CODEWITHSUNDEEP
c++libmemcachedmemcachedb
Andres Felipe
Andres Felipe

Reputation: 684

Unable to connect to memcachedb using libmemcached

I'm writing a small C++ application that is able to connect to a memcachedb instance using the libmemcached C++ API.

The memcachedb instance was created using the following command:

memcachedb -m 64 -p 21201 -A 4096 -u memcachedb -l 127.0.0.1 -H /var/lib/memcachedb -f /var/lib/memcachedb/default.db -U off

I am able to retrieve the server STATS by doing:

telnet 127.0.0.1 21201
stats
STAT pid 1972
STAT uptime 124936
STAT time 1428898165
STAT version 1.2.0
STAT pointer_size 64
STAT rusage_user 5.687019
STAT rusage_system 13.329336
STAT ibuffer_size 512
STAT curr_connections 5
STAT total_connections 12
STAT connection_structures 6
STAT cmd_get 4
STAT cmd_set 4
STAT get_hits 4
STAT get_misses 0
STAT bytes_read 236
STAT bytes_written 2171
STAT threads 4
END

To retrieve the server STATS programatically I am using the following code:

#include <libmemcached/memcached.hpp>

#include <string>

#include <stdio.h>
#include <string.h>

using namespace std;
using namespace memcache;

int main(int argc, char **argv) {

    std::string host(argv[1]);
    int port;
    istringstream(std::string(argv[2])) >> port;

    printf("%s\n", "Instantiating client");

    Memcache first_client;

    printf("Adding server host=[%s] port=[%d]\n", host.c_str(), port);
    first_client.addServer(host, port);

    printf("%s\n", "Getting server STATS");
    map<string, map<string, string> > my_stats;
    bool gotStats = first_client.getStats(my_stats);

    if (gotStats) {
        printf("%s\n", "Got STATS");
    } else {
        printf("%s\n", "Unable to get STATS");
    }

    return EXIT_SUCCESS;
}

I am compiling the application using the following Makefile:

CXXFLAGS =  -O2 -g -Wall -fmessage-length=0
OBJS =      MemcachedExample.o
LIBS =      -lmemcached
TARGET =    MemcachedExample

$(TARGET):  $(OBJS)
$(CXX) -o $(TARGET) $(OBJS) $(LIBS)

all:    $(TARGET)

clean:
rm -f $(OBJS) $(TARGET)

When I execute the application using the following command:

MemcachedExample 127.0.0.1 21201

It fails to retrieve the server STATS:

Instantiating client
Adding server host=[127.0.0.1] port=[21201]
Getting server STATS
Unable to get STATS

What am I doing wrong? I would appreciate any insight.

Upvotes: 0

Views: 416

Answers (1)

Andres Felipe
Andres Felipe

Reputation: 684

Looking at the tcpdump as pointed by @Rastmaj I found that in the dump for the connection via my program there were no packets related with the connection I was trying to establish.

This made me take look at the code of the C++ interface for the C Client Library for memcached. I found that the empty constructor was initializing the memcached_st object using memcached("", 0) while the constructor receiving hostname and port arguments not only initialized the memcached_st object using memcached("", 0) but also added a server using the memcached_server_add function.

Though there is no apparent reason why does it work when adding a server just after initializing the memcached_st object, since the C++ method memcache::Memcache::addServer does exactly the same, the solution I found was simply changing:

Memcache first_client;

for

Memcache first_client("--SERVER=" + host + ":" + std::string(argv[2]));

This solves my problem but I still lack an explanation of this behavior, further insights are appreciated.

Upvotes: 0

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