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A correct guess should usually add progress, not finish a round. The server should mark the selected candidate as accepted, persist that progress, and declare completion only when the participant has found every item in the round’s target set. That distinction drives the data model, endpoint contract, retry behavior, authorization checks, and transaction boundaries.
The safest design keeps targets and progress on the server, accepts one candidate identifier per request, and computes the result from stored state. A client may display a success message, but completion—and any reward, dialog, or unlock—must be recorded as a durable server-side transition.
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What is the correct completion rule for a guessing round?
The completion predicate is set equality: a round is complete only when the accumulated correct selections equal the entire target set. One accepted answer changes progress from, for example, two of five targets to three of five; it does not satisfy the completion predicate.
Let T be the server-owned target set and C be the set of distinct correct candidates already accepted for the attempt. A robust predicate normalizes both sets by removing duplicates, sorting, and comparing both cardinality and members:
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completed = normalize(C).length === normalize(T).length
&& every member of normalize(T) is in normalize(C)Checking only the count is unsafe. A corrupted or duplicated progress list could have the same length as the target set while containing the wrong members. Comparing normalized membership prevents that class of false completion.
This rule also separates two concepts that are often incorrectly collapsed into one response field: answer correctness and round status. An answer can be correct while the attempt remains active; an attempt can be completed only after the final missing target is accepted.
What state should the server store?
Keep the information needed to evaluate every future request in durable server-side records. A practical attempt or round model contains the following fields.
- Identity and ownership: an attempt ID, participant or owner ID, and any group or round identifier.
- Target set: the candidate identifiers that must be found for this attempt.
- Candidate set: the identifiers currently selectable by the participant, if it differs from the target set.
- Accumulated correct set: distinct target identifiers already accepted.
- Per-group answer records: the selected identifier, outcome, request key, and timestamps for each answerable group.
- Status: such as
active,completed,abandoned_stale,abandoned_unavailable, orfailed. - Target version: the content version captured when the attempt began.
- Lifecycle timestamps: creation, last activity, completion, abandonment, and update times.
- Completion side-effect identifier: a durable reference to a reward grant, dialog-opening event, unlock, or other effect that must occur once.
Store sets in a representation that supports uniqueness. A join table with a uniqueness constraint, or a canonicalized array protected by transactional checks, prevents the same accepted candidate from adding progress twice. Per-group records are useful when a group may receive retries or when answering groups out of sequence is forbidden.
How should an answer request be validated?
Accept a narrowly defined request containing an attempt or group identifier and exactly one selected candidate identifier. The server, not the browser, decides whether that identifier is correct.
- Validate the shape. Reject missing, extra, malformed, or duplicated fields. A request that can contain both a candidate and a client-supplied verdict creates an avoidable trust problem.
- Authenticate the caller. Resolve the participant from the session or token rather than trusting an owner ID in the body.
- Authorize the attempt. Confirm that the authenticated participant owns the attempt or is otherwise permitted to answer it.
- Check lifecycle status. Completed, stale, unavailable, and failed attempts follow their documented terminal behavior instead of silently reopening.
- Check membership. Confirm that the selected identifier belongs to the current candidate set and that the group belongs to this attempt.
- Apply ordering rules. If groups must be answered in sequence, compare the requested group with the next required group before changing any state.
- Compare with stored answers and targets. Determine correctness from the server-owned target set and previously recorded answers.
Do not accept fields named correct, result, completed, or similar verdicts as authoritative. They may be ignored or rejected; in either case, the response must be based on the server’s comparison.
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Names are design choices, but every branch should have a documented precondition, write set, and response. The following vocabulary makes client behavior and logs unambiguous.
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| Answer outcome | Meaning | State change |
|---|---|---|
accepted_progress |
The candidate is correct and the target set is not yet complete. | Add one distinct member to accumulated progress; keep the attempt active. |
accepted_completion |
The candidate is correct and the completion predicate is now true. | Persist final progress, mark completed, and record the completion side effect. |
incorrect |
The selected candidate is valid but is not a target for this attempt. | Record the answer if your product needs an audit trail; do not advance progress. |
duplicate_idempotent |
The same accepted request or answer is being retried. | Return the previously stored result without another progress write or reward. |
conflict |
A group already has a recorded answer, but the retry supplies a different candidate. | Reject; preserve the original answer. |
out_of_order |
The request targets a later group while an earlier group is still required. | Reject without advancing the attempt. |
stale |
The target version changed after the attempt began. | Abandon or otherwise terminate according to the stale-content policy. |
Attempt status is separate from answer outcome. For example, an accepted_progress response leaves status active, while accepted_completion changes it to completed. A terminal status should be visible in every response so clients do not infer lifecycle from a single boolean.
How should retries and duplicate answers work?
Retries are normal: mobile clients lose responses, users double-click, and workers replay messages. Repeating an accepted choice must return the durable original result and leave progress unchanged.
Give each submission a request key or another stable answer identity. Persist it with the resulting outcome and use a uniqueness constraint within the attempt. A retry with the same key and payload returns the stored response, including an idempotency indicator. A retry that uses the same group but a different candidate is a conflict, not a new attempt to overwrite history.
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If the client loses a response after the server commits, it can safely resend the request. The server should not grant a reward twice, append a candidate twice, or emit two completion events. Microsoft’s HTTP guidance notes that “POST and PATCH aren’t inherently idempotent”; therefore, a POST-based answer endpoint needs an explicit idempotency-key contract or a processed-request record.
Use method semantics consistently. GET is for reads. POST is appropriate for a command that evaluates an answer, provided duplicate behavior is documented. Return 202 only when processing has been accepted for later work; a synchronously evaluated answer should return the completed operation result rather than pretending it is deferred.
What transaction boundary prevents a false completion?
Evaluate the answer, write progress, change status, and arrange the completion side effect as one logical transition. If one operation commits while another fails, the API can report completion without granting the reward—or grant the reward twice after a retry.
A typical transaction performs these actions in order:
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- Lock the participant/target pair or another contention boundary.
- Read the attempt with a row lock such as
FOR UPDATE. - Verify ownership, status, target version, group ordering, and request identity.
- Handle duplicate and conflicting repeats before comparing a new answer.
- Compare the selected identifier with the server-owned target set.
- Insert the answer record and, when correct, the new progress member.
- Recompute the normalized completion predicate from committed data.
- Update status and write a completion-side-effect record when the predicate becomes true.
- Commit a response payload that can be returned again for the same request key.
The side effect can be committed in the same transaction when the system supports it. Otherwise, use a durable outbox row written in that transaction and an idempotent worker that marks the effect delivered. The worker must use the side-effect identifier as its deduplication key. A local implementation that locks rows is evidence of an approach, not proof that every concurrent sequence is safe; production verification still requires controlled concurrent requests and assertions about rows, status, and effects.
What happens when targets change during an attempt?
Mutable content needs an explicit policy. Record the target’s content version when the attempt starts, then compare it with the current version on every answer.
Abandon a stale attempt
With this policy, a version mismatch changes an active attempt to abandoned_stale and returns a stale result. The participant must start a new attempt against the new target set. This avoids silently judging old answers against new content.
Use an immutable snapshot
An alternative is to copy the target set into the attempt and continue evaluating against that snapshot even after the source content changes. This can provide a stable experience, but it requires a retention and access policy for the copied content.
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How do authorization and exposed fields affect the design?
Authorize every read and mutation against both the participant and the attempt owner. An unguessable attempt ID is not permission. A caller who can guess or obtain another participant’s ID must still receive an authorization failure and no target, progress, or answer details.
Use explicit response DTOs rather than serializing database rows. Expose only fields the participant needs: current status, progress counts or permitted identifiers, the outcome of the submitted answer, and recovery information for a conflict or stale attempt. Keep internal target metadata, ownership keys, and worker state out of the public response. Microsoft guidance likewise recommends explicit validation and control over exposed fields.
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Is REST enough for a stateful guessing game?
For ordinary round operations, yes. Persist the state in a database and expose HTTP requests for starting an attempt, reading progress, and submitting an answer. REST-style operations fit sessions, inventories, friends, leaderboards, and similar game services.
A persistent connection becomes justified when the product needs continuous shared interaction: live turns among several players, presence, server-pushed countdowns, or immediate events for spectators. AWS for Games summarizes the trade-off as: “Some services need a persistent connect, but many can use REST APIs.” A stateful domain does not require a stateful network connection; durable server state and ordinary request/response transport can coexist.
| Decision axis | Request/response REST | Persistent connection |
|---|---|---|
| Round submission | One HTTP command per answer; easy to retry and audit. | Useful when answers are part of a continuously synchronized session. |
| Server push | Requires polling or a separate notification channel. | Natural for live turns, presence, and event broadcasts. |
| State durability | Still requires database transactions. | Still requires durable storage; a socket is not a database. |
| Operational complexity | Usually simpler scaling and recovery. | Connection lifecycle, reconnect, and fan-out behavior add complexity. |
What should a concrete endpoint contract look like?
Keep the command small and make the response describe the resulting state. For example, a request can contain one attempt ID, one group ID, one selected candidate ID, and one idempotency key:
POST /attempts/{attemptId}/answers
{
"groupId": "group-17",
"selectedCandidateId": "candidate-42",
"idempotencyKey": "8f3..."
}A successful response should include the answer outcome, resulting attempt status, whether progress changed, the durable progress view permitted to that participant, and a completion-side-effect identifier when applicable. A duplicate response should contain the same outcome and an explicit indication that it was replayed. A conflict response should identify the group and explain that its recorded answer cannot be replaced; it should not reveal another participant’s data or hidden target details.
Document status codes alongside domain outcomes. Authentication and authorization failures should not be confused with an incorrect guess. Validation errors should identify the malformed field. A stale or unavailable attempt should be terminal according to the contract, allowing the client to stop retrying the old command.
How can you verify the transition model?
Test transitions as stored outcomes, not only as messages shown by a client. The following matrix covers the failure modes most likely to create double progress or false completion.
- Repeated accepted intermediate choice: progress remains unchanged and the stored result is returned.
- Different choice for a recorded group: the API returns
conflictand preserves the original answer. - Later group answered early: the API returns
out_of_orderwithout advancing progress. - Target edited mid-attempt: the documented stale policy is applied, with no accidental completion.
- Target made unavailable: access terminates as unavailable and grants nothing.
- Lost response followed by retry: the same durable outcome is returned and no side effect is duplicated.
- Concurrent submissions: controlled requests produce one consistent progress set, one final status, and at most one completion effect.
Include assertions on database rows, uniqueness constraints, emitted events, and reward records. A green HTTP response alone cannot prove that the transition is atomic.
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Which design choices should be settled before implementation?
Write these decisions into the API contract before clients depend on incidental behavior.
- Is completion based on the full target set, or does a product intentionally allow an earlier threshold?
- Are groups strictly ordered, or may participants answer any group?
- How long are idempotency keys and processed-request records retained?
- Does a stale attempt abandon immediately, or does it use an immutable snapshot?
- What happens to an answer after completion: return the stored completion result, or report a terminal status without re-evaluating?
- Which completion side effect is guaranteed, and what identifier proves it was delivered?
- Which progress fields may the participant read, and which remain private?
- Does the transport need live push, or are ordinary HTTP operations sufficient?
Making these choices explicit prevents clients from depending on a single correct: true flag whose meaning changes from one request to the next. The durable contract is the transition: precondition, server decision, writes, side effect, and replay behavior.
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Frequently Asked Questions
Should a completed attempt accept another answer request?
Treat completion as terminal for normal submissions. A retry carrying the original request key can return the stored completion result, while a new answer should receive a documented terminal response rather than reopening or mutating the attempt.
Can the client calculate progress for an instant user interface?
It may display an optimistic count, but the server response remains authoritative. Reconcile the interface with the returned progress and status, especially after reconnects, rejected guesses, or retries.
Where should a reward be granted if the answer transaction is short?
Record a completion effect identifier with the state transition, then grant the reward synchronously or through an outbox worker keyed by that identifier. This makes a retry safe even when delivery or the original response fails.
What should a participant do after receiving a stale result?
Stop submitting answers for that attempt, refresh the current content and authorization state, and create a new attempt if permitted. Do not replay old answers against the changed target set.
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