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9DESCRIPTION 9DESCRIPTION
10 (Note that in this manual, "thread" refers to real threads as 10 (Note that in this manual, "thread" refers to real threads as
11 implemented by the Coro module, not to the built-in windows process 11 implemented by the Coro module, not to the built-in windows process
12 emulation which unfortunately is also called "threads") 12 emulation which unfortunately is also called "threads")
13 13
14 This module "patches" DBD::mysql database handles so that they do not 14 This module replaces the I/O handlers for a database connection, with
15 block the whole process, but only the thread that they are used in. 15 the effect that "patched" database handles no longer block the all
16 threads of a process, but only the thread that does the request.
16 17
17 This can be used to make parallel sql requests using Coro, or to do 18 This can be used to make parallel sql requests using Coro, or to do
18 other stuff while mysql is rumbling in the background. 19 other stuff while mysql is rumbling in the background.
19 20
20 CAVEAT 21 CAVEAT
21 Note that this module must be linked against exactly the same 22 Note that this module must be linked against exactly the same (shared,
23 possibly not working with all OSes) libmysqlclient library as
22 libmysqlclient library as DBD::mysql, otherwise it will not work. 24 DBD::mysql, otherwise it will not work.
25
26 Also, this module requires a header file that apparently isn't installed
27 everywhere (violite.h), and therefore comes with it's own copy, which
28 might or might not be compatible to the violite.h of your library - when
29 in doubt, make sure all the libmysqlclient header files are installed
30 and delete the violite.h header that comes with this module.
31
32 On the good side, this module does a multitude of checks to ensure that
33 the libray versions match on the binary level, so on incompatibilities
34 you should expect an exception when trying to unblock a handle, rather
35 than data corruption.
23 36
24 Also, while this module makes database handles non-blocking, you still 37 Also, while this module makes database handles non-blocking, you still
25 cannot run multiple requests in parallel on the same database handle. If 38 cannot run multiple requests in parallel on the same database handle. If
26 you want to run multiple queries in parallel, you have to create 39 you want to run multiple queries in parallel, you have to create
27 multiple database connections, one for each thread that runs queries. 40 multiple database connections, one for each thread that runs queries.
41 Not doing so can corrupt your data - use a Coro::Semaphore to protetc
42 access to a shared database handle when in doubt.
28 43
29 If you make sure that you never run two or more requests in parallel, 44 If you make sure that you never run two or more requests in parallel,
30 you cna freely share the database handles between threads, of course. 45 you can freely share the database handles between threads, of course.
31
32 Also, this module uses a number of "unclean" techniques (patching an
33 internal libmysql structure for one thing) and was hacked within a few
34 hours on a long flight to Malaysia.
35
36 It does, however, check whether it indeed got the structure layout
37 correct, so you should expect perl exceptions or early crashes as
38 opposed to data corruption when something goes wrong.
39 46
40 SPEED 47 SPEED
41 This module is implemented in XS, and as long as mysqld replies quickly 48 This module is implemented in XS, and as long as mysqld replies quickly
42 enough, it adds no overhead to the standard libmysql communication 49 enough, it adds no overhead to the standard libmysql communication
43 routines (which are very badly written). 50 routines (which are very badly written, btw.). In fact, since it has a
51 more efficient buffering and allows requests to run in parallel, it
52 often decreases the actual time to run many queries considerably.
44 53
45 For very fast queries ("select 0"), this module can add noticable 54 For very fast queries ("select 0"), this module can add noticable
46 overhead (around 15%) as it tries to switch to other coroutines when 55 overhead (around 15%, 7% when EV can be used) as it tries to switch to
47 mysqld doesn't deliver the data instantly. 56 other coroutines when mysqld doesn't deliver the data immediately,
57 although, again, when running queries in parallel, they will usually
58 execute faster.
48 59
49 For most types of queries, there will be no overhead, especially on 60 For most types of queries, there will be no extra latency, especially on
50 multicore systems where your perl process can do other things while 61 multicore systems where your perl process can do other things while
51 mysqld does its stuff. 62 mysqld does its stuff.
63
64 LIMITATIONS
65 This module only supports "standard" mysql connection handles - this
66 means unix domain or TCP sockets, and excludes SSL/TLS connections,
67 named pipes (windows) and shared memory (also windows). No support for
68 these connection types is planned, either.
69
70CANCELLATION
71 Cancelling a thread that is within a mysql query will likely make the
72 handle unusable. As far as Coro::Mysql is concerned, the handle can be
73 safely destroyed, but it's not clear how mysql itself will react to a
74 cancellation.
75
76FUNCTIONS
77 Coro::Mysql offers a single user-accessible function:
52 78
53 $DBH = Coro::Mysql::unblock $DBH 79 $DBH = Coro::Mysql::unblock $DBH
54 This function takes a DBI database handles and "patches" it so it 80 This function takes a DBI database handles and "patches" it so it
55 becomes compatible to Coro threads. 81 becomes compatible to Coro threads.
56 82
57 After that, it returns the patched handle - you should always use 83 After that, it returns the patched handle - you should always use
58 the newly returned database handle. 84 the newly returned database handle.
59 85
86 It is safe to call this function on any database handle (or just
87 about any value), but it will only do anything to DBD::mysql
88 handles, others are returned unchanged. That means it is harmless
89 when applied to database handles of other databases.
90
91 It is also safe to pass "undef", so code like this is works as
92 expected:
93
94 my $dbh = DBI->connect ($database, $user, $pass)->Coro::Mysql::unblock
95 or die $DBI::errstr;
96
97USAGE EXAMPLE
98 This example uses PApp::SQL and Coro::on_enter to implement a function
99 "with_db", that connects to a database, uses "unblock" on the resulting
100 handle and then makes sure that $PApp::SQL::DBH is set to the
101 (per-thread) database handle when the given thread is running (it does
102 not restore any previous value of $PApp::SQL::DBH, however):
103
104 use Coro;
105 use Coro::Mysql;
106 use PApp::SQL;
107
108 sub with_db($$$&) {
109 my ($database, $user, $pass, $cb) = @_;
110
111 my $dbh = DBI->connect ($database, $user, $pass)->Coro::Mysql::unblock
112 or die $DBI::errstr;
113
114 Coro::on_enter { $PApp::SQL::DBH = $dbh };
115
116 $cb->();
117 }
118
119 This function makes it possible to easily use PApp::SQL with
120 Coro::Mysql, without worrying about database handles.
121
122 # now start 10 threads doing stuff
123 async {
124
125 with_db "DBI:mysql:test", "", "", sub {
126 sql_exec "update table set col = 5 where id = 7";
127
128 my $st = sql_exec \my ($id, $name),
129 "select id, name from table where name like ?",
130 "a%";
131
132 while ($st->fetch) {
133 ...
134 }
135
136 my $id = sql_insertid sql_exec "insert into table values (1,2,3)";
137 # etc.
138 };
139
140 } for 1..10;
141
142SEE ALSO
143 Coro, PApp::SQL (a user friendly but efficient wrapper around DBI).
144
145HISTORY
146 This module was initially hacked together within a few hours on a long
147 flight to Malaysia, and seems to have worked ever since, with minor
148 adjustments for newer libmysqlclient libraries.
149
60AUTHOR 150AUTHOR
61 Marc Lehmann <schmorp@schmorp.de> 151 Marc Lehmann <schmorp@schmorp.de>
62 http://home.schmorp.de/ 152 http://home.schmorp.de/
63 153

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