如果你是指的 active_record 的连接池 connection_pool 的话，在源码 https://github.com/rails/rails/blob/v5.0.7/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb#L345 里。

@connections存了当前进程从数据库获取到的总连接；

@available存@connections中空闲的，可分配给线程的连接；

@thread_cached_conns是个 hash，存了@connections中已分配给线程的连接。

# rails c例子
# 连接池最多可以有多少个连接
ActiveRecord::Base.connection_pool.instance_eval { @size }

# 连接池目前已经从数据库获得了多少个连接
ActiveRecord::Base.connection_pool.instance_eval { @connections.size }

# 当前线程占用一个连接
User.first
ActiveRecord::Base.connection_pool.instance_eval { @connections.size }

# 新起一个线程，又占用一个连接
Thread.new {User.first; sleep}
ActiveRecord::Base.connection_pool.instance_eval { @connections.size }

# 查看每个线程占用的连接
ActiveRecord::Base.connection_pool.instance_eval { @thread_cached_conns.keys }

# 释放当前线程的连接
ActiveRecord::Base.connection_pool.release_connection
ActiveRecord::Base.connection_pool.instance_eval { @connections.size }
ActiveRecord::Base.connection_pool.instance_eval { @available.instance_eval{@queue.size} }

如果你是指数据库当前本身有多少个连接的话，mysql 的话，我查到是如下的 sql

ActiveRecord::Base.connection.exec_query("show status where `variable_name` = 'Threads_connected';")

建议独立监控

就是要单独监控

谢谢哈，源代码看了很久都不太清楚

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