This is a pretty subjective post. I'm sharing my perspective, taking into account years of experience building backend and frontend with user experience in mind.
If you do not want to read this article, then watch the recording of the live stream about the same:
Everything we hear is an opinion, not a fact. Everything we see is a perspective, not the truth. ― Marcus Aurelius
This blog post is the first of a series of blog posts about WebSocket I'm working on.
What is WebSocket
It is a pretty old protocol used for duplex communication over TCP connection. It was standardized in 2011. Yes, ten years ago means it is old, super old.
So why do I even mention it in 2021?
It is very widely adopted and will not go away anytime soon because tooling support is excellent and serves its purpose well. Just remind yourself when HTTP/2 showed up and how many years it took everyone to migrate. It would not happen without the strong support and push from all the big players.
Sure, there is HTTP/2 multiplexing and protocols like Mercure or GraphQL Subscription. There is also RFC8441 for WebSocket and HTTP/2 and some tools already adopted it, like Envoy or Jetty. Nevertheless, WebSocket is here to stay.
Anyway, the future of WebSocket has nothing to do with this post. This post is for the AsyncAPI community looking into the AsyncAPI spec because of WebSockets now, no matter the protocol's future.
Websocket use case
- Do you like to see in Slack that someone is typing a response?
- Do you like it when a user interface updates without page refresh?
- Do you like it when your client app knows there are updates available for display?
That is what WebSocket is for. You establish a long-living connection between client and server. Through such a connection, the client can send a stream of messages to the server, and this is possible the other way around at the same time.
One could say: I don't need WebSocket to achieve that. I could just set up a data polling with REST API. Just ask the API every few seconds if there are updates.
Sadly this is not a joke. Engineers do it. Some engineers just take shortcuts, mostly because deadlines hunt them down.
HTTP polling was presented very well in Shrek's famous Are we there yet? scene.
Don't go that path. Do not perform unnecessary connections to your servers and create more and more traffic with more and more resource consumption. Wasting resources is bad and makes Shrek angry. WebSocket changes a lot there:
Figure 1: HTTP Pull vs WebSocket vs Shrek.
Why AsyncAPI
When building a WebSocket API on a server, you might have some additional needs:
- Want to document the API for the team that writes a client app, Web UI, Desktop app, or Mobile app.
- Want to have a way to specify the format of the messages that the server supports to validate them in the runtime.
- Want to generate a server or/and a client? If not for final production use, then for sure for prototyping and testing.
WebSocket described with AsyncAPI
When I google for some public WebSocket API to play with, I find mostly currency trading products:
I'm sorry if you expected me to describe Shrek's API interface using AsyncAPI. It would be fun, but only fun, and I'd also like to teach you something.
I will write an AsyncAPI document for Kraken API after playing with the API and basing it on the current documentation.
Playing with WebSocket API
The best way to play with a WebSocket API is through a CLI. Who didn't hear about curl in the REST API world? For WebSocket, I would recommend websocat. Kraken's API is partially public without authorization which is just great because to play with it, you do not have to set up an account to get an authorization token.
- Install websocat. For other installation options, check out this list.
brew install websocat - Establish connection with the API:
websocat wss://ws.kraken.com - Ping the API to see if it responds. Just type the below message and hit Enter:
{"event": "ping"} - Now subscribe to the event ticker stream that sends messages with currency price. Just type the below message and hit Enter:
{ "event": "subscribe", "pair": [ "XBT/USD", "XBT/EUR" ], "subscription": { "name": "ticker" }} - You should now see a constant stream of data sent by the server. You do not have to ask the API every second for an update, as the update is pushed to you.
1{"event":"heartbeat"} 2[340,{"a":["45520.10000",6,"6.78103490"],"b":["45520.00000",0,"0.00185230"],"c":["45520.10000","0.01643250"],"v":["1397.95434819","5589.12101024"],"p":["44883.49461","44062.07654"],"t":[14350,66782],"l":["43607.60000","42770.80000"],"h":["45811.10000","45811.10000"],"o":["43659.30000","44709.10000"]},"ticker","XBT/EUR"] 3[340,{"a":["45520.10000",5,"5.84803490"],"b":["45492.50000",0,"0.09374582"],"c":["45492.50000","0.00625418"],"v":["1398.10526819","5589.26685876"],"p":["44883.56109","44062.11477"],"t":[14359,66790],"l":["43607.60000","42770.80000"],"h":["45811.10000","45811.10000"],"o":["43659.30000","44709.10000"]},"ticker","XBT/EUR"] 4{"event":"heartbeat"} 5[340,{"a":["45503.80000",1,"1.00000000"],"b":["45496.20000",0,"0.01426600"],"c":["45496.20000","0.00109400"],"v":["1398.10636219","5589.26295766"],"p":["44883.56157","44062.11447"],"t":[14360,66788],"l":["43607.60000","42770.80000"],"h":["45811.10000","45811.10000"],"o":["43659.30000","44709.90000"]},"ticker","XBT/EUR"] 6{"event":"heartbeat"}
Now you know how to interact with the Kraken API. Now let's try to describe it using AsyncAPI.
Describing API using AsyncAPI
I'll explain, in detail, how to describe Websocket API with AsyncAPI in another blog post that will be part of the series. Why? I don't want to make this post super lengthy and discourage others from reading it. Let us learn step by step.
For now, I will throw here a full AsyncAPI document I created for the Kraken API. You can also open it up in the AsyncAPI Studio and compare with their current documentation
Familiarize with below before you look at the AsyncAPI document:
- AsyncAPI describes the API interface between the client and the server. In other words, the AsyncAPI document is for the user of the API. It does not describe what the server does but what the user can do with the API.
- Kraken API is quite complex. It has some beta servers, some private messages, and messages closely related to vocabulary specific for currency trading. I dropped all of those from my research not to overcomplicate things. In other words, the AsyncAPI file that you can see below is not a complete document.
- Websocket protocol is very flexible, and therefore you can implement the server in many different ways. There is no standard way of doing things, like there is no common way of doing things with AsyncAPI. We can only make some generic assumptions looking at existing implementations:
- Your server has one entry point, just one endpoint that you communicate with to gain access to the API. It can be a path with some dynamic values, as some data id. It can also be nothing, no path at all, like in the case of below Kraken API. These entry points are channels in AsyncAPI document. Commonly, Websocket API has just one channel that user can send messages to and receive messages at the same time
- AsyncAPI publish and subscribe operations translates to messages user can send to the API and messages user will receive from the API. Depending on API complexity, sometimes you have an API that sends only one message. You can also have a situation where you can send to the server multiple different messages, and also receive different messages in response. This is when you need to use oneOf as I did in document for Kraken API.
- Current AsyncAPI limitation is that you cannot specify that once the user sends (publish) message ping, the pong message is a reply. Look at this thread to participate in an ongoing discussion about request/reply pattern support in AsyncAPI. In the below document, you will notice that for such a use case, I use AsyncAPI specification extensions (x-response).
Message to Kraken API developers and technical writers
In case you want to continue the work I started on the AsyncAPI document for Kraken API, feel free to do that. I'm happy to help, just let me know. Reach me out in our AsyncAPI Slack workspace.
1asyncapi: 2.0.0
2
3info:
4 title: Kraken Websockets API
5 version: '1.8.0'
6 description: |
7 WebSockets API offers real-time market data updates. WebSockets is a bidirectional protocol offering fastest real-time data, helping you build real-time applications. The public message types presented below do not require authentication. Private-data messages can be subscribed on a separate authenticated endpoint.
8
9 ### General Considerations
10
11 - TLS with SNI (Server Name Indication) is required in order to establish a Kraken WebSockets API connection. See Cloudflare's [What is SNI?](https://www.cloudflare.com/learning/ssl/what-is-sni/) guide for more details.
12 - All messages sent and received via WebSockets are encoded in JSON format
13 - All decimal fields (including timestamps) are quoted to preserve precision.
14 - Timestamps should not be considered unique and not be considered as aliases for transaction IDs. Also, the granularity of timestamps is not representative of transaction rates.
15 - At least one private message should be subscribed to keep the authenticated client connection open.
16 - Please use REST API endpoint [AssetPairs](https://www.kraken.com/features/api#get-tradable-pairs) to fetch the list of pairs which can be subscribed via WebSockets API. For example, field 'wsname' gives the supported pairs name which can be used to subscribe.
17 - Cloudflare imposes a connection/re-connection rate limit (per IP address) of approximately 150 attempts per rolling 10 minutes. If this is exceeded, the IP is banned for 10 minutes.
18 - Recommended reconnection behaviour is to (1) attempt reconnection instantly up to a handful of times if the websocket is dropped randomly during normal operation but (2) after maintenance or extended downtime, attempt to reconnect no more quickly than once every 5 seconds. There is no advantage to reconnecting more rapidly after maintenance during cancel_only mode.
19
20servers:
21 public:
22 url: ws.kraken.com
23 protocol: wss
24 description: |
25 Public server available without authorization.
26 Once the socket is open you can subscribe to a public channel by sending a subscribe request message.
27 private:
28 url: ws-auth.kraken.com
29 protocol: wss
30 description: |
31 Private server that requires authorization.
32 Once the socket is open you can subscribe to private-data channels by sending an authenticated subscribe request message.
33
34 The API client must request an authentication "token" via the following REST API endpoint "GetWebSocketsToken" to connect to WebSockets Private endpoints. For more details read https://support.kraken.com/hc/en-us/articles/360034437672-How-to-retrieve-a-WebSocket-authentication-token-Example-code-in-Python-3
35
36 The resulting token must be provided in the "token" field of any new private WebSocket feed subscription:
37 ```
38 {
39 "event": "subscribe",
40 "subscription":
41 {
42 "name": "ownTrades",
43 "token": "WW91ciBhdXRoZW50aWNhdGlvbiB0b2tlbiBnb2VzIGhlcmUu"
44 }
45 }
46 ```
47
48channels:
49 /:
50 publish:
51 description: Send messages to the API
52 operationId: processReceivedMessage
53 message:
54 oneOf:
55 - $ref: '#/components/messages/ping'
56 - $ref: '#/components/messages/subscribe'
57 - $ref: '#/components/messages/unsubscribe'
58
59 subscribe:
60 description: Messages that you receive from the API
61 operationId: sendMessage
62 message:
63 oneOf:
64 - $ref: '#/components/messages/pong'
65 - $ref: '#/components/messages/heartbeat'
66 - $ref: '#/components/messages/systemStatus'
67 - $ref: '#/components/messages/subscriptionStatus'
68
69components:
70 messages:
71 ping:
72 summary: Ping server to determine whether connection is alive
73 description: Client can ping server to determine whether connection is alive, server responds with pong. This is an application level ping as opposed to default ping in websockets standard which is server initiated
74 payload:
75 $ref: '#/components/schemas/ping'
76 x-response:
77 $ref: '#/components/messages/pong'
78 heartbeat:
79 description: Server heartbeat sent if no subscription traffic within 1 second (approximately)
80 payload:
81 $ref: '#/components/schemas/heartbeat'
82 pong:
83 summary: Pong is a response to ping message
84 description: Server pong response to a ping to determine whether connection is alive. This is an application level pong as opposed to default pong in websockets standard which is sent by client in response to a ping
85 payload:
86 $ref: '#/components/schemas/pong'
87 systemStatus:
88 description: Status sent on connection or system status changes.
89 payload:
90 $ref: '#/components/schemas/systemStatus'
91 examples:
92 - payload:
93 connectionID: 8628615390848610000
94 event: systemStatus
95 status: online
96 version: 1.0.0
97 subscribe:
98 description: Subscribe to a topic on a single or multiple currency pairs.
99 payload:
100 $ref: '#/components/schemas/subscribe'
101 examples:
102 - payload:
103 event: subscribe
104 pair:
105 - XBT/USD
106 - XBT/EUR
107 subscription:
108 name: ticker
109 - payload:
110 event: subscribe
111 subscription:
112 name: ownTrades
113 token: WW91ciBhdXRoZW50aWNhdGlvbiB0b2tlbiBnb2VzIGhlcmUu
114 x-response:
115 $ref: '#/components/messages/subscriptionStatus'
116 unsubscribe:
117 description: Unsubscribe, can specify a channelID or multiple currency pairs.
118 payload:
119 $ref: '#/components/schemas/subscribe'
120 examples:
121 - payload:
122 event: unsubscribe
123 pair:
124 - XBT/EUR
125 - XBT/USD
126 subscription:
127 name: ticker
128 - payload:
129 event: unsubscribe
130 subscription:
131 name: ownTrades
132 token: WW91ciBhdXRoZW50aWNhdGlvbiB0b2tlbiBnb2VzIGhlcmUu
133 x-response:
134 $ref: '#/components/messages/subscriptionStatus'
135 subscriptionStatus:
136 description: Subscription status response to subscribe, unsubscribe or exchange initiated unsubscribe.
137 payload:
138 $ref: '#/components/schemas/subscriptionStatus'
139 examples:
140 - payload:
141 channelID: 10001
142 channelName: ohlc-5
143 event: subscriptionStatus
144 pair: XBT/EUR
145 reqid: 42
146 status: unsubscribed
147 subscription:
148 interval: 5
149 name: ohlc
150 - payload:
151 errorMessage: Subscription depth not supported
152 event: subscriptionStatus
153 pair: XBT/USD
154 status: error
155 subscription:
156 depth: 42
157 name: book
158
159 schemas:
160 ping:
161 type: object
162 properties:
163 event:
164 type: string
165 const: ping
166 reqid:
167 $ref: '#/components/schemas/reqid'
168 required:
169 - event
170 heartbeat:
171 type: object
172 properties:
173 event:
174 type: string
175 const: heartbeat
176 pong:
177 type: object
178 properties:
179 event:
180 type: string
181 const: pong
182 reqid:
183 $ref: '#/components/schemas/reqid'
184 systemStatus:
185 type: object
186 properties:
187 event:
188 type: string
189 const: systemStatus
190 connectionID:
191 type: integer
192 description: The ID of the connection
193 status:
194 $ref: '#/components/schemas/status'
195 version:
196 type: string
197 status:
198 type: string
199 enum:
200 - online
201 - maintenance
202 - cancel_only
203 - limit_only
204 - post_only
205 subscribe:
206 type: object
207 properties:
208 event:
209 type: string
210 const: subscribe
211 reqid:
212 $ref: '#/components/schemas/reqid'
213 pair:
214 $ref: '#/components/schemas/pair'
215 subscription:
216 type: object
217 properties:
218 depth:
219 $ref: '#/components/schemas/depth'
220 interval:
221 $ref: '#/components/schemas/interval'
222 name:
223 $ref: '#/components/schemas/name'
224 ratecounter:
225 $ref: '#/components/schemas/ratecounter'
226 snapshot:
227 $ref: '#/components/schemas/snapshot'
228 token:
229 $ref: '#/components/schemas/token'
230 required:
231 - name
232 required:
233 - event
234 unsubscribe:
235 type: object
236 properties:
237 event:
238 type: string
239 const: unsubscribe
240 reqid:
241 $ref: '#/components/schemas/reqid'
242 pair:
243 $ref: '#/components/schemas/pair'
244 subscription:
245 type: object
246 properties:
247 depth:
248 $ref: '#/components/schemas/depth'
249 interval:
250 $ref: '#/components/schemas/interval'
251 name:
252 $ref: '#/components/schemas/name'
253 token:
254 $ref: '#/components/schemas/token'
255 required:
256 - name
257 required:
258 - event
259 subscriptionStatus:
260 type: object
261 oneOf:
262 - $ref: '#/components/schemas/subscriptionStatusError'
263 - $ref: '#/components/schemas/subscriptionStatusSuccess'
264 subscriptionStatusError:
265 allOf:
266 - properties:
267 errorMessage:
268 type: string
269 required:
270 - errorMessage
271 - $ref: '#/components/schemas/subscriptionStatusCommon'
272 subscriptionStatusSuccess:
273 allOf:
274 - properties:
275 channelID:
276 type: integer
277 description: ChannelID on successful subscription, applicable to public messages only.
278 channelName:
279 type: string
280 description: Channel Name on successful subscription. For payloads 'ohlc' and 'book', respective interval or depth will be added as suffix.
281 required:
282 - channelID
283 - channelName
284 - $ref: '#/components/schemas/subscriptionStatusCommon'
285 subscriptionStatusCommon:
286 type: object
287 required:
288 - event
289 properties:
290 event:
291 type: string
292 const: subscriptionStatus
293 reqid:
294 $ref: '#/components/schemas/reqid'
295 pair:
296 $ref: '#/components/schemas/pair'
297 status:
298 $ref: '#/components/schemas/status'
299 subscription:
300 required:
301 - name
302 type: object
303 properties:
304 depth:
305 $ref: '#/components/schemas/depth'
306 interval:
307 $ref: '#/components/schemas/interval'
308 maxratecount:
309 $ref: '#/components/schemas/maxratecount'
310 name:
311 $ref: '#/components/schemas/name'
312 token:
313 $ref: '#/components/schemas/token'
314 interval:
315 type: integer
316 description: Time interval associated with ohlc subscription in minutes.
317 default: 1
318 enum:
319 - 1
320 - 5
321 - 15
322 - 30
323 - 60
324 - 240
325 - 1440
326 - 10080
327 - 21600
328 name:
329 type: string
330 description: The name of the channel you subscribe too.
331 enum:
332 - book
333 - ohlc
334 - openOrders
335 - ownTrades
336 - spread
337 - ticker
338 - trade
339 token:
340 type: string
341 description: base64-encoded authentication token for private-data endpoints.
342 depth:
343 type: integer
344 default: 10
345 enum:
346 - 10
347 - 25
348 - 100
349 - 500
350 - 1000
351 description: Depth associated with book subscription in number of levels each side.
352 maxratecount:
353 type: integer
354 description: Max rate-limit budget. Compare to the ratecounter field in the openOrders updates to check whether you are approaching the rate limit.
355 ratecounter:
356 type: boolean
357 default: false
358 description: Whether to send rate-limit counter in updates (supported only for openOrders subscriptions)
359 snapshot:
360 type: boolean
361 default: true
362 description: Whether to send historical feed data snapshot upon subscription (supported only for ownTrades subscriptions)
363 reqid:
364 type: integer
365 description: client originated ID reflected in response message.
366 pair:
367 type: array
368 description: Array of currency pairs.
369 items:
370 type: string
371 description: Format of each pair is "A/B", where A and B are ISO 4217-A3 for standardized assets and popular unique symbol if not standardized.
372 pattern: '[A-Z\s]+\/[A-Z\s]+'Personal note
If you can, if you are in a planning phase, new project, etc., then start designing your architecture with AsyncAPI. Don't do the mistake of coding first and then trying to figure out how to describe it with AsyncAPI 😅
I recommend you also read another article from the series about WebSocket: Creating AsyncAPI for WebSocket API - Step by Step.