Variable substitution

Variable substitution is a small, reusable building block: resolve ${key} placeholders (with optional ${key:-default} fallbacks) from a pluggable source. It is not tied to any one subsystem — the scheduler uses it for ${...} variables in cron/interval expressions, and you can use it anywhere you want configurable, runtime-changeable string values.

The pieces

• IVariableSource — the pluggable backend: bool TryGetValue(string key, out string? value). The shipped adapter is ConfigurationVariableSource (over IConfiguration); other sources (a dictionary, environment, a custom store) implement the same seam.
• VariableSubstitution — the resolver. A missing, empty, or whitespace value is treated as unset so an inline :- default can recover.

Both live in Elarion.Abstractions.Substitution.

Two models

Whole-value — a string is either a literal or a single placeholder. Useful for optional, settable attributes; an unresolved placeholder yields null.

IVariableSource source = new ConfigurationVariableSource(configuration);

VariableSubstitution.Resolve("plain", source);            // "plain" (literal)
VariableSubstitution.Resolve("${Jobs:Interval:-15s}", source); // configured value, else "15s"
VariableSubstitution.Resolve("${Jobs:Interval}", source); // null when unset (no default)
VariableSubstitution.ResolveRequired("${Jobs:Interval}", source); // throws when unset

Embedded — replace every ${...} inside a larger template (a connection string, a path, a message):

VariableSubstitution.Substitute("postgres://${db:host}:${db:port:-5432}/app", source);
// → "postgres://db.local:5432/app"

Embedded substitution throws when a placeholder is unset and has no default; nesting (${a:${b}}) is not supported.

Dependency injection

Register a default source so any subsystem can inject one:

builder.Services.AddElarionVariableSubstitution(); // IVariableSource → ConfigurationVariableSource

Because it reads IConfiguration, it transparently sees appsettings, environment variables, and — if you add the settings IConfiguration adapter — database-backed settings, with runtime changes as those providers reload. Register a different IVariableSource first to override the source.

Observable sources

A source that can signal change implements IObservableVariableSource (IChangeToken Watch()). ConfigurationVariableSource is observable — Watch() bridges to the configuration reload token, so a provider reload (appsettings change, or a settings-backed provider reload) propagates to consumers. A static source (a dictionary) simply doesn't implement it.

How the scheduler uses it

Scheduled jobs resolve ${...} variables in their schedule (interval, cron, time zone, enabled flag) on every occurrence, so changing a variable changes the schedule on its next fire. The scheduler injects an IVariableSource (config-backed by default), so pointing it at a settings-backed IConfiguration makes job schedules runtime-changeable with no scheduler-specific wiring — the substitution concept is shared, not duplicated.

Because the default source is observable, the scheduler goes further than next-fire pickup: when a watched variable changes it reschedules affected recurring jobs immediately, re-resolving each job whose variables changed and replacing its queued occurrence (a job with a literal schedule, or one whose variables didn't change, is left untouched; a fixed-delay job mid-run reschedules itself on completion). So shortening a job's interval, changing its cron, or enabling a ${...}-disabled job takes effect at once rather than after the (possibly long) current interval.