How to combine websockets and http to create a RES

问题:

I am thinking about buildning a REST API with both websockets and http where I use websockets to tell the client that new data is available or provide the new data to the client directly.

Here are some different ideas of how it could work:
ws = websocket

Idea A:

1. David get all users with GET /users
2. Jacob add a user with POST /users
3. A ws message is sent to all clients with info that a new user exist
4. David recive a message by ws and calls GET /users

Idea B:

1. David get all users with GET /users
2. David register to get ws updates when a change is done to /users
3. Jacob add a user with POST /users
4. The new user is sent to David by ws

Idea C:

1. David get all users with GET /users
2. David register to get ws updates when a change is done to /users
3. Jacob add a user with POST /users and it gets the id 4
4. David receive the id 4 of the new user by ws
5. David get the new user with GET /users/4

Idea D:

1. David get all users with GET /users
2. David register to get ws updates when changes is done to /users.
3. Jacob add a user with POST /users
4. David receive a ws message that changes is done to /users
5. David get only the delta by calling GET /users?lastcall='time of step one'

Which alternative is the best and what are the pros and cons?
Is it another better 'Idea E'?
Do we even need to use REST or is ws enought for all data?

Edit
To solve problems with data getting out of sync we could provide the header
"If-Unmodified-Since"
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Unmodified-Since
or "E-Tag"
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag
or both with PUT requests.

回答1:

Idea B is for me the best, because the client specifically subscribes for changes in a resource, and gets the incremental updates from that moment.

Do we even need to use REST or is ws enought for all data?

Please check: WebSocket/REST: Client connections?

回答2:

I don't know Java, but I worked with both Ruby and C on these designs...

Funny enough, I think the easiest solution is to use JSON, where the REST API simply adds the method data (i.e. method: "POST") to the JSON and forwards the request to the same handler the Websocket uses.

The underlying API's response (the response from the API handling JSON requests) can be translated to any format you need, such as HTML rendering... though I would consider simply returning JSON for most use cases.

This helps encapsulate the code and keep it DRY while accessing the same API using both REST and Websockets.

As you might infer, this design makes testing easier, since the underlying API that handles the JSON can be tested locally without the need to emulate a server.

Good Luck!

P.S. (Pub/Sub)

As for the Pub/Sub, I find it best to have a "hook" for any update API calls (a callback) and a separate Pub/Sub module that handles these things.

I also find it more resource friendly to write the whole data to the Pub/Sub service (option B) instead of just a reference number (option C) or an "update available" message (options A and D).

In general, I also believe that sending the whole user list isn't effective for larger systems. Unless you have 10-15 users, the database call might be a bust. Consider the Amazon admin calling for a list of all users... Brrr....

Instead, I would consider dividing this to pages, say 10-50 users a page. These tables can be filled using multiple requests (Websocket / REST, doesn't matter) and easily updated using live Pub/Sub messages or reloaded if a connection was lost and reestablished.

EDIT (REST vs. Websockets)

As For REST vs. Websockets... I find the question of need is mostly a subset of the question "who's the client?"...

However, once the logic is separated from the transport layer, than supporting both is very easy and often it makes more sense to support both.

I should note that Websockets often have a slight edge when it comes to authentication (credentials are exchanged once per connection instead of once per request). I don't know if this is a concern.

For the same reason (as well as others), Websockets usually have an edge with regards to performance... how big an edge over REST depends on the REST transport layer (HTTP/1.1, HTTP/2, etc').

Usually these things are negligible when it comes time to offer a public API access point and I believe implementing both is probably the way to go for now.

回答3:

To summarize your ideas:

A: Send a message to all clients when a user edits data on the server. All users then request an update of all data.
-This system may make a lot of unnecessary server calls on behalf of clients who are not using the data. I don't recommend producing all of that extra traffic as processing and sending those updates could become costly.

B: After a user pulls data from the server, they then subscribe to updates from the server which sends them information about what has changed.
-This saves a lot of server traffic, but if you ever get out of sync, you're going to be posting incorrect data to your users.

C: Users who subscribe to data updates are sent information about which data has been updated, then fetch it again themselves.
-This is the worst of A and B in that you'll have extra round trips between your users and servers just to notify them that they need to make a request for information which may be out of sync.

D: Users who subscribe to updates are notified when any changes are made and then request the last change made to the server.
-This presents all of the problems with C, but includes the possibility that, once out of sync, you may send data that will be nonsense to your users which might just crash the client side app for all we know.

I think that this option E would be best:
Every time data changes on the server, send the contents of all the data to the clients who have subscribed to it. This limits the traffic between your users and the server while also giving them the least chance of having out of sync data. They might get stale data if their connection drops, but at least you wouldn't be sending them something like Delete entry 4 when you aren't sure whether or not they got the message that entry 5 just moved into slot 4.

Some Considerations:

• How often does the data get updated?
• How many users need to be updated each time an update occurs?
• What are your transmission costs? If you have users on mobile devices with slow connections, that will affect how often and how much you can afford to send to them.
• How much data gets updated in a given update?
• What happens if a user sees stale data?
• What happens if a user gets data out of sync?

Your worst case scenario would be something like this: Lots of users, with slow connections who are frequently updating large amounts of data that should never be stale and, if it gets out of sync, becomes misleading.

回答4:

Another option is to use Firebase Cloud Messaging:

Using FCM, you can notify a client app that new email or other data is available to sync.

How does it work?

An FCM implementation includes two main components for sending and receiving:

• A trusted environment such as Cloud Functions for Firebase or an app server on which to build, target and send messages.
• An iOS, Android, or Web (JavaScript) client app that receives messages.

Client registers its Firebase key to a server. When updates are available, server sends push notification to the Firebase key associated with the client. Client may receive data in notification structure or sync it with a server after receiving a notification.

回答5:

Generally you might have a look at current "realtime" web frameworks like MeteorJS which tackle exactly this problem.

Meteor in specific works more or less like your example D with subscriptions on certain data and deltas being sent out after changes only to the affected clients. Their protocol used is called DDP which additionally sends the deltas not as overhead prone HTML but raw data.

If websockets are not available fallbacks like long polling or server sent events can be used.

If you plan to implement it yourself i hope these sources are some kind of inspiration how this problem has been approached. As already stated the specific use case is important

回答6:

The answer depends on your use case. For the most part though I've found that you can implement everything you need with sockets. As long as you are only trying to access your server with clients who can support sockets. Also, scale can be an issue when you're using only sockets. Here are some examples of how you could use just sockets.

Server side:

socket.on('getUsers', () => {
    // Get users from db or data model (save as user_list).
    socket.emit('users', user_list );
})
socket.on('createUser', (user_info) => {
    // Create user in db or data model (save created user as user_data).
    io.sockets.emit('newUser', user_data);
})

Client side:

socket.on('newUser', () => {
    // Get users from db or data model (save as user_list).
    socket.emit('getUsers');
})
socket.on('users', (users) => {       
    // Do something with users
})

This uses socket.io for node. I'm not sure what your exact scenario is but this would work for that case. If you need to include REST endpoints that would be fine too.

回答7:

With all great information all the great people added before me.

I found that eventually there is no right or wrong, its simply goes down to what suits your needs:

lets take CRUD in this scenario:

WS Only Approach:

Create/Read/Update/Deleted information goes all through the websocket.    
--> e.g If you have critical performance considerations ,that is not 
acceptable that the web client will do successive REST request to fetch 
information,or if you know that you want the whole data to be seen in 
the client no matter what was the event ,  so just send the CRUD events 
AND DATA inside the websocket.

WS TO SEND EVENT INFO + REST TO CONSUME THE DATA ITSELF

Create/Read/Update/Deleted , Event information is sent in the Websocket,
giving the web client information that is necessary to send the proper 
REST request to fetch exactly the thing the CRUD that happend in server.

e.g. WS sends UsersListChangedEvent {"ListChangedTrigger: "ItemModified" , "IdOfItem":"XXXX#3232" , "UserExtrainformation":" Enough info to let the client decide if it relevant for it to fetch the changed data"}

I found that using WS [Only for using Event Data] and REST [To consume the data ]is better because:

[1] Separation between reading and writing model, Imagine you want to add some runtime information when your data is retrieved when its read from REST , that is now achieved because you are not mixing Write & Read models like in 1.

[2] Lets say other platform , not necessarily web client will consume this data. so you just change the Event trigger from WS to the new way, and use REST to consume the data.

[3] Client do not need to write 2 ways to read the new/modified data. usually there is also code that reads the data when the page loads , and not
through the websocket , this code now can be used twice , once when page loads , and second when WS triggered the specific event.

[4] Maybe the client do not want to fetch the new User because its showing currently only a view of old Data[E.g. users] , and new data changes is not in its interest to fetch ?

回答8:

I personally have used Idea B in production and am very satisfied with the results. We use http://www.axonframework.org/, so every change or creation of an entity is published as an event throughout the application. These events are then used to update several read models, which are basically simple Mysql tables backing one or more queries. I added some interceptors to the event processors that update these read models so that they publish the events they just processed after the data is committed to the DB.

Publishing of events is done through STOMP over web sockets. It is made very simple is you use Spring's Web Socket support (https://docs.spring.io/spring/docs/current/spring-framework-reference/html/websocket.html). This is how I wrote it:

@Override
protected void dispatch(Object serializedEvent, String topic, Class eventClass) {
    Map<String, Object> headers = new HashMap<>();
    headers.put("eventType", eventClass.getName());
    messagingTemplate.convertAndSend("/topic" + topic, serializedEvent, headers);
}

I wrote a little configurer that uses Springs bean factory API so that I can annotate my Axon event handlers like this:

@PublishToTopics({
    @PublishToTopic(value = "/salary-table/{agreementId}/{salaryTableId}", eventClass = SalaryTableChanged.class),
    @PublishToTopic(
            value = "/salary-table-replacement/{agreementId}/{activatedTable}/{deactivatedTable}",
            eventClass = ActiveSalaryTableReplaced.class
    )
})

Of course, that is just one way to do it. Connecting on the client side may look something like this:

var connectedClient = $.Deferred();

function initialize() {
    var basePath = ApplicationContext.cataDirectBaseUrl().replace(/^https/, 'wss');
    var accessToken = ApplicationContext.accessToken();
    var socket = new WebSocket(basePath + '/wss/query-events?access_token=' + accessToken);
    var stompClient = Stomp.over(socket);

    stompClient.connect({}, function () {
        connectedClient.resolve(stompClient);
    });
}


this.subscribe = function (topic, callBack) {
    connectedClient.then(function (stompClient) {
        stompClient.subscribe('/topic' + topic, function (frame) {
            callBack(frame.headers.eventType, JSON.parse(frame.body));
        });
    });
};

initialize();

回答9:

i prefer the A, it allows client the flexibility whether or not to update the existing data.

also with this method, implementation and access control becomes much more easier.

for example you can simply broadcast the userUpdated event to all users, this saves having a client list for do specific broadcasts and the Access Controls and Authentications applied for your REST Route wont have to change to reapplied again because the client is gonna make a GET request again.

Many things depends on the what kind of application you are making.