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A guessing round should not become completed merely because one answer is correct. The server should record that accepted choice, add it to the participant’s accumulated correct set, and finish the round only when that set contains every item in the target set. The client submits one candidate identifier at a time; it never declares an answer correct or tells the server that the round is finished. Reliable implementations also define what happens when a request is retried, arrives out of order, races with another request, or targets content that has changed. Model those cases as explicit state transitions, and make the completion side effect part of the same durable operation.
What state should a stateful guessing round keep?
A round needs durable state for both the content being solved and the progress already made. Keeping only the latest answer, or trusting the browser to remember found items, makes retries and recovery ambiguous.
| Field | Purpose | Important rule |
|---|---|---|
id |
Stable identifier for the attempt or round. | Use it for lookups, but never treat an unguessable ID as proof of authorization. |
| Owner or participant | Identifies the player allowed to read and mutate the attempt. | Check ownership on every request. |
| Target set | The complete set of answers that must be found. | Store or reference the server-side set; do not accept a client-supplied target. |
| Candidate set | Identifiers that can legally be selected for the current round. | Reject identifiers outside the current set before checking correctness. |
| Accumulated correct set | Accepted target members already found. | Set semantics prevent a retry from adding the same member twice. |
| Per-group answer records | The selected candidate and result for each group, image, or question slot. | They let the server distinguish an idempotent repeat from a conflicting replacement. |
| Status | Lifecycle state such as active or completed. |
Only documented transitions may change it. |
| Target version | Version of the content used when the attempt started. | Compare it with the current version before accepting an answer. |
| Timestamps | Creation, update, completion, abandonment, and expiry information. | Use server time for ordering and audit records. |
| Completion-side-effect ID | Identifier for a reward, unlock, notification, or other effect caused by completion. | Make it idempotent so a retry cannot grant the effect twice. |
Keep the target and progress sets on the server. A browser may display progress optimistically, but the durable response from the server is authoritative after reconnects, refreshes, or device changes.
How do you define completion when one correct answer is only progress?
Define completion as a predicate over the accumulated correct set, not as the result of the latest answer. If T is the target set and C is the accumulated correct set, the round is complete only when C = T.
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normalizedTarget = normalize(T)
normalizedCorrect = normalize(C)
completed = size(normalizedTarget) == size(normalizedCorrect)
and normalizedTarget == normalizedCorrectNormalization should remove duplicate identifiers, sort them into a stable order, and then compare both membership and size. Comparing only counts can produce a false completion if the sets contain different members; comparing only an unsorted serialized representation can produce a false negative when the same members arrive in a different order.
The transition for a correct answer therefore has two possible outcomes:
accepted_progress: the candidate is correct, the accumulated set grows, and the attempt remainsactive.accepted_completion: the candidate is correct, the accumulated set now equals the target set, and the attempt becomescompletedtogether with its completion side effect.
An incorrect candidate does not change the accumulated set. A response can still include the resulting status and progress count, but the server must not let the client set either value.
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Accept exactly one candidate identifier plus the attempt or group context needed to locate the answer slot. Correctness, completion, and progress are computed from server-side data.
POST /attempts/{attemptId}/answers
Content-Type: application/json
Idempotency-Key: 6b2f...
{
"group_id": "group-17",
"candidate_id": "candidate-42"
}The request contract should reject a body containing fields such as correct, result, completed, or a replacement progress set. Those fields are assertions by an untrusted caller, not inputs to the business decision.
- Validate shape. Require the attempt identifier, the group identifier when the game uses groups, and one candidate identifier. Reject missing, duplicated, or extra values according to the API schema.
- Authorize ownership. Resolve the authenticated participant and verify that the attempt belongs to that participant. An unguessable URL does not replace an authorization check.
- Check lifecycle. An already completed, abandoned, or failed attempt cannot accept a new answer. Return its documented terminal outcome rather than silently reopening it.
- Check content version and availability. Compare the attempt’s stored target version with the current version and verify that the target is still available to this participant.
- Check candidate membership. The candidate must belong to the current candidate set and be valid for the requested group.
- Read the prior group record. This determines whether the request is new, an exact repeat, or a conflicting replacement.
- Evaluate correctness. Compare the candidate with the server-side answer mapping.
- Write the answer and progress atomically. If the new progress completes the target, persist the terminal status and completion side-effect record in the same transaction or through a durable outbox.
A response should expose the resulting attempt state and whether progress changed. For example:
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{
"attempt_id": "attempt-91",
"outcome": "accepted_progress",
"status": "active",
"progress_changed": true,
"progress": {"correct": 3, "target": 5}
}Do not expose answer keys or unrelated participant fields. Define response DTOs deliberately so internal columns cannot leak when the schema evolves.
Which statuses and answer outcomes make transitions unambiguous?
Names are design choices, but every status and outcome needs a precondition, a write set, and a stable response meaning.
| State or outcome | Meaning | Typical transition |
|---|---|---|
active |
The attempt can accept a valid answer. | Created from a new attempt; remains active after incorrect or partial correct answers. |
completed |
The accumulated correct set equals the target set and the completion effect is recorded. | Reached only by accepted_completion. |
abandoned_stale |
The target changed after the attempt began. | Set when the stored target version differs from the current version under an abandon-on-change policy. |
abandoned_unavailable |
The participant can no longer access the target. | Set when availability or access rules terminate the attempt. |
failed |
An unrecoverable processing or business failure was recorded. | Use only for documented failure conditions; do not use it as a catch-all for validation errors. |
accepted_progress |
New correct answer increased progress without finishing. | Answer record and accumulated set are written. |
accepted_completion |
New correct answer finished the target. | Answer, terminal status, and side-effect record are committed together. |
incorrect |
Candidate was valid but did not match the answer. | Record the result if the game permits retries; do not advance progress. |
duplicate_idempotent |
The same accepted request was received again. | Return the stored result without another progress write or side effect. |
conflict |
A group already has a different recorded choice. | Reject the replacement and preserve the original answer. |
out_of_order |
The request targets a later group before its predecessor is resolved. | Reject without changing progress or answer records. |
stale |
The target version no longer matches the attempt. | Return the stale policy result, usually after marking the attempt abandoned. |
Document whether each outcome is represented by a normal success response or a client error. The key requirement is that callers can tell “nothing changed because this was a safe retry” from “nothing changed because your request conflicts with recorded state.”
How should retries, duplicates, and out-of-order answers behave?
Retries are normal in mobile and unreliable-network clients, so duplicate behavior belongs in the API contract rather than in ad hoc client code.
| Request situation | Required behavior |
|---|---|
| Repeat the same accepted candidate for the same group | Return the original outcome, leave progress unchanged on the second request, and mark the response as idempotent or replayed. |
| Submit a different candidate for a recorded group | Return conflict; do not overwrite the first answer unless the product explicitly supports corrections through a separate operation. |
| Submit a later group before an earlier required group | Return out_of_order and leave the attempt untouched. |
| Retry after the client lost the response | Use the same idempotency key or answer identity and return the durable stored result, including the same completion-side-effect identifier. |
| Retry after the attempt became terminal | Return the terminal state and stored result; never reopen the round implicitly. |
HTTP methods do not make a business operation idempotent automatically. Microsoft Learn explicitly notes that “POST and PATCH aren’t inherently idempotent.” If answer submission uses POST, store a processed-request record keyed by an idempotency key scoped to the authenticated participant and operation. The record should contain the request fingerprint and serialized outcome, so reusing a key with different content is itself a conflict.
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How do you make the state transition safe under concurrency?
Make the answer decision, progress update, terminal status, and side-effect record one atomic unit. A transaction that writes only the answer row can still double-count progress or issue two rewards when two requests arrive together.
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- Begin a database transaction.
- Load and lock the participant/target pair used by the attempt, then load the attempt row with a row-level lock such as
FOR UPDATE. - Check ownership, status, target version, availability, group ordering, and any existing answer record while the lock is held.
- Branch to duplicate, conflict, out-of-order, stale, incorrect, progress, or completion handling.
- Insert or update the answer record and accumulated progress exactly once.
- If completion occurred, insert a unique side-effect record with a stable key.
- Commit, then return the committed outcome.
Locking strategy must match the database schema and transaction isolation. A local implementation that uses row locks is not proof that every concurrent sequence is safe in production; exercise controlled concurrent requests and assert the resulting rows, status, and side effects. Unique constraints on an idempotency key, attempt/group pair, and completion-effect key provide a second line of defense when application code is retried.
What should happen when the target changes during an attempt?
Choose and publish a target-consistency policy before shipping. The two defensible choices are abandoning a stale attempt or freezing an immutable snapshot.
| Policy | How it works | Trade-off |
|---|---|---|
| Abandon on version change | Store the target version at attempt creation; on each answer, compare it with the current version. If different, mark the active attempt abandoned_stale and return a skipped or stale result. |
Prevents answers against obsolete content, but the player must start over. |
| Immutable attempt snapshot | Copy the target and answer mapping into the attempt at creation and always evaluate against that snapshot. | Preserves continuity, but requires storage and a policy for whether old content remains legally or editorially usable. |
Do not confuse content consistency with authorization. A target can be unchanged yet no longer available to a participant, or it can be changed while the participant remains authorized. Return distinct reasons such as stale content versus unavailable access so clients can offer the right recovery path.
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Authorize every read and mutation against both the authenticated participant and the attempt owner. A random-looking attempt identifier prevents casual enumeration but does not establish permission.
- Derive the participant from the authentication context, not from a writable request field.
- Check ownership before revealing whether an attempt exists, unless your error policy intentionally uses a uniform not-found response.
- Validate group and candidate membership against the authorized attempt.
- Return only fields defined by the endpoint DTO; do not serialize the full attempt, answer key, internal version metadata, or another participant’s identifiers.
- Apply the same checks to administrative reads, exports, and replay endpoints.
Separate validation errors, authorization failures, stale content, and terminal-state responses. That separation gives clients a safe way to retry, restart, or display an access message without inferring sensitive information.
Is REST enough, or does a guessing game need a persistent connection?
Ordinary HTTP requests are appropriate for a turn-based guessing round whose state is stored durably. A persistent connection is justified when the product requires continuous shared turns, presence, server-pushed events, or very low-latency multiplayer interaction.
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| Requirement | Suitable transport | Reason |
|---|---|---|
| Submit an answer, read progress, view history | REST over HTTP | Each operation has a clear request and response and can recover after reconnects. |
| Friends, inventory, leaderboard, or session operations | REST over HTTP | These are discrete resource operations with durable state. |
| Live shared turns, presence, typing, or server-pushed timers | Persistent connection plus a durable command model | The connection reduces polling, but the database still needs authoritative transitions and replay protection. |
A persistent socket does not remove the need for idempotency, authorization, version checks, or transactional writes. It only changes how events are delivered.
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How do you guarantee the completion side effect?
Opening a dialog in the browser is not completion. Granting a reward, unlocking content, or emitting a “round finished” event must be tied to the server’s durable transition.
For a simple system, write the terminal status and a unique completion-effect record in one database transaction. A worker can then process that record and mark it delivered. For a larger system, use a transactional outbox: commit the attempt update and outbox message together, let an idempotent worker deliver the effect, and record the delivery result.
- Give the effect a deterministic key derived from the attempt and completion event.
- Make the consumer ignore a key it has already processed.
- Return the effect identifier in the completion response so a client retry can correlate the same result.
- Do not send an irreversible external reward before the database transaction commits.
- Provide an operator or replay path for a failed delivery without changing the completed attempt back to active.
What verification matrix should you run before release?
Test transitions and side effects, not just individual HTTP status codes. The following cases expose the bugs that a happy-path test misses.
- Repeated intermediate choice: submit an accepted answer twice; verify that the second response returns the stored result and progress is unchanged.
- Conflicting repeat: submit a different candidate for the same recorded group; verify a conflict and preservation of the first answer.
- Out-of-order answer: answer a later group early; verify rejection with no progress or answer-row change.
- Target edit: change the target version mid-attempt; verify the selected stale policy and that no completion is granted from obsolete content.
- Unavailable target: revoke access; verify termination without a completion effect.
- Lost response retry: commit an answer, discard the client response, then retry with the same key; verify the same outcome and one side effect.
- Concurrent submissions: issue controlled simultaneous requests for the same and different groups; assert final progress, status, answer rows, and effect count.
- Terminal replay: resend an answer after completion; verify the completed state is returned and the round is not reopened.
Capture database assertions and emitted-effect counts in these tests. A passing response code alone cannot prove that the round did not double-count or double-grant.
What does a complete round look like in practice?
Assume a target contains five members and the participant has correctly found two. A correct third answer produces accepted_progress, stores the third member, and leaves the attempt active. A retry of that answer produces duplicate_idempotent with no change. After the fifth distinct correct member is accepted, normalization shows equal target and progress sets; the server commits completed and one completion-effect record. A sixth request cannot add progress, even if the client still displays an old candidate list.
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This sequence illustrates why “the latest answer was correct” is not a completion rule. Completion is a property of the accumulated state after a successful transition.
Implementation checklist for a production API
- Define the target, candidate, progress, answer-record, status, and version fields before writing handlers.
- Keep correctness and completion decisions on the server.
- Normalize sets and compare both members and cardinality.
- Specify duplicate, conflict, out-of-order, stale, unavailable, and terminal behavior.
- Use an idempotency key or durable processed-request record for answer submission.
- Authorize every operation by authenticated participant and attempt ownership.
- Lock the rows needed for the transition and enforce uniqueness in the database.
- Commit progress and completion side effects atomically, or use a transactional outbox with an idempotent worker.
- Record a target version and choose abandon-versus-snapshot behavior.
- Use REST for discrete operations; add a persistent connection only for genuine live-interaction requirements.
- Verify stored rows and side effects under retries and controlled concurrency.
Frequently Asked Questions
Should a duplicate answer return HTTP 200 or HTTP 409?
Either can be correct if the contract is consistent. Many APIs return the original successful representation with an idempotency indicator because the requested state already exists; reserve a conflict response for a different candidate submitted for the same recorded group. Document the choice so client retry code does not treat a safe replay as a failure.
What should an idempotency key contain?
Use a high-entropy client-generated value, and scope it to the authenticated participant, endpoint, and attempt. Store a fingerprint of the relevant request fields; if the same key is reused with a different candidate or group, reject it instead of returning the earlier result for unrelated data.
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How long should abandoned attempts and processed-request records be retained?
Retention should follow your audit, support, and privacy requirements rather than an arbitrary game rule. Keep enough history to investigate disputed rewards and late retries, then expire or anonymize records with a documented job while preserving uniqueness for any still-replayable keys.
Can a player change an earlier answer after submitting it?
Treat corrections as a separate, explicitly authorized operation if the game needs them. It must define whether later groups are invalidated, how progress is recalculated, and how already-issued completion effects are reversed; silently overwriting an answer makes retries indistinguishable from edits.
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