[Pljava-dev] Is it safe to use multi-threaded PL/Java in single-threaded postgres process?

So, I really think this question has been definitively answered. If you can produce a test case of your concerns which demonstrates the issue, we can discuss it. But the simple answer to your question is simply: yes. Pl/java obeys the rules and works according to spec. It does the right thing in the right way using the best practices.

> On Mar 15, 2014, at 8:56 PM, MauMau <maumau307 at gmail.com> wrote:
>
> I got the reply in pgsql-hackers about this. According to Tom's comment, I'm worried about PL/Java's design.
>
> http://www.postgresql.org/message-id/11347.1394545919 at sss.pgh.pa.us
>
> Is there any rationale that it's safe for PL/Java to:
>
> * load multi-threaded Java VM in single-threaded postgres process (which is what Tom is concerned about)
>
> * make multi-threaded Java VM load single-threaded pljava.so (which is what JNI specification says should not be done).
>
>
> ----- Original Message ----- From: "MauMau" <maumau307 at gmail.com>
> To: <pljava-dev at lists.pgfoundry.org>
> Sent: Saturday, March 08, 2014 7:21 PM
> Subject: Is it safe to use multi-threaded PL/Java in single-threaded postgres process?
>
>
>> Hello,
>>
>> Is PL/Java safe to use in terms of its threading design? I've just asked pgsql-hackers about this and am waiting for response, adding the sentence:
>>
>> "To put the question in other words, is it safe to load a multi-threaded PL library in the single-threaded backend process, if the PL only calls SPI in the main thread?"
>>
>> My understanding is:
>>
>> * PL/Java (pljava.so) is linked with the JNI (Java Native Interface) library, libjvm.so, in JRE. libjvm.so is linked with libpthread.so, because Java VM is multi-threaded. SO, "ldd pljava.so" shows libjvm.so and libpthread.so. pljava.so doesn't seem to be built for multh-threading --- none of -mt, -D_REENTRANT or -D_POSIX_C_SOURCE is specified when building it.
>>
>> * When the application calls Java stored function, pljava.so calls a function in libjvm.so to create a JVM in the backend process, then invokes the user-defined Java method in the main thread. The user-defined Java method calls JDBC methods to access database. The JDBC method calls are translated to backend SPI function calls through JNI.
>>
>> * The main thread can create Java threads using Java Thread API, and those threads can call JDBC methods. However, PL/Java intercepts JDBC method calls and serializes SPI calls. So, only one thread calls SPI functions at a time. I'm wondering if this is the reason why PL/Java is safe for use.
>>
>>
>> What I'm concerned about is whether multi-threaded code (Java VM) can run safely in a single-threaded code (postgres). I cannot point out what can be a particular problem with PL/Java, but in general, the mixture of single-threaded code and multi-threaded one seems to cause trouble around handling errno, memory and file handles/pointers.
>>
>> FYI, JNI specification says that the code called from Java VM should be built for multi-threading as follows. But postgres is not.
>>
>> http://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/design.html#wp9502
>>
>> [Excerpt]
>> Compiling, Loading and Linking Native Methods
>> Since the Java VM is multithreaded, native libraries should also be compiled and linked with multithread aware native compilers. For example, the -mt flag should be used for C++ code compiled with the Sun Studio compiler. For code complied with the GNU gcc compiler, the flags -D_REENTRANT or -D_POSIX_C_SOURCE should be used. For more information please refer to the native compiler documentation.
>
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