📄 otl3_stream_pooling.htm
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<H1>OTL 4.0, OTL stream pooling</H1>
<P>OTL stream pooling is a new mechanism which allows to turn inefficently
managed otl_stream's into something of a greater value as far as performance
is concerned. Instances of otl_stream variables get saved in a stream pool,
when they get closed, for further reuse in the program. Typically, the
program may have a handful of otl_stream variables, defined locally in
functions. The streams get instantiated and closed all over again, especially
when functions get called repeatedly. </P>
<P>Each instantiation of a stream triggers re-parsing of the stream's SQL
statement on the database back end. It is a relatively expensive operation.
The stream pooling mechanism alleviates that burden and simplifies coding
techniques. </P>
<P>When a stream variable gets closed, the actual instance of the otl_stream
gets saved in the stream pool. Then if there is a request to a similar
OTL stream, the OTL stream instance can be assigned to the otl_stream variable,
which, may be, in its turn, either a local [in function] variable, or allocated
in the heap. The <I>similarity</I> of the streams is defined as <I>stream
buffer size</I> + <I>SQL statement text</I>, meaning that streams with
the same SQL statements and buffer sizes will be saved in the same bucket
in the stream pool. </P>
<P>Say, three instances of <I>similar</I> streams of <buf_size1+SQLStm1>
were allocated and then all of them were closed. The pool will have one
entry point for all the three and the actual bodies of the instances will
be placed in the same bucket (see the diagram below). After that, the functions,
in which the streams had been created, get called on the second iteration
of a loop, and the corresponding otl_stream variables get instantiated
again. The second time around, the instances of the similar streams, which
were previously saved in the pool, will be pulled out of the pool and re-assigned
to the variables, until the variables get closed again.</P>
<P>In some cases, an instance of a stream, if it is, say, used only once,
can bypass the pool and be destroyed without saving it in the pool. The
otl_stream::<A HREF="otl3_stream_class.htm#close">close</A>() function
has extended functionality to allow the developer to do that.</P>
<P>If the stream pooling is undesirable, then the OTL code, which implements
the pooling, can be turned off (not turned on at all). This is controlled
by a conditional compilation <A HREF="otl3_compile.htm#OTL_STREAM_POOLING_ON">directive</A>,
since OTL is a C++ template as well as macro library with a lot of conditional
compilation directives in it, for efficient code generation.</P>
<P>The stream pool has a fixed maximum size, that can be changed by the
otl_connect::<A HREF="otl3_connect_class.htm#set_stream_pool_size">set_stream_pool_size</A>()
function. And, as it already should be obvious from the description above,
the stream pool is a property of the <A HREF="otl3_connect_class.htm">otl_connect</A>
object. </P>
<P>The implementation of the stream pool was made with the help of STL
containers, maps and vectors in particular. Therefore, #define <A HREF="otl3_compile.htm#OTL_STL">OTL_STL</A>
needs to be enabled in order to turn on the OTL code in the STL compliance
mode.</P>
<P>From the statistic that I (<A HREF="resume.htm">Sergei Kuchin</A>) was
able to gather, the stream pooling has very little overhead (2-4%), compared
to the use of otl_stream's, which are kept open always, and don't get closed.
This is due to the fact that the std::map<>, as an STL data
container, is performant enough.</P>
<P><IMG SRC="pool.gif" HEIGHT=664 WIDTH=795><BR>
<HR WIDTH="100%"></P>
<CENTER><PRE><A HREF="home.htm">Go Home</A> </PRE></CENTER>
<P>Copyright © 1996, 2008, Sergei Kuchin, email: <A HREF="mailto:skuchin@aceweb.com">skuchin@aceweb.com</A>,
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