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.
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, you can attempt a manual memcache flush. The memcache protocol flush command allows you to immediately flag all stored values as invalid.
/bin/echo -e 'flush_all\nquit' | nc -q1 $(hostname -s) 11211
If the command completes normally, the output should simply be OK.