Discord

Discord maintains message consistency across web, mobile, and desktop clients through a WebSocket-based event streaming architecture that broadcasts message creates, edits, and deletes to all connected clients in a channel. The system uses operational transformation or CRDT-like conflict resolution to handle concurrent edits, with server-authoritative validation ensuring only the originating user or moderators can modify messages. Latency is typically <100ms for message delivery within a guild.

Discord implements a guild-scoped role hierarchy system where permissions are computed as a bitfield (64-bit integer) combining role permissions, channel-specific overwrites, and user-specific overwrites. The permission resolution algorithm walks the role hierarchy (ordered by position) and applies overwrites in precedence order: explicit channel denies override allows, then explicit allows. This is evaluated server-side on every action (message send, channel access, member management) with caching at the client for UI purposes.

Discord maintains an audit log for all guild actions (member joins/leaves, role changes, channel creation/deletion, message deletions, bans, etc.) with metadata (actor, target, timestamp, reason). The audit log is queryable via API with filters (action type, user ID, target ID) and returns paginated results. Each audit log entry includes the action type (enum), actor ID, target ID, changes (before/after values), and optional reason. The system retains audit logs for 90 days. Bots can listen to audit log events via the AUDIT_LOG_ENTRY_CREATE event (requires audit log read permission).

Discord guilds can upload custom emoji (static PNG/JPEG or animated GIF) and stickers (PNG, APNG, or Lottie JSON) that members can use in messages and reactions. Emoji and stickers are stored per-guild with metadata (name, ID, animated flag, roles that can use it). The system validates file size (emoji: 256KB, stickers: 512KB), dimensions, and format. Custom emoji can be restricted to specific roles. Emoji and stickers are cached on Discord's CDN and served globally. The system supports emoji aliases (e.g., ':smile:' for standard emoji) and autocomplete for custom emoji.

Discord guilds can generate invite links (URLs like discord.gg/XXXXX) with configurable metadata (max uses, expiration time, temporary membership flag). Invites are tracked server-side with metadata (creator, creation date, uses, max uses, expiration). The system broadcasts INVITE_CREATE and INVITE_DELETE events when invites are created/revoked. Invites can be temporary (user is removed from guild when they go offline) or permanent. The system supports vanity URLs (custom guild URLs like discord.gg/myguild) for verified guilds. Invite metadata is queryable via API.

Discord broadcasts user presence (online, idle, do not disturb, offline) and activity status (playing, streaming, listening, watching) to all guild members in real-time via PRESENCE_UPDATE events. Presence is computed client-side based on user activity (keyboard/mouse input, app focus) and sent to Discord's gateway. The system aggregates presence across all connected devices (web, mobile, desktop) and shows the most active status. Custom status messages (e.g., 'In a meeting') can be set by users and are broadcast alongside presence. Bots can query user presence via the GUILD_MEMBER_PROFILE endpoint.

Discord provides a slash command system where commands are registered via HTTP API with parameter schemas (name, type, required/optional flags, choices). When a user types '/', the client fetches registered commands and renders an autocomplete UI. On submission, Discord sends an INTERACTION_CREATE event (via WebSocket or HTTP webhook) containing the command name, parameters, and context. Bots respond with INTERACTION_RESPONSE (deferred, immediate, or modal) within 3 seconds or the interaction times out. This replaces prefix-based commands (e.g., '!help') with a discoverable, type-safe interface.

Discord's voice system uses a peer-to-peer (P2P) or server-relayed UDP connection for audio streaming. Clients negotiate codec support (Opus, H.264 for video) via the VOICE_STATE_UPDATE event, then establish a UDP connection to a voice server. Audio is encrypted using XSalsa20-Poly1305 (libsodium) with per-packet nonces. The system handles jitter, packet loss, and latency through adaptive bitrate and forward error correction. Voice activity detection (VAD) is performed client-side to reduce bandwidth when users are silent.

Discord supports structured message embeds (JSON objects with title, description, fields, images, thumbnails, author, footer) that are rendered natively in the client with consistent styling. Embeds support markdown in text fields, inline fields with flexible layout, and embedded images/videos. The system validates embed schemas server-side (max 6000 characters total, max 25 fields) and caches rendered HTML/CSS on Discord's CDN. Bots construct embeds programmatically and send them as part of message payloads; the client renders them without additional parsing.

Discord webhooks are HTTP endpoints that accept POST requests with message payloads (content, embeds, files) and deliver them to a channel without requiring a bot to be online. Webhooks are created per-channel with a unique URL and optional avatar/username override. The system supports both synchronous delivery (webhook waits for HTTP 200) and asynchronous delivery (Discord queues and retries). Webhooks can be triggered by external services (GitHub, monitoring tools, etc.) to post notifications, logs, or alerts. Rate limiting is per-webhook (10 requests/second by default).

Discord provides APIs to add/remove members from guilds, assign/revoke roles, and manage member metadata (nickname, avatar, flags). Member operations are performed via HTTP API with guild ID and user ID. Role assignment is atomic (add/remove a single role) or bulk (replace all roles). The system maintains member state (join date, roles, permissions) server-side and broadcasts changes to all connected clients via GUILD_MEMBER_UPDATE events. Member pruning (removing inactive members) can be scheduled with configurable inactivity thresholds.

Discord allows users to add emoji reactions to messages, which are tracked server-side with user IDs and emoji identifiers (Unicode or custom guild emoji). The system maintains a list of users who reacted with each emoji, queryable via API. Reactions trigger MESSAGE_REACTION_ADD and MESSAGE_REACTION_REMOVE events (via WebSocket) that bots can listen to. Reactions are limited to 20 unique emoji per message (configurable per guild). The system supports both standard Unicode emoji and custom guild emoji (animated or static).

Discord threads are sub-conversations spawned from a message, allowing users to discuss a topic without cluttering the main channel. Threads are created from a message (parent message ID) and have their own message history, member list, and notification settings. Threads can be public (visible to all channel members) or private (invite-only). The system maintains thread metadata (name, creation date, member count, last message timestamp) and broadcasts thread events (THREAD_CREATE, THREAD_UPDATE, THREAD_DELETE) to connected clients. Threads auto-archive after 1 hour of inactivity (configurable).

Discord guilds can create scheduled events with metadata (title, description, start/end time, location or voice channel, event type: stage, voice, external). Events are displayed in a guild calendar and members can RSVP (interested, attending, not attending). The system broadcasts event updates (GUILD_SCHEDULED_EVENT_CREATE, UPDATE, DELETE) and sends notifications to members who RSVP'd. Events can be linked to voice channels (auto-start when event begins) or external locations (e.g., Twitch streams). Event reminders are sent 15 minutes before start time.