Develop Reference

Using Redis Stream to Block HTTP response via HTTP long polling in Spring Boot App

I have a spring boot web application with the functionality to update an entity called StudioLinking. This entity describes a temporary, mutable, descriptive logical link between two IoT devices for which my web app is their cloud service. The Links between these devices are ephemeral in nature, but the StudioLinking Entity persists on the database for reporting purposes. StudioLinking is stored to the SQL based datastore in the conventional way using Spring Data/ Hibernate. From time to time this StudioLinking entity will be updated with new information from a Rest API. When that link is updated the devices need to respond (change colors, volume, etc). Right now this is handled with polling every 5 seconds but this creates lag from when a human user enters an update into the system and when the IoT devices actually update. It could be as little as a millisecond or up to 5 seconds! Clearly increasing the frequency of the polling is unsustainable and the vast majority of the time there are no updates at all!
So, I am trying to develop another Rest API on this same application with HTTP Long Polling which will return when a given StudioLinking entity is updated or after a timeout. The listeners do not support WebSocket or similar leaving me with Long Polling. Long polling can leave a race condition where you have to account for the possibility that with consecutive messages one message may be "lost" as it comes in between HTTP requests (while the connection is closing and opening, a new "update" might come in and not be "noticed" if I used a Pub/Sub).
It is important to note that this "subscribe to updates" API should only ever return the LATEST and CURRENT version of the StudioLinking, but should only do so when there is an actual update or if an update happened since the last checkin. The "subscribe to updates" client will initially POST an API request to setup a new listening session and pass that along so the server knows who they are. Because it is possible that multiple devices will need to monitor updates to the same StudioLinking entity. I believe I can acomplish this by using separately named consumers in the redis XREAD. (keep this in mind for later in the question)
After hours of research I believe the way to acomplish this is using redis streams.
I have found these two links regarding Redis Streams in Spring Data Redis:
https://www.vinsguru.com/redis-reactive-stream-real-time-producing-consuming-streams-with-spring-boot/
https://medium.com/#amitptl.in/redis-stream-in-action-using-java-and-spring-data-redis-a73257f9a281
I also have read this link about long polling, both of these links just have a sleep timer during the long polling which is for demonstration purposes but obviously I want to do something useful.
https://www.baeldung.com/spring-deferred-result
And both these links were very helpful. Right now I have no problem figuring out how to publish the updates to the Redis Stream - (this is untested "pseudo-code" but I don't anticipate having any issues implementing this)
// In my StudioLinking Entity
#PostUpdate
public void postToRedis() {
StudioLinking link = this;
ObjectRecord<String, StudioLinking> record = StreamRecords.newRecord()
.ofObject(link)
.withStreamKey(streamKey); //I am creating a stream for each individual linking probably?
this.redisTemplate
.opsForStream()
.add(record)
.subscribe(System.out::println);
atomicInteger.incrementAndGet();
}
But I fall flat when it comes to subscribing to said stream: So basically what I want to do here - please excuse the butchered pseudocode, it is for idea purposes only. I am well aware that the code is in no way indicative of how the language and framework actually behaves :)
// Parameter studioLinkingID refers to the StudioLinking that the requester wants to monitor
// updateList is a unique token to track individual consumers in Redis
#GetMapping("/subscribe-to-updates/{linkId}/{updatesId}")
public DeferredResult<ResponseEntity<?>> subscribeToUpdates(#PathVariable("linkId") Integer linkId, #PathVariable("updatesId") Integer updatesId) {
LOG.info("Received async-deferredresult request");
DeferredResult<ResponseEntity<?>> output = new DeferredResult<>(5000l);
deferredResult.onTimeout(() ->
deferredResult.setErrorResult(
ResponseEntity.status(HttpStatus.REQUEST_TIMEOUT)
.body("IT WAS NOT UPDATED!")));
ForkJoinPool.commonPool().submit(() -> {
//----------------------------------------------
// Made up stuff... here is where I want to subscribe to a stream and block!
//----------------------------------------------
LOG.info("Processing in separate thread");
try {
// Subscribe to Redis Stream, get any updates that happened between long-polls
// then block until/if a new message comes over the stream
var subscription = listenerContainer.receiveAutoAck(
Consumer.from(studioLinkingID, updateList),
StreamOffset.create(studioLinkingID, ReadOffset.lastConsumed()),
streamListener);
listenerContainer.start();
} catch (InterruptedException e) {
}
output.setResult("IT WAS UPDATED!");
});
LOG.info("servlet thread freed");
return output;
}
So is there a good explanation to how I would go about this? I think the answer lies within https://docs.spring.io/spring-data/redis/docs/current/api/org/springframework/data/redis/core/ReactiveRedisTemplate.html but I am not a big enough Spring power user to really understand the terminology within Java Docs (the Spring documentation is really good, but the JavaDocs is written in the dense technical language which I appreciate but don't quite understand yet).
There are two more hurdles to my implementation:
My exact understanding of spring is not at 100% yet. I haven't yet reached that a-ha moment where I really fully understand why all these beans are floating around. I think this is the key to why I am not getting things here... The configuration for the Redis is floating around in the Spring ether and I am not grasping how to just call it. I really need to keep investigating this (it is a huge hurdle to spring for me).
These StudioLinking are short lived, so I need to do some cleanup too. I will implement this later once I get the whole thing up off the ground, I do know it will be needed.

Why don't you use a blocking polling mechanism? No need to use fancy stuff of spring-data-redis. Just use simple blocking read of 5 seconds, so this call might take around 6 seconds or so. You can decrease or increase the blocking timeout.
class LinkStatus {
private final boolean updated;
LinkStatus(boolean updated) {
this.updated = updated;
}
}
// Parameter studioLinkingID refers to the StudioLinking that the requester wants to monitor
// updateList is a unique token to track individual consumers in Redis
#GetMapping("/subscribe-to-updates/{linkId}/{updatesId}")
public LinkStatus subscribeToUpdates(
#PathVariable("linkId") Integer linkId, #PathVariable("updatesId") Integer updatesId) {
StreamOperations<String, String, String> op = redisTemplate.opsForStream();
Consumer consumer = Consumer.from("test-group", "test-consumer");
// auto ack block stream read with size 1 with timeout of 5 seconds
StreamReadOptions readOptions = StreamReadOptions.empty().block(Duration.ofSeconds(5)).count(1);
List<MapRecord<String, String, String>> records =
op.read(consumer, readOptions, StreamOffset.latest("test-stream"));
return new LinkStatus(!CollectionUtils.isEmpty(records));
}

Getting DB IO issue in java code because of ASYNC_NETWORK_IO

DB team of our application has reported ASYNC_NETWORK_IO issue along with a big but optimized query that gets executed in around 36Sec and brings around 6,44,000 rows.
The main reasons behind this might be one of these:
A. Problem with the client application
B. Problems with the network – (but we have Ethernet speed 1 GB)
So, might be this is a code side issue because The session must wait for the client application to process the data received from SQL Server in order to send the signal to SQL Server that it can accept new data for processing.
This is a common scenario that may reflect bad application design, and is the most often cause of excessive ASYNC_NETWORK_IO wait type values
and here is the how we are getting data from DB in code.
try {
queue.setTickets(jdbcTemplate.query(sql, params, new QueueMapper()));
} catch (DataAccessException e) {
logger.error("get(QueueType queueType, long ticketId)", e);
}
Can anyone advise me on this?
Thanks in advance.

How to balance the load when building cache from App Engine?

I currently have the following situation, which has bothered me for a couple of months right now.
The case
I have build a Java (FX) application which serves as a cash registry for my shop. The application contains a lot of classes (such as Customer, Customer, Transaction etc.), which are shared with the server API. The server API is hosted on Google App Engine.
Because we also have an online shop, I have chosen to build the cache of the entire database on startup of the application. To do this I call the GET of my Data API for each class/table:
protected QueryBuilder performGet(HttpServletRequest req, HttpServletResponse res)
throws ServletException, IOException, ApiException, JSONException {
Connection conn = connectToCloudSQL();
log.info("Parameters: "+Functions.parameterMapToString(req.getParameterMap()));
String tableName = this.getTableName(req);
log.info("TableName: "+tableName);
GetQueryBuilder queryBuilder = DataManager.executeGet(conn, req.getParameterMap(), tableName, null);
//Get the correct method to create the objects
String camelTableName = Functions.snakeToCamelCase(tableName);
String parsedTableName = Character.toUpperCase(camelTableName.charAt(0)) + camelTableName.substring(1);
List<Object> objects = new ArrayList<>();
try {
log.info("Parsed Table Name: "+parsedTableName);
Method creationMethod = ObjectManager.class.getDeclaredMethod("create"+parsedTableName, ResultSet.class, boolean.class);
while (queryBuilder.getResultSet().next()) {
//Create new objects with the ObjectManager
objects.add(creationMethod.invoke(null, queryBuilder.getResultSet(), false));
}
log.info("List of objects created");
creationMethod = null;
}
catch (Exception e) {
camelTableName = null;
parsedTableName = null;
objects = null;
throw new ApiException(e, "Something went wrong while iterating through ResultSet.", ErrorStatus.NOT_VALID);
}
Functions.listOfObjectsToJson(objects, res.getOutputStream());
log.info("GET Request succeeded");
//Clean up objects
camelTableName = null;
parsedTableName = null;
objects = null;
closeConnection(conn);
return queryBuilder;
}
It simples gets every row from the requested table in my Cloud SQL database. Then it creates the objects, with the class that is shared with the client application. Lastly, it converts these classes to JSON using GSON. Some of my tables have 10.000+ rows, and then it takes approx. 5-10 sec to do this.
At the client, I convert this JSON back to a list of objects by using the same shared class. First I load the essential classes sequentially (because else the application won't start), and after that I load the rest of the classes in the background with separate threads.
The problem
Every time I load up the cache, there is a chance (1 on 4) that the server responds with a DeadlineExceededException on some of the bigger tables. I believe this has something to do with Google App Engine not being able to fire up a new instance in time, and therefore the computation time exceeds the limit.
I know it has something to do with loading the objects in background threads, because these all start at the same time. When I delay the start of these threads with 3 seconds, the error occurs a lot less, but is still present. Because the application loads 15 classes in the background, delaying them is not ideal because the application will only work partly until it is done. It is also not an option to load everything before starting, because this will take more than 2 minutes.
Does anyone know how to set up some load balancing on Google App Engine for this? I would like to solve this server side.

You clearly have an issue with warm up requests and a query that takes quite long. You have the usual options:
Do some profiling and reduce the cost of your method invocations
use caching (memcache) to cache some of the result
If those options don't work for you, you should parallelize your computations. One thing that comes to my mind is that you could reliably reduce request times if you simply split your request into multiple parallel requests like so:
Let's say your table contains 5k rows.
Then you create 50 requests with each handleing 100 rows.
Aggregate the results on server or client side and respond
It'll be quite tough to do this on just the server side but it should be possible if your now (much) smaller taks return within a couple of seconds.
Alternatively you could return a job id at once and make the client poll for the result in a couple of seconds. This would however require a small change on the client side. It's the better option though imho, especially if you want to use a task queue for creating your response.

Can I use Thread.sleep in a servlet to add random delays for the local server to answer my api call?

My deployed server has sometimes long response times, while working and developing at localhost all calls are really fast.
This has made my application enter unexpected behaviour once deployed a few times due to problems with resource loading taking too long.
I'd like to simulate in my local tests the bad connection with my real server, therefore I want to add a random delay to every request-response and my first thought was to use Thread.sleep in the servlet:
protected void doPost(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
//add delay before processing request
if (DELAY > 0){
int delay;
if (RANDOMIZE){
delay = Random.nextInt(DELAY);
} else {
delay = DELAY;
}
try {
Thread.sleep(delay);
} catch (InterruptedException e1) {
logger.error(e1);
}
}
...
However I have read that one should not use Thread.sleep() inside a servlet, but the context of such discouragement and their solutions are drastically different from my case, can I use thread.sleep() in this context?
EDIT: This is of course only for local and for the client to be strained a bit in the local tests... I just want to simulate the bad network I've encountered in reality!

I think this whole approach is flawed. I wouldn't introduce a random delay (how are you going to repeat test cases?). You can introduce a Thread.sleep(), but I wouldn't. Would this be in your production code ? Is it configurable ? What happens if it's accidentlally turned on in production ?
I would rather set up a test server with the exact characteristics of your production environment. That way you can not only debug effectively, but build a regression test suite that will allow you to develop effectively, knowing how the application will perform in production.
Perhaps the one concession to the above is to introduce network delays (as appropriate) between client and server if your users are geographically diverse. That's often done using a hardware device on the network and wouldn't affect your code or configuration.

I did this to get delay :
response.setContentType("text/html;charset=UTF-8");
try (PrintWriter out = response.getWriter()) {
out.println("<meta http-equiv=\"Refresh\" content=\"3;url=home.jsp/\">");
}
Remember that in content=\"3;url=home.jsp/\", 3 is the delay seconds and home.jsp is the page you want to go to after the given seconds.

Multiple Connectors on a Jetty Server

I'm trying to run a Jetty Server that can have a number of people connect to the server and see a list of print outs. I want everybody who connects to see the same values printed out.
For instance, if I have a single list keeping track of the time and I want 5 or so people to be able to go to my website (e.g. localhost:8080/time) and have them all see what time it is every 30 seconds, how would i set that up?
What I have:
I am using Jetty.
I created a single server bound to port 8080.
I created my own handler that extends AbstractHandler
this writes to the screen saying when an event has transpired (i.e. 30 seconds have passed)
If two people connect to this page, they each see a print out every minute (that is it switches back and forth letting each person know when every other event has transpired)
If 3 people connect, only two can stay connected and the third just spins getting no output to the screen
I have not set up an Connectors of my own since my attempts to do so have been unsuccessful and i'm not sure how I understand if that is the solution to my problem.
Any help would be much appreciated and if anybody has some idea but needs some clarification on what I am doing I would be glad to give more details.
Thanks!
Handler code:
#Override
public void handle(String target, Request request, HttpServletRequest httpServletRequest, HttpServletResponse httpServletResponse) throws IOException, ServletException
{
httpServletResponse.setContentType("text/html;charset=utf-8");
httpServletResponse.setStatus(HttpServletResponse.SC_OK);
request.setContextPath("/time");
request.setHandled(true);
while (true) {
synchronized(Main.list) {
while (!Main.list.isEmpty()) {
Double time = Main.list.get(0);
httpServletResponse.getWriter().println("<h1>The time now is " + time + "</h1>");
httpServletResponse.flushBuffer();
Main.list.remove(0);
}
try {
Main.list.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
So the list object is a static ArrayList defined in the Main class that I wake up (i.e. notify) every 30 seconds. Hopefully this helps someone understand more what I am talking about as i'm not sure what I could change in the handler...

How are you feeding clients into your handler? Browsers have limits to the number of connections are made to to a particular host, perhaps your seeing that.
there is nothing intrinsically wrong that handler code aside from it being a generally odd thing to see in a handler