I have single threaded server written in C that accepts TCP/UDP connections based on EPOLL and supports plugins for the multitude of protocol layers we need to support. That bit is fine.
Due to the single threaded nature, I wanted to implement a database layer that could utilize the same EPOLL architecture rather then separately iterating over all of the open connections.
We use MariaDB and the MariaDB connector that supports non blocking functions in it's API.
https://mariadb.com/kb/en/mariadb/using-the-non-blocking-library/
But what I'm finding is not what I expected, and what I was expecting is described below.
First I fire the mysql_real_connect_start() and if it returns zero we dispatch the query immediately as this indicates no blocking was required, although this never happens.
Otherwise, I fetch the file descriptor that seems to be immediate and register it with EPOLL and bail back to the main EPOLL loop waiting for events.
s = mysql_get_socket(mysql);
if(s > 0)
{
brt_socket_set_fds(endpoint, s);
struct epoll_event event;
event.data.fd = s;
event.events = EPOLLRDHUP | EPOLLIN | EPOLLET | EPOLLOUT;
s = epoll_ctl(efd, EPOLL_CTL_ADD, s, &event);
if (s == -1) {
syslog(LOG_ERR, "brd_db : epoll error.");
// handle error.
}
...
So, then some time later I do get the EPOLLOUT indicating the socket has been opened.
And I dutifully call mysql_real_connect_cont() but at this stage it is still returning a non-zero value, indicating I must wait longer?
But then that is the last EPOLL event I get, except for the EPOLLRDHUP when I guess the MariaDB hangs up after 10 seconds.
Can anyone help me understand if this idea is even workable?
Thanks... Thanks... so much Thanks.
OK for anyone else that lands here, I fixed it or rather un-broke it.
Notice that - from the examples - the returned status from _start / _cont calls are passed in as a parameter to the next _cont. Turns out this is critical.
The status contains flags MYSQL_WAIT_READ, MYSQL_WAIT_WRITE, MYSQL_WAIT_EXCEPT, MYSQL_WAIT_TIMEOUT, and if not passed to the next _cont my guess is you are messing with the _cont state-machine.
I was not saving the state of status between different places where _start and _cont were being called.
struct MC
{
MYSQL *mysql;
int status;
} MC;
...
// Initial call
mc->status = mysql_real_connect_start(&ret, mc->mysql, host, user, password, NULL, 0, NULL, 0);
// EPOLL raised calls.
mc->status = mysql_real_connect_cont(&ret, mc->mysql, mc->status);
if(mc->status) return... // keep waiting check for errors.
Related
I am using gorm v1 (ORM), go version 1.14
DB connection is created at the start of my app
and that DB is being passed throughout the app.
I have a complex & long functionality.
Let's say I have 10 sets of queries to run and the order doesn't matter.
So, what I did was
go queryset1(DB)
go queryset2(DB)
...
go queryset10(DB)
// here I have a wait, maybe via channel or WaitGroup.
Inside queryset1:
func queryset1(db *gorm.DB, /*wg or errChannel*/){
db.Count() // basic count query
wg.Done() or errChannel <- nil
}
Now, the problem is I encounter the error :1040 "too many connections" - Mysql.
Why is this happening? Does every go routine create a new connection?
If so, is there a way to check this & "live connections" in mysql
(Not the show status variables like connection)
How can I concurrently query the DB?
Edit:
This guy has the same problem
The error is not directly related to go-gorm, but to the underlying MySQL configuration and your initial connection configuration. In your code, you can manage the following parameters during your initial connection to the database.
maximum open connections (SetMaxOpenConns function)
maximum idle connections (SetMaxIdleConns function)
maximum timeout for idle connections (SetConnMaxLifetime function)
For more details, check the official docs or this article how to get the maximum performance from your connection configuration.
If you want to prevent a situation where each goroutine uses a separate connection, you can do something like this:
// restrict goroutines to be executed 5 at a time
connCh := make(chan bool, 5)
go queryset1(DB, &wg, connCh)
go queryset2(DB, &wg, connCh)
...
go queryset10(DB, &wg, connCh)
wg.Wait()
close(connCh)
Inside your queryset functions:
func queryset1(db *gorm.DB, wg *sync.WaitGroup, connCh chan bool){
connCh <- true
db.Count() // basic count query
<-connCh
wg.Done()
}
The connCh will allow the first 5 goroutines to write in it and block the execution of the rest of the goroutines until one of the first 5 goroutines takes the value from the connCh channel. This will prevent the situations where each goroutine will start it's own connection. Some of the connections should be reused, but that also depends on the initial connection configuration.
I am new to ReactiveX and reactive programming in general. I need to implement a retry mechanism for Couchbase CAS operations, but the example on the Couchbase website shows a retryWhen which seems to retry indefinitely. I need to have a retry limit and retry count somewhere in there.
The simple retry() would work, since it accepts a retryLimit, but I don't want it to retry on every exception, only on CASMismatchException.
Any ideas? I'm using the RxJava library.
In addition to what Simon Basle said, here is a quick version with linear backoff:
.retryWhen(notification ->
notification
.zipWith(Observable.range(1, 5), Tuple::create)
.flatMap(att ->
att.value2() == 3 ? Observable.error(att.value1()) : Observable.timer(att.value2(), TimeUnit.SECONDS)
)
)
Note that "att" here is a tuple which consists of both the throwable and the number of retries, so you can very specifically implement a return logic based on those two params.
If you want to learn even more, you can peek at the resilient doc I'm currently writing: https://gist.github.com/daschl/db9fcc9d2b932115b679#retry-with-delay
retryWhen is clearly a little bit more complicated than simple retry, but here's the gist of it:
you pass a notificationHandler function to retryWhen which takes an Observable<Throwable> and outputs an Observable<?>
the emission of this returned Observable determine when retry should occur or stop
so, for each occurring Exception in the original stream, if the handler's one emits 1 item, there'll be 1 retry. If it emits 2 items, there'll be 2...
as soon as the handler's stream emits an error, retry is aborted.
Using this, you can both:
work only on CasMismatchExceptions: just have your function return an Observable.error(t) in other cases
retry only for a specific number of times: for each exception, flatMap from an Observable.range representing the max number of retries, have it return an Observable.timer using the retry # if you need increasing delays.
Your use case is pretty close to the one in RxJava doc here
reviving this thread since in the Couchbase Java SDK 2.1.2 there's a new simpler way to do that: use the RetryBuilder:
Observable<Something> retryingObservable =
sourceObservable.retryWhen(
RetryBuilder
//will limit to the relevant exception
.anyOf(CASMismatchException.class)
//will retry only 5 times
.max(5)
//delay doubling each time, from 100ms to 2s
.delay(Delay.linear(TimeUnit.MILLISECONDS, 2000, 100, 2.0))
.build()
);
When using MySQL in C you free the memory using the MySQL API, like so:
MYSQL* connection = NULL;
connection = mysql_init(NULL);
// Stuff...
mysql_close(connection);
But Splint doesn't know that mysql_close is in fact freeing memory, so I get this error:
Fresh storage connection not released before return
A memory leak has been detected. Storage allocated locally is
not released before the last reference to it is lost. (Use
-mustfreefresh to inhibit warning)
How do I tell Splint that mysql_close is deallocating memory? A special annotation to the mysql.h file?
EDIT: OK, maybe the releases *p annotation, if that can be used in a header file. Will try.
EDIT 2: Added /*#releases *sock#*/ to mysql.h, but now get this error:
Releases clauses includes *sock of non-dynamically allocated
type MYSQL
A declaration uses an invalid annotation. (Use -annotationerror to inhibit
warning)
This is the signaure of mysql_close:
void STDCALL mysql_close(/*#notnull#*/ MYSQL *sock) /*#releases *sock#*/;
I believe that proper annotation would be:
void STDCALL mysql_close(/*#special#*/ /*#notnull#*/ MYSQL *sock)
/*#releases sock#*/;
The key, that you have missed is /*#special#*/ annotation, that is required to "activate" so called state clauses. From Splint's documentation, 7.4 State Clauses:
The /*#special#*/ annotation is used to mark a parameter, global
variable, or return value that is described using state clauses.
I am using the following code to read an error message from a byte array and it works fine the first time but if I try to access it the second time it throws an error:
errorData = process.standardError.readUTFBytes(process.standardError.bytesAvailable);
StandardError is of type InboundPipe?
The error is:
Error: Error #3212: Cannot perform operation on a NativeProcess that is not running.
even though the process is running (process.running is true). It's on the second call to readUTFBytes that seems to be the cause.
Update:
Here is the code calling the same call one after another. The error happens on the next line and process.running has not changed from true. Happens on the second call.
errorData = process.standardError.readUTFBytes(process.standardError.bytesAvailable);
errorData = process.standardError.readUTFBytes(process.standardError.bytesAvailable);
I also found out the standardError is a InboundPipe instance and implements IDataInput.
Update 2:
Thanks for all the help. I found this documentation when viewing the bytesAvailable property.
[Read Only] Returns the number of bytes of data available for reading
in the input buffer. User code must call bytesAvailable to ensure that
sufficient data is available before trying to read it with one of the
read methods.
When I call readUTFBytes() it resets the bytes available to 0. So when I read it a second time and there are no bytes available it causes the error. The error is or may be incorrect in my opinion or the native process.running flag is incorrect.
I looked into seeing if it has a position property and it does not, at least not in this instance.
Could you try to set position to zero before reading process, especially before repetitive access:
Moves, or returns the current position, in bytes, of the file pointer into the ByteArray object. This is the point at which the next call to a read method starts reading or a write method starts writing.
//ByteArray example
var source: String = "Some data";
var data: ByteArray = new ByteArray();
data.writeUTFBytes(source);
data.position = 0;
trace(data.readUTFBytes(data.bytesAvailable));
data.position = 0;
trace(data.readUTFBytes(data.bytesAvailable));
This was a tricky problem since the object was not a byte array although it looks and acts like one (same methods and almost same properties). It is an InboundPipe that also implements IDataInput.
I found this documentation when viewing the bytesAvailable property.
[Read Only] Returns the number of bytes of data available for reading
in the input buffer. User code must call bytesAvailable to ensure that
sufficient data is available before trying to read it with one of the
read methods.
When I call readUTFBytes() it resets the bytes available to 0. So when I call it a second time and there are no bytes available it causes the error. The error is or may be incorrect in my opinion or the native process.running flag is incorrect although I have reason to believe it's the former.
The solution is to check bytesAvailable before calling read operations and store the value if it needs to be accessed later.
if (process.standardError.bytesAvailable) {
errorData = process.standardError.readUTFBytes(process.standardError.bytesAvailable);
errorDataArray.push(errorData);
}
I looked into seeing if it has a position property and it does not, at least not in this instance.
I've got code that's performing HTTP requests using WinInet API's asynchronously. In general, my code works, but I'm confused about the 'right' way to do things. In the documentation for InternetReadFile(), it states:
To ensure all data is retrieved, an application must continue to call
the InternetReadFile function until the function returns TRUE and the
lpdwNumberOfBytesRead parameter equals zero.
but in asynchronous mode, it may (or may not) return false, and an error of ERROR_IO_PENDING, indicating it'll do the work asynchronously, and call my callback when finished. If I read the documentation literally, it seems that the asynchronous calls could also just do a partial read of the requested buffer, and require the caller to keep calling InternetReadFile until a read of 0 bytes is encountered.
A typical implementation using InternetReadFile() synchronously would look something like this:
while(InternetReadFile(Request, Buffer, BufferSize, &BytesRead) && BytesRead != 0)
{
// do something with Buffer
}
but with the possibility that any one call to InternetReadFile() could signal that it's going to do the work asynchronously (and perhaps read part, but not all of your request), it becomes much more complicated. If I turn to MSDN sample code for guidance, the implementation is simple, simply calling InternetReadFile() once, and expecting a single return having read the entire requested buffer either instantly or asynchronously. Is this the correct way to use this function, or is MSDN Sample Code ignoring the possibility that InternetReadFile() will only read part of the requested buffer?
After a more careful reading of the asynchronous example, I see now that it is reading repeatedly until a successful read of 0 bytes is encountered. So to answer my own question, you must call InternetReadFile() over and over again, and be prepared for either a synchronous or asynchronous response.
Reading InternetReadFile() repeatedly until it returns TRUE and BytesRead is 0 is a correct way to use InternetReadFile(), but not enough if you work asynchronously.
As MSDN says
When running asynchronously, if a call to InternetReadFile does not result in a completed transaction, it will return FALSE and a subsequent call to GetLastError will return ERROR_IO_PENDING. When the transaction is completed the InternetStatusCallback specified in a previous call to InternetSetStatusCallback will be called with INTERNET_STATUS_REQUEST_COMPLETE.
So InternetReadFile() may return FALSE and set the last error to ERROR_IO_PENDING value if you work in asynchronous mode.
When InternetSetStatusCallback will be called again with INTERNET_STATUS_REQUEST_COMPLETE, the lpvStatusInformation parameter will contain the address of an INTERNET_ASYNC_RESULT structure (see InternetStatusCallback callback function). The INTERNET_ASYNC_RESULT.dwResult member will contain the result of asynchronous operation (TRUE or FALSE since you called InternetReadFile) and the INTERNET_ASYNC_RESULT.dwError will contain a error code only if dwResult is FALSE.
If dwResult is TRUE then your Buffer contains data read from Internet, and the BytesRead contains the number of bytes read asynchronously.
So one of the most important things when you work asynchronously, the Buffer and the BytesRead must be persistent between InternetStatusCallback calls, i.e. must not be allocated on the stack. Otherwise it has undefined behaviour, causes memory corruption, etc.