2018/06/24 15:25
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local res = ngx.location.capture(uri, options)



context: rewrite_by_lua*, access_by_lua*, content_by_lua*

Issues a synchronous but still non-blocking Nginx Subrequest using uri.

Nginx's subrequests provide a powerful way to make non-blocking internal requests to other locations configured with disk file directory or any other nginx C modules like ngx_proxyngx_fastcgingx_memcngx_postgresngx_drizzle, and even ngx_lua itself and etc etc etc.

Also note that subrequests just mimic the HTTP interface but there is no extra HTTP/TCP traffic nor IPC involved. Everything works internally, efficiently, on the C level.

Subrequests are completely different from HTTP 301/302 redirection (via ngx.redirect) and internal redirection (via ngx.exec).

You should always read the request body (by either calling ngx.req.read_body or configuring lua_need_request_body on) before initiating a subrequest.

This API function (as well as ngx.location.capture_multi) always buffers the whole response body of the subrequest in memory. Thus, you should use cosockets and streaming processing instead if you have to handle large subrequest responses.

Here is a basic example:

res = ngx.location.capture(uri)

Returns a Lua table with 4 slots: res.statusres.headerres.body, and res.truncated.

res.status holds the response status code for the subrequest response.

res.header holds all the response headers of the subrequest and it is a normal Lua table. For multi-value response headers, the value is a Lua (array) table that holds all the values in the order that they appear. For instance, if the subrequest response headers contain the following lines:

Set-Cookie: a=3
 Set-Cookie: foo=bar
 Set-Cookie: baz=blah

Then res.header["Set-Cookie"] will be evaluated to the table value {"a=3", "foo=bar", "baz=blah"}.

res.body holds the subrequest's response body data, which might be truncated. You always need to check the res.truncated boolean flag to see if res.body contains truncated data. The data truncation here can only be caused by those unrecoverable errors in your subrequests like the cases that the remote end aborts the connection prematurely in the middle of the response body data stream or a read timeout happens when your subrequest is receiving the response body data from the remote.

URI query strings can be concatenated to URI itself, for instance,

res = ngx.location.capture('/foo/bar?a=3&b=4')

Named locations like @foo are not allowed due to a limitation in the nginx core. Use normal locations combined with the internal directive to prepare internal-only locations.

An optional option table can be fed as the second argument, which supports the options:

  • method specify the subrequest's request method, which only accepts constants like ngx.HTTP_POST.
  • body specify the subrequest's request body (string value only).
  • args specify the subrequest's URI query arguments (both string value and Lua tables are accepted)
  • ctx specify a Lua table to be the ngx.ctx table for the subrequest. It can be the current request's ngx.ctx table, which effectively makes the parent and its subrequest to share exactly the same context table. This option was first introduced in the v0.3.1rc25 release.
  • vars take a Lua table which holds the values to set the specified Nginx variables in the subrequest as this option's value. This option was first introduced in the v0.3.1rc31 release.
  • copy_all_vars specify whether to copy over all the Nginx variable values of the current request to the subrequest in question. modifications of the nginx variables in the subrequest will not affect the current (parent) request. This option was first introduced in the v0.3.1rc31 release.
  • share_all_vars specify whether to share all the Nginx variables of the subrequest with the current (parent) request. modifications of the Nginx variables in the subrequest will affect the current (parent) request. Enabling this option may lead to hard-to-debug issues due to bad side-effects and is considered bad and harmful. Only enable this option when you completely know what you are doing.
  • always_forward_body when set to true, the current (parent) request's request body will always be forwarded to the subrequest being created if the body option is not specified. The request body read by either ngx.req.read_body() or lua_need_request_body on will be directly forwarded to the subrequest without copying the whole request body data when creating the subrequest (no matter the request body data is buffered in memory buffers or temporary files). By default, this option is false and when the body option is not specified, the request body of the current (parent) request is only forwarded when the subrequest takes the PUT or POST request method.

Issuing a POST subrequest, for example, can be done as follows

res = ngx.location.capture(
     '/foo/bar', { method = ngx.HTTP_POST, body = 'hello, world' } )

See HTTP method constants methods other than POST. The method option is ngx.HTTP_GET by default.

The args option can specify extra URI arguments, for instance,

ngx.location.capture('/foo?a=1', { args = { b = 3, c = ':' } } )

is equivalent to


that is, this method will escape argument keys and values according to URI rules and concatenate them together into a complete query string. The format for the Lua table passed as the args argument is identical to the format used in the ngx.encode_args method.

The args option can also take plain query strings:

ngx.location.capture('/foo?a=1', { args = 'b=3&c=%3a' } } )

This is functionally identical to the previous examples.

The share_all_vars option controls whether to share nginx variables among the current request and its subrequests. If this option is set to true, then the current request and associated subrequests will share the same Nginx variable scope. Hence, changes to Nginx variables made by a subrequest will affect the current request.

Care should be taken in using this option as variable scope sharing can have unexpected side effects. The argsvars, or copy_all_vars options are generally preferable instead.

This option is set to false by default

location /other {
     set $dog "$dog world"; echo "$uri dog: $dog"; } location /lua { set $dog 'hello'; content_by_lua_block { res = ngx.location.capture("/other", { share_all_vars = true }); ngx.print(res.body) ngx.say(ngx.var.uri, ": ", } }

Accessing location /lua gives

/other dog: hello world
/lua: hello world

The copy_all_vars option provides a copy of the parent request's Nginx variables to subrequests when such subrequests are issued. Changes made to these variables by such subrequests will not affect the parent request or any other subrequests sharing the parent request's variables.

location /other {
     set $dog "$dog world"; echo "$uri dog: $dog"; } location /lua { set $dog 'hello'; content_by_lua_block { res = ngx.location.capture("/other", { copy_all_vars = true }); ngx.print(res.body) ngx.say(ngx.var.uri, ": ", } }

Request GET /lua will give the output

/other dog: hello world
/lua: hello

Note that if both share_all_vars and copy_all_vars are set to true, then share_all_vars takes precedence.

In addition to the two settings above, it is possible to specify values for variables in the subrequest using the vars option. These variables are set after the sharing or copying of variables has been evaluated, and provides a more efficient method of passing specific values to a subrequest over encoding them as URL arguments and unescaping them in the Nginx config file.

location /other {
     content_by_lua_block {
         ngx.say("dog = ",  ngx.say("cat = ",  }  }   location /lua {  set $dog '';  set $cat '';  content_by_lua_block {  res = ngx.location.capture("/other",  { vars = { dog = "hello", cat = 32 }});   ngx.print(res.body)  }  }

Accessing /lua will yield the output

dog = hello
cat = 32

The ctx option can be used to specify a custom Lua table to serve as the ngx.ctx table for the subrequest.

location /sub {
     content_by_lua_block { = "bar"; } } location /lua { content_by_lua_block { local ctx = {} res = ngx.location.capture("/sub", { ctx = ctx }) ngx.say(; ngx.say(; } }

Then request GET /lua gives


It is also possible to use this ctx option to share the same ngx.ctx table between the current (parent) request and the subrequest:

location /sub {
     content_by_lua_block { = "bar"; } } location /lua { content_by_lua_block { res = ngx.location.capture("/sub", { ctx = ngx.ctx }) ngx.say(; } }

Request GET /lua yields the output


Note that subrequests issued by ngx.location.capture inherit all the request headers of the current request by default and that this may have unexpected side effects on the subrequest responses. For example, when using the standard ngx_proxymodule to serve subrequests, an "Accept-Encoding: gzip" header in the main request may result in gzipped responses that cannot be handled properly in Lua code. Original request headers should be ignored by setting proxy_pass_request_headersto off in subrequest locations.

When the body option is not specified and the always_forward_body option is false (the default value), the POST and PUTsubrequests will inherit the request bodies of the parent request (if any).

There is a hard-coded upper limit on the number of concurrent subrequests possible for every main request. In older versions of Nginx, the limit was 50 concurrent subrequests and in more recent versions, Nginx 1.1.x onwards, this was increased to 200 concurrent subrequests. When this limit is exceeded, the following error message is added to the error.log file:

[error] 13983#0: *1 subrequests cycle while processing "/uri"

The limit can be manually modified if required by editing the definition of the NGX_HTTP_MAX_SUBREQUESTS macro in the nginx/src/http/ngx_http_request.h file in the Nginx source tree.

Please also refer to restrictions on capturing locations configured by subrequest directives of other modules.

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