Considerations for Designing and Running an Application in the Cloud

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Application Design for the Cloud

Applications written in supported application frameworks often run unmodified on Cloud Foundry, if the application design follows a few simple guidelines. Following these guidelines makes an application cloud-friendly, and facilitates deployment to Cloud Foundry and other cloud platforms.

The following guidelines represent best practices for developing modern applications for cloud platforms. For more detailed reading about good app design for the cloud, see The Twelve-Factor App.

For more information on features of HTTP routing handled by the Cloud Foundry router, see the HTTP Routing topic. For more information on the lifecycle of application containers, see the Application Container Lifecycle topic.

Avoid Writing to the Local File System

Applications running on Cloud Foundry should not write files to the local file system for the following reasons:

  • Local file system storage is short-lived. When an application instance crashes or stops, the resources assigned to that instance are reclaimed by the platform including any local disk changes made since the app started. When the instance is restarted, the application will start with a new disk image. Although your application can write local files while it is running, the files will disappear after the application restarts.

  • Instances of the same application do not share a local file system. Each application instance runs in its own isolated container. Thus a file written by one instance is not visible to other instances of the same application. If the files are temporary, this should not be a problem. However, if your application needs the data in the files to persist across application restarts, or the data needs to be shared across all running instances of the application, the local file system should not be used. We recommend using a shared data service like a database or blobstore for this purpose.

For example, instead of using the local file system, you can use a Cloud Foundry service such as the MongoDB document database or a relational database like MySQL or Postgres. Another option is to use cloud storage providers such as Amazon S3, Google Cloud Storage, Dropbox, or Box. If your application needs to communicate across different instances of itself, consider a cache like Redis or a messaging-based architecture with RabbitMQ.

Cookies Accessible across Applications

In an environment with shared domains, cookies might be accessible across applications.

Many tracking tools such as Google Analytics and Mixpanel use the highest available domain to set their cookies. For an application using a shared domain such as example.com, a cookie set to use the highest domain has a Domain attribute of .example.com in its HTTP response header. For example, an application at my-app.shared-domain.example.com might be able to access the cookies for an application at your-app.shared-domain.example.com.

Consider whether you want your applications or tools that use cookies to set and store the cookies at the highest available domain.

Port Limitations

Clients connect to applications running on Cloud Foundry by making requests to URLs associated with the application. Cloud Foundry allows HTTP requests to applications on ports 80 and 443. For more information, see the Routes and Domains topic.

Cloud Foundry also supports WebSocket handshake requests over HTTP containing the Upgrade header. The Cloud Foundry router handles the upgrade and initiates a TCP connection to the application to form a WebSocket connection.

To support WebSockets, the operator must configure the load balancer correctly. Depending on the configuration, clients may have to use a different port for WebSocket connections, such as port 4443. For more information, see the Supporting WebSockets topic.

Cloud Foundry Updates and Your Application

For application management purposes, Cloud Foundry may need to stop and restart your application instances. If this occurs, Cloud Foundry performs the following steps:

  1. Cloud Foundry sends a single termination signal to the root process that your start command invokes.

  2. Cloud Foundry waits 10 seconds to allow your application to cleanly shut down any child processes and handle any open connections.

  3. After 10 seconds, Cloud Foundry forcibly shuts down your application.

Your application should accept and handle the termination signal to ensure that it shuts down gracefully.

Ignore Unnecessary Files When Pushing

By default, when you push an application, all files in the application’s project directory tree are uploaded to your Cloud Foundry instance, except version control or configuration files with the following file extensions:

  • .cfignore
  • _darcs
  • .DS_Store
  • .git
  • .gitignore
  • .hg
  • /manifest.yml
  • .svn

If the application directory contains other files (such as temp or log files), or complete subdirectories that are not required to build and run your application, the best practice is to exclude them using a .cfignore file. (.cfignore is similar to git’s .gitignore, which allows you to exclude files and directories from git tracking.) Especially with a large application, uploading unnecessary files slows down application deployment.

Specify the files or file types you wish to exclude from upload in a text file, named .cfignore, in the root of your application directory structure. For example, these lines exclude the “tmp” and “log” directories.

    tmp/
    log/

The file types you will want to exclude vary, based on the application frameworks you use. The .gitignore templates for common frameworks, available at https://github.com/github/gitignore, are a useful starting point.

Run Multiple Instances to Increase Availability

When a DEA is upgraded, the applications running on it are shut down gracefully, then restarted on another DEA. To avoid the risk of an application being unavailable during a Cloud Foundry upgrade processes, you should run more than one instance of the application.

Using Buildpacks

A buildpack consists of bundles of detection and configuration scripts that provide framework and runtime support for your applications. When you deploy an application that needs a buildpack, Cloud Foundry installs the buildpack on the Droplet Execution Agent (DEA) where the application runs.

For more information, see the Buildpacks topic.

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