Question

我正在使用Goliath（由eventmachine提供支持）和postgres gem pg，目前我正以阻塞的方式使用pg gem：conn.exec('SELECT * FROM products')（例如）我想知道是否有更好的方法连接到postgres数据库？

Answer 1

pg库为PostgreSQL的异步API提供全面支持。我添加了an example如何将其用于samples/目录：

#!/usr/bin/env ruby

require 'pg'

# This is a example of how to use the asynchronous API to query the
# server without blocking other threads. It's intentionally low-level;
# if you hooked up the PGconn#socket to some kind of reactor, you
# could make this much nicer.

TIMEOUT = 5.0 # seconds to wait for an async operation to complete
CONN_OPTS = {
    :host     => 'localhost',
    :dbname   => 'test',
    :user     => 'jrandom',
    :password => 'banks!stealUR$',
}

# Print 'x' continuously to demonstrate that other threads aren't
# blocked while waiting for the connection, for the query to be sent,
# for results, etc. You might want to sleep inside the loop or 
# comment this out entirely for cleaner output.
progress_thread = Thread.new { loop { print 'x' } }

# Output progress messages
def output_progress( msg )
    puts "\n>>> #{msg}\n"
end

# Start the connection
output_progress "Starting connection..."
conn = PGconn.connect_start( CONN_OPTS ) or 
    abort "Unable to create a new connection!"
abort "Connection failed: %s" % [ conn.error_message ] if
    conn.status == PGconn::CONNECTION_BAD

# Now grab a reference to the underlying socket so we know when the
# connection is established
socket = IO.for_fd( conn.socket )

# Track the progress of the connection, waiting for the socket to 
# become readable/writable before polling it
poll_status = PGconn::PGRES_POLLING_WRITING
until poll_status == PGconn::PGRES_POLLING_OK ||
      poll_status == PGconn::PGRES_POLLING_FAILED

    # If the socket needs to read, wait 'til it becomes readable to
    # poll again
    case poll_status
    when PGconn::PGRES_POLLING_READING
        output_progress "  waiting for socket to become readable"
        select( [socket], nil, nil, TIMEOUT ) or
            raise "Asynchronous connection timed out!"

    # ...and the same for when the socket needs to write
    when PGconn::PGRES_POLLING_WRITING
        output_progress "  waiting for socket to become writable"
        select( nil, [socket], nil, TIMEOUT ) or
            raise "Asynchronous connection timed out!"
    end

    # Output a status message about the progress
    case conn.status
    when PGconn::CONNECTION_STARTED
        output_progress "  waiting for connection to be made."
    when PGconn::CONNECTION_MADE
        output_progress "  connection OK; waiting to send."
    when PGconn::CONNECTION_AWAITING_RESPONSE
        output_progress "  waiting for a response from the server."
    when PGconn::CONNECTION_AUTH_OK
        output_progress "  received authentication; waiting for " +
                        "backend start-up to finish."
    when PGconn::CONNECTION_SSL_STARTUP
        output_progress "  negotiating SSL encryption."
    when PGconn::CONNECTION_SETENV
        output_progress "  negotiating environment-driven " +
                        "parameter settings."
    end

    # Check to see if it's finished or failed yet
    poll_status = conn.connect_poll
end

abort "Connect failed: %s" % [ conn.error_message ] unless 
    conn.status == PGconn::CONNECTION_OK

output_progress "Sending query"
conn.send_query( "SELECT * FROM pg_stat_activity" )

# Fetch results until there aren't any more
loop do
    output_progress "  waiting for a response"

    # Buffer any incoming data on the socket until a full result 
    # is ready. 
    conn.consume_input
    while conn.is_busy
        select( [socket], nil, nil, TIMEOUT ) or
            raise "Timeout waiting for query response."
        conn.consume_input
    end

    # Fetch the next result. If there isn't one, the query is 
    # finished
    result = conn.get_result or break

    puts "\n\nQuery result:\n%p\n" % [ result.values ]
end

output_progress "Done."
conn.finish

if defined?( progress_thread )
    progress_thread.kill
    progress_thread.join
end

我建议您阅读PostgreSQL手册的PQconnectStart函数和Asynchronous Command Processing部分的文档，然后将其与上面的示例进行比较。

之前我没有使用过EventMachine，但是如果它允许你注册套接字和回调它变得可读/可写，我认为将数据库调用集成到它中相当容易。

我一直想使用Ilya Grigorik's article on using Fibers to clean up evented code中的想法来使异步API更容易使用，但这还有很长的路要走。如果您对自己感兴趣/有动力，我会a ticket open跟踪它。

Answer 2

是的，你可以从goliath以非阻塞的方式访问postgres。我有同样的需要，并将这个概念证明放在一起：https://github.com/levicook/goliath-postgres-spike

Answer 3

我（不再）非常熟悉Pg，但我没有听说过任何流行的数据库都可以异步连接。因此，您仍需要在查询期间保持与数据库的连接。因此，您仍然需要阻止堆栈中的某些位置。

根据您的应用程序，您可能已经采用了最佳方式。

但是当你处理某种轮询应用程序时（同一个客户端在短时间内发送大量请求）并且将响应输出更为重要，即使它是空的，那么你可以编写一个ruby { {1}}或flull烧结线程或长期存在的进程，代理查询数据库并缓存结果。

例如：来自客户端A的请求.Goliath应用程序使用一些唯一ID处理对数据库进程的查询，并使用'no data yet'响应查询。数据库进程完成查询并将结果保存到具有ID的缓存中。当下一个请求来自同一个客户端时，Goliath发现它已经有等待的查询结果，从缓存中删除结果并响应客户端。同时，它使用数据库进程安排下一个查询，以便更快地准备好。如果下一个请求在最后一个请求完成之前进入，则不会安排新查询（不会将查询相乘）。

这样，您的响应快速且无阻塞，同时仍然可以从DB ASAP提供新数据。当然，它们可能与实际数据有点不同步，但同样，根据应用程序的不同，这可能不是问题。

Answer 4

我们的想法是使用与数据库（Postgresql）的异步适配器和一个公平的Web服务器（Goliath）来获得性能。 Mike Perham去年为Rails 2.3写了PG activerecord adaptor。也许你可以使用它。

另一个例子是，Ilya Grigorik发布了this demo异步Rails堆栈。在这种情况下，事件服务器是Thin，数据库是Mysql。安装演示并尝试使用和不使用EM感知驱动程序的基准测试。差别很大。

使用postgresql gem async

4 个答案: