Module ngx_http_scgi_module

The ngx_http_scgi_module module allows passing requests to an SCGI server.

location / {
    include   scgi_params;
    scgi_pass localhost:9000;
}

Makes outgoing connections to an SCGI server originate from the specified local IP address with an optional port (1.11.2). Parameter value can contain variables (1.3.12). The special value off (1.3.12) cancels the effect of the scgi_bind directive inherited from the previous configuration level, which allows the system to auto-assign the local IP address and port.

The transparent parameter (1.11.0) allows outgoing connections to an SCGI server originate from a non-local IP address, for example, from a real IP address of a client:

scgi_bind $remote_addr transparent;

In order for this parameter to work, it is usually necessary to run nginx worker processes with the superuser privileges. On Linux it is not required (1.13.8) as if the transparent parameter is specified, worker processes inherit the CAP_NET_RAW capability from the master process. It is also necessary to configure kernel routing table to intercept network traffic from the SCGI server.

scgi_buffer_size 4k|8k;

Sets the size of the buffer used for reading the first part of the response received from the SCGI server. This part usually contains a small response header. By default, the buffer size is equal to one memory page. This is either 4K or 8K, depending on a platform. It can be made smaller, however.

scgi_buffering on;

Enables or disables buffering of responses from the SCGI server.

When buffering is enabled, nginx receives a response from the SCGI server as soon as possible, saving it into the buffers set by the scgi_buffer_size and scgi_buffers directives. If the whole response does not fit into memory, a part of it can be saved to a temporary file on the disk. Writing to temporary files is controlled by the scgi_max_temp_file_size and scgi_temp_file_write_size directives.

When buffering is disabled, the response is passed to a client synchronously, immediately as it is received. nginx will not try to read the whole response from the SCGI server. The maximum size of the data that nginx can receive from the server at a time is set by the scgi_buffer_size directive.

Buffering can also be enabled or disabled by passing “yes” or “no” in the “X-Accel-Buffering” response header field. This capability can be disabled using the scgi_ignore_headers directive.

scgi_buffers 8 4k|8k;

Sets the number and size of the buffers used for reading a response from the SCGI server, for a single connection. By default, the buffer size is equal to one memory page. This is either 4K or 8K, depending on a platform.

scgi_busy_buffers_size 8k|16k;

When buffering of responses from the SCGI server is enabled, limits the total size of buffers that can be busy sending a response to the client while the response is not yet fully read. In the meantime, the rest of the buffers can be used for reading the response and, if needed, buffering part of the response to a temporary file. By default, size is limited by the size of two buffers set by the scgi_buffer_size and scgi_buffers directives.

scgi_cache off;

Defines a shared memory zone used for caching. The same zone can be used in several places. Parameter value can contain variables (1.7.9). The off parameter disables caching inherited from the previous configuration level.

scgi_cache_background_update off;

Allows starting a background subrequest to update an expired cache item, while a stale cached response is returned to the client. Note that it is necessary to allow the usage of a stale cached response when it is being updated.

Defines conditions under which the response will not be taken from a cache. If at least one value of the string parameters is not empty and is not equal to “0” then the response will not be taken from the cache:

scgi_cache_bypass $cookie_nocache $arg_nocache$arg_comment;
scgi_cache_bypass $http_pragma    $http_authorization;

Can be used along with the scgi_no_cache directive.

Defines a key for caching, for example

scgi_cache_key localhost:9000$request_uri;

scgi_cache_lock off;

When enabled, only one request at a time will be allowed to populate a new cache element identified according to the scgi_cache_key directive by passing a request to an SCGI server. Other requests of the same cache element will either wait for a response to appear in the cache or the cache lock for this element to be released, up to the time set by the scgi_cache_lock_timeout directive.

scgi_cache_lock_age 5s;

If the last request passed to the SCGI server for populating a new cache element has not completed for the specified time, one more request may be passed to the SCGI server.

scgi_cache_lock_timeout 5s;

Sets a timeout for scgi_cache_lock. When the time expires, the request will be passed to the SCGI server, however, the response will not be cached.

Before 1.7.8, the response could be cached.

Sets an offset in bytes for byte-range requests. If the range is beyond the offset, the range request will be passed to the SCGI server and the response will not be cached.

scgi_cache_methods GET HEAD;

If the client request method is listed in this directive then the response will be cached. “GET” and “HEAD” methods are always added to the list, though it is recommended to specify them explicitly. See also the scgi_no_cache directive.

scgi_cache_min_uses 1;

Sets the number of requests after which the response will be cached.

Sets the path and other parameters of a cache. Cache data are stored in files. The file name in a cache is a result of applying the MD5 function to the cache key. The levels parameter defines hierarchy levels of a cache: from 1 to 3, each level accepts values 1 or 2. For example, in the following configuration

scgi_cache_path /data/nginx/cache levels=1:2 keys_zone=one:10m;

file names in a cache will look like this:

/data/nginx/cache/c/29/b7f54b2df7773722d382f4809d65029c

A cached response is first written to a temporary file, and then the file is renamed. Starting from version 0.8.9, temporary files and the cache can be put on different file systems. However, be aware that in this case a file is copied across two file systems instead of the cheap renaming operation. It is thus recommended that for any given location both cache and a directory holding temporary files are put on the same file system. A directory for temporary files is set based on the use_temp_path parameter (1.7.10). If this parameter is omitted or set to the value on, the directory set by the scgi_temp_path directive for the given location will be used. If the value is set to off, temporary files will be put directly in the cache directory.

In addition, all active keys and information about data are stored in a shared memory zone, whose name and size are configured by the keys_zone parameter. One megabyte zone can store about 8 thousand keys.

Cached data that are not accessed during the time specified by the inactive parameter get removed from the cache regardless of their freshness. By default, inactive is set to 10 minutes.

The special “cache manager” process monitors the maximum cache size set by the max_size parameter, and the minimum amount of free space set by the min_free (1.19.1) parameter on the file system with cache. When the size is exceeded or there is not enough free space, it removes the least recently used data. The data is removed in iterations configured by manager_files, manager_threshold, and manager_sleep parameters (1.11.5). During one iteration no more than manager_files items are deleted (by default, 100). The duration of one iteration is limited by the manager_threshold parameter (by default, 200 milliseconds). Between iterations, a pause configured by the manager_sleep parameter (by default, 50 milliseconds) is made.

A minute after the start the special “cache loader” process is activated. It loads information about previously cached data stored on file system into a cache zone. The loading is also done in iterations. During one iteration no more than loader_files items are loaded (by default, 100). Besides, the duration of one iteration is limited by the loader_threshold parameter (by default, 200 milliseconds). Between iterations, a pause configured by the loader_sleep parameter (by default, 50 milliseconds) is made.

In versions 1.7.3, 1.7.7, and 1.11.10 cache header format has been changed. Previously cached responses will be considered invalid after upgrading to a newer nginx version.

scgi_cache_revalidate off;

Enables revalidation of expired cache items using conditional requests with the “If-Modified-Since” and “If-None-Match” header fields.

scgi_cache_use_stale off;

Determines in which cases a stale cached response can be used when an error occurs during communication with the SCGI server. The directive’s parameters match the parameters of the scgi_next_upstream directive.

The error parameter also permits using a stale cached response if an SCGI server to process a request cannot be selected.

Additionally, the updating parameter permits using a stale cached response if it is currently being updated. This allows minimizing the number of accesses to SCGI servers when updating cached data.

Using a stale cached response can also be enabled directly in the response header for a specified number of seconds after the response became stale (1.11.10). This has lower priority than using the directive parameters.

• The “stale-while-revalidate” extension of the “Cache-Control” header field permits using a stale cached response if it is currently being updated.
• The “stale-if-error” extension of the “Cache-Control” header field permits using a stale cached response in case of an error.

To minimize the number of accesses to SCGI servers when populating a new cache element, the scgi_cache_lock directive can be used.

Sets caching time for different response codes. For example, the following directives

scgi_cache_valid 200 302 10m;
scgi_cache_valid 404      1m;

set 10 minutes of caching for responses with codes 200 and 302 and 1 minute for responses with code 404.

If only caching time is specified

scgi_cache_valid 5m;

then only 200, 301, and 302 responses are cached.

In addition, the any parameter can be specified to cache any responses:

scgi_cache_valid 200 302 10m;
scgi_cache_valid 301      1h;
scgi_cache_valid any      1m;

Parameters of caching can also be set directly in the response header. This has higher priority than setting of caching time using the directive.

• The “X-Accel-Expires” header field sets caching time of a response in seconds. The zero value disables caching for a response. If the value starts with the @ prefix, it sets an absolute time in seconds since Epoch, up to which the response may be cached.
• If the header does not include the “X-Accel-Expires” field, parameters of caching may be set in the header fields “Expires” or “Cache-Control”.
• If the header includes the “Set-Cookie” field, such a response will not be cached.
• If the header includes the “Vary” field with the special value “*”, such a response will not be cached (1.7.7). If the header includes the “Vary” field with another value, such a response will be cached taking into account the corresponding request header fields (1.7.7).
• If the header includes the “Age” field and caching time is set with the “Cache-Control” field, caching time is reduced accordingly (1.27.3).

Processing of one or more of these response header fields can be disabled using the scgi_ignore_headers directive.

scgi_connect_timeout 60s;

Defines a timeout for establishing a connection with an SCGI server. It should be noted that this timeout cannot usually exceed 75 seconds.

scgi_force_ranges off;

Enables byte-range support for both cached and uncached responses from the SCGI server regardless of the “Accept-Ranges” field in these responses.

By default, nginx does not pass the header fields “Status” and “X-Accel-...” from the response of an SCGI server to a client. The scgi_hide_header directive sets additional fields that will not be passed. If, on the contrary, the passing of fields needs to be permitted, the scgi_pass_header directive can be used.

scgi_ignore_client_abort off;

Determines whether the connection with an SCGI server should be closed when a client closes the connection without waiting for a response.

Disables processing of certain response header fields from the SCGI server. The following fields can be ignored: “X-Accel-Redirect”, “X-Accel-Expires”, “X-Accel-Limit-Rate” (1.1.6), “X-Accel-Buffering” (1.1.6), “X-Accel-Charset” (1.1.6), “Expires”, “Cache-Control”, “Set-Cookie” (0.8.44), “Vary” (1.7.7), and “Age” (1.27.3).

If not disabled, processing of these header fields has the following effect:

• “X-Accel-Expires”, “Expires”, “Cache-Control”, “Set-Cookie”, “Vary”, and “Age” set the parameters of response caching;
• “X-Accel-Redirect” performs an internal redirect to the specified URI;
• “X-Accel-Limit-Rate” sets the rate limit for transmission of a response to a client;
• “X-Accel-Buffering” enables or disables buffering of a response;
• “X-Accel-Charset” sets the desired charset of a response.

scgi_intercept_errors off;

Determines whether an SCGI server responses with codes greater than or equal to 300 should be passed to a client or be intercepted and redirected to nginx for processing with the error_page directive.

scgi_limit_rate 0;

Limits the speed of reading the response from the SCGI server. The rate is specified in bytes per second. The zero value disables rate limiting. The limit is set per a request, and so if nginx simultaneously opens two connections to the SCGI server, the overall rate will be twice as much as the specified limit. The limitation works only if buffering of responses from the SCGI server is enabled.

scgi_max_temp_file_size 1024m;

When buffering of responses from the SCGI server is enabled, and the whole response does not fit into the buffers set by the scgi_buffer_size and scgi_buffers directives, a part of the response can be saved to a temporary file. This directive sets the maximum size of the temporary file. The size of data written to the temporary file at a time is set by the scgi_temp_file_write_size directive.

The zero value disables buffering of responses to temporary files.

This restriction does not apply to responses that will be cached or stored on disk.

scgi_next_upstream error timeout;

Specifies in which cases a request should be passed to the next server:

error

an error occurred while establishing a connection with the server, passing a request to it, or reading the response header;

timeout

a timeout has occurred while establishing a connection with the server, passing a request to it, or reading the response header;

invalid_header

a server returned an empty or invalid response;

http_500

a server returned a response with the code 500;

http_503

a server returned a response with the code 503;

http_403

a server returned a response with the code 403;

http_404

a server returned a response with the code 404;

http_429

a server returned a response with the code 429 (1.11.13);

non_idempotent

normally, requests with a non-idempotent method (POST, LOCK, PATCH) are not passed to the next server if a request has been sent to an upstream server (1.9.13); enabling this option explicitly allows retrying such requests;

off

disables passing a request to the next server.

One should bear in mind that passing a request to the next server is only possible if nothing has been sent to a client yet. That is, if an error or timeout occurs in the middle of the transferring of a response, fixing this is impossible.

The directive also defines what is considered an unsuccessful attempt of communication with a server. The cases of error, timeout and invalid_header are always considered unsuccessful attempts, even if they are not specified in the directive. The cases of http_500, http_503, and http_429 are considered unsuccessful attempts only if they are specified in the directive. The cases of http_403 and http_404 are never considered unsuccessful attempts.

Passing a request to the next server can be limited by the number of tries and by time.

scgi_next_upstream_timeout 0;

Limits the time during which a request can be passed to the next server. The 0 value turns off this limitation.

scgi_next_upstream_tries 0;

Limits the number of possible tries for passing a request to the next server. The 0 value turns off this limitation.

Defines conditions under which the response will not be saved to a cache. If at least one value of the string parameters is not empty and is not equal to “0” then the response will not be saved:

scgi_no_cache $cookie_nocache $arg_nocache$arg_comment;
scgi_no_cache $http_pragma    $http_authorization;

Can be used along with the scgi_cache_bypass directive.

Sets a parameter that should be passed to the SCGI server. The value can contain text, variables, and their combination. These directives are inherited from the previous configuration level if and only if there are no scgi_param directives defined on the current level.

Standard CGI environment variables should be provided as SCGI headers, see the scgi_params file provided in the distribution:

location / {
    include scgi_params;
    ...
}

If the directive is specified with if_not_empty (1.1.11) then such a parameter will be passed to the server only if its value is not empty:

scgi_param HTTPS $https if_not_empty;

Sets the address of an SCGI server. The address can be specified as a domain name or IP address, and a port:

scgi_pass localhost:9000;

or as a UNIX-domain socket path:

scgi_pass unix:/tmp/scgi.socket;

If a domain name resolves to several addresses, all of them will be used in a round-robin fashion. In addition, an address can be specified as a server group.

Parameter value can contain variables. In this case, if an address is specified as a domain name, the name is searched among the described server groups, and, if not found, is determined using a resolver.

Permits passing otherwise disabled header fields from an SCGI server to a client.

scgi_pass_request_body on;

Indicates whether the original request body is passed to the SCGI server. See also the scgi_pass_request_headers directive.

scgi_pass_request_headers on;

Indicates whether the header fields of the original request are passed to the SCGI server. See also the scgi_pass_request_body directive.

scgi_read_timeout 60s;

Defines a timeout for reading a response from the SCGI server. The timeout is set only between two successive read operations, not for the transmission of the whole response. If the SCGI server does not transmit anything within this time, the connection is closed.

scgi_request_buffering on;

Enables or disables buffering of a client request body.

When buffering is enabled, the entire request body is read from the client before sending the request to an SCGI server.

When buffering is disabled, the request body is sent to the SCGI server immediately as it is received. In this case, the request cannot be passed to the next server if nginx already started sending the request body.

When HTTP/1.1 chunked transfer encoding is used to send the original request body, the request body will be buffered regardless of the directive value.

scgi_send_timeout 60s;

Sets a timeout for transmitting a request to the SCGI server. The timeout is set only between two successive write operations, not for the transmission of the whole request. If the SCGI server does not receive anything within this time, the connection is closed.

scgi_socket_keepalive off;

Configures the “TCP keepalive” behavior for outgoing connections to an SCGI server. By default, the operating system’s settings are in effect for the socket. If the directive is set to the value “on”, the SO_KEEPALIVE socket option is turned on for the socket.

scgi_store off;

Enables saving of files to a disk. The on parameter saves files with paths corresponding to the directives alias or root. The off parameter disables saving of files. In addition, the file name can be set explicitly using the string with variables:

scgi_store /data/www$original_uri;

The modification time of files is set according to the received “Last-Modified” response header field. The response is first written to a temporary file, and then the file is renamed. Starting from version 0.8.9, temporary files and the persistent store can be put on different file systems. However, be aware that in this case a file is copied across two file systems instead of the cheap renaming operation. It is thus recommended that for any given location both saved files and a directory holding temporary files, set by the scgi_temp_path directive, are put on the same file system.

This directive can be used to create local copies of static unchangeable files, e.g.:

location /images/ {
    root              /data/www;
    error_page        404 = /fetch$uri;
}

location /fetch/ {
    internal;

    scgi_pass         backend:9000;
    ...

    scgi_store        on;
    scgi_store_access user:rw group:rw all:r;
    scgi_temp_path    /data/temp;

    alias             /data/www/;
}

scgi_store_access user:rw;

Sets access permissions for newly created files and directories, e.g.:

scgi_store_access user:rw group:rw all:r;

If any group or all access permissions are specified then user permissions may be omitted:

scgi_store_access group:rw all:r;

scgi_temp_file_write_size 8k|16k;

Limits the size of data written to a temporary file at a time, when buffering of responses from the SCGI server to temporary files is enabled. By default, size is limited by two buffers set by the scgi_buffer_size and scgi_buffers directives. The maximum size of a temporary file is set by the scgi_max_temp_file_size directive.

scgi_temp_path scgi_temp;

Defines a directory for storing temporary files with data received from SCGI servers. Up to three-level subdirectory hierarchy can be used underneath the specified directory. For example, in the following configuration

scgi_temp_path /spool/nginx/scgi_temp 1 2;

a temporary file might look like this:

/spool/nginx/scgi_temp/7/45/00000123457

See also the use_temp_path parameter of the scgi_cache_path directive.