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    Memcache monitoring and flush using the nc command

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    Most Drupal website administrators are interested in quickly monitoring memcache's statistics, and the nc command (on some systems, netcat) is a useful command for interacting with memcache servers.

    Here's how you can view and monitor your memcache statistics:

    Find the servers

    As a preliminary step, you'll need a list of the memcache servers that Drupal is using. As an Acquia Cloud customer, you can find your memcache servers by navigating to the Application > Environment > Servers page in the Acquia Cloud UI. Find the "web" server(s) that have "Memcache" and note their full DNS name (e.g. "web-123.prod.hosting.acquia.com").

    If you have a Drupal 6 or Drupal 7 application, you can use the following Drush command to look up the memcache_servers variable:

    drush vget memcache_servers

    The output should look something like this:

    memcache_servers: Array (
  [server-1234.prod.hosting.acquia.com:11211] => default
  [server-5678.prod.hosting.acquia.com:11211] => default
)

    This array is automatically populated on Acquia Cloud servers using memcache.

    The format of the servers' array is hostname:port => bin. In the sample output, the default memcache bin is split across two memcache hosts — server-1234.prod.hosting.acquia.com and server-5678.prod.hosting.acquia.com, using port 11211 on each host.

    View memcache statistics

    The Memcached protocol provides a stats command that can provide some insight into a few dozen server variables. Many of these server variables count the number of times a particular event occurred since the service was launched.

    In this example, we echo two commands together: stats, followed by quit (separated by a newline). The -q1 option tells the nc command to not disconnect immediately. Without this option, nc might close the connection before it delivers the output.

    /bin/echo -e 'stats\nquit' | nc -q1 [memcache hostname] [port number]

    Here's an example using server information from the previous example:

    /bin/echo -e 'stats\nquit' | nc -q1  $(hostname -s) 11211

    The output of the preceding command should look similar to the following:

    STAT pid 1216 
STAT uptime 2472376 
STAT time 1373572082 
STAT version 1.4.5 
STAT pointer_size 64 
STAT rusage_user 66.790000 
STAT rusage_system 110.900000 
STAT curr_connections 3 
STAT total_connections 297279 
STAT connection_structures 10 
STAT cmd_get 2115470 
STAT cmd_set 92126 
STAT cmd_flush 0 
STAT get_hits 1918776 
STAT get_misses 196694 
STAT delete_misses 806 
STAT delete_hits 1566 
STAT incr_misses 0 
STAT incr_hits 19 
STAT decr_misses 0 
STAT decr_hits 0 
STAT cas_misses 0 
STAT cas_hits 0 
STAT cas_badval 0 
STAT auth_cmds 0 
STAT auth_errors 0 
STAT bytes_read 346432348 
STAT bytes_written 3932785939 
STAT limit_maxbytes 67108864 
STAT accepting_conns 1 
STAT listen_disabled_num 0 
STAT threads 1 
STAT conn_yields 0 
STAT bytes 46704290 
STAT curr_items 21955 
STAT total_items 92127 
STAT evictions 13531 
STAT reclaimed 0 
END

    Monitor memcache statistics

    You can use the watch command to monitor all of these statistics in real time. For example:

    watch -d -n 10 "/bin/echo -e 'stats\nquit' | nc -q1  $(hostname -s) 11211"

    Adding the -d option to the watch command highlights the parts of the output that have changed since the last output. The -n option sets a delay interval; in this example, it's set to 10 seconds.

    Flush memcache

    In most cases, when a particular part of a Drupal page appears "stuck" in cache, you should use the drush cache-clear command. This command can clear a specific cache that appears to be problematic. In extremely rare circumstances you may need to manually trigger a memcache flush. For Drupal 7 websites, you may need to trigger a registry rebuild function. (For more information, see the Registry Rebuild module on drupal.org.) If these processes fail, and you have ruled out the reverse-proxy cache, you can attempt a manual memcache flush. The memcache protocol flush command allows you to immediately flag all stored values as invalid.

    Here's an example:

    /bin/echo -e 'flush_all\nquit' | nc -q1 $(hostname -s) 11211

    If the command completes normally, the output should simply be OK.

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