Typical system design diagrams for back-end services like Uber involves a proxy and web socket server connection to the client.
I'm curious why only web sockets (and long polling) are considered for these modern web designs. If the demand is for a location update service from a mobile app that constantly pushes location updates to the server, why don't people block out a custom tcp or udp connection between the iOS client and the server for example?
Tcp connection is really what websockets uses under the hood, but with a raw TCP connection, you have way more mature libraries that you can leverage (Netty, Kernel-bypass, FPGA)
Udp seems even better since it's stateless and recoverable during disconnections. If it's a one way stream of location updates, it seems to serve the purpose just fine.
Thoughts?
The main point of using Websockets is that it plays well with existing firewalls, proxies and other limitations. It is not uncommon that devices are used in restricted networks which only allow access to web and mail. It is also nice that it also provides a message semantic (TCP is only a byte stream) and that support for TLS is nicely integrated too. While "raw" TCP might have less overhead, the actual overhead of Websockets is fairly small. And often the overhead of the non-binary payloads (i.e. JSON, XML) is much higher which makes the additional small overhead of Websockets irrelevant.
Is there a reason why HTTP2 is required when websocket is already available?
Nathan Aw (Singapore)
Why do we need HTTP/2 when we have WebSockets? Well why did we need WebSockets when we have TCP? Or even IP? Protocols are basically agreed standards that can be implemented by independent parties.
WebSockets are good for two way communication but are mostly unstructured on top of that and application specific. HTTP is (mostly) a series of one-way requests to the server (ask for a resource, receive an answer) — though HTTP/2 enhances that slightly with HTTP/2 push and the binary framing layer could in theory be used more for proper two way push. So the full two way nature of WebSockets — the very thing they are good at — is not really needed for most HTTP use cases.
Looking at HTTP, it has various extras that WebSockets does not have. Including defined methods, headers and compression. This allows a well-defined understanding between various HTTP implementations to facilitate communications for its use case including features like multiplexing, caching, compression, redirects, error handling... etc. If you had to reinvent all of those on top of WebSockets (which is a very raw protocol), you’d end up with an HTTP/2 like protocol.
Could HTTP/2 have used WebSockets to act as it’s underlying transport layer? Possibly, but that’s an unnecessary extra level of abstraction (IP->TCP->WS->HTTP2->HTTP), not to mention that websockets are often established over HTTP initially. HTTP is big enough to have its own transport protocol so in fact they’ve gone the other way and specified WebSockets over HTTP/2.
Finally, it should also be noted that HTTP/2 does not make Web Sockets obselete either. They are different and with different advantages and disadvantages.
I have recently come across a term 'Reactive sockets'. Up until this point, I used to think websockets are the way to go for full fledged asynchronous style.
So what are reactive sockets.
This link (http://rsocket.io/) even talks about comparison over websockets.
What is RSocket?
RSocket implements the Reactive Streams specification over the network boundary. It is an application-level communication protocol with framing, session resumption, and backpressure built-in that works over the network.
RSocket is transport agnostic. RSocket can be run over Websockets, TCP, HTTP/2, and Aeron.
How does RSocket differ from Websockets?
Websockets do not provide application-level backpressure, only TCP-based byte-level backpressure. Websockets also only provide framing they do not provide application semantics. It is up to the developer to build out an application protocol for interacting with the websocket.
RSocket provides framing, application semantics, application-level backpressure, and it is not tied to a specific transport.
For more information on the motivations behind the creation of RSocket checkout the motivations doc on the RSocket site.
Both WebSocket and RSocket are protocols which feature bi-directional, multiplex, duplex communication. But both work at different levels.
WebSocket is a low level communication protocol layered over TCP. It defines how a stream of bytes is transformed into frames. But WebSocket message itself does not have instructions about how to route or process it. Therefore, we need messaging protocols that operate on top of websocket, at application level, to achieve two-way communication.
RSocket is a fully reactive application level protocol which runs over byte stream transports such as TCP, WebSocket, UDP or other. It provides application flow control over the network to prevent outages and increase resiliency. RSocket employs the idea of asynchronous stream processing with non-blocking back-pressure, in which a failing component will, rather than simply dropping traffic, communicate its stress to upstream components, getting them to reduce the load.
Websocket
TLDR: L4 protocol, TCP for web.
Websocket is single bytestream, frames based protocol with very compact header.
It relies on web/http in each important protocol aspect: http based handshake (full roundtrip, not ideal for latency), text frames in addition to binary ones, compression support (for low-throughput/high latency connections),
mandatory frame content masking for compatibility with legacy non-tls http proxies.
Frames may be fragmented for better memory utilization by client and server;
Flow control is byte level only from TCP, and is not propagated to userspace.
Because websocket is just single bytestream transport, It needs full application protocol on top to be useful, and application level flow control scheme to be scalable.
Adoption wise, stable websocket implementation is available for most OSes / architectures, protocol is supported by all browsers and is go-to solution if any traffic needs to survive internet hop.
RSocket. Theory
TLDR: L5 protocol, primarily cloud/datacenter communications with excellent
throughput/latency characteristic: huge throughput while maintaining latency < few millis.
RSocket is session layer protocol, offers multiplexed flow controlled streams of binary messages over any transport capable of transferring bytes in order (tcp, unix sockets, also websocket).
Low latency is cornerstone, protocol has several capabilities for this:
2 levels of flow control: Reactive-Streams on individual stream level,
request leasing on connection level. Leasing is feature to control number of active streams by responder side using service & connection latency stats.
Instant handshake: client may send requests immediately after initial setup message.
Message fragmentation helps with reducing server memory pressure and improves latency for large messages (if done properly, see RSocket. Practice below).
Session resumption: reduced latency on client reconnection.
Because binary streaming interactions / multiplexing are available out-of-the-box, It is trivial to implement application RPC on top - only data serialization/deserialization is needed (mstreams-rpc using protobuf data encoding).
The protocol is semantically compatible with http2, which means It is also compatible with GRPC (given protobuf is used for message encoding).
RSocket. Practice
Only useful on JVM because that's where reactive streams are popular and practically useful with several
stable implementations: rxjava, project-reactor, smallrye-mutiny.
RSocket/RSocket-java is based on project-reactor from springboot.
Natural expectation would be best-in-class throughput, unfortunately RSocket/RSocket-java did not get this
right so performs worse
than 10+ year older GRPC (its predecessor Stubby was in use from ~2001) on top of http2: chatty web protocol.
Fragmentation: no server memory use or latency improvement because RSocket/RSocket-java
implemented It in pointless way - frames are always reassembled before passing
downstream.
GRPC compatibility: absent.
Advice for 2022: better stick with GRPC.
I've read that HTTP is not suitable as a messaging protocol in several places such as here in reference to RabbitMQ.
I assume that there's a technical reason for this and that it's not a mere opinion. I've looked through the AMQP spec for example and can't see any reason why HTTP + Web Sockets can't work. In fact, something seems to be in the works for AMQP over Web Sockets. Furthermore, I've looked at the STOMP protocol which does use HTTP + Web Sockets and can't see any significant limitations (other than a small performance hit).
What technical characteristic does HTTP + Web Sockets lack that makes it unsuitable as a messaging protocol?
UPDATE:
This is what I was looking for: Crossbar.IO - a WAMP message broker. I needed a message broker that I can easily connect to from a browser and have not been satisfied with RabbitMQ (over STOMP) or HiveMQ (MQTT).
HTTP is request/response based, what makes it difficult to work in a publisher/subscriber fashion. Basically, you can either poll the source of messages for new ones, or create another local endpoint where the other end push messages to you.
WebSocket is different. Despite of starting as a HTTP request, it switches straightaway to a persistent, full-duplex connection, where both end can push data. Basically, in this case HTTP is only used as protocol to negotiate the connection, once negotiated WebSocket uses its own protocol to transfer data.
UPDATE: We are clear that HTTP is not a messaging protocol, since it is request/response. WebSockets, although it allows pushing data from both ends, it is not a messaging protocol neither. It defines a way of framing data, but there are not defined semantic or grammar to subscribe to topics or any operation about messaging. For example WAMP is an actual messaging protocol for websockets.
I'm learning about HTTP/2 protocol. It's a binary protocol with small message frames. It allows stream multiplexing over single TCP connection. Conceptually it seems very similar to WebSockets.
Are there plans to obsolete websockets and replace them with some kind of headerless HTTP/2 requests and server-initiated push messages? Or will WebSockets complement HTTP/2?
After just getting finished reading RFC 7540, HTTP/2 does obsolete websockets for all use cases except for pushing binary data from the server to a JS webclient. HTTP/2 fully supports binary bidi streaming (read on), but browser JS doesn't have an API for consuming binary data frames and AFAIK such an API is not planned.
For every other application of bidi streaming, HTTP/2 is as good or better than websockets, because (1) the spec does more work for you, and (2) in many cases it allows fewer TCP connections to be opened to an origin.
PUSH_PROMISE (colloquially known as server push) is not the issue here. That's just a performance optimization.
The main use case for Websockets in a browser is to enable bidirectional streaming of data. So, I think the OP's question becomes whether HTTP/2 does a better job of enabling bidirectional streaming in the browser, and I think that yes, it does.
First of all, it is bi-di. Just read the introduction to the streams section:
A "stream" is an independent, bidirectional sequence of frames
exchanged between the client and server within an HTTP/2 connection.
Streams have several important characteristics:
A single HTTP/2 connection can contain multiple concurrently open
streams, with either endpoint interleaving frames from multiple
streams.
Streams can be established and used unilaterally or shared by
either the client or server.
Streams can be closed by either endpoint.
Articles like this (linked in another answer) are wrong about this aspect of HTTP/2. They say it's not bidi. Look, there is one thing that can't happen with HTTP/2: After the connection is opened, the server can't initiate a regular stream, only a push stream. But once the client opens a stream by sending a request, both sides can send DATA frames across a persistent socket at any time - full bidi.
That's not much different from websockets: the client has to initiate a websocket upgrade request before the server can send data across, too.
The biggest difference is that, unlike websockets, HTTP/2 defines its own multiplexing semantics: how streams get identifiers and how frames carry the id of the stream they're on. HTTP/2 also defines flow control semantics for prioritizing streams. This is important in most real-world applications of bidi.
(That wrong article also says that the Websocket standard has multiplexing. No, it doesn't. It's not really hard to find that out, just open the Websocket RFC 6455 and press ⌘-F, and type "multiplex". After you read
The protocol is intended to be extensible; future versions will likely introduce additional concepts such as multiplexing.
You will find that there is 2013 draft extension for Websocket multiplexing. But I don't know which browsers, if any, support that. I wouldn't try to build my SPA webapp on the back of that extension, especially with HTTP/2 coming, the support may never arrive).
Multiplexing is exactly the kind of thing that you normally have to do yourself whenever you open up a websocket for bidi, say, to power a reactively updating single page app. I'm glad it's in the HTTP/2 spec, taken care of once and for all.
If you want to know what HTTP/2 can do, just look at gRPC. gRPC is implemented across HTTP/2. Look specifically at the half and full duplex streaming options that gRPC offers. (Note that gRPC doesn't currently work in browsers, but that is actually because browsers (1) don't expose the HTTP/2 frame to the client javascript, and (2) don't generally support Trailers, which are used in the gRPC spec.)
Where might websockets still have a place? The big one is server->browser pushed binary data. HTTP/2 does allow server->browser pushed binary data, but it isn't exposed in browser JS. For applications like pushing audio and video frames, this is a reason to use websockets.
Edit: Jan 17 2020
Over time this answer has gradually risen up to the top (which is good, because this answer is more-or-less correct). However there are still occasional comments saying that it is not correct for various reasons, usually related to some confusion about PUSH_PROMISE or how to actually consume message-oriented server -> client push in a single page app.
If you need to build a real-time chat app, let's say, where you need to broadcast new chat messages to all the clients in the chat room that have open connections, you can (and probably should) do this without websockets.
You would use Server-Sent Events to push messages down and the Fetch api to send requests up. Server-Sent Events (SSE) is a little-known but well supported API that exposes a message-oriented server-to-client stream. Although it doesn't look like it to the client JavaScript, under the hood your browser (if it supports HTTP/2) will reuse a single TCP connection to multiplex all of those messages. There is no efficiency loss and in fact it's a gain over websockets because all the other requests on your page are also sharing that same TCP connection. Need multiple streams? Open multiple EventSources! They'll be automatically multiplexed for you.
Besides being more resource efficient and having less initial latency than a websocket handshake, Server-Sent Events have the nice property that they automatically fall back and work over HTTP/1.1. But when you have an HTTP/2 connection they work incredibly well.
Here's a good article with a real-world example of accomplishing the reactively-updating SPA.
From what I understood HTTP/2 is not a replacement for websocket but aims to standardize SPDY protocol.
In HTTP/2, server-push is used behind the scene to improve resource loading by the client from the browser. As a developer, you don't really care about it during your development. However, with Websocket, the developer is allowed to use API which is able to consume and push message with an unique full-duplex connection.
These are not the same things, and they should complement each other.
I say Nay (Websockets aren't obsolete).
The first and most often ignored issue is that HTTP/2 push isn't enforceable and might be ignored by proxies, routers, other intermediaries or even the browser.
i.e. (from the HTTP2 draft):
An intermediary can receive pushes from the server and choose not to forward them on to the client. In other words, how to make use of the pushed information is up to that intermediary. Equally, the intermediary might choose to make additional pushes to the client, without any action taken by the server.
Hence, HTTP/2 Push can't replace WebSockets.
Also, HTTP/2 connections do close after a while.
It's true that the standard states that:
HTTP/2 connections are persistent. For best performance, it is expected that clients will not close connections until it is determined that no further communication with a server is necessary (for example, when a user navigates away from a particular web page) or until the server closes the connection.
But...
Servers are encouraged to maintain open connections for as long as possible but are permitted to terminate idle connections if necessary. When either endpoint chooses to close the transport-layer TCP connection, the terminating endpoint SHOULD first send a GOAWAY (Section 6.8) frame so that both endpoints can reliably determine whether previously sent frames have been processed and gracefully complete or terminate any necessary remaining tasks.
Even if the same connection allows for pushing content while it is open and even if HTTP/2 resolves some of the performance issues introduced by HTTP/1.1's 'keep-alive'... HTTP/2 connections aren't kept open indefinitely.
Nor can a webpage re-initiate an HTTP/2 connection once closed (unless we're back to long-pulling, that is).
EDIT (2017, two years later)
Implementations of HTTP/2 show that multiple browser tabs/windows share a single HTTP/2 connection, meaning that push will never know which tab / window it belongs to, eliminating the use of push as a replacement for Websockets.
EDIT (2020)
I'm not sure why people started downvoting the answer. If anything, the years since the answer was initially posted proved that HTTP/2 can't replace WebSockets and wasn't designed to do so.
Granted, HTTP/2 might be used to tunnel WebSocket connections, but these tunneled connections will still require the WebSocket protocol and they will effect the way the HTTP/2 container behaves.
The answer is no. The goal between the two are very different. There is even an RFC for WebSocket over HTTP/2 which allows you to make multiple WebSocket connections over a single HTTP/2 TCP pipe.
WS over HTTP/2 will be a resource conservation play by decreasing the time to open new connections and allowing for more communication channels without the added expense of more sockets, soft IRQs, and buffers.
https://datatracker.ietf.org/doc/html/draft-hirano-httpbis-websocket-over-http2-01
Well, to quote from this InfoQ article:
Well, the answer is clearly no, for a simple reason: As we have seen above, HTTP/2 introduces Server Push which enables the server to proactively send resources to the client cache. It does not, however, allow for pushing data down to the client application itself. Server pushes are only processed by the browser and do not pop up to the application code, meaning there is no API for the application to get notifications for those events.
And so HTTP2 push is really something between your browser and server, while Websockets really expose the APIs that can be used by both client (javascript, if its running on browser) and application code (running on server) for transferring real time data.
As of today, no.
HTTP/2, compared to HTTP, allows you to maintain a connection with a server. From there, you can have multiple streams of data at the same time. The intent is that you can push multiple things at the same time even without the client requesting it. For example, when a browser asks for a index.html, the server might want to also push index.css and index.js. The browser didn't ask for it, but the server might provide it without being asked because it can assume you're going to want in a few seconds.
This is faster than the HTTP/1 alternative of getting index.html, parsing it, discovering it needs index.js and index.css and then building 2 other requests for those files. HTTP/2 lets the server push data the client hasn't even asked for.
In that context, it's similar to WebSocket, but not really by design. WebSocket is supposed to allow a bi-directional communication similar to a TCP connection, or a serial connection. It's a socket where both communicate with each other. Also, the major difference is that you can send any arbitrary data packets in raw bytes, not encapsulated in HTTP protocol. The concepts of headers, paths, query strings only happen during the handshake, but WebSocket opens up a data stream.
The other difference is you get a lot more fine-tuned access to WebSocket in Javascript, whereas with HTTP, it's handled by the browser. All you get with HTTP is whatever you can fit in XHR/fetch(). That also means the browser will get to intercept and modify HTTP headers without you being able to control it (eg: Origin, Cookies, etc). Also, what HTTP/2 is able to push is sent to the browser. That means JS doesn't always (if ever) know things are being pushed. Again, it makes sense for index.css and index.js because the browser will cache it, but not so much for data packets.
It's really all in the name. HTTP stands for HyperText Transfer Protocol. We're geared around the concept of transferring assets. WebSocket is about building a socket connection where binary data gets passed around bidirectionally.
The one we're not really discussing is SSE (Server-Sent Events). Pushing data to the application (JS) isn't HTTP/2's intent, but it is for SSE. SSE gets really strengthened with HTTP/2. But it's a not a real replacement for WebSockets when what's important is the data itself, not the variable endpoints being reached. For each endpoint in with WebSocket a new data stream is created, but with SSE it's shared between the already existing HTTP/2 session.
Summarized here are the objectives for each:
HTTP - Respond to a request with one asset
HTTP/2 - Respond to a request with multiple assets
SSE - Respond with a unidirectional text (UTF-8) event stream
WebSocket - Create a bidirectional binary data stream
Message exchange and simple streaming(not audio, video streaming) can be done via both Http/2 multiplexing and WebSockets. So there is some overlap, but WebSockets have well established protocol, a lot of frameworks/APIs and less headers overhead.
Here is nice article about the topic.
No, WebSockets are not obsolete. However, HTTP/2 breaks websockets as defined for HTTP/1.1 (mostly by forbidding protocol updates using the Upgrade header). Which is why this rfc:
https://datatracker.ietf.org/doc/html/rfc8441
defines a websocket bootstrapping procedure for HTTP/2.
For the time being April 2020, HTTP/2 is not making WebSockets obsolete. The greatest advantage of WebSockets over HTTP2 is that
HTTP/2 works only on Browser Level not Application Level
Means that HTTP/2 does not offer any JS API like WebSockets to allow communication and transfer some kind of JSON or other data to server directly from Application (e.g. Website). So, as far as I believe, HTTP/2 will only make WebSockets obsolete if it starts offering API like WebSockets to talk to server. Till that it is just updated and faster version of HTTP 1.1.
No HTTP/2 does not make websockets obsolete, but SSE over HTTP/2 offers a viable alternative. The minor caveat is that SSE does not support unsolicited events from server to client (and neither does HTTP/2): i.e. the client has to explicitly subscribe by creating an EventSource instance specifying the event source endpoint. So you may have to slightly reorganise how the client arranges for events to be delivered - I can't think of a scenario where this is actually a technical barrier.
SSE works with HTTP/1.1. But HTTP/2 makes using SSE generally viable and competitive with websockets in terms of efficiency, instead of practically unusable in the case of HTTP/1.1. Firstly, HTTP/2 multiplexes many event source connections (or rather "streams" in HTTP/2 terms) onto a single TCP connection where as in HTTP/1.1 you'd need one connection for each. According to the HTTP/2 spec, millions of streams can be created per connection by default with the recommended (configurable) minimum being 100, where as browsers maybe severly limited in the number of TCP connections they can make. Second reason is efficiency: many streams in HTTP/2 is requires much less overhead than the many connections required in HTTP/1.1.
One final thing is, if you want to replace websockets with SSE your forgoing some of the tools / middlewares built on top of websockets. In particular I'm thinking of socket.io (which is how a lot of people actually use websockets), but I'm sure there is a ton more.