Async vs Inline Adapter for Low-Latency Background Jobs in Rails

Low-Latency Background Job Processing Trade-off

Rails applications requiring sub-second response times face a critical choice when configuring Active Job adapters. The 'inline' adapter executes jobs immediately within the request thread, eliminating queue latency but blocking the entire request-response cycle. Conversely, the 'async' adapter uses an in-process thread pool that defers job execution without external dependencies, though jobs are lost on process restart.

For applications serving API endpoints where user-perceived latency must remain under 100ms, the inline adapter guarantees no background processing delay. However, this approach risks request timeouts when jobs perform database operations or external API calls, particularly under Puma's request timeout constraints.

The async adapter offers a middle ground for development environments but introduces uncertainty about job completion during deployments. Production deployments using async will permanently lose queued jobs unless a persistent adapter like Sidekiq is implemented.

Given these constraints, which adapter should be selected when the primary requirement is maintaining low request latency while ensuring job completion survives process restarts?