Published at 2011-02-03 19:05:28
There are many web and GUI interfaces available for accessing Memcached server instances, my personal favourite is phpMemCacheAdmin. Sometimes however you may need to access a Memcached instance from the command line.
In this tutorial, I will show you how to connect to a Memcached instances using Telnet, how to get statistics on the server, and how to read and delete data from the cache.
To make a connection to Memcached using Telnet, use the following command:
[jcollins@redhat memcached]$ telnet localhost 11211 Trying 127.0.0.1... Connected to localhost. Escape character is '^]'.
If at any time you wish to terminate the Telnet session, simply type "quit" and hit return:
quit Connection closed by foreign host. [jcollins@redhat memcached]$
You can get an overview of the important statistics of your Memcached server by running the stats command once connected:
stats STAT pid 22622 STAT uptime 69300 STAT time 1296733424 STAT version 1.2.5 STAT pointer_size 32 STAT rusage_user 0.117982 STAT rusage_system 0.145977 STAT curr_items 3 STAT total_items 10 STAT bytes 1174 STAT curr_connections 2 STAT total_connections 13 STAT connection_structures 3 STAT cmd_get 10 STAT cmd_set 10 STAT get_hits 9 STAT get_misses 1 STAT evictions 0 STAT bytes_read 4593 STAT bytes_written 7388 STAT limit_maxbytes 268435456 STAT threads 1 END
Some useful information is returned, such as the uptime for the server, the version of Memcached installed, the total number of items in the cache, and the amount of client connections to the instance.
According to the Memcached FAQ:
Memory is allocated in chunks internally and constantly reused. Since memory is broken into different size slabs, you do waste memory if your items do not fit perfectly into the slab the server chooses to put it in.
So Memcached allocates your data into different "slabs" (think of these as partitions) of memory automatically, based on the size of your data, which in turn makes memory allocation more optimal.
To list the slabs in the instance you are connected to, use the stats slab command:
stats slabs STAT 1:chunk_size 88 STAT 1:chunks_per_page 11915 STAT 1:total_pages 1 STAT 1:total_chunks 11915 STAT 1:used_chunks 11915 STAT 1:free_chunks 0 STAT 1:free_chunks_end 11914 STAT 6:chunk_size 296 STAT 6:chunks_per_page 3542 STAT 6:total_pages 1 STAT 6:total_chunks 3542 STAT 6:used_chunks 3541 STAT 6:free_chunks 1 STAT 6:free_chunks_end 3541 STAT 7:chunk_size 376 STAT 7:chunks_per_page 2788 STAT 7:total_pages 1 STAT 7:total_chunks 2788 STAT 7:used_chunks 2788 STAT 7:free_chunks 0 STAT 7:free_chunks_end 2787 STAT 8:chunk_size 472 STAT 8:chunks_per_page 2221 STAT 8:total_pages 1 STAT 8:total_chunks 2221 STAT 8:used_chunks 2220 STAT 8:free_chunks 1 STAT 8:free_chunks_end 2218 STAT active_slabs 4 STAT total_malloced 4193552 END
A more useful command is the stats items, which will give you a list of slabs which includes a count of the items store within each slab:
stats items STAT items:1:number 1 STAT items:1:age 35 STAT items:1:evicted 0 STAT items:1:outofmemory 0 STAT items:7:number 1 STAT items:7:age 69596 STAT items:7:evicted 0 STAT items:7:outofmemory 0 STAT items:8:number 2 STAT items:8:age 69627 STAT items:8:evicted 0 STAT items:8:outofmemory 0 END
Now that you know how to list slabs, you can browse inside each slab to list the items contained within by using the stats cachedump command, which has the following signature:
stats cachedump [slab ID] [number of items, 0 for all items]
stats cachedump 1 0 ITEM testkey [9 b; 1296857316 s] END
..tells us that there is one item in the slab 1 with the key testkey. If we want to get the actual value of that item, we can use the get [key] command:
get testkey VALUE testkey 0 9 test data END
Finally, if you want to delete an item from the cache you can use the delete [key] command:
END delete testkey DELETED
Memcached is an amazing piece of software in terms of its many potential applications to increase web application performance. While powerful, it is simple to use and configure, and I find the API very intuitive. I am looking forward to using it on a many projects in the future.