今天在寫zabbix storm job監控腳本的時候用到了python的redis模塊，之前也有用過，但是沒有過多的了解，今天看了下相關的api和源碼,看到有ConnectionPool的實現，這裡簡單說下。
在ConnectionPool之前，如果需要連接redis，我都是用StrictRedis這個類，在源碼中可以看到這個類的具體解釋：
redis.StrictRedis Implementation of the Redis protocol.This abstract class provides a Python interface to all Redis commands and an
implementation of the Redis protocol.Connection and Pipeline derive from this, implementing how the commands are sent and received to the Redis server
使用的方法：
r=redis.StrictRedis(host=xxxx, port=xxxx, db=xxxx)
r.xxxx()
有了ConnectionPool這個類之後，可以使用如下方法
pool = redis.ConnectionPool(host=xxx, port=xxx, db=xxxx)
r = redis.Redis(connection_pool=pool)
這裡Redis是StrictRedis的子類
簡單分析如下：
在StrictRedis類的__init__方法中，可以初始化connection_pool這個參數，其對應的是一個ConnectionPool的對象：
class StrictRedis(object):
........
def __init__(self, host='localhost', port=6379,
db=0, password=None, socket_timeout=None,
socket_connect_timeout=None,
socket_keepalive=None, socket_keepalive_options=None,
connection_pool=None, unix_socket_path=None,
encoding='utf-8', encoding_errors='strict',
charset=None, errors=None,
decode_responses=False, retry_on_timeout=False,
ssl=False, ssl_keyfile=None, ssl_certfile=None,
ssl_cert_reqs=None, ssl_ca_certs=None):
if not connection_pool:
..........
connection_pool = ConnectionPool(**kwargs)
self.connection_pool = connection_pool
在StrictRedis的實例執行具體的命令時會調用execute_command方法，這裡可以看到具體實現是從連接池中獲取一個具體的連接，然後執行命令，完成後釋放連接：
# COMMAND EXECUTION AND PROTOCOL PARSING
def execute_command(self, *args, **options):
"Execute a command and return a parsed response"
pool = self.connection_pool
command_name = args[0]
connection = pool.get_connection(command_name, **options) #調用ConnectionPool.get_connection方法獲取一個連接
try:
connection.send_command(*args) #命令執行，這裡為Connection.send_command
return self.parse_response(connection, command_name, **options)
except (ConnectionError, TimeoutError) as e:
connection.disconnect()
if not connection.retry_on_timeout and isinstance(e, TimeoutError):
raise
connection.send_command(*args)
return self.parse_response(connection, command_name, **options)
finally:
pool.release(connection) #調用ConnectionPool.release釋放連接
在來看看ConnectionPool類：
class ConnectionPool(object):
...........
def __init__(self, connection_class=Connection, max_connections=None,
**connection_kwargs): #類初始化時調用構造函數
max_connections = max_connections or 2 ** 31
if not isinstance(max_connections, (int, long)) or max_connections < 0: #判斷輸入的max_connections是否合法
raise ValueError('"max_connections" must be a positive integer')
self.connection_class = connection_class #設置對應的參數
self.connection_kwargs = connection_kwargs
self.max_connections = max_connections
self.reset() #初始化ConnectionPool 時的reset操作
def reset(self):
self.pid = os.getpid()
self._created_connections = 0 #已經創建的連接的計數器
self._available_connections = [] #聲明一個空的數組，用來存放可用的連接
self._in_use_connections = set() #聲明一個空的集合，用來存放已經在用的連接
self._check_lock = threading.Lock()
.......
def get_connection(self, command_name, *keys, **options): #在連接池中獲取連接的方法
"Get a connection from the pool"
self._checkpid()
try:
connection = self._available_connections.pop() #獲取並刪除代表連接的元素，在第一次獲取connectiong時，因為_available_connections是一個空的數組，
會直接調用make_connection方法
except IndexError:
connection = self.make_connection()
self._in_use_connections.add(connection) #向代表正在使用的連接的集合中添加元素
return connection
def make_connection(self): #在_available_connections數組為空時獲取連接調用的方法
"Create a new connection"
if self._created_connections >= self.max_connections: #判斷創建的連接是否已經達到最大限制，max_connections可以通過參數初始化
raise ConnectionError("Too many connections")
self._created_connections += 1 #把代表已經創建的連接的數值+1
return self.connection_class(**self.connection_kwargs) #返回有效的連接，默認為Connection(**self.connection_kwargs)
def release(self, connection): #釋放連接，鏈接並沒有斷開，只是存在鏈接池中
"Releases the connection back to the pool"
self._checkpid()
if connection.pid != self.pid:
return
self._in_use_connections.remove(connection) #從集合中刪除元素
self._available_connections.append(connection) #並添加到_available_connections 的數組中
def disconnect(self): #斷開所有連接池中的鏈接
"Disconnects all connections in the pool"
all_conns = chain(self._available_connections,
self._in_use_connections)
for connection in all_conns:
connection.disconnect()
execute_command最終調用的是Connection.send_command方法，關閉鏈接為 Connection.disconnect方法，而Connection類的實現：
class Connection(object):
"Manages TCP communication to and from a Redis server"
def __del__(self): #對象刪除時的操作，調用disconnect釋放連接
try:
self.disconnect()
except Exception:
pass
核心的鏈接建立方法是通過socket模塊實現：
def _connect(self):
err = None
for res in socket.getaddrinfo(self.host, self.port, 0,
socket.SOCK_STREAM):
family, socktype, proto, canonname, socket_address = res
sock = None
try:
sock = socket.socket(family, socktype, proto)
# TCP_NODELAY
sock.setsockopt(socket.IPPROTO_TCP, socket.TCP_NODELAY, 1)
# TCP_KEEPALIVE
if self.socket_keepalive: #構造函數中默認 socket_keepalive=False，因此這裡默認為短連接
sock.setsockopt(socket.SOL_SOCKET, socket.SO_KEEPALIVE, 1)
for k, v in iteritems(self.socket_keepalive_options):
sock.setsockopt(socket.SOL_TCP, k, v)
# set the socket_connect_timeout before we connect
sock.settimeout(self.socket_connect_timeout) #構造函數中默認socket_connect_timeout=None，即連接為blocking的模式
# connect
sock.connect(socket_address)
# set the socket_timeout now that we're connected
sock.settimeout(self.socket_timeout) #��造函數中默認socket_timeout=None
return sock
except socket.error as _:
err = _
if sock is not None:
sock.close()
.....
關閉鏈接的方法：
def disconnect(self):
"Disconnects from the Redis server"
self._parser.on_disconnect()
if self._sock is None:
return
try:
self._sock.shutdown(socket.SHUT_RDWR) #先shutdown再close
self._sock.close()
except socket.error:
pass
self._sock = None
可以小結如下
1）默認情況下每創建一個Redis實例都會構造出一個ConnectionPool實例，每一次訪問redis都會從這個連接池得到一個連接，操作完成後會把該連接放回連接池（連接並沒有釋放）,可以構造一個統一的ConnectionPool，在創建Redis實例時，可以將該ConnectionPool傳入，那麼後續的操作會從給定的ConnectionPool獲得連接，不會再重復創建ConnectionPool。
2）默認情況下沒有設置keepalive和timeout，建立的連接是blocking模式的短連接。
3）不考慮底層tcp的情況下，連接池中的連接會在ConnectionPool.disconnect中統一銷毀。
Ubuntu 14.04下Redis安裝及簡單測試 http://www.linuxidc.com/Linux/2014-05/101544.htm
Redis集群明細文檔 http://www.linuxidc.com/Linux/2013-09/90118.htm
Ubuntu 12.10下安裝Redis（圖文詳解）+ Jedis連接Redis http://www.linuxidc.com/Linux/2013-06/85816.htm
Redis系列-安裝部署維護篇 http://www.linuxidc.com/Linux/2012-12/75627.htm
CentOS 6.3安裝Redis http://www.linuxidc.com/Linux/2012-12/75314.htm
Redis安裝部署學習筆記 http://www.linuxidc.com/Linux/2014-07/104306.htm
Redis配置文件redis.conf 詳解 http://www.linuxidc.com/Linux/2013-11/92524.htm
Redis 的詳細介紹：請點這裡
Redis 的下載地址：請點這裡
