memcached client way of working

Dustin Sallings dustin at
Sun May 6 20:25:19 UTC 2007

On May 6, 2007, at 4:13, reffael caspi wrote:

>   I don’t see where the client sends parallel requests to all the  
> buckets.

	It's up to the client to decide how it wants its operations  
processed.  I can't speak to the design of the clients you were  
looking at, but this is a basic overview of mine:

	There is a single IO thread handling all IO for all connections.

	Each connection has an input queue, a read queue, and a write queue.

	On the caller side, an operation is constructed, a server is  
selected via the current hash function, that operation is added the  
appropriate server's input queue, and the IO selector is  
interrupted.  Note that in the case of a multi-key get, multiple  
operations may be created (one for each destination server).

	In the IO thread, all IO is handled for available reads and writes  
(as well as connection management), and then it loops.  Before going  
back into the selector, the input queue is processed.

	Input queue processing involves transforming operations into buffers  
(which are placed on the write queue), as well as optimizing  
sequential get operations into a single, deduplicated get op on the  

	Once we have write ops, it's just a matter of the selector informing  
us a given channel is write ready.  In writing, a network buffer is  
filled by concatenating sequential write ops as to send them as  
efficiently as possible.  Write ops whose buffers are drained get a  
state change and then are added to the read op queue waiting for  
their responses.

	Sorry for the excessive background, but I think it's helpful to  
understand the answer to your question with respect to my client.   
For example:

	If I issue two get requests from two threads, one for key ``a'' and  
one for key ``b'' that respectively hash to servers 11 and 25, those  
two will get queued concurrently, both writes will happen around the  
same time (although sequentially), and the selector will be waiting  
for results from both.

	If you want more detail, you can get my client here:


Dustin Sallings

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