Design an IoC/Dependency Injection Framework — System Design Interview Practice

Interview prompt

Design a type-safe inversion-of-control and dependency-injection framework that supports registration, constructor/property injection, scopes, lifecycle hooks, factories, diagnostics, and safe concurrent resolution.

• Define service descriptors, keys/types, lifetimes, scopes, factories, decorators, dependency graph validation, and disposal semantics.
• Detect missing providers and cycles at registration or build time, preserve deterministic precedence, and prevent captive dependencies across scopes.
• Keep resolution thread-safe and low overhead with compiled plans/caches while allowing explicit overrides for tests and plugins.
• Explain diagnostics, lazy/async dependencies, disposal failures, module isolation, compatibility, and safe error reporting.

Requirements and scale assumptions

• Register providers and modules, validate dependency graphs, resolve constructor/property/factory dependencies, and manage singleton/scoped/transient lifetimes.
• Support named/keyed services, decorators, optional/lazy/async resolution, child scopes, disposal, diagnostics, and test overrides.
• Provide deterministic errors, thread safety, module versioning, cycle detection, resource cleanup, and recovery from factory failures.
• Keep compiled resolution overhead within a small constant budget and report graph errors before application startup where possible.
• Support thousands of registrations and concurrent resolutions without global mutable contention or unbounded synchronous work.
• Do not lose committed state; make retries and duplicate events safe.
• Degrade safely when downstream workers, caches, or external dependencies fail.
• 10k registrations and 100k concurrent resolutions across isolated scopes
• Partition by the primary tenant, user, item, or geographic key and isolate hot partitions.
• Keep serving state bounded; retain raw events or durable records for replay and auditing.

• Peak scale: 10k registrations; 100k resolutions — Capacity assumption that drives partitioning and backpressure.
• Latency target: compiled resolve overhead < 1us target — User-facing budget for the primary request or read path.
• Durable boundary: Committed before async — Immutable provider registrations and compiled dependency plans are authoritative; instance caches are scoped runtime state.
• Async boundary: At-least-once workers — Keep Service container/registry pattern, Reflection for auto-wiring, Graph-based dependency resolution off the synchronous path.

Key entities

• ResourceSpecresourceId, tenantId, desiredState, version, policyVersion, updatedAt

  Versioned desired state for a ioc dependency injection framework managed resource.

• OperationoperationId, resourceId, requestHash, step, attempt, status

  Durable ioc dependency injection framework reconciliation operation with per-step progress.

• PolicyVersionpolicyId, scope, version, rules, effectiveAt, status

  Auditable ioc dependency injection framework policy evaluated before provisioning or mutation.

• ReconciliationCheckpointresourceId, provider, observedVersion, cursor, lastError, updatedAt

  Provider-specific ioc dependency injection framework observation and recovery cursor.

Data flow

1. 1. Accept a desired-state commandThe ioc dependency injection framework control plane authenticates the tenant, validates policy and quotas, checks the expected version, and records the desired state.
2. 2. Plan a safe operationA planner turns ioc dependency injection framework desired state into ordered, bounded steps with dependency checks, blast-radius limits, and rollback metadata.
3. 3. Reconcile providers asynchronouslyWorkers apply ioc dependency injection framework operations through provider adapters, persist checkpoints, rate-limit calls, and treat unknown outcomes as observable state.
4. 4. Publish observed healthThe serving projection joins desired and observed ioc dependency injection framework state with operation status, policy version, freshness, and actionable errors.
5. 5. Recover and auditRetries, dead letters, drift detection, and operator approvals repair ioc dependency injection framework resources without losing the original command or provider evidence.

Deep dives and trade-offs

• Desired versus observed stateKeep ioc dependency injection framework desired state separate from provider-observed state and show both to operators. Make every reconciliation step conditional and resumable so a worker crash does not restart unsafe effects. Version policy and resource state so old operations cannot overwrite newer intent.
• Provider failures and unknown outcomesUse provider-specific idempotency tokens and query-after-timeout behavior for ioc dependency injection framework operations. Bound retries with exponential backoff, circuit breakers, and per-provider quotas. Route irreconcilable drift to an approval or quarantine path instead of retrying forever.
• Blast radius and operationsPartition ioc dependency injection framework work by tenant, region, cluster, or resource class and cap concurrent mutations. Audit who changed desired state, which policy allowed it, and what provider evidence was observed. Alert on drift age, operation backlog, failed steps, policy denials, and stale observations.
• Push versus pull reconciliationUse event triggers for fast response and periodic scans for missed events, drift, and recovery. A push-only ioc dependency injection framework controller silently misses changes when a provider event is lost.
• Central control plane versus provider-native controllersKeep policy, intent, and audit centralized while isolating provider-specific application logic behind adapters. A monolithic controller becomes hard to scale and couples unrelated provider failure domains.
• Automatic repair versus approvalAutomate low-risk, reversible ioc dependency injection framework changes and require approval for destructive or high-blast-radius operations. Full automation without policy or blast-radius controls can turn a transient signal into a widespread outage.