I am building a web app to display on my iPad to control my raspberry pi acting as an audio recorder. Part of the need is to maintain an event source open so that the server can send Server Side Events. A specific instance of the app can grab control of the recording process, but will loose control if the server sees sse link closes. This is just protection against a client disappearing and leaving the control held (control of the process does needed to be renewed at least every 5 minutes - but I don't really want to wait that long in the normal case of someone just closing the browser tab.)
Part of my need is to push the browser to the background so I can then open up the camera and record a video.
I built this app and had it almost working see https://github.com/akc42/pi_record.git (master branch).
Until I pushed the browser to the background and found IOS shut down the page and broke the sse link.
I tried restructuring to use a private web worker to manage the sse link, massing messages between the web worker and the main javascript thread - again almost working (see workers branch of above repository). But that got shutdown too!
My last thought is to use a service worker, but how to structure the app?
Clearly the service worker must act as a client to the server for the server side events. It must keep the connection open, but it also needs to keep track of multiple tabs in the browser which may or may not try and grab control of the interface, and only allow one tab to do so.
I can think of three approaches - but its difficult to see which is better. At least I have never even seen any mention of approach 2 and 3 below , but it seems to me that one of these two might actually be the simplest.
Approach 1
Move the code I have now for separate web workers into the service worker. However we will need to add to the message passing some form of ID between window and service. So I can record which tab actually grabbed control of the interface and therefore exclude other tabs from doing so (ie simulate a failed attempt to take control).
As far as I can work out MessageEvent.ports[0] could be a unique object which I could store in a Map somewhere, but I am not entirely convinced that the MessageChannel wouldn't close if the browser moved to the background.
Approach 2
have a set of phantom urls in the service worker that simulate all the different message types (and parameters) that where previously sent my the tab to its private web worker.
The fetch event provides a clientid (which I can use to difference between who actually grabbed control) and which I can use to then do Clients.get(clientid).postMessage() (or Clients.matchAll when a broadcast response is needed)
Code would be something like
self.addEventListener('fetch', (event) => {
const requestURL = new URL(event.request.url);
if (/^\/api\//.test(requestURL.pathname)) {
event.respondWith(fetch(event.request)); //all api requests are a direct pass through
} else if (/^\/service\//.test(requestURL.pathname)) {
/*
process these like a message passing with one extra to say the client is going away.
*/
if (urlRecognised) {
event.respondWith(new Response('OK', {status: 200}));
} else {
event.respondWith(new Response(`Unknown request ${requestURL.pathname}`, {status: 404}));
}
} else {
event.respondWith(async () => {
const cache = await caches.open('recorder');
const cachedResponse = await cache.match(event.request);
const networkResponsePromise = fetch(event.request);
event.waitUntil(async () => {
const networkResponse = await networkResponsePromise;
await cache.put(event.request, networkResponse.clone());
});
// Returned the cached response if we have one, otherwise return the network response.
return cachedResponse || networkResponsePromise;
});
}
});
The top of the the fetch event just passes the standard api requests made by the client straight through. I can't cache these (although I could be more sophisticated and perhaps pre reject those not supported).
The second section matches phantom urls /service/something
The last section is taken from Jake Archibald's offline cookbook and tries to use the cache, but updates the cache in the background if any of the static files have changed.
Approach 3
Similar to the approach above, in that we would have phantom urls and use the clientid as a unique marker, but actually try and simulate a server side event stream with one url.
I'm thinking the code with be more like
...
} else if (/^\/service\//.test(requestURL.pathname)) {
const stream = new TransformStream();
const writer = stream.writeable.getWriter();
event.respondWith(async () => {
const streamFinishedPromise = new Promise(async (resolve,reject) => {
event.waitUntil(async () => {
/* eventually close the link */
await streamFinishedPromise;
});
try {
while (true) writer.write(await nextMessageFromServerSideEventStream());
} catch(e) {
writer.close();
resolve();
}
});
return new Response(stream.readable,{status:200}) //probably need eventstream headers too
}
I am thinking that approach 2 could be the simplest, given where I am now but I am concerned that I can see nothing when searching for how to use service workers that discusses this phantom url approach.
Can anyone comment on any of these approaches and provide guidance on how to best program the tricky bits (for instance does Approach 1 message channel close when the browser is moved to the background on an iPad, or how do you really keep a response channel open, and does that get closed when the browser moves to the background in Approach 3)
The simple truth is that none of these approaches will work. What I didn't realise when I asked the question is that a service worker is re-run by the browser when ever there is something to do and that run only lasts for the length of time of the processing of an event. Although eventWaitUntil can prolong that, the only reference to how long I can find is that the browser is still at liberty to cancel it if it appears it might never close. I can't imagine than in a period of several hours it won't get cancelled. So an Event Source will close effectively terminate its link to the server.
So my only option to achieve what I want is to have the server carry on when the Event Source closes and find some other mechanism to release resources held on behalf of the client
Related
I am using the event store client for .Net and I am struggling to find the correct way to use the client. When I register the client as a singleton in the .Net dependency injection and run my application over an extended period of time memory usage grows continuously with each subscription.
I create and register the client in the following way. A full minimal application that experiences the problem can be found here.
var esdbConnectionString = configuration.GetValue("ESDB_CONNECTION_STRING", "esdb://admin:changeit#localhost:2113?tls=false");
var eventStoreClientSettings = EventStoreClientSettings.Create(esdbConnectionString);
var eventStoreClient = new EventStoreClient(eventStoreClientSettings);
services.AddSingleton(eventStoreClient);
My application has a high number of short streams over an extended period of time
To Reproduce
Steps to reproduce the behavior:
Register EventStoreClient as singleton as reccomended in the documentation.
Subscribe to a very high number of streams over an extended time.
Cancel the CancellationToken sent into the stream subscription and let it be garbage collected.
Watch memory usage of service grow.
How I am creating and subscribing to streams:
var streamName = CreateStreamName();
var payload = new PingEvent { StreamNr = _currentStreamNumber };
var eventData = new EventData(Uuid.NewUuid(), typeof(PingEvent).Name, EventSerialization.SerializeEventData(payload));
await _client.AppendToStreamAsync(streamName, StreamState.Any, new[] { eventData });
var streamCancellationTokenSource = new CancellationTokenSource(TimeSpan.FromMinutes(30));
await _client.SubscribeToStreamAsync(streamName, FromStream.Start, async (sub, evnt, token) =>
{
if (evnt.Event.EventType == "PongEvent")
{
_previousStreamIsDone = true;
streamCancellationTokenSource.Cancel();
}
},
cancellationToken: streamCancellationTokenSource.Token);
Approaches attempted
Registering as Transient or Scoped
If I register the client as Transient or Scoped in .Net DI it is throwing thousands of exceptions internally and causing multiple problems.
Manually handling lifetime of client
By having a singleton service that handles the lifetime of the client I have attempted to every once in a while dispose of the client and create a new one, ensuring that there exists only one instance of the client at the same time. This results in same problem as registering the service as Transient or Scoped.
I am using version 22.0.0 of the Event Store client in .Net 6 against Event Store Database 21.10.0. The problems happens both when running on windows and on the standard aspnet:6.0 linux docker container.
By inspecting the results of these dotnet-dumps the memory growth seem to be happening inside this HashSet of ActiveCalls in the gRPC client.
I am hoping to find a way of using the client that does not lead to memory growth.
In your reproduction the leaked calls are coming from the extra read that you are issuing while processing an event received on the subscription.
There is an open issue (https://github.com/EventStore/EventStore-Client-Dotnet/issues/219) at the moment to deal with this better, but currently if you issue a read but don't consume all the events and don't cancel the read, then the call remains open. In your case this is happening if the slave has managed to reply Pong before the master has issued the read that results from receiving its own Ping in the subscription. That read will then contain the Ping and the Pong, only the Ping is read, and the call remains open.
For now, if you cancel those reads by passing the cancellation token that you are cancelling into the ReadStreamAsync call in ReadFromStartOfStreamToEnd, it should resolve your problem.
In case it's helpful for you, you can see the number of Current Calls live rather than waiting a long time to see the effect on memory:
dotnet-counters monitor --counters "Grpc.Net.Client" -p <processid>
This is a really annoying issue. I am using a third party login in my application. When a user logins in through the third party, it redirects an api call to the server.
ex: /api/signin/github?code=test&state=test
For some strange reason this API call is getting fetched from the service worker instead on the server which handles the login logic.
ex:
Without seeing your service worker's fetch event handler, it's hard to say exactly what code is responsible for that.
In general, though, if there are URLs for which you want to tell the service worker never to respond to, you can just avoid calling event.respondWith(...) when they trigger a fetch. There are lots of ways to avoid doing that, but an early return is straightforward:
self.addEventListener('fetch', (event) => {
const url = new URL(event.request.url);
if (url.pathname === '/api/signin/github') {
// By returning without calling event.respondWith(),
// the request will be handled by the normal browser
// network stack.
return;
}
// Your fetch event response generation logic goes here.
event.respondWith(...);
});
Is it possible to make HTTP requests in background with service worker, when users are not visiting my webpage. I want to make periodic requests to my webpage (e.g. 3 seconds)?
There is a feature called periodicSync, but i didn't understand how to use it.
I've not tried implementing this but for me the clearest overview has been this explanation.
Making periodic requests involves first handling the Service Worker ready event, invoking the periodicSync.register() function with config options. The register() function returns a Promise that allows you to deal with success or rejection of the periodic sync registration.
registration.periodicSync.register()
Pass a 'config' object parameter with the following properties:
tag
minPeriod
powerState
networkState
You may then register listeners against the periodicSync event. E.g (slightly simplified example based on the explanation.
self.addEventListener('periodicsync', function(event) {
if (event.registration.tag == 'my-tag') {
event.waitUntil(doTheWork()); // "do the work" asynchronously via a Promise.
}
else {
// unknown sync, may be old, best to unregister
event.registration.unregister();
}
});
I have a .Net MVC web application and I am using SignalR to implement a progress bar functionality.
I have a View that is making an Ajax POST to an action using JQuery:
$.ajax({
url: actionUrl,
type: 'POST',
data: { ids: ids },
success: function (data) {
...
}
});
The Controller is procesing information inside a loop, and every n iteration is sending a message using signalr to a Hub. The client is connected to the Hub and updates a progress bar with the information in the messages.
I open the connection to the signalr hub with this code:
var connection = new HubConnection("http://localhost/");
connection.Credentials = CredentialCache.DefaultNetworkCredentials;
this.proxy = connection.CreateHubProxy("progressHub");
connection.Start().Wait();
And then send the information with this:
proxy.Invoke("ProgressChanged", taskId, progress);
This is working correctly for the first two updates, but from that on it is not working anymore until the end of the long process, when I receive all the remaining messages. I am receiving something like this:
0% complete
5% complete
... (long pause as the process completes)
10%, 15%, 20%, 25%, etc (all these messages come together)
It is always the first two messages, not a random number of messages.
Do you know any configuration that I may be missing?
I tried adding a sleep after each message and making the action async and adding .Wait() to each message, but it is always the same behaviour.
I tried it in IISExpress and full IIS 8.
Your action method in the controller should be async and call an asynchronous Task. My guess is that while your ajax call is waiting for completion, the call made by signalR are probalby queued until your proccess is complete and end the ajax request.
There are running examples of SignalR, but in those, i have seen that the process is started by the client i.e. every piece of code contains following similar lines
$.connection.hub.start().done(function () {
$('#mybutton').click(function () {
notifier.server.doLongOperation();
});
});
The process on server starts on $('#mybutton').click and then responds.
Is my understanding correct? If yes then is it possible to start the process by Server? I mean Server will push messages to all clients without any triggering from the client side.
This didn't work
var context = GlobalHost.ConnectionManager.GetHubContext<Broadcast>();
context.Clients.All.Send(message);
My bad, method name on client side was incorrect. Problem solved
Yes it is possible to send server initiated "messages" from the server to clients. Note that you have to call a method on the client. Note that it's a RPC/Remoting type of communication.
On the server you'd have a code like this:
Clients.All.Say("Hello World!");
where the client needs to define a function:
myHub.client.say = function (message) {
console.log(message);
});
see the SignalR documentation