Hashing uses port number?

[Chris Newland]

Mikael,

Thanks, this is clear now.

I'll add a user comment to the memcache docs on php.net as they don't
make any mention of this behaviour.

Regards,

Chris


Mikael Johansson wrote:
> The behavior of Memcache::connect() was to always reinitialize the pool
> from scratch regardless of any previous calls to addServer(). Your last
> connect() call would therefore clear out the pool and then add and
> connect 127.0.0.1:11211 making it the only server.
>
> The new behavior of Memcache::connect() (currently in pecl/memcache CVS
> only) is to add the server, much like addServer() except that the server
> is connected immediately instead of automatically-when-needed.
>
> Please note that if you specify a server to addServer() and again to
> connect() it will actually be added to the pool twice. In your later
> example
>
>   
>> $mmc = new Memcache()
>> $mmc->addServer('127.0.0.1', 11211);
>> $mmc->addServer('127.0.0.1', 11212);
>> $mmc->addServer('127.0.0.1', 11213);
>>
>> $mmc->connect('127.0.0.1', 11211);
>>     
>
> Using the new code from CVS, you would have a pool looking as
>
>  127.0.0.1:11211 (1 bucket, connected on demand)
>  127.0.0.1:11212 (1 bucket, connected on demand)
>  127.0.0.1:11213 (1 bucket, connected on demand)
>  127.0.0.1:11211 (1 bucket, forcibly connected)
>
> //Mikael
>
> Chris Newland wrote:
>   
>> Hi Mikael,
>>
>> Using your suggestion to not use connect() solved the problem but I
>> don't understand why the data is not evenly spread when I use connect().
>> I'm using random set of keys to test this behaviour.
>>
>> ====================
>> No Connect
>> ====================
>>
>> $mmc = new Memcache()
>> $mmc->addServer('127.0.0.1', 11211);
>> $mmc->addServer('127.0.0.1', 11212);
>> $mmc->addServer('127.0.0.1', 11213);
>>
>> $val =
>> "doifgsidfhgilshdflghsldifglisdfhgilhsdlfighsldfighslkhvmilemhrilcftmsilemhg";
>>       
>> for ($i = 0; $i < 100; $i++)
>> {          
>>     $key = rand(0, 10000000);
>>
>>     $mmc->set($key, $val);   
>> }
>>
>> Result as expected
>>
>> 127.0.0.1:11211 curr_items = 28
>> 127.0.0.1:11212 curr_items = 38
>> 127.0.0.1:11213 curr_items = 34
>>
>> ============================
>> But when I use connect:
>> ============================
>>
>> $mmc = new Memcache()
>> $mmc->addServer('127.0.0.1', 11211);
>> $mmc->addServer('127.0.0.1', 11212);
>> $mmc->addServer('127.0.0.1', 11213);
>>
>> $mmc->connect('127.0.0.1', 11211);
>>
>> $val =
>> "doifgsidfhgilshdflghsldifglisdfhgilhsdlfighsldfighslkhvmilemhrilcftmsilemhg";
>>       
>> for ($i = 0; $i < 100; $i++)
>> {           
>>
>>     $key = rand(0, 10000000);
>>
>>     $mmc->set($key, $val);    
>>
>> }
>>
>> Result
>>
>> 127.0.0.1:11211 curr_items = 100
>> 127.0.0.1:11212 not present in result of getExtendedStats()
>> 127.0.0.1:11213 not present in result of getExtendedStats()
>>
>> Using connect() seems to make it ignore the other servers.
>>
>> Regards,
>>
>> Chris
>>
>> Mikael Johansson wrote:
>>
>>     
>>>> Each call to Memcache::addServer() adds $weight (default = 1) number of
>>>> buckets to the list of possible servers to select for a given key. When
>>>> selecting a server to serve a given key, the key is hashed using (more
>>>> or less) crc32(key)%total_number_of_buckets.
>>>>
>>>> The short answer is that yes, the port is considered in the hashing
>>>> algorithm. If your all your data is stored on the first server it's
>>>> because all the keys you are using map to that specific servers buckets.
>>>> Using memcache_connect() or only addServer() shouldn't make a
>>>> difference, though the hignest performance can be found using the
>>>> extension as
>>>>
>>>>  $mmc = new Memcache()
>>>>  $mmc->addServer('node01.example.com', 11211);
>>>>  $mmc->addServer('node02.example.com', 11211);
>>>>  $mmc->addServer('node03.example.com', 11211);
>>>>
>>>>  $mmc->set('foo', 'bar');
>>>>
>>>> Since addServer() will not connect to the server until actually needed
>>>> (such as when the server is selected for a set/get operation)
>>>>
>>>> //Mikael
>>>>
>>>> Chris Newland wrote:
>>>>   
>>>>         
>>>   
>>>       
>>>>>> Hi,
>>>>>>
>>>>>> I'm trying to simulate my production environment by running multiple
>>>>>> memcached servers on my dev machine (on different ports).
>>>>>>
>>>>>> I can add the servers to the pool, and getExtendedStats for all of them
>>>>>> ok but when I try to write data, it only goes to the memcached that I
>>>>>> have connected to. (Using getExtendedStats curr_items to determine this).
>>>>>>
>>>>>> Can someone let me know if the port is considered in the hashing algorithm?
>>>>>>
>>>>>> I hope not as that would be an easy explanation.
>>>>>>
>>>>>> I'm using Debian Sarge, memcached 2.0.4, PHP PECL memcache client.
>>>>>>
>>>>>> Many thanks,
>>>>>>
>>>>>> Chris
>>>>>>     
>>>>>>             
>>>>     
>>>>         
>>
>>     
>
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