Getting started with memcached-libmemcached


Memcached is the free, high performance, open source distributed caching system. It was designed to alleviate a high number of database queries by caching the data in memory. Since memcached is a distributed caching system the application data is distributed across servers. Data is inserted and retrieved from the distributed cache using a key,value pair.

Memcached uses a consistent hashing scheme to distribute the keys across the servers. The consistent hashing algorithm handles server crashes and servers joining-in by redistributing the keys across the necessary servers.

This article focuses on getting started with memcached-libmecached and making the process as painless as possible. After you have downloaded and installed memcached & libmemcached you are good to go.

First start 4 memcached servers
$ memcached -p 11221 &
$ memcached -p 11222 &
$ memcached -p 11223 &
$ memcached -p 11223 &

They start on the local host. (For full options check memcached -help)
Verify they are running using ps -ef.

libmemcached is the C client which can be used to connect to the memcached servers which you have started above.
A snippet of the libmemcached code client_test1.c is shown
client_test1.c
….
const char *server_string= “localhost:11221, localhost:11222, localhost:11223, localhost:11224″;
memc= memcached_create(NULL);
servers= memcached_servers_parse(server_string);
rc= memcached_server_push(memc, servers);
rc= memcached_flush(memc, 0);
rc= memcached_set(memc, key, strlen(key),in_value, strlen(in_value),(time_t)0, (uint32_t)0);
rc= memcached_append(memc, key, strlen(key),” the”, strlen(” the”),(time_t)0, (uint32_t)0);
rc= memcached_append(memc, key, strlen(key),” people here”, strlen(” people here”), time_t)0, (uint32_t)0);
out_value= memcached_get(memc, key, strlen(key),&value_length, &flags, &rc);
printf(“Out value is: %s\n”,out_value);
memcached_server_list_free(servers);
free(out_value);
….

When you execute this client you should see
$ Out value is: We the people here

You can check which server the key is stored by doing
$ memdump –servers localhost:11221
fig
$memdump –servers localhost:11222
$
This shows that the key data is stored in the 1st servers localhost:11221

Now assume that we store a lot more data through client_test2.c
client test2.c
…..
const char *server_string= “localhost:11221, localhost:11222, localhost:11223, localhost:11224″;
memc= memcached_create(NULL);
servers= memcached_servers_parse(server_string);
rc= memcached_server_push(memc, servers);
rc= memcached_flush(memc, 0);
for (i=0; i < 100; i++)
{
sprintf(str,”%d”,i);
sprintf(str1,”%d”,2*i);
printf(“String %s string1 %s\n”,str,str1);
printf(“reached here\n”);
rc= memcached_set(memc,str, strlen(str), str1, strlen(str1),(time_t)0, (uint32_t)0);
test_true(rc == MEMCACHED_SUCCESS);
}
for(i=0; i < 10; i++)
{
printf(“Input value:”);
scanf(“%s”,testvalue);
printf(“Value to search for %s”,testvalue);
value= (uint32_t *)memcached_get(memc, testvalue, strlen(testvalue), &value_length, &flags, &rc)
test_true(rc == MEMCACHED_SUCCESS);
printf(“Value is %s\n”,value);
}
…..

After executing this when we dump the key values from the servers we will see
$ memdump –servers localhost:11221
97
94
92
89
….
….

Similarly
$ memdump –servers localhost:11222
99
98
91
87
86
….
….

Hence the keys are hashed across servers. The consistent hashing mechanism takes O(log(n)) to get to cache server as against a naive hashing scheme which would take O(1).

Happy memcaching …

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One thought on “Getting started with memcached-libmemcached

  1. Pingback: yorkr pads up for the Twenty20s:Part 3:Overall team performance against all oppositions! | Giga thoughts …

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