Scheduler

The Scheduler is an in-memory delayed task queue that is part of the HolaCloud ecosystem. It provides time-based task enqueuing, lease-based reservation for worker safety, and snapshot monitoring via Server-Sent Events (SSE). It is designed for short-lived delayed work, not recurring scheduling, automatic redelivery, or shared persistence.

Key Features

In-Memory Task Storage

Tasks are held in memory for fast scheduling and dispatch. The service is optimized for high-throughput, low-latency workloads where tasks are processed shortly after their scheduled time.

Time-Based Scheduling

Each task carries a future (or delay) field that controls when it becomes available. Tasks are dormant until their scheduled time elapses, at which point they become visible to workers.

Lease-Based Reservation

When a worker reserves a task, the task enters a lease period defined by worktime. If the worker completes and acknowledges the task within the lease, it is removed. If the lease expires, the task becomes available for reservation by another worker. This guarantees at-least-once processing without manual intervention.

SSE Streaming

The /schedulers/{id}/tasks/stream endpoint delivers SSE snapshot events. Workers and monitoring dashboards can subscribe to receive the current scheduled and inflight task lists.

Labeling and Filtering

Tasks can be annotated with labels as an array of strings at enqueue time. List operations support filtering by label, making it easy to inspect tasks by project, priority, or worker group.

Use Cases

Delayed Dispatch

Schedule one-off delayed work. Each enqueue call creates a task that becomes available at the specified time.

Delayed Job Processing

Enqueue jobs with a delay, for example sending a reminder email 24 hours after a user signs up. The Scheduler holds the task in memory until its time comes.

Worker Coordination

Multiple workers can reserve and process tasks from the same scheduler. The lease mechanism prevents concurrent processing while the lease is active; if the lease expires, the task becomes available again.