LISTEN registers the current session as a listener on the notification channel named channel. If the current session is already registered as a listener for this notification channel, nothing is done.
Whenever the command NOTIFY channel is invoked, either by this session or another one connected to the same database, all the sessions currently listening on that notification channel are notified, and each will in turn notify its connected client application.
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The method a client application must use to detect notification events depends on which PostgreSQL application programming interface it uses. With the libpq library, the application issues LISTEN as an ordinary SQL command, and then must periodically call the function PQnotifies to find out whether any notification events have been received. Other interfaces such as libpgtcl provide higher-level methods for handling notify events; indeed, with libpgtcl the application programmer should not even issue LISTEN or UNLISTEN directly. See the documentation for the interface you are using for more details.
There is a race condition when first setting up a listening session: if concurrently-committing transactions are sending notify events, exactly which of those will the newly listening session receive? The answer is that the session will receive all events committed after an instant during the transaction's commit step. But that is slightly later than any database state that the transaction could have observed in queries. This leads to the following rule for using LISTEN: first execute (and commit!) that command, then in a new transaction inspect the database state as needed by the application logic, then rely on notifications to find out about subsequent changes to the database state. The first few received notifications might refer to updates already observed in the initial database inspection, but this is usually harmless.
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When both AIO and sendfile are enabled on Linux,AIO is used for files that are larger than or equal tothe size specified in the directio directive,while sendfile is used for files of smaller sizesor when directio is disabled.
By default, multi-threading is disabled, it should beenabled with the--with-threads configuration parameter.Currently, multi-threading is compatible only with theepoll,kqueue,andeventport methods.Multi-threaded sending of files is only supported on Linux.
Sets buffer size for reading client request body.In case the request body is larger than the buffer,the whole body or only its part is written to atemporary file.By default, buffer size is equal to two memory pages.This is 8K on x86, other 32-bit platforms, and x86-64.It is usually 16K on other 64-bit platforms.
Determines whether nginx should save the entire client request bodyinto a file.This directive can be used during debugging, or when using the$request_body_filevariable, or the$r->request_body_filemethod of the modulengx_http_perl_module.
Defines a directory for storing temporary files holding client request bodies.Up to three-level subdirectory hierarchy can be used under the specifieddirectory.For example, in the following configuration
Defines a timeout for reading client request body.The timeout is set only for a period between two successive read operations,not for the transmission of the whole request body.If a client does not transmit anything within this time, therequest is terminated with the408 (Request Time-out)error.
Sets buffer size for reading client request header.For most requests, a buffer of 1K bytes is enough.However, if a request includes long cookies, or comes from a WAP client,it may not fit into 1K.If a request line or a request header field does not fit intothis buffer then larger buffers, configured by thelarge_client_header_buffers directive,are allocated.
Sets the maximum allowed size of the client request body.If the size in a request exceeds the configured value, the413 (Request Entity Too Large)error is returned to the client.Please be aware thatbrowsers cannot correctly displaythis error.Setting size to 0 disables checking of clientrequest body size.
Allows accurate tuning of per-connection memory allocations.This directive has minimal impact on performanceand should not generally be used.By default, the size is equal to256 bytes on 32-bit platforms and 512 bytes on 64-bit platforms.
Controls whether header fields with invalid names should be ignored.Valid names are composed of English letters, digits, hyphens, and possiblyunderscores (as controlled by the underscores_in_headersdirective).
Disables keep-alive connections with misbehaving browsers.The browser parameters specify whichbrowsers will be affected.The value msie6 disables keep-alive connectionswith old versions of MSIE, once a POST request is received.The value safari disables keep-alive connectionswith Safari and Safari-like browsers on macOS and macOS-likeoperating systems.The value none enables keep-alive connectionswith all browsers.
Sets the maximum number and size ofbuffers used for reading large client request header.A request line cannot exceed the size of one buffer, or the414 (Request-URI Too Large)error is returned to the client.A request header field cannot exceed the size of one buffer as well, or the400 (Bad Request)error is returned to the client.Buffers are allocated only on demand.By default, the buffer size is equal to 8K bytes.If after the end of request processing a connection is transitionedinto the keep-alive state, these buffers are released.
Limits the rate of response transmission to a client.The rate is specified in bytes per second.The zero value disables rate limiting.The limit is set per a request, and so if a client simultaneously openstwo connections, the overall rate will be twice as muchas the specified limit.
When lingering_close is in effect,this directive specifies the maximum time during which nginxwill process (read and ignore) additional data coming from a client.After that, the connection will be closed, even if there will bemore data.
The ssl parameter (0.7.14) allows specifying that allconnections accepted on this port should work in SSL mode.This allows for a more compact configuration for the server thathandles both HTTP and HTTPS requests.
The http2 parameter (1.9.5) configures the port to acceptHTTP/2 connections.Normally, for this to work the ssl parameter should bespecified as well, but nginx can also be configured to accept HTTP/2connections without SSL.
The listen directivecan have several additional parameters specific to socket-related system calls.These parameters can be specified in anylisten directive, but only once for a givenaddress:port pair.
If a location is defined by a prefix string that ends with the slash character,and requests are processed by one ofproxy_pass,fastcgi_pass,uwsgi_pass,scgi_pass,memcached_pass, orgrpc_pass,then the special processing is performed.In response to a request with URI equal to this string,but without the trailing slash,a permanent redirect with the code 301 will be returned to the requested URIwith the slash appended.If this is not desired, an exact match of the URI and location could bedefined like this:
Limits the maximum allowed number of ranges in byte-range requests.Requests that exceed the limit are processed as if there were nobyte ranges specified.By default, the number of ranges is not limited.The zero value disables the byte-range support completely.
Enables or disables resetting timed out connectionsand connectionsclosedwith the non-standard code 444 (1.15.2).The reset is performed as follows.Before closing a socket, theSO_LINGERoption is set on it with a timeout value of 0.When the socket is closed, TCP RST is sent to the client, and all memoryoccupied by this socket is released.This helps avoid keeping an already closed socket with filled buffersin a FIN_WAIT1 state for a long time.
The optional status_zone parameter (1.17.1)enablescollectionof DNS server statistics of requests and responsesin the specified zone.The parameter is available as part of ourcommercial subscription.
If the directive is set to a non-zero value, nginx will try to minimizethe number of send operations on client sockets by using eitherNOTE_LOWAT flag of thekqueue methodor the SO_SNDLOWAT socket option.In both cases the specified size is used.
Sets a timeout for transmitting a response to the client.The timeout is set only between two successive write operations,not for the transmission of the whole response.If the client does not receive anything within this time,the connection is closed.
In this configuration, sendfile() is called withthe SF_NODISKIO flag which causes it not to block on disk I/O,but, instead, report back that the data are not in memory.nginx then initiates an asynchronous data load by reading one byte.On the first read, the FreeBSD kernel loads the first 128K bytesof a file into memory, although next reads will only load data in 16K chunks.This can be changed using theread_ahead directive.
During searching for a virtual server by name,if the name matches more than one of the specified variants,(e.g. both a wildcard name and regular expression match), the first matchingvariant will be chosen, in the following order of priority: 152ee80cbc
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