I want to implement push notifications to my client application working on windows mobile with compact framework 3.5.
For accessing remote data it uses servicestack/protobuf on the server (self hosted in my server application) and standard httpwebrequests (servicestack is not supported on CF) with protobuf on the clients.
My idea is to use httprequests with long timeouts, where connection is kept open and the notification sent as answer when available.
Considering less than 300 operating clients, my question, before starting to code this (and eventually fail miserably) is: is it acceptable for a servicestack selfhosted server to keep 300 connections continuously open? plus the "normal" opening/closing calls.
Thanks!
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I am developing a cloud-based back-end HTTP service that will be exposed for integration with some on-prem systems. Client systems are custom-made by external vendors, they are back-end systems with their own databases. These systems are deployed in companies of our clients, we don't have access to them and don't control them. We are providing vendors our API specifications and they implement client code.
The data format which my service exchanges with clients is based on XML and follows a certain standard. Vendors implement their client systems in different programming languages and new vendors will appear over time. I want as many of clients to be able to work with my service as possible.
Most of my service API is REST-like: it receives HTTP requests, processes them, and sends back HTTP responses.
Additionally, my service accumulates some data state changes and needs to regularly push this data to client systems. Because of the below limitations, this use-case does not seem to fit the traditional client-server HTTP request-response model.
Due to the nature of the business, the client systems cannot afford to have their own HTTP API endpoints open and so my service can't establish an outbound HTTP connection to them for delivering data state notifications. I.e. use of WebHooks is not an option.
At the same time my service stakeholders need recorded acknowledgment that data state notifications were accepted by the client system, therefore fire-and-forget systems like Amazon SNS don't seem to apply.
I was considering few approaches to this problem but I'm not sure if I'm missing some simple options or some technologies that already address the problem. Hence this question.
The question text updated: options moved to my own answer.
Related questions and resources
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https://www.smashingmagazine.com/2018/02/sse-websockets-data-flow-http2/
I eventually found answers to my question myself and with some help from my team. For people like me who come here with a question "how do I arrange notifications delivery from my service to its clients" here's an overview of available options.
WebHooks
This is when the client opens endpoint iself. The service calls client's endpoints whenever the service has some notification to deliver. This way the client also acts as a service and so the client and the service swap roles during notification delivery.
With WebHooks the client must be able to open the endpoint with a well-known address. This is complicated if the client's software is working behind NAT or firewall or if the client is Browser or a mobile application.
The service needs to be prepared that client's WebHook endpoints may not always be online and may not always be healthy.
Another issue is flow control: special measures should be taken in the service not to overwhelm the client with high volume of connections, requests and/or data.
Polling
In this case the client is still the client and the service is still the service, unlike WebHooks. The service offers an endpoint where the client can continuously request new notifications. The advantage of this option is that it does not change connection direction and request-response direction and so it works well with HTTP-based services.
The caveat is that polling API should have some rich semantics to be reasonably reliable if loss of notifications is not acceptable. Good examples could be Google Pub/Sub pull and Amazon SQS.
Here are few considerations:
Receiving and deleting notification should be separate operations. Otherwise, if the service deletes notification just before giving it to the client and the client fails to process the notification, the notification will be lost forever. When deletion operation is separate from receiving, the client is forced to do deletion explicitly which normally happens after successful processing.
In case the client received the notification and has not yet deleted it, it might be undesirable to let the same notification to be processed by some other actor (perhaps a concurrent process of the same client). Therefore the notification must be hidden from receiving after it was first received.
In case the client failed to delete the notification in reasonable time because of error, network loss or process crash, the service has to make notification visible for receiving again. This is retry mechanism which allows the notification to be ultimately processed.
In case the service has no notifications to deliver, it should block the client's call for some time by not delivering empty response immediately. Otherwise, if the client polls in a loop and response comes immediately, the loop iteration will be short and clients will make excessive requests to the service increasing network, parsing load and requests counts. A nice-to have feature is for the service to unblock and respond to the client as soon as some notification appears for delivery. This is sometimes called "long polling".
HTTP Server-sent Events
With HTTP Server-sent Events the client opens HTTP connection and sends a request to the service, then the service can send multiple events (notifications) instead of a single response. The connection is long-living and the service can send events as soon as they are ready.
The downside is that the communication is one-way, the client has no way to inform the service if it successfully processed the event. Because this feedback is absent, it may be difficult for the service to control the rate of events to prevent overwhelming the client.
WebSockets
WebSockets were created to enable arbitrary two-way communication and so this is viable option for the service to send notifications to the client. The client can also send processing confirmation back to the service.
WebSockets have been around for a while and should be supported by many frameworks and languages. WebSocket connection begins as HTTP 1.1 connection and so WebSockets over HTTPS should be supported by many load balancers and reverse proxies.
WebSockets are often used with browsers and mobile clients and more rarely in service-to-service communication.
gRPC
gRPC is similar to WebSockets in a way that it enables arbitrary two-way communication. The advantage of gRPC is that it is centered around protocol and message format definition files. These files are used for code generation that is essential for client and service developers.
gRPC is used for service-to-service communication plus it is supported for Browser clients with grpc-web.
gRPC is supported on multiple popular programming languages and platforms, yet the support is narrower than for HTTP.
gRPC works on top of HTTP/2 which might cause difficulties with reverse proxies and load balancers around things like TLS termination.
Message queue (PubSub)
Finally, the service and the client can use a message queue as a delivery mechanism for notifications. The service puts notifications on the queue and the client receives them from the queue. A queue can be provided by one of many systems like RabbitMQ, Kafka, Celery, Google PubSub, Amazon SQS, etc. There's a wide choice of queuing systems with different properties and choosing one is a challenge on its own. The queue can also be emulated by using database for example.
It has to be decided between the service and the client who owns the queue, i.e. who pays for it. Either way, the queuing system and the queue should be available whenever the service needs to push notifications to it otherwise notifications will be lost (unless the service buffers them internally, with another queue).
Queues are typically used for service-to-service communication but some technologies also allow Browsers as clients.
It is worth noting that an "implicit" internal queue might be used on the service side in other options listed above. One reason is to prevent loss of notifications when there's no client available to receive them. There are many other good reasons like letting clients handle notifications at their pace, allowing to maximize processing throughput, allowing to handle spiky traffic with fixed capacity.
In this option the queue is used "explicitly" as delivery mechanism, i.e. the service does not put any other mechanism (HTTP, gRPC or WebSocket endpoint) in front of the queue and lets the client receive notifications from the queue directly.
Message passing is popular in organizing microservice communications.
Common considerations
In all options it has to be decided whether the loss of notifications is tolerable for the service, the client and the business. Some simpler technical choices are possible if it is ok to lose notifications due to processing errors, unavailability, etc.
It is valuable to have a monitoring for client processing errors from the service side. This way service owners know which clients are more broken without having to ask them.
If the queue is used (implicitly or explicitly) it is valuable to monitor the length of the queue and the age of the oldest notifications. It lets service owners judge how stale data may be in the client.
In case the delivery of notification is organized in a way that notification gets deleted only after a successful processing by the client, the same notification could be stuck in infinite receive loop when the client fails to process it. Such notification is sometimes called "poison message". Poison messages should be removed by the service or the queuing system to prevent clients being stuck in infinite loop. A common practice is to move poison messages to a special place, sometimes called "dead letter queue", for the later human intervention.
One alternative to WebSockets for the problem of server→client notifications with acks from the client seems to be gRPC.
It supports bidirectional communication between server and client in bidirectional streaming mode.
It works on top of HTTP 2.0. In our case functioning over HTTP ports is essential.
There are client and server generators for multiple popular languages and platforms. A nice thing is that I can share protocol definition file with vendors and can be sure my service and their clients will talk the same language.
Drawbacks:
Not as many languages and platforms are supported compared to HTTP. Alternative C from the question will be more accessible if based on HTTP 1.1. WebSockets have also been around longer and I would expect broader adoption than gRPC.
Not all gRPC implementations seem to currently support XML format for data according to FAQ. In order to transport XML my service and its clients will have to transfer XML message as byte arrays inside of gRPC protobuf message.
With gRPC, TLS termination cannot be done on general-purpose HTTP 1.1 load balancer. An application-layer HTTP/2-aware reverse proxy (load balancer) such as Traefik is required.
There are approaches like this and this to allow HTTP 1.1 compatible protocols but they have their own restrictions like limited amount of available clients or necessary client customizations.
For a while now I have been implementing a RESTful API in the design of my project because in my case it is very useful for others to be able to interact with the data in a consistent format (and I find REST to be a clean way of handling requests). I am now trying to not only have my current REST API for my resources, but the ability to expose some pieces of information via a bidirectional websocket connection.
Upon searching for a good .net library to use that implements the websocket protocol, I did find out about SignalR. There was a few problems I had with it (maybe specific to my project?)
I want to be able to initialize a web socket connection through my
existing REST API. (I don't know the proper practice to do this, but
I figured a custom header would work fine) I would like them (the
client) to be able to close the connection and get a http response
back (101?) to signify its completion.
The problem I had with SignalR was:
that there was no clean way outside of a hub instance to get a user's connection id and map it to a external controller where the rest call made affects what piece of data gets broadcasted to the specific client (I don't want to use external memory)
the huge reliance on client side code. I really want to make this process as simple to the client and handle the majority of the work on the server side (which I had hoped modifying my current rest api would accomplish). The only responsibility I see of a client is to disconnect peacefully.
So now the question..
Is there a good server side websocket library for .net that implements the latest web socket protocol? The client can use any client library that adheres to the protocol. What is the best practice to incorporate both web socket connections and a restful api?
ASP.NET supports WebSockets itself if you have IIS8 (only Windows 8/2012 and further). SignalR is just a polyfill,
If you do not have IIS8, you can use external WebSocket frameworks like mine: http://vtortola.github.io/WebSocketListener/
Cheers.
We have a requirement to to support 10k+ users, where every user initiate a request and waits for a response from the server (the response can take as long as 20-30 seconds to arrive). it is only one request from the client, and after a long processing by the server, a response will be transmitted and then the connection will disconnect.
in the background, the server will do some DB search and wait for other background processes to notify on completion before responding to the client.
after doing some research i figured out we will need to use something like the atmosphere framework to support websockets/sse event/long polling along with an asynchronous server like netty (=> nettosphere) or jetty.
As for my experience - mostly Java EE world and Tomcat server.
my questions are:
what will be easier to implement in regard to my experience and our requirement: atmosphere + netty or atmoshphere+jetty? which one can scale better, has an easier learning curve and easier to implement other java technologies?
how do u implement in atmosphere a response that is sent only to the originating client and not broadcast to the rest of the clients? (all the examples i found are broadcast).
how can i implement in netty (or jetty) when using the atmosphere framework our response? i.e., the client send a request, after it is received in the server some background processes are run, and when they finish i need to locate the connection and transmit the response. is that achievable?
Some thoughts:
At 10k+ users, with 20-30 second response latency, you likely hit file descriptor limits if using just 1 network interface. Consider a solution that uses multiple network interfaces.
Your description of your request/response can be handled entirely with standard Servlet 3.0, standard HTTP/1.1, Async request handling, and large timeouts.
If your clients are web browsers, and you don't start sending a response from the server until the 20-30 second window, you might hit browser idle timeouts.
Atmosphere and Cometd do the same things, supporting long duration connections, with connection technique fallbacks, and with logical channel APIs.
I believe the AKKA framework will handle this sort of need. I am looking at using it to handle scaling issues possibly with a RabbitMQ to help off load work to potentially other servers that may be added later to scale as needed.
I am implementing a comet using AsyncHttpHandlers in my current asp.net application. According to my implementation client initially sends Notification Hook request to server (with its user id) on AsyncHttpHandler, and on server side I maintain a Global (Application level) dictionary of userid(key) and IAynsResult (value). So when ever a request is received to send notification to a user I just pick the matching IAsyncResult from the Global Dictionary and send response to the client user.
My concern is, is maintaing a Dictionary of Userid and IAsyncResult at Application level a good design? I feel it will put a lot of load on the server, at the time of high traffic.
Is there any other way I can achieve the comet. or what will be the good design to achieve comet for high traffic scenarios.
That depends on the number of requests to the server, IAsyncResult request utilizes the process's ThreadPool which automatically manages the number of worker threads. These threads are assigned a task, run them to completion, then are returned to the ThreadPool for reuse.
The ThreadPool is used by other aspects of .NET, and provides a limited number of threads. If you overuse it, there is the possibility your tasks will be blocked waiting for others to complete!
So basically, running a comet server on an ASP.net needs either a strong hardware, or application distribution over several servers.
I would recommend HTML5 WebSocket, which is the W3C API for implementing sockets in HTML and is easier to setup on an ASP.net server:
SignalR for .net 4
ASP.net WebSocket API for .net 4.5
I'm pretty sure this isn't possible with HTTP 1.1 or webservices, but just want to double check with you guys (and thus will probably be switching this application to WCF).
I want to send a message from the server an asp.net webservice is running on, to the client consuming it. Is this possible without polling (IE an interrupt based model)?
In WCF, you do have a thing called duplex bindings, which allows the server to call back into the client at a specific address.
See the MSDN documentation on duplex channels for a first impression of what those are.
I don't think you can do this with ASMX.
Marc