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You can let people send files peer to peer from a browser without installing a separate app: use a Cloudflare Worker and Durable Object to coordinate WebRTC setup, then send the file over a WebRTC data channel. “Zero-install” describes the user experience, not a serverless or dependency-free design. A signaling service, compatible browsers, and working network connectivity are still required.
How browser-based peer-to-peer file transfer works
WebRTC can establish a direct connection between two browsers, but it does not specify how those browsers exchange the information needed to find and connect to each other. MDN puts it plainly: “The most important thing to know about the signaling process for WebRTC: it is not defined in the specification.” The application chooses that signaling mechanism; it could use WebSocket, HTTPS requests, or another channel. MDN Web Docs: Signaling and video calling also discusses the session lifecycle in its WebRTC session-lifetime guide.
In this design, the Worker routes each room to a named Durable Object. That object acts as a small, room-scoped rendezvous point: it passes setup messages between participants but is not the route for file contents once a peer data channel is established. Cloudflare describes Durable Objects as globally named, stateful coordination objects with attached durable, strongly consistent storage; a single object can coordinate multiple clients. Cloudflare: What are Durable Objects?
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How do I send files peer to peer in a browser?
- Create or join a room. The web app obtains an unpredictable room identifier and opens a WebSocket to a Worker endpoint. Treat the identifier as one part of room security, not a substitute for authorization.
- Route the room to its Durable Object. The Worker uses the room identifier to find the corresponding named Durable Object. The object accepts both participants’ server-side WebSockets and associates each connection with its room and participant identity.
- Exchange WebRTC setup messages. The browsers send session descriptions—the offer and answer—and ICE candidates through the object. The application should validate message type, size, rate, room membership, and sender and recipient before forwarding messages. Durable Objects provide coordination, not automatic application-level authorization.
- Open a peer data channel and send file chunks. After ICE connectivity succeeds, the browsers transfer file data over a WebRTC data channel. The application protocol should handle progress, backpressure, completion, cancellation, and integrity checks. Signaling documentation does not establish a particular file-transfer protocol, throughput, or performance guarantee.
- Close or expire the room. End the signaling room when the transfer finishes, or expire it according to an application-defined policy. Do not assume a room map held only in memory is durable state.
Can WebRTC work without installing an app?
Yes, the user-facing client can be a web app opened in a compatible browser. But the application still needs a signaling mechanism and an available network path between peers. “Zero-install” does not mean the transfer works offline, that every network permits a direct connection, or that the service has no server-side component.
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The signaling transport is an application choice. WebSocket suits interactive, two-way setup messages and trickled ICE candidates because either side can send updates over an open connection. Other transports, including HTTPS requests, can also exchange signaling information; the trade-offs include connection overhead, latency, server push, reconnection behavior, and operational complexity. WebRTC itself does not mandate WebSocket. See MDN’s signaling overview and session-lifetime guide.
What does the Durable Object handle—and what does it not?
The object gives each room a common coordination point: both peers connect to the same object, and it forwards the setup messages needed to negotiate WebRTC. Once the peer data channel is established, file payloads travel between the peers rather than through that signaling flow. This separation can keep the signaling service focused on brief coordination rather than file delivery.
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It does not guarantee that a direct peer route can be established. Some network conditions require a TURN relay to carry traffic. TURN is distinct from signaling: signaling exchanges connection setup information, while a relay can carry data when a direct path is unavailable. The sources cited here do not establish a particular TURN provider’s availability, pricing, or suitability for a file-transfer service, so those details must be checked with the provider before choosing a connectivity design.
How WebSocket hibernation affects room state
For WebSockets accepted by a Durable Object, Cloudflare recommends considering its WebSocket Hibernation API. Eligible objects can leave memory while accepted client connections remain open; an incoming event wakes the object. Because in-memory state is reset, reconstruct room state from durable storage or retain only the small per-connection information needed in WebSocket attachments. Cloudflare documents a 16,384-byte maximum serialized attachment size; file contents and large transfer state do not belong there. See Cloudflare’s WebSocket best practices.
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Hibernation is conditional, not a general promise that an object can sleep with any workload. Cloudflare says an object cannot hibernate while it relies on scheduled setTimeout or setInterval callbacks, has unfinished I/O, uses a standard WebSocket API connection, or is still processing an event. Eligible objects hibernate after 10 seconds without an incoming request or event, according to Cloudflare’s lifecycle documentation; the runtime controls the transition, so this is not a guaranteed timer. Cloudflare: Durable Object lifecycle
Hibernation is not the same as shutdown. A deployment, runtime decision, or other shutdown can terminate WebSockets. Clients therefore need bounded-backoff reconnection, room rejoining, and renegotiation when required. The application must also define what happens to authorization and a partially transferred file after a reconnect; WebRTC signaling alone does not provide automatic transfer resumption.
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How many connections can one Durable Object handle?
Cloudflare documents a maximum of 32,768 WebSocket connections per Durable Object when using the Hibernation API. That is a documented ceiling, not a recommended room size or a capacity guarantee: CPU and memory can constrain practical concurrency below it. Cloudflare Durable Object state API
Design object identity around the coordination unit—usually one bounded transfer room—so participants in a room reach the same object and unrelated rooms map to separate objects. Avoid sending unrelated application traffic to one singleton. Room-ID entropy, abuse controls, cleanup, reconnect races, duplicate messages, and authorization are application-specific design concerns. Test expected signaling bursts rather than treating the connection ceiling as a performance target.
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Security and reliability decisions to make
- Room access: Make room identifiers difficult to guess, and decide how participants are authorized to join. A room ID alone should not silently grant unlimited access.
- Message handling: Enforce message types, size limits, rate limits, and sender and recipient checks before forwarding offers, answers, or ICE candidates.
- State recovery: Decide which room details must survive hibernation and how they are restored. Keep transient in-memory maps out of assumptions about durable state.
- Reconnect behavior: Rejoin the room and renegotiate when connections are lost. Specify whether a failed or interrupted transfer restarts, rather than implying it resumes automatically.
- Transfer integrity: Define chunking, progress, backpressure, cancellation, and verification in the application protocol. The signaling architecture does not supply these file-transfer behaviors.
- Connectivity fallback: Evaluate whether users need TURN support for networks where a direct path cannot be established. The signaling server is not a substitute for a relay.
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