Обсуждение: multiple threads using one connection

Hi,

I have multi-thread application written in Ada which uses APQ. All 
threads use the same connection (can't afford one connection per 
thread). I compiled libpq with --enable-thread-safty option.
When two threads try to execute a query at the same time one of them is 
blocked and even when the other one finishes it is not unblocked.

I wrote simple test in c to find out is it APQ fault. But the result was 
the same. Am I doing something wrong?

My libpq version is 8.0.4 and here's the code of this test:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include "libpq-fe.h"
#include <pthread.h>

/* for ntohl/htonl */
#include <netinet/in.h>
#include <arpa/inet.h>

PGconn     *conn;
 static void
exit_nicely(PGconn *conn)
{ //PQfinish(conn);
}


void *run(void *ptr)
{ char *message; PGresult   *res; message = (char *)ptr; res = PQexec(conn, "select * from foo;");
 if (PQresultStatus(res) != PGRES_TUPLES_OK) {   fprintf(stderr, "select failed: %s\n", PQerrorMessage(conn));
PQclear(res);  exit_nicely(conn);   return; }
 
 PQclear(res); printf ("%s completed\n", message);

}
 int
main(int argc, char **argv)
{ const char *conninfo; pthread_t thread1, thread2; char *message1 = "Thread 1"; char *message2 = "Thread 2"; int
iret1,iret2;
 
   //TODO conninfo = "dbname =  user =  password = ";
 /* Make a connection to the database */ conn = PQconnectdb(conninfo);
 /* Check to see that the backend connection was successfully made */ if (PQstatus(conn) != CONNECTION_OK) {
fprintf(stderr,"Connection to database failed: %s",       PQerrorMessage(conn));   exit_nicely(conn); } else {
fprintf(stderr,"Connected.\n"); }
 
 PGresult   *res; res = PQexec(conn, "drop table foo;create table foo(id int);");
 if (PQresultStatus(res) != PGRES_COMMAND_OK) {   fprintf(stderr, "create table failed: %s\n", PQerrorMessage(conn));
PQclear(res);  exit_nicely(conn);   return; }
 
 iret1 = pthread_create( &thread1, NULL, run, (void*) message1); iret2 = pthread_create( &thread2, NULL, run, (void*)
message2);
 pthread_join( thread1, NULL); pthread_join( thread2, NULL); /* close the connection to the database and cleanup */
PQfinish(conn);
 return 0;
}

Robert Gruszka wrote:
> Hi,
> 
> I have multi-thread application written in Ada which uses APQ. All 
> threads use the same connection (can't afford one connection per 
> thread). I compiled libpq with --enable-thread-safty option.
> When two threads try to execute a query at the same time one of them is 
> blocked and even when the other one finishes it is not unblocked.

The libpq API is not really intended to be used by multiple threads
simultaneously (unless you make sure only one thread is submitting a
query and waiting for the result at any time).

The --enable-thread-safety flag is merely to ensure that the calls to
other libraries (Kerberos, etc) are handled in a thread-safe manner.  It
doesn't have any effect on the thread safety of the libpq API itself.

-- 
Alvaro Herrera                                http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

Alvaro Herrera wrote:
> Robert Gruszka wrote:
> > Hi,
> > 
> > I have multi-thread application written in Ada which uses APQ. All 
> > threads use the same connection (can't afford one connection per 
> > thread). I compiled libpq with --enable-thread-safty option.
> > When two threads try to execute a query at the same time one of them is 
> > blocked and even when the other one finishes it is not unblocked.
> 
> The libpq API is not really intended to be used by multiple threads
> simultaneously (unless you make sure only one thread is submitting a
> query and waiting for the result at any time).
> 
> The --enable-thread-safety flag is merely to ensure that the calls to
> other libraries (Kerberos, etc) are handled in a thread-safe manner.  It
> doesn't have any effect on the thread safety of the libpq API itself.

Just to clarify, libpq is thread-safe is each thread uses its own
connection object.

--  Bruce Momjian   http://candle.pha.pa.us SRA OSS, Inc.   http://www.sraoss.com
 + If your life is a hard drive, Christ can be your backup. +

On Wed, 1 Mar 2006 05:17 am, Bruce Momjian wrote:
> Just to clarify, libpq is thread-safe is each thread uses its own
> connection object.

Hope it's okay if I clarify this a little further, Bruce...

Robert, I think you *should* be able to use the same connection from multiple 
threads so long as you do your own work to avoid concurrent use of the 
connection. I'm sorry, I know less than nothing about Ada, but in C terms 
you'd be using a mutex, acquiring it before your thread issues a statement on 
the connection, reading back all data etc., then releasing the mutex. Other 
threads can then acquire the mutex and use the connection. I assume Ada 
provides some similar sort of concurrency control.

On a side note, the preferred way is to use a connection per thread... and 
given the overhead of thread creation, ISTM the cost of a connection wouldn't 
be that high... but I assume you have some good reason for doing it this way. 
I suppose a connection pool might be another way you could handle this 
requirement.

I say it *should* work because I know this works with ECPG, and AFAIK it uses 
libpq calls, so should be similar for your language binding, but please test 
it with a sample app before assuming that I know what I'm talking about. Can 
someone correct me if I have this wrong? It's been a while since I had to 
think about this stuff...

Regards, Philip.

-- 

"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it." - Brian W. Kernighan

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