How does swoole determine whether fd is connected?

What is a heartbeat?

As the name suggests, heartbeat is a criterion for judging whether a thing is alive or dead. In swoole, heartbeat is used to judge whether a connection is normal or disconnected.

(Recommended Learning: SWOOLE Video Tutorial )

We all know that a five -dollar group logo a network connection and create There are three handshakes for a connection, and four handshakes for disconnecting a connection. Regardless of whether the server or the client initiates the closing of the connection, the process of waving four times will be completed. In this way, everything is perfect. The system recycles the fd, and the application layer can also handle related matters through the onClose callback.

What is fd?

The scientific name of fd is a file descriptor. In the Unix philosophy that everything is a file, this fd is the identifier exposed by the system layer to the business layer to represent a five-tuple network connection.

You can simply understand it as an index. By operating this fd, the system layer can find the corresponding connection and perform a series of operations, such as sending data to the network pupil, closing the connection, etc.

Why does the heart beat?

Just mentioned, if we want to close a connection, we can initiate the operation of closing the connection on fd at the business layer. Take swoole as an example:

$server->close($fd);

Under normal circumstances, It will complete the four waves (swoole will have an onClose callback), and the system will recycle the fd to be allocated to other connections.

Why does the system recycle fd? Because fd resources are limited, they must be reused.

But in some cases, such as suddenly unplugging the network cable or digging out the optical cable during Lanxiang drills, the server cannot sense the abnormality of the connection, but in fact the connection has failed. If there is no Recycling mechanism, this type of connection will use up all fd, causing the system to no longer accept new connection requests, so there is a heartbeat mechanism.

What is the heartbeat mechanism?

The heartbeat mechanism is a method provided by the business layer to determine whether a connection is alive, so that the system can determine whether a connection has failed. There are generally two implementation methods:

1: The client regularly sends a heartbeat packet to tell the server that I am still alive. The server regularly checks the list of all clients to see whether the time of their last heartbeat packet has passed. If it is too long, it is considered that there is no heartbeat, the connection is determined to be dead, and the connection is actively closed.

2: The server regularly asks all clients, are you still alive? If you are alive, give me a feedback. Clients who do not get feedback will be killed without mercy.

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