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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesA Socket.IO app deploys successfully on Dokku when three pieces agree: your Node process runs as a web process, it listens on the port Dokku assigns, and nginx forwards the WebSocket upgrade on the /socket.io/ path. Miss any one of those and ordinary HTTP may work while real-time connections fail.
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What you are deploying
Dokku runs your Node.js application behind its nginx reverse proxy. A browser first reaches nginx, nginx routes the request to the Dokku process, and Socket.IO then negotiates either a WebSocket connection or its HTTP long-polling fallback. The reverse proxy must preserve that negotiation; a normal HTTP proxy configuration is not sufficient.
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This guide assumes a VPS with Dokku already installed, a Node.js application that already runs locally, and DNS pointing your intended hostname at the server. VPS provisioning, DNS setup, TLS issuance and the exact deploy workflow vary by Dokku release and were not established in the available documentation, so verify those parts against the documentation installed with your version.
Prepare the Node.js application
Create one HTTP server for Express and Socket.IO
Attach Socket.IO to the same HTTP server that serves your application. The important deployment detail is that the listener uses Dokku’s supplied port and binds to an externally reachable interface:
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const http = require('http');
const express = require('express');
const { Server } = require('socket.io');
const app = express();
app.get('/', (_req, res) => res.send('ok'));
const server = http.createServer(app);
const io = new Server(server, {
// Match this path in the nginx configuration.
path: '/socket.io/'
});
io.on('connection', (socket) => {
socket.emit('ready', { id: socket.id });
});
const port = process.env.PORT;
if (!port) throw new Error('PORT was not provided');
server.listen(port, '0.0.0.0', () => {
console.log(`listening on ${port}`);
});Do not hard-code a public port in application code. Dokku’s port-management documentation advises letting Dokku derive the value from its port mapping rather than manually setting PORT. Because behavior can differ between Dokku releases and deployment methods, confirm the effective value in your installed version and log the port at startup.
Make the start command explicit
Declare a production start script in package.json:
{
"scripts": {
"start": "node server.js"
}
}Then add a Procfile at the repository root:
web: npm startDokku routes web traffic to the process type named web. On a first deploy, that process type is normally scaled to one instance; without it, Dokku has no web process to send requests to. If your entry file or package-manager command differs, change only the command after web:.
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Create the Dokku app and deploy it
Use a unique app name
On the VPS, create an app name that matches the name you will use in the Dokku Git remote. The exact command syntax should be checked against your Dokku release:
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dokku apps:create my-socket-appFrom your local project, add the Dokku remote in the form used by your server and push the branch you deploy:
git remote add dokku dokku@your-vps-host:my-socket-app
git push dokku mainThese commands are the conventional Dokku Git deployment form; if your installation uses a different SSH account, branch policy or remote name, use the values configured by that installation. Watch the build and release output for confirmation that dependencies installed, the web process started and a port was assigned.
Confirm the process and mapping before debugging nginx
Inspect the app’s process and proxy information with the reporting commands provided by your Dokku version. You want to see a running web process and a proxy mapping to the port on which the Node process is listening:
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dokku ps:report my-socket-app
dokku apps:report my-socket-appIf the process exits immediately, read the application log first. Typical causes are a missing production dependency, a start script that points at the wrong file, or code that calls listen with an undefined or hard-coded port.
Handle the port correctly
Port configuration is the part of this deployment that most often differs between tutorials. Dokku’s current port-management guidance says not to set PORT manually; Dokku obtains the internal port from its proxy mapping. A third-party example instead sets PORT=3000, but that approach is lower-confidence and may conflict with the installed release.
| Situation | What to do | Why |
|---|---|---|
Application reads process.env.PORT |
Prefer Dokku’s generated port and verify it with the app report. | Dokku can keep the process and proxy mapping consistent. |
Application hard-codes 3000 or another port |
Change the code to read process.env.PORT. |
A hard-coded listener may not match Dokku’s routing target. |
| You are following an older port-map example | Treat DOKKU_PROXY_PORT_MAP=http:80:5000 as a dated example, not a universal default. |
Port mappings and defaults can change between Dokku versions. |
A tutorial tells you to run dokku config:set APP PORT=3000 |
Do not apply it blindly; check the installed Dokku port-management documentation first. | Manual PORT settings can override or contradict Dokku’s mapping. |
The reliable test is agreement among three values: the port printed by your Node process, the internal target shown by Dokku, and the upstream target in nginx. If any two differ, fix that mismatch before changing Socket.IO settings.
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Configure nginx to pass the WebSocket upgrade
Required headers for a dedicated Socket.IO location
Socket.IO’s reverse-proxy guidance and Dokku’s WebSocket tutorial both require HTTP/1.1 plus the WebSocket upgrade headers. Dokku’s example also disables proxy buffering for the WebSocket location. A representative location is:
location /socket.io/ {
proxy_pass http://YOUR_DOKKU_UPSTREAM;
proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $scheme;
proxy_buffering off;
proxy_read_timeout 600s;
}YOUR_DOKKU_UPSTREAM is illustrative: use the upstream name or mapped backend generated by your Dokku installation. Do not assume that a backend port such as 5000 is correct for your app. Keep the /socket.io/ path aligned with the path option in your Socket.IO server and client.
Dokku’s tutorial uses a dedicated WebSocket upstream/location (often named with a -ws suffix). That arrangement makes the upgrade headers explicit while leaving ordinary HTTP handling separate. Apply the change through the nginx customization mechanism supported by your Dokku version rather than editing a generated file that Dokku will overwrite on the next deploy.
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Serve HTTP and WebSocket traffic in one location
If you want one location to handle both normal requests and upgrade requests, nginx can select the connection header from $http_upgrade:
map $http_upgrade $connection_upgrade {
default upgrade;
'' close;
}
location / {
proxy_pass http://YOUR_DOKKU_UPSTREAM;
proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection $connection_upgrade;
proxy_set_header Host $host;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $scheme;
proxy_read_timeout 600s;
}The map directive belongs in nginx’s http context, not inside a location. A dedicated /socket.io/ location is easier to audit; the mapped single-location form is useful when your application intentionally handles both traffic types through one route.
Set a timeout for long-lived connections
Reverse proxies may close an idle upstream connection after roughly 60 seconds by default. A Socket.IO connection can legitimately remain quiet for longer, so raise proxy_read_timeout for the WebSocket route (the examples use 600 seconds) and choose a value compatible with your application’s heartbeat interval. A timeout only controls how long nginx waits for upstream activity; it does not repair a missing upgrade header or an incorrect port.
Test the deployment in the right order
- Test ordinary HTTP. Open the app’s public URL and confirm that the route served by Express responds. If it fails, fix the process, domain or port mapping before testing WebSockets.
- Inspect the browser handshake. In browser developer tools, open the Network panel, filter for
socket.io, and reload the page. A successful WebSocket transport shows an upgrade request; polling requests may appear first while Socket.IO negotiates transports. - Check the response path. The client and server must use the same Socket.IO path, normally
/socket.io/. A client pointed at/wswill not reach a location configured for/socket.io/. - Read both logs. A 502 from nginx usually indicates an unavailable process or wrong upstream. A 400/404 on the Socket.IO endpoint commonly indicates a path, host or upgrade-routing mismatch. Application logs reveal whether the connection callback runs.
- Retest after every configuration change. Reload the supported nginx/Dokku configuration, reconnect the client, and verify that the generated configuration still contains the upgrade headers after a redeploy.
Troubleshoot the common failure modes
| Symptom | Likely cause | Correction |
|---|---|---|
| HTTP works, but the browser stays on polling or reports a failed upgrade | Missing HTTP/1.1, Upgrade or Connection header. |
Add all three directives to the Socket.IO location and reload the supported nginx configuration. |
| 502 Bad Gateway | The web process is stopped, listening on another port, or the upstream target is wrong. |
Check process status, startup logs, Dokku’s reported mapping and the nginx upstream. |
404 on /socket.io/ |
The client path and server path differ, or a more specific route intercepts the request. | Use the same path on both Socket.IO instances and route that path explicitly. |
| Connections drop after about one minute of inactivity | nginx’s idle upstream timeout is too short. | Increase proxy_read_timeout for the long-lived route and ensure Socket.IO heartbeats are functioning. |
| It works locally but not through the hostname | The local process is healthy, but DNS, Dokku routing or nginx is not forwarding the public request. | Verify the hostname reaches this VPS, then test HTTP, the reported port mapping and the upgrade request separately. |
| The process starts and then exits | The start command, production dependency installation or environment validation failed. | Run the exact production start command, inspect Dokku logs and ensure process.env.PORT is present at startup. |
Scaling beyond one instance
A single web process is the simplest topology and avoids cross-instance coordination. Socket.IO’s long-polling fallback is commonly associated with a need for sticky sessions when clients can move between instances, but the available Dokku material does not confirm how session affinity is configured for a particular Dokku release. Do not scale this app out solely by increasing the process count until you have verified Dokku’s routing behavior and Socket.IO’s adapter/session requirements for your version.
If you do test multiple instances, monitor both transports rather than testing only upgraded WebSocket connections. A setup that appears healthy for clients that remain on one upgraded connection can still fail for polling clients that make repeated HTTP requests.
Quick Recap
Production checklist
- The application creates one HTTP server and attaches Socket.IO to it.
- The listener reads
process.env.PORTand binds to0.0.0.0. package.jsonhas a working productionstartscript.- The root
Procfiledeclaresweb: npm start(or your equivalent command). - Dokku reports a running
webprocess and a matching proxy mapping. - You have verified the installed Dokku release’s port-management behavior instead of copying an unversioned
PORTsetting. - The nginx Socket.IO route uses HTTP/1.1,
Upgrade,Connectionand disabled buffering. - The client and server agree on
/socket.io/(or another explicitly configured path). proxy_read_timeoutis long enough for your heartbeat and idle-connection policy.- You have tested both the public HTTP response and the browser’s Socket.IO network handshake after deployment.
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