Benchmarked at one million concurrent WebSocket connections and 30 million delivered messages per minute on a single server, Centrifugo is a self-hosted real-time messaging server that integrates with any backend language — Node.js, Python, Go, Ruby, PHP, Java — without requiring architectural changes to your existing application. It handles persistent connections over WebSocket, HTTP-streaming, Server-Sent Events, gRPC, and experimental WebTransport, routing messages through a channel-based PUB/SUB model with automatic reconnection recovery that replays missed messages from hot channel history. Built-in horizontal scaling operates through Redis (including Redis Cluster, AWS ElastiCache, Valkey, KeyDB, and DragonflyDB), NATS, or PostgreSQL brokers, allowing multi-node deployments without external message queues. The server provides JWT-based authentication using HMAC or RSA keys, granular channel permissions, online presence tracking showing who is currently subscribed to a channel, and an RPC proxy that forwards client calls to your backend over HTTP or gRPC. The embedded ReactJS admin web interface displays real-time connection statistics, node information, and provides an interactive API console for publishing messages, managing subscriptions, and inspecting channel history. Official client SDKs are available for JavaScript, Dart, Go, Swift, and Java, with unidirectional transports requiring no SDK at all. Centrifugo also supports delta updates to minimize bandwidth, connection event proxying, and Prometheus-compatible metrics for Grafana monitoring. Running on a dedicated VPS on RepoCloud with guaranteed CPU, RAM, and SSD, full root SSH access, and a browser serial console. Apache 2.0 licensed.

AnyCable moves your WebSocket connections out of your application process and into a dedicated realtime server, so live chat, presence, and notifications keep flowing while your app code focuses on business logic. Rails teams feel it first: the Action Cable channels, subscriptions, and broadcasts already in production keep working unchanged once the endpoint points at AnyCable, which handles authentication, subscriptions, and connection churn by calling back into your application over RPC only when channel code needs to run. Anything that can send an HTTP request can publish to connected clients, and teams outside Rails use standalone mode from Node, Python, Go, or PHP services, including Laravel applications looking for a Reverb or Pusher replacement. Broadcast traffic arrives through Redis Pub/Sub, durable Redis Streams, or the HTTP API, so background jobs and other services push messages without holding a single socket open. Operators watch connected clients, broadcast volume, and RPC failures through Prometheus metrics, and can disconnect remote clients or inspect active streams without a restart. Signed-in users stay safe because every connection request passes through your existing authentication before the server accepts it. Running on a dedicated VPS on RepoCloud with guaranteed CPU, RAM, and SSD, full root SSH access, and a browser serial console. MIT licensed.

Mercure is a realtime hub that pushes updates from your backend straight to browsers over Server-Sent Events, so live dashboards, chat rooms, and activity feeds refresh the moment data changes. Subscribers connect through the browser's built-in EventSource API and choose topics with exact matchers for single resources or URL Pattern matchers for whole families such as /books/:id. Publishers send one form-encoded POST to the hub endpoint, and Mercure fans each update out to every matching connection instantly. JWT access tokens grant publish and subscribe rights topic by topic, so private updates reach only entitled clients while public topics can stay open to anonymous subscribers. If a connection drops, the client reconnects with its last event ID and Mercure replays the updates it missed from stored history. Subscription events announce every arrival and departure, so your application gets live presence and a current list of who is connected. A built-in debug UI shows messages flowing through the hub alongside active subscriptions, turning realtime bugs into something you can watch happen. Optional end-to-end encryption protects sensitive payloads, and any backend able to send an HTTP POST can publish, whatever language it runs. Running on a dedicated VPS on RepoCloud with guaranteed CPU, RAM, and SSD, full root SSH access, and a browser serial console. AGPL-3.0 licensed.

In continuous open-source development since 2009, Etherpad is the original really-real-time collaborative editor - used by Wikimedia, governments, EU public-sector institutions, and tens of thousands of self-hosters. Its core idea is visible authorship: every keystroke is attributed with author colors, every revision preserved, and the timeslider lets you scrub through a document's entire history character by character. Multiple people type into the same pad and see each other's changes instantly - it scales to thousands of simultaneous editors per pad. The base install is deliberately lightweight; capability comes from roughly 290 plugins installable from the admin web UI: comments, images, tables, drawing, video chat via WebRTC, math rendering, code highlighting, and authentication via OAuth, LDAP, or OpenID. AI is pointedly a plugin, not a default - you choose the model and infrastructure, or never turn it on. There is no telemetry. For integrators, an HTTP API (with OpenAPI definitions at /api/openapi.json) manages pads, users, and groups for embedding in your own applications, and the ueberDB abstraction layer supports PostgreSQL, MySQL, Redis, MongoDB, and SQLite backends. Full data export is built in, the format is open, it is translated into 105 languages, and it runs on anything from a Raspberry Pi to a server farm. Apache 2.0 licensed, Node.js based.

Real-time collaborative Markdown behind your own firewall: HedgeDoc (formerly CodiMD, descended from HackMD's open-source edition) keeps team notes on team infrastructure. Share a note's URL and collaborators are editing together instantly - live cursors, changes appearing keystroke by keystroke - in a three-mode interface that toggles between raw Markdown, rendered preview, and side-by-side split. The Markdown dialect is extended where engineers need it: Mermaid, Graphviz, and Vega-Lite diagrams, MathJax for equations, syntax-highlighted code blocks, embedded content, and a presentation mode that turns a note into reveal.js slides with a single YAML header. A dropdown permission system controls each note - freely editable, limited to signed-in users, or locked read-only - and published notes become clean read-only pages for wider distribution. Revisions track every change with the ability to revert to any earlier version. The AGPL-3.0 codebase is light enough to run on a Raspberry Pi and deploys via Docker with PostgreSQL, MySQL, or SQLite. Authentication covers LDAP, SAML, OAuth2, and email. It deliberately stays a focused document editor - no page trees or kanban - and does that one job with excellent keyboard-first ergonomics for meeting notes, RFCs, runbooks, and shared scratchpads.

An entire backend in a single Go executable: PocketBase embeds SQLite with realtime subscriptions, authentication and user management, file storage, and an admin dashboard, all behind a REST-ish API. SQLite runs in WAL mode, which outperforms client-server databases for the read-heavy workloads typical of small and mid-sized apps. Authentication supports email/password, one-time passwords, and 15+ OAuth2 providers including Google, Apple, and GitHub, with stateless tokens. Clients subscribe to record changes over server-sent events, and official JavaScript and Dart SDKs cover web, mobile, and Flutter frontends. Collections, rules, and API access permissions are managed visually in the admin UI. When you need custom logic, extend it with JavaScript hooks running in the embedded JS VM of the prebuilt binary, or import PocketBase as a Go library and compile custom business logic into your own single-file backend. File storage attaches uploads to records with thumbnail generation for images and optional S3-compatible external storage. All state lives in one pb_data directory, so backup is a directory copy and upgrade is replacing a binary - one of the lowest-maintenance backends you can run. The contrast with Firebase is the point: where usage-based pricing scales with reads, writes, and bandwidth, PocketBase runs the entire backend at flat hosting cost, and the data is a plain SQLite file you can copy anywhere. MIT-licensed.

With over 107,000 GitHub stars and adoption by teams at Mozilla, 1Password, Peerlist, and thousands of startups, Supabase has become the most widely deployed open-source Firebase alternative — built entirely on PostgreSQL rather than a proprietary NoSQL store. Every project provisions a dedicated Postgres database with 40+ preinstalled extensions, auto-generating REST APIs via PostgREST, GraphQL via pg_graphql, and real-time change streams via an Elixir-based WebSocket engine that supports database change listeners, user presence tracking, and arbitrary broadcast messaging. Authentication covers email and password, phone OTP, magic links, and 20+ social providers including Google, GitHub, Apple, Azure, and SAML SSO, all enforced through Postgres Row Level Security policies that run inside the database itself. S3-compatible object storage delivers files through a CDN spanning 285+ cities with on-the-fly image transformations powered by imgproxy. Edge Functions run globally distributed TypeScript on the Deno runtime with Node.js compatibility and access to over two million NPM packages. The pgvector extension stores, indexes, and queries vector embeddings for AI-powered semantic search and retrieval-augmented generation workflows. Supabase Studio provides a browser-based dashboard with a table editor, SQL editor, schema visualizer, and log explorer. Official client SDKs ship for JavaScript, Python, Flutter, Swift, Kotlin, C#, Go, and Ruby. The self-hosted Docker Compose stack runs fourteen coordinated services including Kong API gateway, Supavisor connection pooler, and Logflare analytics. Running on a dedicated VPS on RepoCloud with guaranteed CPU, RAM, and SSD, full root SSH access, and a browser serial console. Apache 2.0 licensed.

Backed by over 56,000 GitHub stars and a growing ecosystem of client and server SDKs covering Flutter, React Native, Swift, Kotlin, Python, Node.js, Deno, and more, Appwrite provides a complete backend platform that replaces Firebase and Supabase with a fully self-hostable architecture. The authentication module supports email and password, phone OTP, magic links, OAuth providers including Google, GitHub, Apple, and Facebook, anonymous sessions, and JWT token management with configurable session policies. The database service provides a document-oriented store with collections, indexes, query filters, pagination, and real-time subscriptions that push changes to connected clients instantly via WebSocket. The storage service handles file uploads with built-in image manipulation including cropping, resizing, format conversion, and compression, served through a CDN-ready URL structure. Serverless functions execute custom backend logic triggered by HTTP requests, scheduled cron jobs, or database events, with runtime support for Node.js, Python, PHP, Ruby, Dart, Swift, Kotlin, and Java. The messaging service enables push notifications, SMS, and email delivery through configurable providers including APNS, FCM, Twilio, Vonage, Mailgun, and Sendgrid. Team management with role-based permissions, webhook integrations, and health monitoring APIs complete the platform. The admin console provides a web-based dashboard for managing all services, monitoring usage metrics, and configuring project settings. Deployment runs via Docker Compose with MariaDB for metadata and configurable storage adapters. Running on a dedicated VPS on RepoCloud with guaranteed CPU, RAM, and SSD, full root SSH access, and a browser serial console. BSD 3-Clause licensed.

Spica skips the month of backend plumbing every new project starts with: data models, CRUD endpoints, logins, file storage, and an admin panel nobody wants to build. It hands you that whole layer as a running platform, so your first day is spent modeling your actual product. Define your data in Buckets and the REST API, with a GraphQL API alongside it, is generated automatically, complete with rules on each bucket as an entry-level security layer. Add custom policies so each user role reaches exactly the data and technical areas it should, then place your business logic in cloud functions triggered by events. Content teams get a back-office they can actually use: customized bucket views with a few clicks, inline image editing, data history, and localization for editors working in more than one language. Identities cover your users and editors, storage serves your files, and webhooks push events out to tools like Slack or Zapier when something changes. Run it as a backend engine for an application you are building, or purely as a headless CMS feeding a frontend that lives somewhere else. Running on a dedicated VPS on RepoCloud with guaranteed CPU, RAM, and SSD, full root SSH access, and a browser serial console. AGPL-3.0 licensed.

Stateful serverless actors that run indefinitely, sleep when idle, and persist state across restarts without external database round trips. Rivet provides a vendor-neutral alternative to Cloudflare Durable Objects, delivering long-running processes with co-located in-memory state and per-actor SQLite databases on any infrastructure you choose. The Rust engine comprises four packages: Pegboard for actor orchestration, Gasoline for durable execution, Guard for traffic routing via Envoy, and Epoxy implementing multi-region KV storage through EPaxos consensus. Each actor maintains instant-access in-memory state plus a dedicated SQLite database for relational queries, backed by RocksDB on single nodes or PostgreSQL with NATS pub/sub for multi-node clusters. RivetKit SDKs in TypeScript, Rust, and Python support built-in WebSocket connections, task queues, scheduling, and CRDT-based real-time collaboration. The v2.3 rewrite moved the core runtime from JavaScript to native Rust via WebAssembly, eliminating main-thread blocking across Node.js, Bun, Deno, and Cloudflare Workers. A built-in dashboard provides actor inspection with real-time Prometheus metrics. Deploys as a single Docker container on port 6420 with optional PostgreSQL for persistence. Available on RepoCloud with a dedicated VPS under the Apache 2.0 license.

Half the architecture sketches on the internet trace back to Excalidraw - the MIT-licensed virtual whiteboard whose hand-drawn aesthetic made technical diagramming feel approachable, at roughly 85,000 GitHub stars. The infinite canvas offers rectangles, ellipses, diamonds, arrows with smart binding and labels, free-draw, text, images, and an eraser, with full undo/redo, zoom, dark mode, and keyboard-first ergonomics. Community shape libraries add thousands of pre-built elements - AWS architecture icons, flowchart stencils, UI wireframe kits - and everything exports to PNG, SVG, the clipboard, or the open .excalidraw JSON format that keeps drawings diffable and portable. Live collaboration works on a share-a-link model with live cursors and a laser pointer for presenting, and it is end-to-end encrypted by design: the room key travels in the URL hash, which never reaches the server, so the WebSocket relay only ever sees ciphertext. The architecture is remarkably light - the app is a static bundle served by Nginx, drawings persist locally in the browser, and the stateless excalidraw-room relay handles multiplayer - so a self-hosted deployment gives unlimited boards and collaborators with near-zero resource cost, replacing per-editor whiteboard subscriptions.

A Node.js server, a large shared canvas, and a URL - WBO (Whiteboard Ophir) is collaborative whiteboarding reduced to its essence. There are no accounts and no setup - to collaborate, you send someone the board's link, and every stroke appears for all connected users in real time over WebSockets, with cursor positions shared so you can see where collaborators are working. Board state persists automatically and continuously, so a diagram drawn in today's lesson is still there next week at the same URL. Boards come in three flavors: a public free-for-all, private boards with random unguessable names, and named boards with custom URLs shared by anyone who knows the name. The tools cover teaching and brainstorming needs - pencil, straight lines, rectangles, ellipses, text annotations, eraser, a full color palette with brush sizes - and boards export as SVG or PNG. Despite the simplicity, the server is production-minded: JWT authentication gates board access with granular capabilities (open, edit, and clear as separate permissions), rate limiting caps per-client message volume, reverse-proxy and subpath deployment are supported, and OpenTelemetry provides metrics, logs, and traces. It works on tablets and touch devices, speaks multiple languages, and consumes minimal resources. AGPL-licensed.