Overlapping Reconnection Attempts

[mikehudson2]

In https://socket.io/docs/v4/client-options/#timeout default 'timeout'=20,000 and default 'reconnectionDelay' = 5,000. If we use those defaults and set 'reconnectionDelayMax' = 5,000, does that mean that if the client is disconnected at t=0ms and stay disconnected for >15,000ms, then at t=15,000 until t=19,999 there will be 4 'live' reconnection attempts? If this is the case is that problematical?

[darrachequesne]

The timeout value is used when establishing the low-level connection to the server. For example, if the WebSocket connection cannot be established (but without triggering the onerror() handler), the connection attempt is cancelled and the next connection attempt is tried after reconnectionDelay ms.

So, in the worst case with reconnectionDelay: 1000 and randomizationFactor: 0:

t	event
0	1st connection attempt
20000	connection attempt failure (after timeout)
21000	2nd connection attempt (after reconnectionDelay)
41000	connection attempt failure (after timeout)
43000	3rd connection attempt (after reconnectionDelay * 2)
63000	connection attempt failure
...

Also, the reconnection delay increases over time, with a maximum value of reconnectionDelayMax.

[mikehudson2]

If the socket is connected, and the client subsequently disconnects (e.g. WiFi off) then we see a 'disconnect' event of reason 'ping timeout'. The client then immediately tries to reconnect and fails, generating a 'connect_error' followed by a 2nd reconnection attempt at 1,000, then a 3rd at 3,000 etc.. Each failed reconnection has its own 'connect_error'. The value of 'timeout' is irrelevant.

Does this mean that there are two types of failed reconnections?
i) Reconnection attempts which fail to establish a handshake with the server and thereby cause 'connect_errors'?; and
ii) Reconnections which successfully handshake with the server but do not receive a ping from it within a certain period?

Why is 'timeout' relevant in (ii) but not in (i)? In (ii) is 'timeout' effectively how long the client should wait for the first ping from the server?

(reconnectionDelay:1000, reconnectionDelayMax: not set, randomizationFactor:0, timeout 20000)

[darrachequesne]

The timeout value is only relevant in a few cases:

• in HTTP long-polling mode, if the server handles the HTTP request but keeps it open without sending the handshake payload
• in WebSocket mode, if the server accepts the WebSocket connection but does not send the handshake payload

Once the handshake payload is received, the low-level connection is considered up and running, and the timeout value is not used anymore. (maybe openTimeout would be a better name for this option)

Source: https://github.com/socketio/socket.io-client/blob/89175d0481fc7633c12bb5b233dc3421f87860ef/lib/manager.ts#L354-L360