# Low-Level Design: Blog Platform — Content Management, Comments, and SEO-Friendly URLs

Source: https://www.techinterview.org/post/3233465905/lld-blog-platform/
Updated: 2026-04-17 · techinterview.org

## Core Entities

**User**: user_id, username (unique, URL-safe), email, password_hash, display_name, bio, avatar_url, role (READER, AUTHOR, EDITOR, ADMIN), created_at. **Post**: post_id, author_id, title, slug (unique, URL-safe: "my-first-post"), body (Markdown or HTML), excerpt (auto-generated or manual), status (DRAFT, PUBLISHED, ARCHIVED), featured_image_url, published_at, created_at, updated_at, view_count, reading_time_minutes. **Tag**: tag_id, name, slug. **PostTag**: post_id, tag_id. **Comment**: comment_id, post_id, author_id (nullable for guests), author_name (for guests), author_email, body, status (PENDING, APPROVED, SPAM), parent_id (nullable for threading), created_at. **PostRevision**: revision_id, post_id, author_id, body_snapshot, title_snapshot, created_at, change_summary.

## SEO-Friendly URLs and Slug Generation

Posts are accessed via /author-username/post-slug (e.g., /john/how-to-design-systems). Slug generation: convert title to lowercase, replace spaces with hyphens, strip non-alphanumeric characters except hyphens, truncate to 100 characters. Ensure uniqueness per author: if slug already exists, append -2, -3, etc. Redirect handling: when a post's title (and thus slug) changes, keep the old slug mapping in a PostSlug table: (slug, post_id, is_primary). The old slug returns HTTP 301 to the new slug. Never return 404 for previously valid URLs -- search engines penalize broken links. Canonical URL: set the canonical link tag to the primary slug URL to consolidate SEO credit if multiple paths reach the same post. RSS feed: generate /feed.xml for each author and globally. Cache the RSS XML (regenerate only on new post publish). Sitemap: generate /sitemap.xml with all published posts, tags, and author pages. Refresh daily. Submit to Google Search Console.

## Draft and Publishing Workflow


```
class PostService:
    def publish_post(self, post_id: int, publisher_id: int) -> Post:
        post = self.repo.get_post(post_id)
        if post.status != PostStatus.DRAFT:
            raise InvalidStatusError()
        if post.author_id != publisher_id and not self.is_editor(publisher_id):
            raise PermissionError()

        # Compute auto-fields
        post.reading_time_minutes = self._estimate_reading_time(post.body)
        post.excerpt = post.excerpt or self._generate_excerpt(post.body)
        post.published_at = datetime.utcnow()
        post.status = PostStatus.PUBLISHED

        self.repo.save(post)

        # Clear post cache
        self.cache.delete(f"post:{post.slug}")
        self.cache.delete("recent_posts")
        self.cache.delete(f"author_posts:{post.author_id}")

        # Trigger async jobs
        self.events.publish("post.published", {
            "post_id": post.post_id,
            "author_id": post.author_id
        })
        return post
```

The post.published event triggers: notification to subscribers (newsletter send), ping to search engines (IndexNow API for instant indexing), social media auto-post (if configured), CDN cache warmup.

## Comments with Moderation

Comment workflow: (1) Guest/user submits comment → status=PENDING. (2) Spam check: run through Akismet API (industry standard for blog spam). If spam: status=SPAM (not shown). (3) Auto-approve trusted commenters: users who have had 2+ comments approved are auto-approved (status=APPROVED immediately). First-time commenters wait for moderation. (4) Author/editor moderation queue: PENDING comments are visible in the admin dashboard. Approve or mark as spam. Email notification to the post author when a new comment awaits moderation. Threading: store parent_id on each comment. Load top-level comments first; load replies on expand. Flat representation in the database, tree representation in the UI. Comment count: cache the approved comment count per post in Redis. Increment on approve, decrement on delete. Avoid counting in SQL on every page load. Anti-spam: rate limit by IP (max 3 comments per 10 minutes), require JavaScript challenge (honeypot field), validate email format for guests.

## Post Discovery and Recommendations

Tag-based discovery: posts are tagged with topics. Tag page (/tag/system-design) shows all posts for a tag sorted by published_at DESC. Index: (tag_id, published_at DESC) on PostTag join with Posts. Author page (/john) shows all of an author's published posts. Related posts: after reading a post, show 3-5 related posts. Simple approach: posts sharing the most tags with the current post. Query: find posts sharing any tag, rank by number of shared tags, return top 5. More sophisticated: TF-IDF similarity on post content (compute offline, store in a related_posts cache). Cache related posts per post_id for 24 hours. Trending posts: posts with the most views in the last 7 days. Maintain a sorted set in Redis: ZINCRBY trending:posts 1 post_id on each page view. Refresh daily from the database for accuracy (Redis count is approximate). Full-text search: Elasticsearch index for searching across titles, bodies, and tags. Autocomplete suggestions from a completion suggester on post titles.

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