I am currently working on performance improvements for a React-based SPA. Most of the more basic stuff is already done so I started looking into more advanced stuff such as service workers.
The app makes quite a lot of requests on each page (most of the calls are not to REST endpoints but to an endpoint that basically makes different SQL queries to the database, hence the amount of calls). The data in the DB is not updated too often so we have a local cache for the responses, but it's obviously getting lost when a user refreshes a page. This is where I wanted to use the service worker - to keep the responses either in cache store or in IndexedDB (I went with the second option). And, of course, the cache-first approach does not fit here too well as there is still a chance that the data may become stale. So I tried to implement the stale-while-revalidate strategy: fetch the data once, then if the response for a given request is already in cache, return it, but make a real request and update the cache just in case.
I tried the approach from Jake Archibald's offline cookbook but it seems like the app is still waiting for real requests to resolve even when there is a cache entry to return from (I see those responses in Network tab).
Basically the sequence seems to be the following: request > cache entry found! > need to update the cache > only then show the data. Doing the update immediately is unnecessary in my case so I was wondering if there is any way to delay that? Or, alternatively, not to wait for the "real" response to be resolved?
Here's the code that I currently have (serializeRequest, cachePut and cacheMatch are helper functions that I have to communicate with IndexedDB):
self.addEventListener('fetch', (event) => {
// some checks to get out of the event handler if certain conditions don't match...
event.respondWith(
serializeRequest(request).then((serializedRequest) => {
return cacheMatch(serializedRequest, db.post_cache).then((response) => {
const fetchPromise = fetch(request).then((networkResponse) => {
cachePut(serializedRequest, response.clone(), db.post_cache);
return networkResponse;
});
return response || fetchPromise;
});
})
);
})
Thanks in advance!
EDIT: Can this be due to the fact that I put stuff into IndexedDB instead of cache? I am sort of forced to use IndexedDB instead of the cache because those "magic endpoints" are POST instead of GET (because of the fact they require the body) and POST cannot be inserted into the cache...
Related
I want to be able to call an HTTP endpoint (that I own) from an Azure Function at the end of the Azure Function request.
I do not need to know the result of the request
If there is a problem in the HTTP endpoint that is called I will log it there
I do not want to hold up the return to the client calling the initial Azure Function
Offloading the call of the secondary WebApi onto a background job queue is considered overkill for this requirement
Do I simply call HttpClient.PutAsync without an await?
I realise that the dependencies I have used up until the point that the call is made may well not be available when the call returns. Is there a safe way to check if they are?
My answer may cause some controversy but, you can always start a background task and execute it that way.
For anyone reading this answer, this is far from recommended. The OP has been very clear that they don't care about exceptions or understanding what sort of result the request is returning ...
Task.Run(async () =>
{
using (var httpClient = new HttpClient())
{
await httpClient.PutAsync(...);
}
});
If you want to ensure that the call has fired, it may be worth waiting for a second or two after the call is made to ensure it's actually on it's way.
await Task.Delay(1000);
If you're worried about dependencies in the call, be sure to construct your payload (i.e. serialise it, etc.) external to the Task.Run, basically, minimise any work the background task does.
I placed this code inside a Route::get() method only to test it quicker. So this is how it looks:
use Illuminate\Support\Facades\Cache;
Route::get('/cache', function(){
$lock = Cache::lock('test', 4);
if($lock->get()){
Cache::put('name', 'SomeName'.now());
dump(Cache::get('name'));
sleep(5);
// dump('inside get');
}else{
dump('locked');
}
// $lock->release();
});
If you reach this route from two browsers (almost)at the same time. They both will respond with the result from dump(Cache::get('name'));. Shouldn't the second browser respond be "locked"? Because when it calls the $lock->get() that is supposed to return false? And that because when the second browser tries to reach this route the lock should be still set.
That same code works just fine if the time required for the code after the $lock = Cache::lock('test', 4) to be executed is less than 4. If you set the sleep($sec) when $sec<4 you will see that the first browser reaching this route will respond with the result from Cache::get('name') and the second browser will respond with "locked" as expected.
Can anyone explain why is this happening? Isn't it suppose that any get() method to that lock, expect the first one, to return false for that amount of time the lock has been set? I used 2 different browsers but it works the same with 2 tabs from the same browser too.
Quote from the 5.6 docs https://laravel.com/docs/5.6/cache#atomic-locks:
To utilize this feature, your application must be using the memcached or redis cache driver as your application's default cache driver. In addition, all servers must be communicating with the same central cache server.
Quote from the 5.8 docs https://laravel.com/docs/5.8/cache#atomic-locks:
To utilize this feature, your application must be using the memcached, dynamodb, or redis cache driver as your application's default cache driver. In addition, all servers must be communicating with the same central cache server.
Quote from the 8.0 docs https://laravel.com/docs/8.x/cache#atomic-locks:
To utilize this feature, your application must be using the memcached, redis, dynamodb, database, file, or array cache driver as your application's default cache driver. In addition, all servers must be communicating with the same central cache server.
Apparently, they have been adding support for more drivers to make use of this lock functionality. Check which Cache driver you are using and if it fits the support list of your Laravel version.
There is likely an atomicity issue here where the cache driver you are using is not able to lock a file atomically. What should happen is that when a process (i.e. a php request) is writing to the lock file, all other processes requiring the lock file should at least wait until the lock file available to be read again. If not, they read the lock file before it has been written to, which obviously causes a race condition.
I saw this question I asked, well now I can say that the problem I was trying to solve here was not because of the atomic lock. The problem here is the sleep method. If the time provided to the sleep method is bigger than the time that a lock will live, it means when the next request it's able to hit the route the lock time will expire(will be released). And that's because let's say you have defined a route like this:
Route::get('case/{value}', function($value){
if($value){
dump('hit-1');
}else{
sleep(5);
dump('hit-0');
}
});
And you open two browser tabs with the same URL that hits this route something like:
127.0.0.1:8000/case/0
and
127.0.0.1:8000/case/1
It will show you that the first route will take 5sec to finish execution and even if the second request is sent almost at the same time with the first request, still it will wait to finish the first one and then run. This means the second request will last 5sec(from the first request) plus the time it took to run.
Back to the asked question the lock time will expire by the time the second request will get it or said differently run the $lock->get() statement.
I'm trying to synchronize my POSTs to an endpoint in Angular. I did see some examples of doing a synchronized GET but had trouble understanding the examples well enough to apply them to POSTs.
The POSTs are pretty simple, at least from my perspective as the front-end developer. I send an object with an parent group ID and a sub group ID to a /parentgroups endpoint. On the backend, however, async calls cause the data to get overwritten.
Apologies for lack of an example, but I am pretty far from having one that's close to working how I need. My code is still async and a loop over calls to $http.post().
You actually cannot do real synchronous (as in blocking) http calls in Angular, it forces you do use async. If you can't do it with callbacks then you have a problem with your architecture that the entire team should focus on solving ASAP. If your current architecture requires the frontend to do blocking calls then your architecture is quite simply broken and needs to be fixed.
Anyway, while I recommend against it you could always register your request in a list, and then in each callback you pop the next request from the list and run it. That way you can just keep pushing requests into the list without knowing how many there will be. Something like this (untested, but the general principle should work):
var requestList = [];
requestList.push(function() {
$http.post('/someUrl', {})
.success(function(data, status, headers, config) {
// Remove the next request from list and call it
requestList.shift()();
});
});
requestList.push(function() {
$http.post('/someOtherUrl', {})
.success(function(data, status, headers, config) {
// Remove the next request from list and call it
requestList.shift()();
});
});
// Start the first request
requestList.shift()();
This is fairly clean, but still a bit of a hack. It would probably work fine but I would be taking a good long look at why the API forces you to do something like this.
I have an express app running with Sequelize.js as an ORM. My express app receives requests from my main Rails app, and because of the cross-domain policy, these requests are performed with getJSON.
On the client, the request is fired when the user hits a key.
Everything goes fine and express logs the queries being performed (and json being served) each time the user hits the key. Even trying to hit quickly it performs ok. But, whenever I leave the key pressed (or maybe several clients hitting the key very quickly), as it starts firing lots of requests, at some moment the server just hangs, all the requests from that point on are left pending (I see that in the Network tab of Chrome Dev Tools), and they slowly start to timeout. I have to reboot the server to make it respond again.
The server code for my request is:
models.Comment.findAllPublic(req.params.pId, req.params.sId, function(comments){
var json = comments.map(function(comment){
var com = {};
['user_id','user_avatar', 'user_slug', 'user_name', 'created_at', 'text', 'private', 'is_speaker_note'].forEach(function(key){
com[key]=comment[key];
});
return com;
});
res.json({comments: json});
});
And the findAllPublic method from the Comment model (this is a Sequelize model) is:
findAllPublicAndMyNotes: function(current_user, presentationId, slideId, cb){
db.query("SELECT * FROM `comments` WHERE commentable_type='Slide' AND commentable_id=(SELECT id from `slides` where `order_in_presentation`="+slideId+" AND `presentation_id`="+presentationId+") AND (`private` IS FALSE OR (`private` IS TRUE AND `user_id`="+current_user+" AND `is_speaker_note` IS FALSE))",self.Comment).on('success', cb).on('failure',function(err){console.log(err);});
}
How to avoid the server from getting stuck? Am I leaving some blocking code in the request that may slowly hang the server as new requests are made?
At first I thought it could be a problem because of the "forEach" when composing the json object from the Sequelize model, but I also tried leaving the callback for the mysql query empty, just responding empty json and it also got frozen.
Maybe it is a problem of the mysql connector? When the server gets stuck I can normally run the mysql console and perform queries on my database and it also responds, so I don't know if that's the problem.
I know I could just control the key event to prevent it from firing too many requests when the key gets pressed for a long time, but the problem seems to appear also when several clients hit the key repeatedly and concurrently.
Any thoughts? Thanks in advance for the help :D
Two things:
It seems like you have some path where res.render is not being called. It could be that the database you're connecting to is dropping the connection to your Express server after the absurd number of requests and the callback is never fired (and there's no database.on('close', function() { // Handle disconnect from DB, perhaps auto-restarting }) code to catch it.
Your client-side code should detect when an AJAX request on keypress is still pending while a new one is being started, and cancel the old one. I'm guessing getJSON is a jQuery method? Assuming it's jQuery's, then you need something like the following
.
var currKeyRequest = null;
function callOnKeyUp() {
var searchText = $('#myInputBox').value;
if(currKeyRequest) {
currKeyRequest.reject();
currKeyRequest = null;
}
currKeyRequest = $.getJSON('path/to/server', function(json) {
currKeyRequest = null;
// Use JSON code
});
}
This way, you reduce the load on the client, the latency of the autocomplete functionality (but why not use the jQuery UI autocomplete if that's what you're after?), and you can save the server from some of the load as well if the keypresses are faster than handshaking with the server (possible with a good touch-typist a few hours flight away).
I have read some posts about this topic and the answers are comet, reverse ajax, http streaming, server push, etc.
How does incoming mail notification on Gmail works?
How is GMail Chat able to make AJAX requests without client interaction?
I would like to know if there are any code references that I can follow to write a very simple example. Many posts or websites just talk about the technology. It is hard to find a complete sample code. Also, it seems many methods can be used to implement the comet, e.g. Hidden IFrame, XMLHttpRequest. In my opinion, using XMLHttpRequest is a better choice. What do you think of the pros and cons of different methods? Which one does Gmail use?
I know it needs to do it both in server side and client side.
Is there any PHP and Javascript sample code?
The way Facebook does this is pretty interesting.
A common method of doing such notifications is to poll a script on the server (using AJAX) on a given interval (perhaps every few seconds), to check if something has happened. However, this can be pretty network intensive, and you often make pointless requests, because nothing has happened.
The way Facebook does it is using the comet approach, rather than polling on an interval, as soon as one poll completes, it issues another one. However, each request to the script on the server has an extremely long timeout, and the server only responds to the request once something has happened. You can see this happening if you bring up Firebug's Console tab while on Facebook, with requests to a script possibly taking minutes. It is quite ingenious really, since this method cuts down immediately on both the number of requests, and how often you have to send them. You effectively now have an event framework that allows the server to 'fire' events.
Behind this, in terms of the actual content returned from those polls, it's a JSON response, with what appears to be a list of events, and info about them. It's minified though, so is a bit hard to read.
In terms of the actual technology, AJAX is the way to go here, because you can control request timeouts, and many other things. I'd recommend (Stack overflow cliche here) using jQuery to do the AJAX, it'll take a lot of the cross-compability problems away. In terms of PHP, you could simply poll an event log database table in your PHP script, and only return to the client when something happens? There are, I expect, many ways of implementing this.
Implementing:
Server Side:
There appear to be a few implementations of comet libraries in PHP, but to be honest, it really is very simple, something perhaps like the following pseudocode:
while(!has_event_happened()) {
sleep(5);
}
echo json_encode(get_events());
The has_event_happened function would just check if anything had happened in an events table or something, and then the get_events function would return a list of the new rows in the table? Depends on the context of the problem really.
Don't forget to change your PHP max execution time, otherwise it will timeout early!
Client Side:
Take a look at the jQuery plugin for doing Comet interaction:
Project homepage: http://plugins.jquery.com/project/Comet
Google Code: https://code.google.com/archive/p/jquerycomet/ - Appears to have some sort of example usage in the subversion repository.
That said, the plugin seems to add a fair bit of complexity, it really is very simple on the client, perhaps (with jQuery) something like:
function doPoll() {
$.get("events.php", {}, function(result) {
$.each(result.events, function(event) { //iterate over the events
//do something with your event
});
doPoll();
//this effectively causes the poll to run again as
//soon as the response comes back
}, 'json');
}
$(document).ready(function() {
$.ajaxSetup({
timeout: 1000*60//set a global AJAX timeout of a minute
});
doPoll(); // do the first poll
});
The whole thing depends a lot on how your existing architecture is put together.
Update
As I continue to recieve upvotes on this, I think it is reasonable to remember that this answer is 4 years old. Web has grown in a really fast pace, so please be mindful about this answer.
I had the same issue recently and researched about the subject.
The solution given is called long polling, and to correctly use it you must be sure that your AJAX request has a "large" timeout and to always make this request after the current ends (timeout, error or success).
Long Polling - Client
Here, to keep code short, I will use jQuery:
function pollTask() {
$.ajax({
url: '/api/Polling',
async: true, // by default, it's async, but...
dataType: 'json', // or the dataType you are working with
timeout: 10000, // IMPORTANT! this is a 10 seconds timeout
cache: false
}).done(function (eventList) {
// Handle your data here
var data;
for (var eventName in eventList) {
data = eventList[eventName];
dispatcher.handle(eventName, data); // handle the `eventName` with `data`
}
}).always(pollTask);
}
It is important to remember that (from jQuery docs):
In jQuery 1.4.x and below, the XMLHttpRequest object will be in an
invalid state if the request times out; accessing any object members
may throw an exception. In Firefox 3.0+ only, script and JSONP
requests cannot be cancelled by a timeout; the script will run even if
it arrives after the timeout period.
Long Polling - Server
It is not in any specific language, but it would be something like this:
function handleRequest () {
while (!anythingHappened() || hasTimedOut()) { sleep(2); }
return events();
}
Here, hasTimedOut will make sure your code does not wait forever, and anythingHappened, will check if any event happend. The sleep is for releasing your thread to do other stuff while nothing happens. The events will return a dictionary of events (or any other data structure you may prefer) in JSON format (or any other you prefer).
It surely solves the problem, but, if you are concerned about scalability and perfomance as I was when researching, you might consider another solution I found.
Solution
Use sockets!
On client side, to avoid any compatibility issues, use socket.io. It tries to use socket directly, and have fallbacks to other solutions when sockets are not available.
On server side, create a server using NodeJS (example here). The client will subscribe to this channel (observer) created with the server. Whenever a notification has to be sent, it is published in this channel and the subscriptor (client) gets notified.
If you don't like this solution, try APE (Ajax Push Engine).
Hope I helped.
According to a slideshow about Facebook's Messaging system, Facebook uses the comet technology to "push" message to web browsers. Facebook's comet server is built on the open sourced Erlang web server mochiweb.
In the picture below, the phrase "channel clusters" means "comet servers".
Many other big web sites build their own comet server, because there are differences between every company's need. But build your own comet server on a open source comet server is a good approach.
You can try icomet, a C1000K C++ comet server built with libevent. icomet also provides a JavaScript library, it is easy to use as simple as:
var comet = new iComet({
sign_url: 'http://' + app_host + '/sign?obj=' + obj,
sub_url: 'http://' + icomet_host + '/sub',
callback: function(msg){
// on server push
alert(msg.content);
}
});
icomet supports a wide range of Browsers and OSes, including Safari(iOS, Mac), IEs(Windows), Firefox, Chrome, etc.
Facebook uses MQTT instead of HTTP. Push is better than polling.
Through HTTP we need to poll the server continuously but via MQTT server pushes the message to clients.
Comparision between MQTT and HTTP: http://www.youtube.com/watch?v=-KNPXPmx88E
Note: my answers best fits for mobile devices.
One important issue with long polling is error handling.
There are two types of errors:
The request might timeout in which case the client should reestablish the connection immediately. This is a normal event in long polling when no messages have arrived.
A network error or an execution error. This is an actual error which the client should gracefully accept and wait for the server to come back on-line.
The main issue is that if your error handler reestablishes the connection immediately also for a type 2 error, the clients would DOS the server.
Both answers with code sample miss this.
function longPoll() {
var shouldDelay = false;
$.ajax({
url: 'poll.php',
async: true, // by default, it's async, but...
dataType: 'json', // or the dataType you are working with
timeout: 10000, // IMPORTANT! this is a 10 seconds timeout
cache: false
}).done(function (data, textStatus, jqXHR) {
// do something with data...
}).fail(function (jqXHR, textStatus, errorThrown ) {
shouldDelay = textStatus !== "timeout";
}).always(function() {
// in case of network error. throttle otherwise we DOS ourselves. If it was a timeout, its normal operation. go again.
var delay = shouldDelay ? 10000: 0;
window.setTimeout(longPoll, delay);
});
}
longPoll(); //fire first handler