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NAME |
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Coro::Mysql - let other threads run while doing mysql requests |
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SYNOPSIS |
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use Coro::Mysql; |
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my $DBH = Coro::Mysql::unblock DBI->connect (...); |
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DESCRIPTION |
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(Note that in this manual, "thread" refers to real threads as |
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implemented by the Coro module, not to the built-in windows process |
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emulation which unfortunately is also called "threads") |
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This module replaces the I/O handlers for a database connection, with |
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the effect that "patched" database handles no longer block the all |
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threads of a process, but only the thread that does the request. |
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This can be used to make parallel sql requests using Coro, or to do |
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other stuff while mysql is rumbling in the background. |
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CAVEAT |
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Note that this module must be linked against exactly the same (shared, |
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possibly not working with all OSes) libmysqlclient library as |
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DBD::mysql, otherwise it will not work. |
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Also, this module requires a header file that apparently isn't installed |
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everywhere (violite.h), and therefore comes with it's own copy, which |
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might or might not be compatible to the violite.h of your library - when |
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in doubt, make sure all the libmysqlclient header files are installed |
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and delete the violite.h header that comes with this module. |
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On the good side, this module does a multitude of checks to ensure that |
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the libray versions match on the binary level, so on incompatibilities |
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you should expect an exception when trying to unblock a handle, rather |
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than data corruption. |
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Also, while this module makes database handles non-blocking, you still |
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cannot run multiple requests in parallel on the same database handle. If |
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you want to run multiple queries in parallel, you have to create |
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multiple database connections, one for each thread that runs queries. |
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Not doing so can corrupt your data - use a Coro::Semaphore to protetc |
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access to a shared database handle when in doubt. |
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If you make sure that you never run two or more requests in parallel, |
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you can freely share the database handles between threads, of course. |
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SPEED |
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This module is implemented in XS, and as long as mysqld replies quickly |
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enough, it adds no overhead to the standard libmysql communication |
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routines (which are very badly written, btw.). In fact, since it has a |
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more efficient buffering and allows requests to run in parallel, it |
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often decreases the actual time to run many queries considerably. |
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For very fast queries ("select 0"), this module can add noticable |
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overhead (around 15%, 7% when EV can be used) as it tries to switch to |
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other coroutines when mysqld doesn't deliver the data immediately, |
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although, again, when running queries in parallel, they will usually |
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execute faster. |
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For most types of queries, there will be no extra latency, especially on |
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multicore systems where your perl process can do other things while |
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mysqld does its stuff. |
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LIMITATIONS |
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This module only supports "standard" mysql connection handles - this |
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means unix domain or TCP sockets, and excludes SSL/TLS connections, |
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named pipes (windows) and shared memory (also windows). No support for |
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these connection types is planned, either. |
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CANCELLATION |
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Cancelling a thread that is within a mysql query will likely make the |
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handle unusable. As far as Coro::Mysql is concerned, the handle can be |
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safely destroyed, but it's not clear how mysql itself will react to a |
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cancellation. |
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FUNCTIONS |
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Coro::Mysql offers a single user-accessible function: |
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$DBH = Coro::Mysql::unblock $DBH |
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This function takes a DBI database handles and "patches" it so it |
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becomes compatible to Coro threads. |
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After that, it returns the patched handle - you should always use |
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the newly returned database handle. |
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It is safe to call this function on any database handle (or just |
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about any value), but it will only do anything to DBD::mysql |
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handles, others are returned unchanged. That means it is harmless |
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when applied to database handles of other databases. |
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It is also safe to pass "undef", so code like this is works as |
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expected: |
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my $dbh = DBI->connect ($database, $user, $pass)->Coro::Mysql::unblock |
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or die $DBI::errstr; |
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USAGE EXAMPLE |
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This example uses PApp::SQL and Coro::on_enter to implement a function |
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"with_db", that connects to a database, uses "unblock" on the resulting |
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handle and then makes sure that $PApp::SQL::DBH is set to the |
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(per-thread) database handle when the given thread is running (it does |
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not restore any previous value of $PApp::SQL::DBH, however): |
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use Coro; |
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use Coro::Mysql; |
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use PApp::SQL; |
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sub with_db($$$&) { |
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my ($database, $user, $pass, $cb) = @_; |
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my $dbh = DBI->connect ($database, $user, $pass)->Coro::Mysql::unblock |
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or die $DBI::errstr; |
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Coro::on_enter { $PApp::SQL::DBH = $dbh }; |
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$cb->(); |
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} |
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This function makes it possible to easily use PApp::SQL with |
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Coro::Mysql, without worrying about database handles. |
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# now start 10 threads doing stuff |
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async { |
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with_db "DBI:mysql:test", "", "", sub { |
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sql_exec "update table set col = 5 where id = 7"; |
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my $st = sql_exec \my ($id, $name), |
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"select id, name from table where name like ?", |
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"a%"; |
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while ($st->fetch) { |
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... |
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} |
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my $id = sql_insertid sql_exec "insert into table values (1,2,3)"; |
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# etc. |
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}; |
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} for 1..10; |
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SEE ALSO |
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Coro, PApp::SQL (a user friendly but efficient wrapper around DBI). |
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HISTORY |
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This module was initially hacked together within a few hours on a long |
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flight to Malaysia, and seems to have worked ever since, with minor |
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adjustments for newer libmysqlclient libraries. |
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AUTHOR |
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Marc Lehmann <schmorp@schmorp.de> |
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http://home.schmorp.de/ |
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