This is a queue-size-ware version of memcacheq (http://memcachedb.org/memcacheq/) from xunxin and agentzh originally developed for Yahoo! China's webpage information extraction cluster based on WebKit.

It's opensourced here meant to be useful for others and for the original author of memcacheq's potential consideration of merging back to the mainstream.

This fork adds the ability to limit and query the length of queues and to specify a length limit when creating a queue.

Here's the usage of this fork.

• Server side

$ memcacheq -u my_user -p 5000 -B 4064 -A 4096 -r -c 1024 -m 64 -N
-H /path/to/my/bdbdata/base

where -u indicates the user who runs this deamon, -p specifies a port number that this daemon listens to, -B specifies the message body length in bytes (but actually you can only store messages of size tens of bytes smaller that that), -A specifies the underlying BDB page size, -m specifies the in-memory cache size, -N uses BDB's no-sync feature to gain more speed at the cost of consistency, and -H specifies the on-disk storage location. See memcacheq's command-line usage for more information.

To start the memcacheq daemon:

If there's corruption in your memcacheq's underlying BDB database, try the following command:

$ /path/to/your/bdb/bin/db_recover -h /path/to/your/bdbdata/base

• Client side

We use pseudo libmemcached C code to demonstrate the client-side usage.

1. To create a queue:

memcached_add(memc, queue_name, size_limit);

where "size_limit" is the maximum number of elements in that queue named "queue_name".

Note that the number of queues is limited by the underlying BDB queue storage.

2. To remove a queue:

memcached_delete(memc, queue_name);

3. To insert an element to a queue:

memcached_set(memc, queue_name, element_content);

Once the queue has already reached the length limit, it returns the standard memcached "NOT STORED" exception code.

4. To read an element from a queue:

element_content = memcached_get(memc, queue_name);

Once the queue is already empty, it returns a standard memcached "NOT FOUND" exception code.

5. To monitor the queues' state in a certain memcacheq server (here we use a shell command to illustrate):

$ echo stats queue | nc 10.62.100.35:11211

where the memcacheq daemon listens the 11211 port at 10.62.100.35. A typical instance of the output might be:

STAT bbsdetails 0 2000000
STAT bbslists 72 2000000
STAT comment 3 1234567
STAT done 0 1234567
STAT initial 25 1234567
STAT pagecat 5006 1234567
STAT preprocessed 10 500
END

where the second column in the listing specify the names of the queues, the third column the current size of the queues, and the forth column the size limit of the queues.

Note that the queue size is stored separately in contrast to real-time counting, so it might be kinda out of sync if the daemon exits abnormally and the -N option is specified when starting the daemon.

We usually use the command-line utilities as well as the high-level Perl 5 library provided by the Queue::Memcached::Buffered module opensourced here:

http://github.com/agentzh/queue-memcached-buffered

Below is the original memcacheq documentation in README:

=============================================== MemcacheQ - Simple Queue Service over Memcache

Features

• damn simple
• very fast
• multiple queue
• concurrent well
• memcache protocol compatible

Getting Started

Download

See: http://code.google.com/p/memcacheq/downloads/list

Installation

See: http://memcachedb.org/memcacheq/INSTALL.html

Please take a look at 'ChangLog' file in the distribution, see what's new.

Commands

Only two commands are used to operate the queue:

Append a message to the tail of queue::

set 0 <message_len>\r\n \r\n STORED\r\n

Note: MQ will create a new queue automatically if your queue is not existed. The original 'expire time' field is ignored by server.

Consume a message from the head of queue::

get \r\n VALUE <message_len>\r\n \r\n END\r\n

Examples

Assuming you are using PHP memcachehttp://www.php.net/memcache::

Limitation

The message body is stored in Berkeley DB with fixed length. Any message that is shorter than the declared length will automatically be padded with space character (0x20 in the ASCII character set).

In Berkeley DB, as the official document refers,

"For the Queue access method, the record length must be enough smaller than the database's page size that at least one record plus the database page's metadata information can fit on each database page."

"The minimum page size is 512 bytes, the maximum page size is 64K bytes, and the page size must be a power-of-two."

So we have a limit on the message body size with a max of a bit less than 64K.

Other tips

use 'stats queue' to see your current queues::

$ telnet 127.0.0.1 22201 Trying 127.0.0.1... Connected to localhost. Escape character is '^]'. stats queue STAT test1 STAT test2 STAT test3 STAT test4 END

delete a queue::

$ telnet 127.0.0.1 22201 Trying 127.0.0.1... Connected to localhost. Escape character is '^]'. delete test1 DELETED

'db_stat' a queue to see how many records now in::

$ cd $ /usr/local/BerkeleyDB.4.7/bin/db_stat -d test1 Mon Sep 22 20:25:56 2008 Local time 42253 Queue magic number 4 Queue version number 1024 Fixed-length record size 0x20 Fixed-length record pad 4096 Underlying database page size 131072 Underlying database extent size 100000 Number of records in the database 33334 Number of database pages 2048 Number of bytes free in database pages (99% ff) 1 First undeleted record 100001 Next available record number

Feedback

MemcacheDB mailing list now hosts on Google Group: http://groups.google.com/group/memcachedb

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Last updated by Steve Chuhttp://stvchu.org: 09/22/2008