数据库连接池的database.yml单线程的Rails应用程序正确设置 [英] Correct setting of database connection pool database.yml for single-threaded Rails applications

问题描述

我想知道在Rails的database.yml以下设置：

I was wondering about the following setting in the Rails database.yml:

默认为ActiveRecord的的连接池数据库连接的数量设置为5：

By default the number of database connections for the connection pool of ActiveRecord is set to 5:

development:
  ...
  pool: 5

不过，在默认情况下，轨道3是单线程的。 为什么你需要5个连接默认？

But by default, Rails 3 is single threaded. Why would you need 5 connections by default?

据我了解，一个单线程的Rails应用程序不能在一次触发多个数据库操作，为什么要那么做，你需要保持更多的连接打开？

As far as I understand, a single threaded Rails app can't trigger multiple database operations at once, why would do you need to keep more connection open?

我会假设，2个连接将使感，让你随时拥有，即使另一个超时一个活动连接，但抱着五个连接似乎有点奇怪我。

I would assume that 2 connections would make sense, so you always have one active connection even if the other one times out, but holding five connections seems a little odd to me.

我缺少的东西？

更新 如果任何人是好奇，我刚刚发现了一个承诺，解释它： <一href="https://github.com/rails/rails/commit/b700153507b7d539a57a6e3bcf03c84776795051">https://github.com/rails/rails/commit/b700153507b7d539a57a6e3bcf03c84776795051

UPDATE If anyone else is curious, I just found a commit that explains it: https://github.com/rails/rails/commit/b700153507b7d539a57a6e3bcf03c84776795051

其实这些默认设置都没有任何意义，它是固定的，因为测试套件，但随后暂时恢复（一年前）。

In fact these default settings don't make any sense, it was fixed but then temporarily reverted (one year ago) because of the test suite.

推荐答案

连接池像杂种/乘客/等单线程服务器的主要好处是，连接建立/保持在一个机架处理程序之外的主轨要求加工。这允许，因为它是用不同的方式来建立一次与多次的连接。我们的目标是重复使用所建立的连接，并尽量减少连接的数目。这应该prevent其请求之间给定的请求处理周期内重新连接，甚至可能（如果我没有记错）。

Manage Connections

The major benefit of connection pooling for a single-thread server like Mongrel/Passenger/etc is that the connection is established/maintained in a Rack handler outside the main Rails request processing. This allows for a connection to be established once vs. many times as it's used in different ways. The goal is to re-use the established connection and minimize the number of connections. This should prevent having to reconnect within a given request processing cycle and possibly even between requests (if I recall correctly).

虽然大多数用例（杂种/乘客）是单线程的，只能使用一次的单一连接 - 有JRuby和环境具有完全多线程的支持/应用服务器。因为2.2的Rails已经线程安全的。

Although most use cases (Mongrel/Passenger) are single threaded and can only use a single connection at a time - there is JRuby and environments/app servers that have full multi-threaded support. Rails has been thread safe since 2.2

• 连接池是ActiveRecord的内部处理的，因此所有应用程序服务器应该表现基本相同。

• Connection pooling is handled inside of ActiveRecord, so all application servers should behave basically the same.

数据库连接池开始是空的，并创建根据需求在一段时间的连接。这个池默认的最大大小为5，配置在database.yml中。

The database connection pool starts out empty and creates connections over time according to demand. The maximum size of this pool defaults to 5 and is configured in database.yml.

请求，并从该池的用户共享连接。请求检查出一个连接在第一时间，它需要访问数据库，然后检查连接回在请求的结束。

Requests and users share connections from this pool. A request checks out a connection the first time it needs to access the database and then checks the connection back in at the end of the request.

如果您使用Rails.threadsafe！模式，那么多线程可能会 访问的同时多个连接，所以根据 请求负载，你可能有多个线程争几 连接。

If you use Rails.threadsafe! mode, then multiple threads might be accessing multiple connections at the same time, so depending on the request load you might have multiple threads contending for a few connections.

您可以相应地改变，如果你使用的是单线程应用程序。默认值是5按照大多数用户的需要，因为现在每天的正常的，有一个多线程的应用程序。

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