# Survey and Forms System Low-Level Design: Dynamic Form Builder, Response Storage, and Analytics Source: https://www.techinterview.org/post/3233469734/lld-survey-system/ Updated: 2026-04-17 · techinterview.org ## Form Schema A form is defined as a document with ordered questions, each with a type and optional configuration: ``` forms { form_id UUID PK owner_id UUID FK title VARCHAR(255) description TEXT questions JSONB -- ordered array of question objects settings JSONB status ENUM(DRAFT, ACTIVE, CLOSED) created_at TIMESTAMP } ``` Question types: `text` (short/long), `single_choice`, `multi_choice`, `rating` (1-N scale), `likert` (agree/disagree matrix), `matrix`, `file_upload`, `date`. Each question object: ``` { "question_id": "q1", "type": "single_choice", "text": "How did you hear about us?", "required": true, "options": ["Search", "Social media", "Friend"], "conditional_logic": {"show_if": {"question_id": "q0", "operator": "eq", "value": "Yes"}}, "validation_rules": {"min_length": 10} } ``` Settings include: `one_response_per_user`, `anonymous`, `close_after_date`, `max_responses`. ## Conditional Logic Questions can be shown or hidden based on previous answers. The condition evaluates client-side for instant feedback — no round trip needed. Supported operators: `eq`, `neq`, `contains`, `gt`, `lt`. Example: show question 5 only if question 3 equals "Yes". The form renderer evaluates all conditions after each answer change and updates question visibility accordingly. Skipped (hidden) questions are excluded from the submitted response payload. ## Response Schema Each form submission produces a response record: ``` responses { response_id UUID PK form_id UUID FK respondent_id UUID FK nullable -- null if anonymous anonymous_token VARCHAR(64) nullable answers JSONB -- {"q1": "Search", "q3": ["A","B"]} started_at TIMESTAMP submitted_at TIMESTAMP nullable completion_time_secs INT nullable partial BOOLEAN DEFAULT true } ``` Answers are stored as a flat JSON map from question_id to value. File upload answers store the S3 object key. ## Partial Save and Resume Progress is saved automatically as the respondent fills out the form. Every 30 seconds (or on tab blur), the client sends the current answers to: ``` PUT /responses/{response_id}/partial ``` The `partial` flag remains true until final submission. A resumption token (the response_id) is stored in the browser's localStorage and a session cookie. Returning to the form URL loads the partial response and restores the respondent's position in the form. ## Duplicate Prevention For authenticated forms (`one_response_per_user = true`), a unique constraint on `(form_id, respondent_id)` prevents double submission. For anonymous forms, duplicate prevention uses a combination of browser fingerprint (User-Agent + screen resolution hash) and IP address stored in `anonymous_token`. This is a soft signal — not cryptographically enforced — but deters casual duplicates. The form owner can configure strictness. ## Result Aggregation Aggregations are computed per question type: - **Single / multi choice:** frequency distribution — count of each option selected; percentage of total responses - **Rating:** mean, median, standard deviation, histogram of values - **Text:** word frequency (tokenize, stop-word filter, count); optional sentiment analysis via NLP service - **Likert / matrix:** mean score per row/statement Aggregates are computed incrementally: on each new submission, update running totals stored in a `form_aggregates` cache table. This avoids full table scans for large response sets. The cache is invalidated and rebuilt if responses are deleted. ## Real-Time Response Count A Redis counter tracks total submissions per form: ``` INCR form:responses:{form_id} ``` The live counter is displayed in the form owner's dashboard without querying the database. On reaching `max_responses`, the form status is atomically set to CLOSED and subsequent submissions are rejected with a 410 Gone response. ## Export CSV export produces one row per response, one column per question. Column headers use question text (truncated). Multi-choice answers are serialized as semicolon-delimited values. Export supports filters: date range, completion status (submitted only vs. including partials). For cross-tabulation reports, a pivot export groups responses by one question and shows distribution of another — useful for "segment by role, show rating distribution." ## File Upload Questions When a file upload question is encountered, the client requests a pre-signed S3 URL from the server: ``` POST /forms/{form_id}/upload-url → { "upload_url": "https://s3.../...", "key": "forms/{form_id}/responses/{response_id}/files/q5/{filename}" } ``` The client uploads directly to S3 using the pre-signed URL. The S3 key is stored in the response's answers JSON. File size limits and allowed MIME types are validated server-side when the pre-signed URL is generated. ## Quota Management Before accepting a new submission, the system checks whether `max_responses` has been reached and whether `close_after_date` has passed. Both checks use the Redis counter and a simple timestamp comparison, avoiding database reads on the hot submission path. If either condition is met, the submission is rejected with a human-readable error message shown to the respondent. See also: [Netflix Interview Guide 2026: Streaming Architecture, Recommendation Systems, and Engineering Excellence](https://www.techinterview.org/post/3233460274/netflix-interview-guide-2026-streaming-architecture-recommendation-systems-and-engineering-excellence/) See also: [Atlassian Interview Guide](https://www.techinterview.org/post/3233459919/atlassian-interview/) See also: [LinkedIn Interview Guide 2026: Social Graph Engineering, Feed Ranking, and Professional Network Scale](https://www.techinterview.org/post/3233460278/linkedin-interview-guide-2026-social-graph-engineering-feed-ranking-and-professional-network-scale/)