Microsoft Orleans

Diagnostic Output Budget

Keep native test progress and ANSI visible; use the detected runner's supported flags (--progress on --ansi on for MTP/TUnit), not MTP switches on VSTest. Keep console logs at Warning or higher and one concise final summary. Do not replay progress redraws, successful-test output, or Information/Debug/Trace logs into model context. On failure/crash, show only the failing test/resource, root error, and relevant stack frames; deduplicate and cap each diagnostic response at 80 lines / 8 KiB. Never dump entire console/host/browser logs, HTML, TRX, or crash artifacts. Keep necessary artifacts size-bounded outside context, link them, and inspect exact bounded excerpts. Preserve the runner exit code through capture/filtering; disclose truncation. Silence alone does not establish a hang.

Start With Purpose

Do not begin with an Orleans API. First state:

1. the business identity that owns the behavior;
2. the invariant and what must survive activation or cluster failure;
3. the required consistency, query shape, acknowledgement, durability, replay, fan-out, and timing.

Then select the smallest Orleans primitive whose guarantees match those requirements. Reject Orleans when the problem is primarily shared-memory computation, a finite batch, relational querying, or global coordination with few independent entities.

Inspect package versions for version-sensitive work. Orleans 10.3.1 ships Microsoft.Orleans.DurableJobs* and Microsoft.Orleans.Journaling* as 10.3.1-alpha.1; treat them as experimental until that status changes.

Orleans 10.3 makes Newtonsoft storage enforce the type allow-list, changes RPC telemetry keys, and requires custom grain-context activators to apply configurators before construction. Prefer generated/allowed types over permissive JSON and update affected tests, dashboards, and activators together. 10.3.1 services analyzer contract identities and documents placement hints.

Mental Model

• A grain is a virtual actor with stable identity, behavior, and optional state, not a process, row, DTO, controller, or background job.
• A grain reference is a location-transparent address. Getting a reference does not create a durable record or prove that an activation exists.
• An activation is an ephemeral in-memory execution instance. Orleans creates, places, moves, deactivates, and recreates it. Never equate activation lifetime with entity lifetime.
• A normal grain has at most one activation in the cluster by default and processes turns one at a time. This makes the grain a natural owner of per-identity invariants.
• A silo hosts activations. Silos form a cluster; external clients or a co-hosted IGrainFactory call grains.
• Calls are asynchronous messages even when they look like C# method calls. Network failure, timeout, serialization, retries, and duplicate side effects still matter.
• A grain can model a digital twin when its identity and behavior map to a real or domain entity; that is a use case, not every grain's definition.
• Orleans gives logical ownership and turn-based execution. It does not make external side effects transactional, turn arbitrary data into a queryable database, or provide exactly-once execution by default.

Workflow

1. Inspect the solution, Orleans version, hosting topology, grain interfaces, providers, and tests.
2. Identify domain identities and invariants. Prefer many independent, bounded entities over global coordinator grains.
3. Choose state, communication, and time-based work from the tables below.
4. Keep default non-reentrant scheduling and placement until a measured requirement justifies a change.
5. Configure providers independently and test the failure model the design depends on.

Choose the State Owner

Primitive	Purpose	Choose it when	Do not use it as
Activation fields	Fast, temporary state for one activation	The value is derived, cached, disposable, or safe to rebuild after deactivation/failure	Durable truth
IPersistentState<T>	Durable current state owned by one grain identity	The grain needs a bounded snapshot loaded on activation and explicitly written after commands	A general query database or cross-grain table
External database/repository	Queryable, indexed, relational, bulk, shared, or externally owned data	The system needs joins, search, reporting, set-based updates, independent access, or an existing system of record	A replacement for grain ownership when serialized per-entity decisions are still required
Grain plus database/read model	Separate command ownership from query/storage concerns	A grain owns invariants and a small control snapshot while a database owns large records, history, projections, or reporting	Two competing sources of truth without an explicit contract
JournaledGrain<TState,TEvent>	Persist domain events and reconstruct state	Audit history, business-event replay, log consistency, or multi-cluster event-sourced replication is a requirement	A default persistence choice for ordinary CRUD state
Orleans.Journaling durable states	Replay durable collection/value operations through a journal	The experimental 10.3 journaling model, durable collections, or durable completion state solves a measured need	Stable default persistence; it is distinct from JournaledGrain business event sourcing
ITransactionalState<T>	ACID, serializable all-or-nothing changes across transactional grain state	A short operation must atomically update multiple grain-owned states and compensation is unacceptable	Long-running workflows or atomicity with arbitrary external systems
Saga/process manager	Durable progress with compensation across steps and external systems	Work is long-running, spans services, waits for events, or cannot share one transaction	Instant atomic commit

Grain State Versus a Database

Use grain state for bounded current state and per-identity invariants. Use a database/read model for joins, search, reporting, bulk work, history, shared access, or an external system of record. When using both, define one authority per field and recovery rules.

Never query or mutate another grain's persistence record behind the grain, or expose provider storage as the public query model merely because it uses SQL/Cosmos/Redis. Before using transactions, try one bounded grain owner; use a saga for external effects or long waits. Read references/persistence-api.md for the full boundary.

Choose Communication

Prefer a direct call for completion to a known owner. Use streams for decoupled provider-backed delivery, response streams for one caller, and broadcast/observers/one-way calls only for loss-tolerant notifications. A timeout does not prove whether an external effect committed; retries require idempotency. Read references/primitive-selection.md for the communication guarantees and execution choices.

Choose Time-Based and Background Work

Primitive	Purpose	Choose it when	Do not choose it when
RegisterGrainTimer	Periodic or one-shot work tied to the current activation	Work is frequent, local to an active grain, and safe to stop on deactivation or silo failure	The schedule must survive reactivation/restart
Reminder	Durable recurring schedule definition associated with a grain identity	Low-frequency recurring work must wake the grain after deactivation or cluster restart	Every missed occurrence must be replayed, timing must be precise, or work is high-frequency
Durable Job	Persistent one-time future delivery to a target grain with cancellation/retry metadata	A delayed command, expiry, notification, or workflow step must execute at least once around a due time	Recurring work, exactly-once side effects, or production work that cannot accept the current alpha package status
[StatelessWorker]	Auto-scaled pool of local stateless grain activations	CPU/transform/routing/pre-aggregation work is not tied to one durable entity	Scheduling or durability; a stateless worker is not a job system
BackgroundService	Continuous loop owned by each host process	A silo/web host must poll or consume an external source and forward work into grains	One global loop across replicas unless duplicates are safe or externally coordinated
IHostedService	Host startup/shutdown action or simpler background component	Initialization or bounded host-lifetime work belongs to standard .NET hosting	Per-entity durable work
Orleans startup task	Fail-fast hook at a specific silo startup stage	Legacy/framework integration truly requires Orleans lifecycle ordering	General background work; prefer BackgroundService or IHostedService
Silo lifecycle participant	Ordered initialization/shutdown of an Orleans component	A provider or runtime service must start at an exact lifecycle stage	Business scheduling
Grain service	Per-silo, cluster-partitioned runtime support service	Every silo hosts a long-lived service and responsibility for grains must be partitioned across silos	An ordinary domain entity, a singleton, or a durable job queue
External scheduler/workflow engine	Scheduling/orchestration outside Orleans	Cross-system workflows, cron calendars, human steps, broad operational control, or mature production guarantees dominate	Per-grain work already solved by a stable Orleans primitive

Timer, Reminder, or Durable Job

Timer means “while this activation lives”; reminder means “wake this grain on a durable recurring schedule”; Durable Job means “invoke this target once around a future time, at least once.” Reminders persist definitions but miss ticks while the cluster is down. Durable Job handlers must be idempotent, and current alpha.1 packages are an explicit architecture risk.

BackgroundService means one loop per host replica, not one loop per cluster. Use a well-known grain or external lease/leader for one logical collector.

Read references/scheduling-and-services.md before implementing timers, reminders, Durable Jobs, hosted/startup tasks, silo lifecycle participants, or grain services.

Choose Execution and Scaling

Use standard grains for stateful identities and stateless workers for fungible work. Apply narrow interleaving attributes only after auditing invariants across every await; grain turns do not make reentrant workflows race-free. Preserve default placement unless a measured locality or compatibility requirement justifies changing it. See references/primitive-selection.md before changing scheduling or placement.

Hosting and Provider Boundaries

Keep concerns separate: clustering discovers silos; the grain directory locates activations; grain/reminder/transaction/job storage persist different records; stream providers carry events while PubSubStore tracks subscriptions; serialization defines wire and persistence compatibility.

In-memory clustering, storage, reminders, streams, Durable Jobs, and journaling are development/test choices unless loss is explicitly acceptable. Configure production providers, credentials, TLS/networking, server GC, graceful shutdown, and health/readiness for the deployment target. In Aspire, declare backing resources in AppHost and register the keyed clients expected by Orleans providers.

Contract and API Rules

• Keep grain interfaces coarse-grained and asynchronous: Task, Task<T>, ValueTask<T>, or supported IAsyncEnumerable<T>.
• Cancellation is cooperative, not proof that work did not finish.
• Use [GenerateSerializer] and stable [Id(N)] values on messages/state. Use [Alias] for durable type identity and [Immutable] only for genuinely immutable values.
• Never reuse removed field IDs; write state explicitly and propagate storage failures.
• Bound external calls and use idempotency plus a saga/outbox when state writes and external effects must be reconciled.

Validate the Chosen Guarantees

• Grain boundaries follow business identity and avoid hot global coordinators.
• State is bounded, and the grain-state-versus-database choice is explicit.
• Every time primitive matches activation, recurrence, durability, and delivery requirements.
• Durable Job handlers/retried commands are idempotent; reminders tolerate missed ticks.
• Broadcast channels, observers, and one-way calls carry only loss-tolerant signals.
• Provider-backed tests prove delivery, ordering, replay, and failover claims.
• Reentrancy tests cover state across await; transactions use transactional storage, [Reentrant], and PerformRead/PerformUpdate.
• Hosted services are reviewed for per-replica duplication.
• Production uses persistent providers, multi-silo tests, and observability wherever the guarantee depends on them.

Load References

Open only the references needed for the selected primitive:

• references/official-docs-index.md — official documentation map
• references/scheduling-and-services.md — scheduled/background/runtime work
• references/grains.md and references/grain-api.md — grain design and APIs
• references/persistence-api.md — state, databases, Journaling, event sourcing, transactions
• references/streaming-api.md — streams, broadcast, observers, response streams
• references/serialization-api.md — serialization and versioning
• references/hosting.md and references/configuration-api.md — hosting, providers, operations
• references/implementation.md and references/testing-patterns.md — runtime and tests
• references/patterns.md, references/anti-patterns.md, and references/examples.md — patterns and samples

Prefer Learn for stable APIs. For Durable Jobs and Orleans.Journaling, use version-tagged package READMEs/public API because Learn does not yet cover them fully.