Maintaining zero downtime for state machines in distributed systems is both challenging and costly, yet it is a critical requirement for many modern applications. This pattern outlines techniques to implement zero downtime state machines, with a particular focus on state machines running in a sequencer based architecture.

Why Zero Downtime State Machines?

Zero downtime state machines are a requirement for distributed systems that are required to be available 24x7. Examples include cryptocurrency exchanges, lending platforms, payment gateways, and other decentralized finance (DeFi) services, where clients expect to submit transactions at any time without interruption. Any downtime can lead to lost revenue, diminished user trust, and a competitive disadvantage.

What's the Problem?

The challenges are in three areas:

• State Machine Logic Changes: State machines must evolve to incorporate new features, fix bugs, or improve performance. Modifying the state machine logic, however, can introduce incompatibilities with the existing state. How can we implement logic changes without affecting the integrity of the existing state?
• State Machine Replication: To ensure high availability, state machines are replicated across multiple nodes in a distributed system. Discrepancies arise when different replicas are running different versions of the state machine logic, potentially leading to inconsistent states and behaviors. How do we maintain consistency and correctness when replicas operate with different logic versions?
• State Machine State Persistence: State machines rely on persistent storage to recover from failures and maintain continuity. When a new version of a state machine reads from stored state data (such as a snapshot from an earlier version), compatibility issues can occur if the state format has changed. How can we design the state persistence layer to support seamless upgrades and backward compatibility?

Techniques

Technique 1: Assessing the Necessity for Zero Downtime

The first technique is to evaluate whether zero downtime is essential for every state machine in your system. Often, only certain state machines require continuous availability. For those that do not need to be operational 24/7, you can plan upgrades during scheduled downtime windows that align with business requirements. This approach allows you to bypass the complexities of achieving zero downtime for less critical state machines, focusing your resources on maintaining uninterrupted service for the ones that truly demand it.

Technique 2: Isolating Transport from Logic

In systems like Aeron Cluster, it’s common to bundle the gateway transport (for example, Artio FIX connectivity) and at least some message-processing logic within a single process. While this integration might simplify initial deployment, it becomes a significant hurdle for achieving zero downtime. Any change to the logic mandates a full process restart, causing service interruptions—a scenario that’s unacceptable for state machines requiring continuous availability.

The Solution: Decouple the transport gateway from the processing service. By separating these components, you allow each to be upgraded independently without affecting the other. The transport gateway handles the movement of messages between external connections and internal state machines, ensuring continuous data flow. The processing service processes these messages (in the case of an Artio FIX gateway, the FIX messages are processed in the processing service), implementing the state machine’s functionality.

Benefits of Separation:

• Continuous Operation During Upgrades: Updates to the logic can be deployed without restarting the transport layer, maintaining uninterrupted service.
• Enhanced Reliability: Isolation reduces the risk of a single component failure impacting the entire system.
• Scalability: Independent components can be scaled according to demand without affecting each other.
• Flexibility in Development: Teams can work on transport and logic separately, accelerating development cycles.

By isolating the transport from the logic, you facilitate seamless upgrades and minimize downtime. This approach significantly improves system reliability and ensures that critical state machines remain available to process transactions at all times.

Technique 3: State Machine Upgrade Strategies

Achieving zero downtime during state machine upgrades requires a well-planned strategy that ensures seamless transitions without disrupting service availability. Here are two effective approaches to consider:

Strategy A: Parallel Deployment with Controlled Switchover

In this strategy, you run the new version of the state machine in parallel with the old one. Both versions process incoming messages simultaneously, keeping their states synchronized. Once the new version has been validated and is ready to take over, you send a termination message to the old version, allowing the new version to assume full responsibility.

Mechanically, this looks like:

1. Deploy New Version in Parallel: Launch the new state machine version alongside the existing one.
2. Synchronize States: Ensure both versions receive and process the same messages to maintain state consistency.
3. Monitor Performance: Observe the new version for any anomalies while it’s running in parallel.
4. Initiate Switchover: Send a control message to gracefully terminate the old version once the new version is confirmed stable.
5. Deactivate Old Version: The old state machine shuts down without affecting ongoing operations.

This approach has the following benefits:

• Seamless Transition: Minimizes risk by thoroughly testing the new version in a live environment before full deployment.
• No Service Interruption: Users experience continuous service without any downtime.
• Easy Rollback: If issues arise, you can revert to the old version quickly since it’s already running.

Strategy B: In-Place Version Switching via Controlled Activation

This approach involves deploying the new state machine version as if it were the old one, maintaining identical behavior initially. The new logic remains dormant until you send a specific message that triggers the switch to the new functionality.

Mechanically, this looks like:

1. Deploy New Version with Old Logic: Release the new version configured to operate exactly like the old one.
2. Maintain Consistent Behavior: Ensure no changes are apparent to users during this phase.
3. Send Activation Message: Dispatch a control message to all instances of the state machine to activate the new logic simultaneously.
4. Switch Logic Seamlessly: The state machine transitions to the new behavior without restarting or downtime.
5. Monitor Post-Switch Performance: Keep an eye on the system to quickly address any unforeseen issues.

This approach has the following benefits:

• Atomic Transition: All nodes switch to the new logic at the same time, preventing state inconsistencies.
• Simplified Deployment: Eliminates the need for complex synchronization mechanisms during rollout.
• Reduced Risk: By initially running the new version with old behavior, you minimize potential disruptions.

Choosing the Right Strategy

The decision between these strategies depends on factors like system complexity, resource availability, and the criticality of services. Use Parallel Deployment when you can afford the extra resources to run two versions simultaneously and want the safety net of an easy rollback. Opt for In-Place Version Switching when resource constraints exist or when simultaneous activation across all nodes is crucial.

With both strategies you should:

• Ensure that the new version can interpret and manage the existing state without errors.
• Rigorously test the new version in a controlled environment before deployment.
• Implement robust monitoring to detect and respond to issues promptly during and after the transition.
• Clearly document the upgrade process and communicate plans to all stakeholders involved.

By implementing a well-defined upgrade strategy, you can maintain zero downtime for your state machines, ensuring continuous availability and reliability of your distributed system. These strategies help you navigate the challenges of evolving state machine logic and state persistence across versions, ultimately contributing to a resilient and scalable architecture.