Without polling how does the Flex application receive push notices?
I assume Tweetdeck didn't do push notifications. I assumed it made API calls on a timer. When I have Tweetdeck and ServiceCapture up at the same time, tweetdeck sure does send a lot of traffic. I assume it is server pings looking for more data to retrieve.
That said, to do push notifications You need a socket server; so that the client is perpetually connected to the server. A few options for servers that support this are Adobe's LiveCycle, BlazeDS, GraniteDS, or WebORB.
I understand that GraniteDS uses a technique called long polling. BlazeDS uses the same approach. LiveCycle--and I believe WebORB--can use RTMP to do push notifications.
Related
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
REST API with active push notifications from server to client
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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.
This is an architectural question. I haven't implemented FCM yet, but as far as I understand someone needs to deploy an XMPP server in a real world scenario which provisions the inventory of the registered device tokens.
In my use case I'd like to just broadcast short messages about important update information, like "XY presenter's session at 15:00 got cancelled" and I'm not interested in the device tokens. My application is a Progressive Web App, so I would use FCM for Web.
The demos I saw so far showed a client receiving the device token, then that specific device token was picked up from the debug environment and used to send the demo message to the client - thus bypassing the need of a deployed stand-alone XMPP server, but just for demo purposes.
I'd want to avoid the use of an XMPP server, I'm not interested in dealing with the device tokens at all - if possible. Firebase's FCM/GCM server have them anyway. My plan is to pick a single topic name for that channel (the only topic what my app would use actually at this point), and push messages to the devices who listen to that topic. Is this a viable plan? I haven't found any mention of this whatsoever. Firebase knows all the tokens internally and it would make the architecture simpler if I don't have to deploy a server.
I don't know how the decomission/expiration of the device tokens would happen on Firebase's side, but that's another issue I'd have to deal with if I'll run my own XMPP server and provision tokens.
To send messages to a device (so-called downstream messages), you need to specify the server key. As its name implies, this key should only be present on a server or in some other trusted environment. So to send messages to devices you will need to run code in a trusted environment.
The server doesn't have to speak the XMPP protocol however. You can also just use HTTP to call the FCM servers. But a server will be needed, simply because sending downstream messages can only be done from a server.
For a simple example of sending device-to-device messages with this approach, see my blog post Sending notifications between Android devices with Firebase Database and Cloud Messaging. It's about Android, but the same approach of using the Firebase Database as a message queue will work across all platforms.
The tricky bit to map will be (as you already mention) the fact that topics are not available to FCM for the web yet. Last time I tested, you could call a server-side end-point to subscribe to a topic, like described in this answer: GCM: How do you subscribe a device to a topic?.
We have an app (Ruby on Rails) that sends push notifications to an android app and we're facing an issue with GCM where some of these notifications are either delayed or never received. Considering the fact that these notifications are being sent to employees and not end-users my questions is:
What the best way to send these push notifications without any delay or drop and is there a way to send push notifications independently from the available services such as GCM and SNS?
Please keep in mind that we're looking for a solutions that can send these notifications without any delay as for the app to perform well the notification must be received within 1 to 20 seconds from the time it was sent.
Thanks in advance for anyone that will reply to this question...
Have a look at "Pushy":
They state the following on their website:
The most reliable push notification gateway, perfect for real-time
applications.
Google Cloud Messaging simply doesn't cut it for time-sensitive,
real-time apps, due to its instability and push notification
throttling.
Pushy works by maintaining a dedicated background socket connection
using the MQTT protocol, an extremely light-weight pub/sub protocol
that consumes very little network bandwidth and battery, which makes
it perfect for mobile.
You can use it for free with up to 100 devices, so you could just give it a try to see, if it fits your needs. I didn't try it.
I am trying to understand how exactly push notification services work.
Are these real push services with constant connection to server or just mimics by polling?
How does a server with heavy traffic maintain so many connections?
In general push notifications work both by establishing a long-lived TCP connection, or using long-polling. The maximum number of connected clients is determined by the server resources.
Take a look at the Socket.io protocol stack for an example. Or better yet, at the XMPP/Jabber protocol, which relies on TCP principally and falls back on long polling.
Fusio is correct. For mobile phones, a single push service is typically used (Google cloud messaging for android, Apple Push Notification Service for Apple/iPhone) to limit the amount of connections from the phone. 3rd party applications register to these services and push messages through them.
What are push notifications and how do they work? Also what is the difference with googles cloud to device messaging? Is c2dm also a push notification? Also do I need Special server setup to implement push notification?are push standard or every device has different implementation?
Basically the server maintains a list of connected clients. Whenever something of interest occurs, the server sends the data/notification/updated state to the connected clients. This is in contrast with pull, where the clients poll the server for changes.