Flotilla - The Petit Fleet

Flotilla is an alternative to fleet that uncouples from CoreOS/etcd and couples to AWS.

Flotilla uses:

• DynamoDb - for persistence, locking and communication
• ELB - to reroute traffic during an upgrade
• IAM - for access control
• KMS - for encryption of environment variables
• CloudFormation - for provisioning resources
• SQS - for communication between components

Like fleet, flotilla distributes systemd units across a cluster of machines.

Unlike fleet, flotilla manages environment variables for scheduled services, and supports weighted distributions of services (for canaries, A/B, load balancing, etc).

Scheduling

Flotilla introduces the concept of a service to distinguish worker instances. An instance is born into a service and advertises membership in DynamoDb.

A scheduler running in the cluster periodically checks for:

• Which units+configurations are defined for each service? What are their weights?
• How many instances are currently being used for this service?

Given these inputs, the scheduler updates assignments for instances to satsify the current configuration.

Currently there is a single scheduler for the entire cluster. DynamoDb parallel scans are the intended scaling path for multiple schedulers.

Deployment

Flotilla workers periodically check for assignments in DynamoDb. If a worker's assignment is changed, it executes the following steps:

1. Acquire deployment lock for service.
2. If ELB is defined:
   1. Request deregistration from ELB.
   2. Wait for ELB deregistration (i.e. drain connections)
3. Stop and unload all existing flotilla units.
4. Start new flotilla units.
5. If ELB is defined:
   1. Request registration to ELB.
   2. Wait for ELB registration (i.e. health check).
6. Release deployment lock for service.

Limitations

Don't use this in production. This implementation is output from 3x8h "hackathon"s.