Module ngx_http_memcached_module

The ngx_http_memcached_module module is used to obtain responses from a memcached server. The key is set in the $memcached_key variable. A response should be put in memcached in advance by means external to nginx.

server {
    location / {
        set            $memcached_key "$uri?$args";
        memcached_pass host:11211;
        error_page     404 502 504 = @fallback;
    }

    location @fallback {
        proxy_pass     http://backend;
    }
}

Makes outgoing connections to a memcached 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 memcached_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 a memcached server originate from a non-local IP address, for example, from a real IP address of a client:

memcached_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 memcached server.

memcached_buffer_size 4k|8k;

Sets the size of the buffer used for reading the response received from the memcached server. The response is passed to the client synchronously, as soon as it is received.

memcached_connect_timeout 60s;

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

Enables the test for the flag presence in the memcached server response and sets the “Content-Encoding” response header field to “gzip” if the flag is set.

memcached_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_response

a server returned an empty or invalid response;

not_found

a response was not found on the server;

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_response are always considered unsuccessful attempts, even if they are not specified in the directive. The case of not_found is never considered an unsuccessful attempt.

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

memcached_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.

memcached_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.

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

memcached_pass localhost:11211;

or as a UNIX-domain socket path:

memcached_pass unix:/tmp/memcached.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.

memcached_read_timeout 60s;

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

memcached_send_timeout 60s;

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

memcached_socket_keepalive off;

Configures the “TCP keepalive” behavior for outgoing connections to a memcached 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.

$memcached_key

Defines a key for obtaining response from a memcached server.