§ Production

We’ve designed Flynn to be very easy to get up and running with quickly. However, there are a variety of things that should be considered as you start using Flynn more.

§ Cluster Requirements

At least three hosts are required to deploy Flynn in a highly available configuration. We do not recommend single-node clusters for production use. A three-host cluster will withstand the loss of one node with little to no impact on availability.

All members of the initial cluster participate in Raft consensus voting, hosts started after the initial bootstrap act as proxies to the consensus cluster. We recommend starting with three or five hosts and adding more hosts when necessary.

Each host should have a minimum of 2GB of memory, and inter-host network packets should have a latency of less than 2ms. Deploying a single Flynn cluster across higher latency WAN links is not recommended, as it can have a significant impact on the stability of cluster consensus.

§ Storage

Flynn uses ZFS to store data. By default, a ZFS pool is created in a sparse file on top of the existing filesystem at /var/lib/flynn/volumes. We don’t recommend keeping this configuration in production, as it is not as reliable as dedicating whole disks to the ZFS pool.

§ Custom ZFS pool

The Flynn install script can be used to create the ZFS pool on the device of your choice instead of a sparse file, using the --zpool-create-device flag:

$ ./install-flynn --zpool-create-device /dev/sdb --zpool-create-options "-f"

If you already have a Flynn cluster running, you can move the existing ZFS pool off of the sparse file by first attaching your disk as a mirror, then detaching the sparse file after it has been replicated to the new disk:

# Attach /dev/sdb1 (specify your disk instead of sdb1) to the flynn-default ZFS pool
$ sudo zpool attach flynn-default /var/lib/flynn/volumes/zfs/vdev/flynn-default-zpool.vdev /dev/sdb1

# Wait for the resilver to copy all data onto the newly added disk
$ sudo zpool status flynn-default
  pool: flynn-default
 state: ONLINE
  scan: resilvered 59.7M in 0h0m with 0 errors on Mon Nov  2 04:02:58 2015

    NAME                                                          STATE     READ WRITE CKSUM
    flynn-default                                                 ONLINE       0     0     0
      mirror-0                                                    ONLINE       0     0     0
        /var/lib/flynn/volumes/zfs/vdev/flynn-default-zpool.vdev  ONLINE       0     0     0
        sdb1                                                      ONLINE       0     0     0

errors: No known data errors

# Detach the sparse file from the ZFS pool and delete it
$ sudo zpool detach flynn-default /var/lib/flynn/volumes/zfs/vdev/flynn-default-zpool.vdev
$ sudo rm /var/lib/flynn/volumes/zfs/vdev/flynn-default-zpool.vdev

§ Blobstore Backend

Flynn stores binary blobs like compiled applications, git repo archives, buildpack caches, and Docker image layers using the blobstore component. The blobstore supports multiple backends: Postgres, Amazon S3, Google Cloud Storage, and Microsoft Azure Storage.

By default, the blobstore uses the built-in Postgres appliance to store these blobs. This works well for light workloads and is the default configuration because it allows Flynn to be deployed anywhere without any external service dependencies.

We recommend using one of the external blobstore backends listed below for production, as they are more reliable than the Postgres backend and have predictable performance without consuming disk space on the Flynn hosts.

§ Amazon S3

To migrate to the S3 backend, you first need to provision a new bucket and create credentials for it. In AWS IAM, create a user with access credentials and add a policy that looks like this:

    "Version": "2012-10-17",
    "Statement": [
            "Effect": "Allow",
            "Action": [
            "Resource": [

Don’t forget to save the credentials when creating the user and set the bucket name in the Resource section before adding the policy.

After creating the S3 bucket and credentials, configure the blobstore to use it as the backend with bucket, region, and access credentials:

flynn -a blobstore env set BACKEND_S3MAIN="backend=s3 region=us-east-1 \
bucket=flynnblobstore access_key_id=$AWS_ACCESS_KEY_ID \

flynn -a blobstore env set DEFAULT_BACKEND=s3main

If the credentials are invalid, the first command will fail, and you can check the logs with flynn -a blobstore log.

Finally, migrate the existing blobs from Postgres to S3 and remove them from Postgres:

flynn -a blobstore run /bin/flynn-blobstore migrate --delete

Or if you’re on a version of Flynn older than v20160924.0:

flynn -a blobstore run /bin/flynn-blobstore-migrate --delete

§ Google Cloud Storage

To migrate to the Google Cloud Storage backend, you first need to create a new Service Account in IAM. The account does not need any roles, but make sure you create a private key and download the JSON version. After creating the account, create a Cloud Storage bucket and add the service account ID as a user with Owner access to the bucket permissions.

After creating the Cloud Storage bucket and credentials, configure the blobstore to use it as the backend, with the bucket name and key file:

flynn -a blobstore env set BACKEND_GCSMAIN="backend=gcs bucket=flynnblobstore" \
BACKEND_GCSMAIN_KEY="$(cat Project-7633a787c43f.json)"

flynn -a blobstore env set DEFAULT_BACKEND=gcsmain

If the credentials are invalid, the first command will fail, and you can check the logs with flynn -a blobstore log.

Finally, migrate the existing blobs from Postgres to Cloud Storage and remove them from Postgres:

flynn -a blobstore run /bin/flynn-blobstore migrate --delete

§ Microsoft Azure Storage

To migrate to the Azure Storage backend, you first need to create a storage account and container. After setting those up, you should have an account name, account key, and container name, which can be used to configure the backend:

flynn -a blobstore env set BACKEND_AZUREMAIN="backend=azure account_key=xxx \
account_name=yyy container=flynnblobstore"

flynn -a blobstore env set DEFAULT_BACKEND=azuremain

If the credentials are invalid, the first command will fail, and you can check the logs with flynn -a blobstore log.

Finally, migrate the existing blobs from Postgres to Azure Storage and remove them from Postgres:

flynn -a blobstore run /bin/flynn-blobstore migrate --delete

§ DNS and Load Balancing

Flynn has a built-in router that handles all incoming HTTP, HTTPS and TCP traffic. It is lightweight and runs on every Flynn host, this allows easy deployment of Flynn without requiring another load balancer in front of Flynn.

We recommend running Flynn with low-TTL round-robin DNS A records pointing at each host. For automatic failover, health checks should be configured that automatically remove a host’s A record if it is unhealthy.

Alternatively, a TCP load balancer for ports 80 and 443 can be configured in front of the Flynn router, this may increase overhead and complexity but can make sense in some environments.

§ Firewalling

A firewall preventing external access must always be configured on or in front of Flynn hosts, only these ports should be allowed:

Internal cross-host cluster communication happens on a variety of UDP and TCP ports and should not be restricted.

Outbound Internet access is required to deploy apps using many of the default buildpacks.

§ Automation

Installation and management of Flynn clusters can be automated using a variety of existing tools. We recommend using tools that you are familiar with to create a repeatable system for deploying Flynn clusters. This will allow you to quickly start new clusters for testing, updates, and failure remediation.

When possible, we recommend deploying immutable infrastructure and deploying new clusters for updates and major configuration changes.

§ Backups

Flynn supports full-cluster backup/restore as well as export/import of individual applications (including their databases).

§ Cluster Backup

To take a full-cluster backup, run flynn cluster backup --file backup.tar. A file name backup.tar will be created with a complete copy of all data and configuration for the cluster. This includes full exports of all data in each database managed by Flynn along with configuration necessary to start a new cluster and restore the data.

§ Cluster Restore

To restore from a full-cluster backup, follow the manual installation instructions and modify the flynn-host bootstrap command to include an extra flag: --from-backup backup.tar. The cluster size does not need to be the same, but the --min-hosts flag and cluster discovery flag should be specified. The CLUSTER_DOMAIN variable is ignored, the domain of the previous cluster will be used.

§ App Export

To export a single app, run flynn -a APPNAME export --file app.tar. A file named app.tar will be created with the app configuration and image, along with a copy of all data stored in associated databases. The app export can be restored to the same cluster under a different name, or a different Flynn cluster.

§ App Import

To import an app, run flynn import --file app.tar. This will create a new app on the cluster, import the configuration, create databases, and import the data from the exported app. If you’d like to provide a new name for the app, the --name flag may be specified. By default a new route is created based on the app name and the cluster domain. To import the old routes in addition to the new route, add the --routes flag.

§ SSH Client Key

Some apps require a SSH private key to download dependencies or submodules while deploying. Flynn supports configuring a single SSH client key for the platform that will be used whenever SSH is used during builds triggered by git push.

To configure the keypair, set the SSH_CLIENT_KEY and SSH_CLIENT_HOSTS environment variables on the built-in gitreceive app:

flynn -a gitreceive env set SSH_CLIENT_HOSTS="$(ssh-keyscan -H github.com)"\
   SSH_CLIENT_KEY="$(cat ~/.ssh/id_rsa)"

§ Monitoring

Flynn provides a status endpoint over HTTP that exposes the health of system components at http://status.$CLUSTER_DOMAIN (for example, https://status.1.localflynn.com). The status endpoint returns a status code along with a more detailed JSON response. If any core components are unhealthy, the HTTP status will be 500.

Requests to the status endpoint require a key parameter (example: https://status.1.localflynn.com/?key=$AUTH_KEY) when they come from IP addresses that are not reserved for private use.

The $AUTH_KEY may be retrieved with this command:

flynn -a status env get AUTH_KEY

§ Debugging

Flynn is a self-hosting system, this allows you to use the flynn and flynn-host tools to introspect, configure, and debug the system. All components with the exception of the flynn-host daemon appear as and can be managed as apps in Flynn.

§ Retrieving logs

Logs can be retrieved using several methods.

The flynn -a $APP_NAME log command will retrieve up to the last 10,000 lines logged by all app processes and can also follow the stream as new log lines are emitted.

The flynn-host log $JOB_ID command on a server will retrieve the logs for a specific job and can also follow the stream for that job. A list of all jobs, including those that are no longer running can be retrieved with the flynn-host ps -a command.

Flynn stores logs from jobs in /var/log/flynn on each host. There is one log file for jobs from each app that has been run on the host. All logs from jobs tagged with the same app ID will go into a file named /var/log/flynn/$APP_ID.log. When a log file reaches 100MB in size, it is rotated and a new file is created. One previous rotated log is kept, for a total of a maximum 200MB of logs per app per host.

Upstart manages the flynn-host daemon and stores the log at /var/log/upstart/flynn-host.log.

The flynn-host collect-debug-info command will collect information about the system it is run on along with recent logs from all apps and the flynn-host daemon. By default it uploads these logs to GitHub’s Gist service, but they can also be saved to a local tarball with the --tarball flag.

§ Internal Databases

The controller, router, and blobstore components store data in a PostgreSQL cluster managed by Flynn. Their databases may be accessed by running flynn -a $APP_NAME pg psql.

§ Updating

There are two ways to update Flynn: in-place and backup/restore. The in-place updater is new and we do not consider it to be stable. The most reliable update method is backup/restore.

§ Backup/Restore

The backup/restore update method involves taking a full backup of the cluster and restoring it to a new cluster with the new version of Flynn.

  1. Take a backup of the cluster with flynn cluster backup --file backup.tar.
  2. Install the new version of Flynn on a new cluster by following the manual installation instructions up to but not including the bootstrap step.
  3. Run the flynn-host bootstrap command from the installation guide with an added flag pointing at the cluster backup file: flynn-host bootstrap --from-backup backup.tar
  4. Update the DNS records that point at the old cluster to point at the new cluster.

§ In-place update

The in-place updater is new and could cause cluster failure. We recommend taking a full backup of the cluster first with flynn cluster backup. There is almost zero downtime during the cluster update, however database clusters may be unavailable for a few seconds while they are updated.

To perform an in-place update of the entire cluster, run flynn-host update.

§ Adding Hosts

Hosts may be added to an existing cluster by running flynn-host init with the discovery token or list of host IPs that was used to start the cluster.

Care should be taken to ensure that the same version of Flynn is installed on all hosts. The installed version of Flynn can be checked with flynn-host version, and the version to install can be specified by setting the --version CLI flag to the desired version when running the install script.

§ Replacing Hosts

If a member of the cluster that is participating in the consensus set becomes permanently unavailable, it must be replaced in order to restore fault tolerance. If you have already added additional hosts to your cluster beyond the members of the consensus set, you can promote one of these peers to the consensus set. Otherwise you will first need to start a new host and join it to the cluster as described in the Adding Hosts section. You can check the Raft consensus status of your cluster hosts by running flynn-host list. If the host is a member its status will be displayed as peer. If not, it will be listed as proxy.

The first step is to demote the old peer from the consensus set. If the peer is already unavailable, use the --force flag. If you are preemptively replacing a peer that is currently working, the force flag should not be used.

To demote the peer, run flynn-host demote --force $PEER_IP.

Once the old peer is removed you can add the new peer to the consensus set with flynn-host promote $PEER_IP. This may take a short time as the new peer replicates data from the Raft leader before starting to service operations.

When the process is complete a discoverd deployment should be run to update the DISCOVERD_PEERS environment variable. You can retrieve the current value with flynn -a discoverd env get DISCOVERD_PEERS. Replace the address of the old peer with the new one and update the value with flynn -a discoverd env set DISCOVERD_PEERS=$PEER_IPS

At this point the host has been replaced successfully and the cluster will regain full fault tolerance.

§ Controller Keys

The Flynn CLI requires a controller authentication key to interact with a cluster. The key is generated at cluster creation time, but it can be changed and new keys can be added. Keys are stored in the environment variable AUTH_KEY, a comma-separated list of authentication keys.

# Generate a new random key
NEW_KEY=$(openssl rand -hex 16)

# Add the new key alongside the existing key
flynn -a controller env set -t web AUTH_KEY=$NEW_KEY,$(flynn -a controller env get AUTH_KEY)

To rotate an authentication key:

# Generate a new random key
NEW_KEY=$(openssl rand -hex 16)

# Add both the old and new keys to just the web process type
flynn -a controller env set -t web AUTH_KEY=$NEW_KEY,$(flynn -a controller env get AUTH_KEY)

# Update internal apps to use the new key
flynn -a docker-receive env set AUTH_KEY=$NEW_KEY CONTROLLER_KEY=$NEW_KEY
flynn -a gitreceive env set CONTROLLER_KEY=$NEW_KEY
flynn -a taffy env set CONTROLLER_KEY=$NEW_KEY
flynn -a dashboard env set CONTROLLER_KEY=$NEW_KEY
flynn -a redis env set CONTROLLER_KEY=$NEW_KEY
flynn -a mariadb env set CONTROLLER_KEY=$NEW_KEY
flynn -a mongodb env set CONTROLLER_KEY=$NEW_KEY

# Set the global key to be the new key
flynn -a controller env set AUTH_KEY=$NEW_KEY

# Unset the web process-specific key
flynn -a controller env unset -t web AUTH_KEY