今天在寫zabbix storm job監控指令碼的時候用到了python的redis模組，之前也有用過，但是沒有過多的瞭解，今天看了下相關的api和原始碼,看到有ConnectionPool的實現，這裡簡單說下。
在ConnectionPool之前，如果需要連線redis，我都是用StrictRedis這個類，在原始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這個引數，其對應的是一個ConnectionPool的物件：
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的例項執行具體的命令時會呼叫execute_command方法，這裡可以看到具體實現是從連線池中獲取一個具體的連線，然後執行命令，完成後釋放連線：
# 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) #呼叫ConnectionPool.get_connection方法獲取一個連線 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) #呼叫ConnectionPool.release釋放連線
在來看看ConnectionPool類：
class ConnectionPool(object):
...........
def __init__(self, connection_class=Connection, max_connections=None,
**connection_kwargs): #類初始化時呼叫建構函式
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 #設定對應的引數
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 #已經建立的連線的計數器
self._available_connections = [] #宣告一個空的陣列，用來存放可用的連線
self._in_use_connections = set() #宣告一個空的集合，用來存放已經在用的連線
self._check_lock = threading.Lock()
.......
def get_connection(self, command_name, *keys, **options): #在連線池中獲取連線的方法
"Get a connection from the pool"
self._checkpid()
try:
connection = self._available_connections.pop() #獲取並刪除代表連線的元素，在第一次獲取connectiong時，因為_available_connections是一個空的陣列，
會直接呼叫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線的集合中新增元素
return connection
def make_connection(self): #在_available_connections陣列為空時獲取連線呼叫的方法
"Create a new connection"
if self._created_connections >= self.max_connections: #判斷建立的連線是否已經達到最大限制，max_connections可以通過引數初始化
raise ConnectionError("Too many connections")
self._created_connections += 1 #把代表已經建立的連線的數值+1
return self.connection_class(**self.connection_kwargs) #返回有效的連線，預設為Connection(**self.connection_kwargs)
def release(self, connection): #釋放連線，連結並沒有斷開，只是存在連結池中
"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) #並新增到_available_connections 的陣列中
def disconnect(self): #斷開所有連線池中的連結
"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最終呼叫的是Connection.send_command方法，關閉連結為 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): #物件刪除時的操作，呼叫disconnect釋放連線 try: self.disconnect() except Exception: pass
核心的連結建立方法是通過socket模組實現：
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: #建構函式中預設 socket_keepalive=False，因此這裡預設為短連線 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) #建構函式中預設socket_connect_timeout=None，即連線為blocking的模式 # connect sock.connect(socket_address) # set the socket_timeout now that we`re connected sock.settimeout(self.socket_timeout) #建構函式中預設socket_timeout=None return sock except socket.error as _: err = _ if sock is not None: sock.close() .....
關閉連結的方法：
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）預設情況下每建立一個Redis例項都會構造出一個ConnectionPool例項，每一次訪問redis都會從這個連線池得到一個連線，操作完成後會把該連線放回連線池（連線並沒有釋放）,可以構造一個統一的ConnectionPool，在建立Redis例項時，可以將該ConnectionPool傳入，那麼後續的操作會從給定的ConnectionPool獲得連線，不會再重複建立ConnectionPool。
2）預設情況下沒有設定keepalive和timeout，建立的連線是blocking模式的短連線。
3）不考慮底層tcp的情況下，連線池中的連線會在ConnectionPool.disconnect中統一銷燬。