HTTP, Scheduling, And Resilience

HTTP, scheduling, and resilience are extension modules layered on top of workflow core, management, runtime, and expressions.

HTTP Module

Start in src/modules/Elsa.Http.

HttpFeature owns:

inbound HTTP endpoint workflows
outbound HTTP request activities
route matching and route table updates
HTTP bookmark payloads
HTTP request/response content parsing and writing
downloadable content handling
file cache and zip management
correlation and workflow instance ID selectors
HTTP activity descriptors and UI hints
HTTP resilience strategy registration
HTTP ingress source registration for graceful shutdown

The public module extension is UseHttp.

Inbound HTTP Workflows

The inbound path is:

flowchart LR
    Request["ASP.NET request"] --> Middleware["HttpWorkflowsMiddleware"]
    Middleware --> RouteTable["IRouteTable / IRouteMatcher"]
    RouteTable --> Lookup["IHttpWorkflowLookupService"]
    Lookup --> Runtime["Workflow runtime"]
    Runtime --> Activity["HttpEndpoint activity"]
    Activity --> Response["HTTP response activity"]

Important files:

Hosts enable middleware with UseWorkflows.

Outbound HTTP

Outbound HTTP activities:

Supporting services include HttpClientFileDownloader, content factories, content parsers, and downloadable content handlers.

HTTP Security And Faults

HTTP endpoint authorization and faults are configurable through HttpFeature:

The module registers authorization services because the authentication-based handler requires them.

HTTP Tests

Good test entry points:

Scheduling Module

Start in src/modules/Elsa.Scheduling.

SchedulingFeature registers:

local scheduler
cron parser
trigger scheduler
bookmark scheduler
workflow scheduler
tenant schedule updater
create-schedules background task
ScheduleWorkflows handlers
Cron trigger payload validator
scheduled-trigger ingress source for graceful shutdown
scheduling activities through workflow management

The public extension is UseScheduling.

Scheduling Concepts

Scheduled workflows typically create trigger or bookmark payloads that the scheduler can wake later. Important files:

The scheduler integrates with tenancy by reacting to tenant activation/deletion events.

Resilience Module

Start in src/modules/Elsa.Resilience and Elsa.Resilience.Core.

ResilienceFeature registers:

activity descriptor modifier for resilient activities
resilience strategy catalog
strategy config evaluator
resilient activity invoker
configuration strategy source
retry attempt recorders/readers
transient exception detector and strategy
FastEndpoints assembly for resilience descriptors/testing endpoints

HTTP registers HttpResilienceStrategy with resilience in HttpFeature.Configure().

Resilience Concepts

Core contracts:

Use resilience when an activity performs IO that can fail transiently. Keep strategy types registered by the owning module.

The resilience testing endpoint GET /elsa/api/simulate-response is included in the module. It requires exec:resilience:simulate-response (or wildcard exec:resilience or exec:*) and is intended for integration-testing resilience strategies, not for production traffic.

Cross-Cutting Graceful Shutdown

HTTP and Scheduling both register runtime ingress sources so graceful shutdown can pause new external work. When adding a new external event source, implement and register an IIngressSource in the owning module, then add runtime tests that prove pause/resume/drain behavior.

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