§ Apps

Applications can be deployed to Flynn using Buildpacks or Docker. This page provides information about the management and configuration of apps on Flynn.

§ Configuration

As suggested in The Twelve-Factor App, Flynn uses environment variables to configure applications.

The flynn env command is used to read and write environment variables.

flynn env set SECRET=thisismysecret

Setting environment variables in Flynn creates a new release, which will restart all of the app’s processes with the new configuration.

§ External Databases

Flynn apps can communicate with the built-in databases as well as databases hosted outside of Flynn. Pass the configuration for the external database in as an environment variable with flynn env.

§ Buildpacks

Flynn uses buildpacks to prepare and build apps deployed with git push. Flynn will automatically select a standard buildpack for most supported languages.

The buildpack can be manually specified in cases where auto-detection is not possible, or overridden when the standard buildpacks are not suitable.

The multi buildpack is included in Flynn and can be used to specify a custom buildpack in addition to allowing the use of multiple buildpacks during a single deploy.

To specify a custom buildpack, create and commit a .buildpacks file with one or more URLs of buildpacks to use:


If you don’t want to add a file to your app’s repository to specify the buildpack, you can also set the BUILDPACK_URL environment variable to specify a custom buildpack:

flynn env set BUILDPACK_URL=https://github.com/ryandotsmith/null-buildpack

§ Deployment

Each time new code is pushed or the app configuration is changed, a new release is created. Flynn deploys releases using a zero-downtime strategy, the new release is started and the old release is only stopped if the new one comes up correctly. If the new release does not come back up or something else goes wrong, the deploy is automatically rolled back and the old release stays running.

§ Cancelling Deploys

Deploys via git push can be cancelled by killing the push process with Ctrl-C or by signalling the process to terminate. The build will be cancelled immediately and the code will not be deployed.

§ Building specific Git branches

To deploy a different branch of the same repository, create a new app using the same git repository but with different remotes:

flynn create myapp-staging --remote staging
flynn -a staging env set FOO=bar
git push staging staging:master

§ Processes

You can get a list of an app’s individual processes using flynn ps. The ID returned can then be passed to flynn kill to kill the process. Flynn will automatically restart any killed processes that are not one-off run jobs.

# Get a list of processes
$ flynn ps
ID                                          TYPE  STATE  CREATED        RELEASE                               COMMAND
host0-52aedfbf-e613-40f2-941a-d832d10fc400  web   up     6 seconds ago  cf39a906-38d1-4393-a6b1-8ad2befe8142  /runner/init start web

# Kill a process
$ flynn kill host-28a16c12-6136-4e06-93b1-2b014147de79
Job host-28a16c12-6136-4e06-93b1-2b014147de79 killed.

§ Logs

Flynn automatically logs everything that app processes write to the standard output and standard error streams. These logs can be retrieved with flynn log, and can be followed in real time with flynn log -f.

§ External Logs

Apps can also stream their logs to remote syslog services using system or client libraries. Most programming languages have built-in support for remote logging, for example Python’s SysLogHandler.

§ Routes

Flynn automatically configures a https://$APPNAME.$CLUSTERDOMAIN route that points at instances of the web process type for each app. Apps must bind to and accept HTTP requests at the port provided in the PORT environment variable to receive traffic.

§ Custom Domains

To add an additional HTTP route, use flynn route add http:

flynn route add http www.example.com

DNS will also need to be configured for the domain, in this example www.example.com should be set to a CNAME to $APPNAME.$CLUSTERDOMAIN.

§ Additional Process Types

Flynn supports serving web traffic from multiple process types. These additional process types must be defined in the Procfile and end in -web. For example, this Procfile:

web: ./server
admin-web: ./admin-server

Routes for the additional process type can be configured by specifying the --service flag:

flynn route add http --service myapp-admin-web admin.example.com


The router can automatically terminate HTTPS traffic, the certificate chain and key are specified with the --tls-cert and --tls-key flags when creating or updating the route. Enabling HTTPS for a route also enables HTTP/2 automatically.

flynn route update http/2b3b2004-38f1-4e68-b856-7d8af3e4c6e1 --tls-cert cert.pem --tls-key cert.key

The certificate file should contain PEM-encoded certificate blocks for the desired certificate followed by any intermediate certificates necessary to chain to a trusted root.

§ Service Discovery

Flynn automatically registers each web process type in service discovery for internal requests that do not go through the router. The service discovery entries are available via DNS. The pattern $APPNAME-$PROCTYPE.discoverd is used for the DNS name, for example myapp-web.discoverd and myapp-admin-web.discoverd. This feature can be used to communicate internally between apps and processes.

§ Limits

Memory and other resource limits can be retrieved and specified using the flynn limit command. For example:

flynn limit set web memory=2GB

§ CPU Shares

CPU shares are relative, the more shares a process has, the higher priority it is. When a host is under load, a job with 2000 milliCPUs will get twice the CPU time as a job with the default of 1000.

flynn limit set web cpu=1500

§ Slugbuilder Limits

Some build processes require a lot of memory. If you encounter a slowness or arbitrary failures during git push deploys, try increasing the memory limit of the slugbuilder process:

flynn limit set slugbuilder memory=4GB

You can also specify a default slugbuilder memory limit globally, set the SLUGBUILDER_DEFAULT_MEMORY_LIMIT environment variable for the apps that handle git push and Dashboard deploys:

flynn -a gitreceive env set $limit
flynn -a taffy env set $limit