Accessing Percona XtraDB Cluster nodes in parallel with MySQL asynchronous calls
This post is followup to Peter’s recent post, “Investigating MySQL Replication Latency in Percona XtraDB Cluster,” in which a question was raised as to whether we can measure latency to all nodes at the same time. It is an interesting question: If we have N nodes, can we send queries to nodes to be executed in parallel?
To answer it, I decided to try a new asynchronous call to send a query to MySQL using a new MySQLnd driver for PHP. In this post I’ll only show how to make these calls, and in following posts how to measure latency to all nodes.
PHP does not provide a way for parallel execution, so this is where an asynchronous call helps. What does this call do? Basically we send a query to MySQL, and we do not wait for result but rather get response later.
The MySQLnd driver has been available since PHP 5.3, and in most part it mimics the standard MySQLi driver and functions it provides. But in addition to that, it also provides a function, mysqli_poll, which unfortunately is marked as “not documented,” however we still can use it — using an example from PHP docs.
So there is my example on how to access Percona XtraDB Cluster nodes in parallel:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 | $reader_hosts = array( "192.88.225.243", "192.88.225.242", "192.88.225.240", "192.88.225.160", "192.88.225.159" ); $all_links=array(); # Establish connections foreach ($reader_hosts as $i) { $mysqli = new mysqli($i, 'root', '', 'test'); if ($mysqli->connect_error) { echo 'Connect Error (' . $mysqli->connect_errno . ') ' . $mysqli->connect_error; }else{ $all_links[]=$mysqli; $mysqli->query("SET wsrep_causal_reads=1"); } } # Run queries in parallel: foreach ($all_links as $linkid => $link) { $link->query("SELECT something FROM tableN WHERE ", MYSQLI_ASYNC); } $processed = 0; do { $links = $errors = $reject = array(); foreach ($all_links as $link) { $links[] = $errors[] = $reject[] = $link; } # loop to wait on results if (!mysqli_poll($links, $errors, $reject, 60)) { continue; } foreach ($links as $k=>$link) { if ($result = $link->reap_async_query()) { $res = $result->fetch_row(); # Handle returned result mysqli_free_result($result); } else die(sprintf("MySQLi Error: %s", mysqli_error($link))); $processed++; } } while ($processed < count($all_links)); |
As conclusion, we see that using PHP with MySQLnd drivers we can execute 5 MySQL asynchronous queries in parallel against 5 different nodes of Percona XtraDB Cluster.
This reminds me of Cassandra’s multiget() and non-blocking socket with select()
Will be interesting for what practical purpose this can be used.
If MySQLnd was coupled with a UI that allowed for data to be pushed via websockets, then it would be a very interesting mechanism for a single request to initiate multiple different fetches and actions, all of which could be returned asynchronously over the socket to a waiting handler.
Great article. We’ve been looking for a way to do this, I wonder if this is available with PDO interface for php/mySQL?
It would seem this would be great for de-aggregating queries of the following type where the final number of rows is small, but the aggregation is terribly slow and could be better done in an array to avoid filesorts and to execute the sub-queries in parallel.