Back atom.xml blog Noodling on Otto's pipeline state machine 21 Nov 2020 otto cicd rust

Recently I have been making good progress with Otto such that I seem to be unearthing one challenging design problem per week. The sketches of Otto pipeline syntax necessitated some internal data structure changes to ensure that to right level of flexibility was present for execution. Otto is designed as a services-oriented architecture, and I have the parser service and the agent daemon which will execute steps from a pipeline. I must now implement the service(s) between the parsing of a pipeline and the execution of said pipeline. My current thinking is that two services are needed: the Orchestrator and the Pipeline State Machine.

For this blog post I discuss much of what the Orchestrator should do other than to mention that I intend Orchestrators to exist to provision resources and launch agents for executing pipelines.

The Pipeline State Machine (PSM) is where the real fun starts. Somewhere inside Otto, something must keep track of the progression of a pipeline from one state to another, ensuring that the right actions are being triggered when certain state transitions occur.

## States

The current structure of the internal pipeline model informs the potential states in the state machine:

---
uuid: 'some'
batches:
- mode: Linear
contexts:
- uuid: 'uuid-context'
properties:
name: 'Build'
environment: {}
steps:
- symbol: 'sh'
uuid: 'uuid-step'
context: 'uuid-context'
parameters:
- 'pwd'
- mode: Linear
contexts:
- uuid: 'uuid2-context'
properties:
name: 'Test'
environment: {}
steps:
- symbol: 'sh'
uuid: 'uuid2-step'
context: 'uuid2-context'
parameters:
- 'make test'


“Batches” are a concept which will exist internally to Otto to help handle parallel stages and other novel groupings of steps. Referring back to the “sketches of syntax” post, a parallel or fanout block would result in a single Batch. Inside that Batch would be a Context for each stage declared, allowing some flexibility between the internal representation of a Pipeline and the user-visible declaration.

I believe that each Pipeline will largely need to progress through the states defined below.

• Pending: requires full model and uuid, basically the output from the Parser.
• for batch in batches
• Auction Started: with list of contexts that have been auctioned
• Auction Completed: with list of contexts and the winning Orchestrator for each context.
• Provisioning: mapping each context to an Orchestrator who should be provisioning the resource(s) necessary to execute that context.
• Execution(context): each context has its own state of: Pending/Running/Failed/Aborted/Unstable/Success
• Batch Complete
• Pipeline Complete(status)

“Auctions” refer to the planned resource auctioning work I wish to explore at a later date; the first version of PSM will likely omit these states.

The requirements for PSM I have in mind are:

• It should receive and store the entire pipeline model (YAML above). I am not yet should what the exact interplay between source control and PSM should be. I have an issue which mentions the service which will ingest GitHub Webhook payloads. My current thinking is that this service should perhaps be responsible for handling the webhook payload and fetching the Ottofile in order to send a request to the Parser and then PSM.
• It should hold the mapping between a given pipeline uuid and the states listed above.
• It should fire events for each state transition.

Some requirements I am not yet certain of are:

• Does it need to know which orchestrator or who is actually executing on a context or batch?
• Should PSM contain a mapping of a commit revision to pipeline uuid to help with de-duplication of pipelines for identical commits?

Looking at the shape of PSM is like a inspecting a building in the distance. I have a general idea of its dimensions and key characteristics, but the details remain blurry no matter how hard I squint.

This phase of Otto’s development has certainly been the most frustrating in months. I’m pushing towards enough service integration to allow for a Otto to perform basic self-hosted CI. To accomplish this I will need:

• A service ingesting webhooks and fishing the Ottofile out of the latest commit on a given branch.
• ☑ The Parser service, which turns an Ottofile into a usable internal model.
• A Pipeline State Machine to manage the execution of the pipeline.
• A basic Orchestrator which can dispatch a local otto-agent with the appropriate arguments.
• ☑ An object store service to contain logs, artifacts, and step libraries.
• ☑ An agent capable of executing steps.
• ☑ Defined steps to check out a source repo.

For the most basic self-hosted implementation, I don’t even think I need a GUI/dashboard or an eventbus, both of which are in the “grand vision.”

Much of what remains requires “big think” time however, which is in short supply. Every time I sit down to the problem, I spend a non-trivial amount of time debating whether I am over-complicating this before I am able to re-convince myself of the approach I am taking here.

The curse of working in Jenkins for so long is that I know how so many CI system design decisions ultimately run into limitations for certain use-cases.

Regardless of how challenging the path ahead appears, I will inch along, slowly but surely. :)

As always, if you’re curious to learn more, you’re welcome to join #otto on the Freenode IRC network, or follow along on GitHub