty is a runtime whose operating model only emerges once an application is placed inside it.

Prepackaged does not mean hardened

systemd-analyze security caddy.service

→ Overall exposure level for caddy.service: 8.8 EXPOSED 🙁

The same treatment as in Part 1, Chapter 11 — but explicitlynot the same file:

sudo mkdir -p /etc/systemd/system/caddy.service.dsudo nano /etc/systemd/system/caddy.service.d/10-hardening.conf

The differences are the actual lesson:

Two assumptions can be verified in practice.

Does Caddy get by withoutCAP_NET_ADMIN? The Debian package grants two capabilities. Keeping only the one for binding privileged ports is enough:

grep -E'^Cap(Prm|Eff|Bnd)' /proc/$(systemctl show -p MainPID --value caddy)/status

CapPrm: 0000000000000400
CapEff: 0000000000000400
CapBnd: 0000000000000400

0x400 is exactly bit 10 —CAP_NET_BIND_SERVICE, nothing else. HTTPS, HTTP/2, HTTP/3, and certificate acquisition keep working unchanged.

Does Caddy tolerateMemoryDenyWriteExecute? Yes. The directive that dismantles every JVM in Part 1, Chapter 11 is unproblematic here.

→ Overall exposure level for caddy.service: 1.5 OK 🙂

After this chapter, one thing holds: Caddy is the only process reachable from the internet. What it forwards is decided in Chapter 3.

Domain and HTTPS

DNS

The prerequisite is an A record pointing to the server’s public address:

dig +short A demo.svenruppert.com# 95.216.150.155

Anyone who additionally publishes an AAAA record has to handle IPv6 end to end — firewall, Caddy, and application. An AAAA record in front of a firewall configured only for IPv4 produces partial outages that affect only some visitors and are therefore hard to track down.

Caddyfile

sudo nano /etc/caddy/Caddyfile

demo.svenruppert.com {
reverse_proxy 127.0.0.1:8080
}

Two lines of substance. No certificate path, no ACME client, no redirect rule, nolisten 443 ssl.

sudo systemctl reload caddy

Automatic certificates

sudo journalctl -u caddy -n30

"msg":"using ACME account"
"msg":"trying to solve challenge","identifier":"demo.svenruppert.com","challenge_type":"http-01"
"msg":"authorization finalized","authz_status":"valid"
"msg":"certificate obtained successfully","identifier":"demo.svenruppert.com"

Five seconds from configuration to a valid certificate. The challenge runs over port 80 — which is why it stands open in the firewall even though the application is meant to be reachable exclusively over HTTPS.

HTTP to HTTPS

curl -sI http://demo.svenruppert.com/| head -3

HTTP/1.1 308 Permanent Redirect
Location: https://demo.svenruppert.com/
Server: Caddy

Nobody configured this redirect. Caddy sets it up as soon as a hostname appears in the Caddyfile.

TLS

echo| openssl s_client -servername demo.svenruppert.com\ -connect demo.svenruppert.com:443 2>/dev/null\| openssl x509 -noout -subject -issuer -dates

subject=CN=demo.svenruppert.com
issuer=C=US, O=Let's Encrypt, CN=YE1
notBefore=Sep 4 08:56:45 2026 GMT
notAfter=Dec 3 08:56:44 2026 GMT

Caddy handles renewal itself, without a cron entry and without additional software.

CAA: who may issue at all

The A record determines where the domain lives. It says nothing aboutwho may issue a certificate for it — by default, the answer is: any publicly trusted certificate authority.

svenruppert.com. CAA 0 issue "letsencrypt.org"

The check is binding for all publicly trusted authorities. The record is resolved from the full name upwards; an entry on the registrable domain covers all subdomains as well.

A record that is too narrow locks you out — and the mistake only shows up at the next renewal, possibly months later. That is why setting it should be followed by an actual reissuance, not just a syntax check.

A production-grade Caddyfile

The minimum works — for production operation, quite a bit is missing:

{
admin off
}
demo.svenruppert.com {
encode zstd gzip
header {
Strict-Transport-Security "max-age=31536000; includeSubDomains"
X-Content-Type-Options "nosniff"
Referrer-Policy "strict-origin-when-cross-origin"
X-Frame-Options "SAMEORIGIN"
Permissions-Policy "geolocation=(), microphone=(), camera=()"
-Server
}
reverse_proxy 127.0.0.1:8080 {
transport http {
read_timeout 0
write_timeout 0
}
}
log {
output file /var/log/caddy/demo.svenruppert.com.log {
roll_size 10MiB
roll_keep 10
}
format json
}
}

-Server removes theServer: Jetty(12.1.12) identifier from the response. No client needs the exact version, but every scanner does — Chapter 4 shows that these scanners actually do come by.

admin off disables Caddy’s administration interface on127.0.0.1:2019. Through it, the entire configuration could be changed without authentication. The price:systemctl reload caddy no longer works afterwards; changes require arestart.

The timeouts in thetransport block are the only item in this file that can actively break Vaadin Push — Chapter 5 explains why.

The pitfall when validating

The obvious sequence is: validate first, then reload.

sudo caddy validate --config /etc/caddy/Caddyfile# "Valid configuration"sudo systemctl reload caddy# fails

open /var/log/caddy/demo.svenruppert.com.log: permission denied

The message is misleading, because the directory does belong tocaddy. A look at the file explains it:

ls -l /var/log/caddy/# -rw------- 1 root root 0 demo.svenruppert.com.log

caddy validate instantiates the log writers and creates the file in the process — undersudo, that means asroot with0600. The service running ascaddy cannot subsequently open its own log file.

The fix: delete the file, reload. Or run the validation as the service account:

sudo -u caddy caddy validate --config /etc/caddy/Caddyfile

What is remarkable about the behavior: Caddy doesnot adopt a configuration it cannot load — the old one kept running, the site stayed reachable. Only arestart took the service down. With a reverse proxy, when in doubt:reload, notrestart.

Forwarded Headers and Scheme Behind Caddy

The application is reachable, the certificate valid. Yet at this point the installation is still wrong — and the mistake is invisible.

What the application sees

Caddy accepts an HTTPS connection from a client somewhere on the internet and establishes anunencrypted connection from the loopback address to Jetty. From the application’s point of view, every request therefore looks like this:

• Scheme:http, nothttps
• Client address:127.0.0.1
• Port: 8080

Three consequences follow, and none of them announces itself as an error:

Absolute URLs are generated incorrectly. Wherever the application builds a complete address — redirects, return addresses, links in emails — it containshttp:// and the internal port.

The access log is worthless. Every request apparently comes from127.0.0.1.

Session cookies lose their protection. A cookie with theSecure flag is only transmitted over encrypted connections. If the application believes the connection is unencrypted, it does not set the flag.

Making the error visible

An access log in Jetty makes the state verifiable:

cd /opt/vaadinappsudo java -jar /opt/jetty/start.jar --add-modules=requestlogsudo nano start.d/requestlog.ini

jetty.requestlog.filePath=/var/lib/vaadinapp/logs/yyyy_mm_dd.request.log

sudo systemctl restart vaadinappcurl -s https://demo.svenruppert.com/ > /dev/nullsudo tail -1 /var/lib/vaadinapp/logs/*.request.log

127.0.0.1 - - [04/Sep/2026:09:56:48 +0000] "GET / HTTP/1.1" 200 198 "" "curl/8.7.1"

Every request — from the office next door or from Australia — appears in the log as127.0.0.1.

The fix

cd /opt/vaadinappsudo java -jar /opt/jetty/start.jar --add-modules=forwardedsudo systemctl restart vaadinapp

The same request afterwards:

80.187.114.255 - - [04/Sep/2026:09:57:11 +0000] "GET / HTTP/1.1" 200 198 "" "curl/8.7.1"

On the Caddy side there is nothing to do. Caddy setsX-Forwarded-For,X-Forwarded-Proto, andX-Forwarded-Host withreverse_proxy on its own.No additional directive is needed — all that is required is that Jetty actually evaluates these headers.

The proof that counts

The effect can be read directly from the real application’s response:

curl -sI https://demo.svenruppert.com/| grep -i set-cookie

set-cookie: JSESSIONID=node0pcg76yqw1hqd1btn7lkvwrmj66.node0; Path=/; Secure

TheSecure flag is set. So Jetty knows that the connection is TLS-terminated — and that is achieved exclusively by this module. Without it, the flag would be missing, and the session cookie would also travel over unencrypted connections.

The trust boundary

Forwarded headers are client-supplied claims. Whoever evaluates them believes what the request says — a client could assert any address.

Here that is harmless,because Jetty listens exclusively on the loopback address (Part 1, Chapter 8). Forged headers can only be sent by someone who is already on the server; everything from outside inevitably passes through Caddy, and Caddy sets the values itself.

Bind address and forwarded-header evaluation are therefore connected: the one decision justifies the other. If you let Jetty listen on0.0.0.0 while also evaluating forwarded headers, you would have a log that anyone is allowed to write to.

An incidental finding that was not planned

A few minutes after the certificate was issued, external addresses appeared in the log:

64.227.32.66 - - [...] "GET /config.json HTTP/1.1" 404 ... "l9scan/2.0 (+https://leakix.net)"
64.227.32.66 - - [...] "GET /telescope/requests HTTP/1.1" 404 ...
64.227.32.66 - - [...] "GET /info.php HTTP/1.1" 404 ...

The certificate was issued at 09:55:17; the first external request arrived at 09:56:28 —one minute and eleven seconds later. After that, it came thick and fast:

09:56:56 GET /console/
09:56:57 GET /server
09:56:58 GET /server-status
09:57:00 GET /about
09:57:01 GET /login.action
09:57:02 GET /___proxy_subdomain_whm/login
09:57:02 GET /v2/_catalog

A cross-section of known vulnerabilities:/login.action looks for vulnerable Confluence,/v2/_catalog for an open container registry,/telescope/requests for a Laravel diagnostic tool left in production,/server-status for an open Apache status page. In total,sixteen different external addresses within a few minutes.

How did they know? From theCertificate Transparency logs. Every issued TLS certificate is published in a public, machine-readable registry — a security feature that makes misissuance detectable. It has a side effect: anyone looking for new hostnames simply reads along. A server is publicly known from the second its certificate is issued, not only once someone passes the address around.

Together with the numbers from Part 0, Chapter 7 — the first login attempt twenty-three seconds after boot — this adds up to a clear picture.

And without this chapter, none of it would have been seen. Before the module was activated, every log line read127.0.0.1. The accesses were there the whole time, just invisible. A log that cannot distinguish the office next door from a scanner network is not a log.

Vaadin Push Behind Caddy

Vaadin keeps the UI state on the server. Changes not triggered by a user action — an incoming record, a progress update, a notification — therefore need a path from the server to the browser. That path is provided by Vaadin Push.

WebSockets

Technically, a push connection begins as an ordinary HTTP request asking for a protocol upgrade:

GET /VAADIN/push?v-r=push HTTP/1.1
Connection: Upgrade
Upgrade: websocket
Sec-WebSocket-Version: 13

If the server answers with101 Switching Protocols, the connection stays open and both sides send whenever they want. Exactly this upgrade is what an intermediate reverse proxy has to pass through.

On the Jetty side

The required module was activated in Part 1, Chapter 7:

ee11-websocket-jakarta

If it is missing, the upgrade fails, and Vaadin falls back to a technique that polls the server at regular intervals. The application then still works — just slower and with more load. That is the unpleasant kind of error: everything looks fine.

Proxying — and what Caddy needs for it

Nothing.

reverse_proxy 127.0.0.1:8080

Caddy passes upgrade connections through transparently withreverse_proxy on its own. There is noproxy_http_version setting, noUpgrade andConnection headers to set manually, no separatelocation block for the push path.

That is worth mentioning because the widespread guides for other reverse proxies contain exactly those lines — and because the official Vaadin documentation on reverse proxies covers only Apache HTTPD and nginx. Caddy does not appear there. Anyone copying from there will search for an equivalent that does not exist, because it is not needed.

Upgrade connections and timeouts

The only item in the Caddyfile that can actively break Push is time limits:

reverse_proxy 127.0.0.1:8080 {
transport http {
read_timeout 0
write_timeout 0
}
}

A push connection is normallysilent. It exists without data flowing — until something happens. A read timeout of 30 seconds terminates it after 30 seconds of quiet.

The error does not show up as an error: the application re-establishes the connection, and everything appears normal. But updates occasionally fail to arrive, and short-lived connections pile up in the log.0 means no limit.

Proof

The push endpoint responds:

curl -s -o /dev/null -w'%{http_code}\n'\'https://demo.svenruppert.com/VAADIN/push?v-r=push'# 200

More reliable is the observation in the browser: in the developer tools, the network requests show a connection to/VAADIN/push with status101 and typewebsocket that stays open.

If it does not stay open, or if instead of101 an ordinary200 response appears and repeats every few seconds, the upgrade has failed — then either the Jetty module is missing or a time limit is kicking in.

What this means for the following parts: The push connection is the most visible expression of what Chapter 10 deals with: the server holds the state of this UI. If the service is restarted, not only does the connection drop — the state behind it is gone.

Configuration Outside the WAR

The artifact from Part 1, Chapter 4 is the same on every server. Whatever differs — storage location, ports, addresses of neighboring systems — must therefore not live inside it.

The reason is not aesthetics. An artifact that contains environment values has to be rebuilt for every environment. Then the thing that was tested is not the thing that is shipped.

The resolution chain

The sample application resolves configuration values in three stages:

privatestaticStringresolve(StringsystemProperty,StringenvVariable,Stringfallback){StringfromSystem=System.getProperty(systemProperty);if(fromSystem!=null&&!fromSystem.isBlank())returnfromSystem;StringfromEnv=System.getenv(envVariable);if(fromEnv!=null&&!fromEnv.isBlank())returnfromEnv;returnfallback;}

A system property beats an environment variable; an environment variable beats the default.

The order is deliberate: the system property is the path for the development machine — it can be passed individually at startup without changing the environment. The environment variable is the path for the server, because it can be fed from a file maintained by someone other than the person performing the start.

app.host → APP_HOST → built-in default
app.port → APP_PORT → built-in default

Environment

On the server, these values come from a file that the unit definition reads:

EnvironmentFile=-/etc/vaadinapp/environment

The leading minus sign means: the filemay be missing. Without it, the application starts with its defaults instead of aborting with an error.

sudo nano /etc/vaadinapp/environment

APP_HOST=127.0.0.1
APP_PORT=8080

sudo chown root:vaadinapp /etc/vaadinapp/environmentsudo chmod640 /etc/vaadinapp/environment

root:vaadinapp with640: the service may read, not write. After a change, restarting the service is all it takes; the artifact remains untouched.

Separating artifact and operational data

That puts three things in three places, each with a different lifecycle:

A release swaps out only the first row. Chapter 8 turns this into a procedure.

One value breaks the pattern

The data location is not set through the environment file but in the unit definition:

-Dapp.storage.dir=/var/lib/vaadinapp/data

The reason is an irregularity in the application: this one key is evaluated only as a system property, not additionally as an environment variable. A lineAPP_STORAGE_DIR=… in the environment file would have no effect.

For operations, this makes no difference — the value simply lives in the unit definition instead of the environment file, and both belong toroot. It is still worth mentioning, because an exception without a substantive reason costs the reader more than it explains: anyone who has understood the chain from the first section expects it everywhere.

Part 1, Chapter 11 showed what happens when this value is not setat all.

What does not belong here: credentials. An environment variable appears in/proc/<PID>/environ and is thus readable by anyone working as the same user. Chapter 7 covers the difference.

Secrets and File Permissions

Configuration and credentials look similar and are therefore often treated the same. They are not: a misplaced port is an operational error; a misplaced access token is a security incident.

Two principles

No secrets in the WAR. The first principle is the simplest. An artifact travels across development machines, version control, staging areas, and backups. Whatever is inside it exists in many places at once.

No secrets as process arguments. The second principle is better known than it is followed. It can be demonstrated in two lines:

# NOT like this:java -Dapi.token=s3cr3t -jar app.jar

# From any other account on the same system:ps aux| grep api.token

The process list is readable by every user on the system. That is not a misconfiguration but normal behavior — process arguments are public.

Three levels

This yields an ordering that leads from bad to good:

The middle level is where many stop — the file has0640, belongs toroot, and that feels safe. The detour via/proc remains open nonetheless, and combined withptrace access to the process memory all the more so. Part 0, Chapter 8 therefore set the corresponding kernel parameter to the strict value.

The third level

systemd can store credentials encrypted and deliver them to the service without them appearing as an environment variable or a readable file:

printf'%s'"$TOKEN"| sudo systemd-creds encrypt --name=api-token -\ /etc/vaadinapp/api-token.credsudo chmod600 /etc/vaadinapp/api-token.cred

# /etc/systemd/system/vaadinapp.service.d/20-credentials.conf[Service]LoadCredentialEncrypted=api-token:/etc/vaadinapp/api-token.cred

The service finds the value at$CREDENTIALS_DIRECTORY/api-token.

Proof

ps aux| grep -c"$TOKEN"# 0tr'\0''\n' < /proc/$(systemctl show -p MainPID --value vaadinapp)/environ\| grep -c"$TOKEN"# 0

And the delivery itself:

ls -l /run/credentials/vaadinapp.service/findmnt -no FSTYPE,OPTIONS /run/credentials/vaadinapp.service

-r--r-----+ 1 root root 28 api-token
tmpfs ro,nosuid,nodev,noexec,nosymfollow,size=1024k,mode=700,noswap

Aread-only tmpfs withnoexec,mode=700, andnoswap. The value never touches the disk in plaintext and does not end up in the swap file. A foreign account getsPermission denied.

Ownership and permissions

/etc/vaadinapp/api-token.cred root:root 0600
/etc/vaadinapp/environment root:vaadinapp 0640
/opt/vaadinapp/webapps/ROOT.war root:vaadinapp 0640
/var/lib/vaadinapp/data vaadinapp:vaadinapp 0700

Exactly one path belongs to the service, and it is the one it has to write to.

A caveat that belongs here

Credential secret file '/var/lib/systemd/credential.secret'
is not located on encrypted media, using anyway.

The virtual machine used here has no TPM. Without one, the decryption key sits as a host key on the same disk. The protection works against accidental disclosure, against plaintext backups, and against the two paths from the proof above —not against someone who already has root privileges on the running system.

That needs saying; otherwise the method promises more than it delivers.

What is not covered

Rotating credentials during operation. A change here means: encrypt a new file, restart the service — with the downtime from Chapter 8. Automated rotation is a topic of its own.

And one final sentence that is missing too often: whoever logs an access token has it in the journal. All the care in delivery is then wasted.

Manual Update

Keeping releases side by side

A deployment that overwrites the old file knows no way back. That is why every release gets its own directory:

/opt/vaadinapp/
├── releases/
│ ├── 2026-09-04-1200/ROOT.war
│ └── 2026-09-07-1442/ROOT.war
└── webapps/
└── ROOT.war ← the active version

sudo install -d -m750 -o root -g vaadinapp /opt/vaadinapp/releases

The archive belongs toroot:vaadinapp. The service may read, not write — anyone who takes over the application can alter neither the active artifact nor its predecessors.

Staging the new WAR

mvn -Pproduction clean packagescp target/ROOT.war sven@demo.svenruppert.com:/home/sven/

STAMP=$(date +%Y-%m-%d-%H%M)sudo install -d -m750 -o root -g vaadinapp /opt/vaadinapp/releases/$STAMPsudo install -m640 -o root -g vaadinapp /home/sven/ROOT.war\ /opt/vaadinapp/releases/$STAMP/rm /home/sven/ROOT.war

Up to this point, nothing has happened. The running application is untouched; the new release is merely staged.

Stop the service, swap the deployment, start the service

sudo systemctl stop vaadinappsudo install -m640 -o root -g vaadinapp\ /opt/vaadinapp/releases/$STAMP/ROOT.war /opt/vaadinapp/webapps/ROOT.warsudo systemctl start vaadinapp

Three commands.install instead ofcp, for the same reason as in Part 1, Chapter 9.

Why no swap during operation

Jetty’s monitoring of thewebapps directory could detect a changed file on its own and redeploy it. That is disabled here, and for good reason: a swap during operation means that two versions of the application briefly exist at the same time — with the same data store.

The explicit stop is more honest. It costs downtime, but the state is unambiguous at every point in time.

What it costs

sudo systemctl restart vaadinapp# Measure the time until the first successful response

The downtime consists of three components: starting the JVM, unpacking the archive tojava.io.tmpdir, and starting the application itself — for this application including opening its data store.

The values come from the run with the slimmed-down archive from Part 1, Chapter 4, measured on 2026-09-07 at release2026-09-07-1627. The comparison with the previous state is instructive, because it refutes a widespread expectation:

A quarter less archive shortens the startup by a good half second — that is the share contributed by unpacking and by scanning the libraries for annotations.Memory consumption, however, does not drop; within measurement noise it is even slightly higher. That is no contradiction: what an archive weighs says little about how many classes the application actually loads at runtime. What was removed were libraries that were never going to be loaded anyway. Shrinking a WAR buys transfer time, disk space, and clarity — not RAM.

For comparison: a minimal web application without its own data store was reachable on the same server after around 2 seconds. The difference is not Jetty; it is the application.

These numbers are the baseline for the parts that follow. It will be interesting to see whether embedded Jetty (Part 3) and the custom runtime from Part 6 measurably shorten the startup.

Four steps that could be automated: build, transfer, swap, restart. Exactly these four steps are what a deployment pipeline replaces — nothing more. Anyone who has run them by hand once knows afterwards where an automation can fail.

Rollback

Keeping the previous artifact

The way back is only possible because Chapter 8 kept the old releases. Without the archive there would be no rollback, only a fresh build from a hopefully matching state of version control — under time pressure, in the middle of an incident.

ls -1 /opt/vaadinapp/releases/

How many releases to keep is a question of disk space. At 21 MB per artifact, ten releases are 210 MB — with 75 GB of disk space, not a serious consideration.

A controlled rollback

sudo systemctl stop vaadinappsudo install -m640 -o root -g vaadinapp\ /opt/vaadinapp/releases/2026-09-04-1200/ROOT.war /opt/vaadinapp/webapps/ROOT.warsudo systemctl start vaadinapp

The same procedure as in Chapter 8, just with an older timestamp.

A rollback costs exactly as much as a deployment

That is the most important statement in this chapter, and it surprises many.

Both operations consist of the same three commands and the same downtime — around six seconds. There isno technical penalty for backing out.

Anyone hesitating because a rollback is supposedly costly hesitates without reason. What is costly is not the way back but the decision: recognizing that something is wrong, and making the call.

Which release is currently active can be determined at any time:

sha256sum /opt/vaadinapp/webapps/ROOT.warsha256sum /opt/vaadinapp/releases/*/ROOT.war

The limit: the data does not come back

Here the symmetry ends, and this is the point that deployment guides like to leave out.

The rollback swapsonly the artifact. The data under/var/lib/vaadinapp/data stays as it is — it is not part of the release and survives every swap.

As long as a new application version only reads and writes what the old one also understood, this is unproblematic. As soon as it changes the storage format — an additional field, a changed structure, a migration at startup — this no longer holds: the old version then encounters data it does not expect.

Two operational rules follow:

Before a release that changes the data, a backup belongs in place. A rollback is then artifactand data.

Whoever runs a migration at startup should know whether it is reversible. If it is not, there is no rollback anymore, only rolling forward — and that requires a working new version.

For the sample application with its embedded store, this means concretely: a rollback across a format change requires the backup of the data directory. Without it, the way back stays open exactly as long as the storage format remains unchanged.

Vaadin Sessions and Restarts

Chapter 8 put a number on the downtime: around six seconds. For a stateless application, that would be the whole story. For a Vaadin application, it is the smaller part.

Server-side UI state

Vaadin keeps the state of the UI on the server. Which view is open, what a form contains, which row of a table is selected — all of that lives in the session, not in the browser. The browser renders and reports events back.

That is the property that defines Vaadin: application logic in Java, without separate state management in the frontend. It has a price, and the price comes due at every restart.

Loss of running sessions

A restart terminates the process. With it, the entire session state disappears.

That isnot a flaw of this deployment. It follows directly from the architecture: whoever keeps state in the server loses it with the server. A stateless REST backend loses nothing on a restart, because it has nothing to lose.

The difference deserves naming, because it shapes expectations. The sample application with login and roles makes it tangible: what is affected is not a counter but a logged-in session in the middle of work.

Impact on users

Concretely, it looks like this: the page reloads, the login is gone, unsaved input is lost. With Push active (Chapter 5), the open connection additionally drops, and the browser tries to re-establish it.

The stored data is unaffected — whatever was saved lives under/var/lib/vaadinapp/data and survives. What is lost is exclusively what had not yet been saved.

Session persistence — and why it is not used here

Jetty can serialize sessions and carry them across a restart. The prerequisite is that theentire session content is serializable.

With Vaadin, that is a well-known sore spot. The session content is the component tree of the UI along with all registered listeners. A single lambda capturing a non-serializable reference is enough to make the serialization fail — and not during development, but at restart in production.

It is doable, but it demands discipline across the entire application code and a test that covers this case. For this series, it is therefore named and not done.

Maintenance windows

The pragmatic answer with one server and manual deployment: restarts are scheduled, not endured.

Remarkably, this decision was already made in Part 0. Whoever enabledunattended-upgrades with automatic reboot there has fixed their maintenance window — namely 3:30 in the morning. Whoever did not enable it has unpatched kernels. Both are defensible; staying undecided is not.

Where this series draws a line

Two instances behind Caddy, updated alternately, would defuse the problem: while one restarts, the other serves. That is an established technique and particularly attractive for a Vaadin application, because it limits the session loss to half the users — provided the proxy keeps sessions on the same instance.

However, it breaks the constraints from Part 1, Chapter 2: one server, no orchestration. That is why it stands here as a boundary and not as a guide.

Why this chapter is in the title

Alongside Push from Chapter 5, this is the second place where a propertyof Vaadin shapes the deployment — and the only one that changes the operating procedure itself. Everything else — Jetty, systemd, Caddy, certificates, hardening — would apply equally to any Java web application.

Result

A Vaadin application is reachable under its own domain name over HTTPS. Four building blocks across two parts, each set up individually, each individually verifiable.

The last step: setting up the application

Deployed does not yet mean usable. The sample application ships with login and roles and therefore needs an initial administrator account — no deployment can deliver that, because it would contain a password.

On first start, the application instead writes a one-time token into its data directory:

sudo cat /var/lib/vaadinapp/data/jcustos/bootstrap.token

token=XXXX-XXXX-XXXX-XXXX-XXXX
createdAt=2026-09-07T12:44:10Z

In the browser, the application then walks through the setup: token, username, password. After that, the file is used up.

The procedure is more remarkable than it looks. The token exists exclusively on the server, readable only by the service account and byroot. Whoever finds the application on the net before it is set up cannot complete the setup — they would need access to the file system for that. Given the scanners from Chapter 4, which come by within a minute, that is no theoretical advantage: a setup page without such protection would be an open door for exactly the span between certificate and first login.

This protection is a property of the application, not of the deployment. It belongs here because it makes the difference between a reachable and an operational installation — and because everyone following this guide will look for it at this point.

What is running now

The numbers

These values are the yardstick that Parts 3 through 5 will have to measure up against.

What became visible along the way

Four observations carry beyond this part.

Individual measures only work in combination, even across parts. The bind address from Part 1, Chapter 8 justifies the forwarded-header evaluation from Chapter 4 — without the one, the other would be either ineffective or dangerous. The kernel parameter from Part 0, Chapter 8 closes the gap that Chapter 7 leaves open. None of these measures stands on its own, and none of them sits where you would look for it first.

Two services, two sets of directives. Caddy and the application run on the same server and still needed different hardening: what breaks the JVM does not bother a program written in Go, and vice versa. Copying a template from one to the other would have damaged both — one rendered inoperative, the other left needlessly exposed.

The certificate is a starting gun. One minute passed between the issuance and the first scanner probing the new name. A domain name in the certificate log is public the moment it comes into existence. Anyone who only starts thinking about hardening afterwards is thinking too late — which is why the service stood hardened at the end of Part 1 before anyone could reach it at all.

A rollback costs nothing. The same three commands, the same downtime. What is expensive is not the way back but the decision — and the data, which does not come along.

What this setup does not deliver

For honesty’s sake, because the following parts pick up exactly here:

There is downtime. Around six seconds per release, and all running sessions are lost in the process.

Four things have to be maintained separately: operating system, JDK, Jetty, and application. Each has its own update cycle, and only one of them is in the application developer’s hands.

The artifact is incomplete. A WAR does not run on its own; it needs a container of the right version with the right modules. What works on the development machine can fail on a differently configured server.

Part 3 starts at exactly this last point.

Outlook on Part 3

The boundary shifts

Part 1, Chapter 1 named the through line: in Parts 1 and 2, the application consists exclusively of the WAR. Jetty, the Java runtime, and the server are environment — present, maintained, updated independently.

Part 3 shifts this boundary for the first time.Jetty becomes a library of the application.

What changes as a result

Instead of an installed servlet container into which an archive is placed, there is an application with its ownmain method. It creates the server, configures the connector, mounts the Vaadin servlet, and starts.

Concretely, several things from Part 1 disappear:

What stays

More remarkable than the differences is how little changes. Caddy remains unchanged — it talks to127.0.0.1:8080 and does not know what runs behind it. The unit definition from Part 1, Chapter 10 essentially changes itsExecStart line. The directory layout, the service account, the hardening, the configuration, and the credentials stay as they are.

That is no coincidence but the result of the separation that Part 0 and Chapters 9 through 11 of Part 1 built up: the operational frame is independent of how the application starts internally.

What is gained and what is given up

Gained is an artifact that is more complete. The Jetty version becomes a decision of the application project and moves into version control — a server with a misconfigured container can no longer damage the application.

Given up is independence. From Part 3 on, a security update for Jetty is an application release: rebuild, redeliver, restart. In Part 1, it was a symlink and a restart, without touching the application.

Which of the two splits is the right one depends on who maintains the server and how often releases ship. Part 6 puts all five variants side by side at the end — with the numbers from Chapter 11 as the starting point.