Nginx error log file Error.log Common errors detailed description

We often encounter a variety of nginx error logs, usually based on some nginx error log can be analyzed for reasons. However, it is not very systematic, here on the Internet to see a data or comparison of the system on the nginx of the Error.log detailed instructions, here record, easy to see after understanding.

Error message	Error description
"Upstream prematurely (premature) Closed connection"	The exception that occurs when the request URI is requested is that the upstream has not returned the user's response to the user's disconnection, and that the system has no effect and can be ignored
"Recv () failed (104:connection reset by Peer)"	(1) The number of concurrent connections to the server exceeds its load, and the server will drop some of the connections; (2) The client shuts down the browser and the server is sending data to the client; (3) The browser stops the stop
"(111:connection refused) while connecting to upstream"	When the user is connected, if the back end upstream hangs or does not pass, you will receive the error
"(111:connection refused) while reading response header from upstream"	When the user reads the data after the connection succeeds, he or she will receive the error if he or she encounters a back end upstream hangs or does not pass
"(111:connection refused) while sending request to upstream"	Nginx and upstream connection after the successful transmission of data, if you encounter the back-end upstream hang or impassability, you will receive this error
"(110:connection timed out) while connecting to upstream"	Timeout on upstream after Nginx connection
"(110:connection timed out) while reading upstream"	Timeout when Nginx read response from upstream
"(110:connection timed out) while reading response header from upstream"	Timeout when Nginx read response headers from upstream
"(110:connection timed out) while reading upstream"	Timeout when Nginx read response from upstream
"(104:connection reset by peer) while connecting to upstream"	Upstream sends a RST to reset the connection
"Upstream sent invalid header while reading response header from upstream"	Invalid response header sent by upstream
"Upstream sent no valid http/1.0 header while reading response header from upstream"	Invalid response header sent by upstream
"Client intended to send too large"	Used to set the maximum value of the client request content that is allowed to be accepted, the default value is 1m,client sent by the body exceeds the set value
"Reopening logs"	User sends KILL-USR1 command
"Gracefully shutting down",	User sends Kill-winch command
"No servers are inside upstream"	Server not configured under upstream
"No live upstreams while connecting to upstream"	The servers under the upstream are all dead.
"Ssl_do_handshake () failed"	SSL handshake failed
"Ssl_write () failed (SSL:) while sending to client"
"(13:permission denied) while reading upstream"
"(98:address already in use) while connecting to upstream"
"(99:cannot assign requested address) while connecting to upstream"
"Ngx_slab_alloc () Failed:no memory in SSL session shared cache"	Ssl_session_cache size is not enough to cause
"Could not add new SSL sessions to the session cache while SSL handshaking"	Ssl_session_cache size is not enough to cause
"Send () failed (111:connection refused)"

The above form is from network data. Here is only recorded, convenient to view later.