> Markdown version of [Troubleshooting](https://vaadin.com/docs/v24/hilla/guides/production/troubleshooting). Section index: [llms.txt](https://vaadin.com/docs/v24/hilla/llms.txt)

# Troubleshooting Production Mode

This article offers guidance in the event that you have problems taking Hilla applications to production.

## <a id="verifying-the-production-mode-artifact"></a>Verifying the Production Mode Artifact

Vaadin Servlet configures itself from the `flow-build-info.json` file, which is located in the `META-INF/VAADIN/config/` resource package. The location of resources differs for different artifact types, for example:

- Spring Boot `JAR` file places resources into the `BOOT-INF/classes/` folder in the `JAR` file;

- `WAR` archive places resources into the `WEB-INF/classes/` folder.

If the production mode has been activated properly, the contents of `flow-build-info.json` should be as follows:

flow-build-info.json

```json
{
  "compatibilityMode": false,
  "productionMode": true,      (1)
  "enableDevServer": false,
  "chunks": {
    "fallback": {
      "jsModules": [
        "@vaadin/vaadin-grid/src/vaadin-grid-tree-toggle.js",
// etc etc
        "frontend://ironListConnector.js"
      ],
      "cssImports": [
      ]
    }
  }
}
```

1. The `productionMode` property is set to `true`.

It’s very important to have this file on the class path in production mode exactly once.

If the file is missing, Vaadin Servlet will use other means to examine whether it’s running in production mode: the value of the `vaadin.productionMode` system property, or the `flow-server-production-mode.jar` being present on the class path, or similar.

Depending on the outcome, Vaadin may throw an exception at runtime:

```
Failed to determine project directory for dev mode...
```

or

```
The compatibility mode is explicitly set to 'false', but there are neither 'flow-build-info.json' nor 'webpack.config.js' files
```

Vaadin may also decide to start in development mode; the Vaadin configuration is then governed by a different set of rules (for example, the `project.basedir` property or similar).

The project `flow-build-info.json` file is generated by the Vaadin plugin, in the `prepare-frontend` Maven goal, then modified in the `build-frontend` task to enable production mode.

If the file is present multiple times on the class path, Vaadin tries to choose the right one, which should come from the main project. Sometimes it’s not possible to do so. In this case, Vaadin prints a warning message and chooses the first `flow-build-info.json` file, which depends on the class path ordering, which in turn may depend on the ordering of files on the file system or in the `WAR`/`JAR` archive.

This can happen when a Vaadin add-on incorrectly includes the `flow-build-info.json` file in the `JAR` file. This is a bug in the add-on packaging that needs to be fixed. The `flow-build-info.json` file should be removed from the add-on’s `JAR` file.

For the reasons stated above, be sure to have the `flow-build-info.json` file on the class path only once, and always coming from your main project.

A file named `META-INF/VAADIN/config/stats.json` is generated by the Maven plugin, as well. It’s important to check for the presence of this file in the resources folder.

When packaging for production a webpack executable is run. This happens in the `build-frontend` Maven goal. Hence, webpack is responsible for packaging everything from `src/main/frontend/` and `node_modules` into the pre-compiled JavaScript files bundle. The bundle is located in the `META-INF/VAADIN/build/` resource folder. The folder contents should look like this (the hash is going to differ on every build):

```
├── vaadin-2-18d67c4ccff7e93b081a.cache.js
├── vaadin-2-18d67c4ccff7e93b081a.cache.js.gz
├── vaadin-3-b0147df339bf18eb7618.cache.js
├── vaadin-3-b0147df339bf18eb7618.cache.js.gz
├── vaadin-4-ee1d2e45569f7eca4292.cache.js
├── vaadin-4-ee1d2e45569f7eca4292.cache.js.gz
├── vaadin-5-5e9292474e82143d0a27.cache.js
├── vaadin-5-5e9292474e82143d0a27.cache.js.gz
├── vaadin-bundle-19a00eae62ad7cddd291.cache.js
├── vaadin-bundle-19a00eae62ad7cddd291.cache.js.gz
├── vaadin-bundle.es5-b1c1a3cc054c62ad7949.cache.js
├── vaadin-bundle.es5-b1c1a3cc054c62ad7949.cache.js.gz
└── webcomponentsjs
    ├── bundles
    │   ├── webcomponents-ce.js
    │   ├── webcomponents-ce.js.map
    │   ├── webcomponents-sd-ce.js
    │   ├── webcomponents-sd-ce.js.map
    │   ├── webcomponents-sd-ce-pf.js
    │   ├── webcomponents-sd-ce-pf.js.map
    │   ├── webcomponents-sd.js
    │   └── webcomponents-sd.js.map
    ├── custom-elements-es5-adapter.js
    ├── LICENSE.md
    ├── package.json
    ├── README.md
    ├── src
    │   └── entrypoints
    │       ├── custom-elements-es5-adapter-index.js
    │       ├── webcomponents-bundle-index.js
    │       ├── webcomponents-ce-index.js
    │       ├── webcomponents-sd-ce-index.js
    │       ├── webcomponents-sd-ce-pf-index.js
    │       └── webcomponents-sd-index.js
    ├── webcomponents-bundle.js
    ├── webcomponents-bundle.js.map
    └── webcomponents-loader.js
```

## <a id="common-issues"></a>Common Issues

- After adding the `flow-server-production-mode` dependency, the application no longer starts

  One potential cause of this problem is that the `build-frontend` of the `flow-maven-plugin` wasn’t executed, either because the plugin is missing from the `pom.xml` or it’s missing configuration. To fix this, add `flow-maven-plugin` to your Maven `build` block (make sure it’s visible in your production mode profile), and enable the `build-frontend` goal.