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Dreadlock

Dreadlock is a simple + fast network-locking daemon. A client can connect to dreadlock over TCP and ensure exclusive access to a particular resource.

Note: due to network partitions and things of that nature, this is only a SOFT guarantee of exclusivity; don't use this program for banking. It is optimized for speedy locking, not safe locking. Something like zookeeper is better for safe locking.

Protocol

It talks over TCP and supports two commands. First, lock:

lock <key> <timeout>\r\n

Issuing lock will block the calling client for timeout milliseconds until either the key is held exclusively by this client or the timeout elapses. Keys must contain only ascii letters and digits.

The response will be either:

l locked\r\n

or

t timeout\r\n

Your client should raise an exception on t timeout (or return an error code, depending on language). Note: timeouts are +/- 100ms, so don't count on deadly accuracy.

Clients will be given the lock as it is released in first-come-first-serve order. If a client's TCP connection to dreadlock drops, it is considered dead, and all locks it holds are immediately released.

To explicitly release a lock when a client is done, use:

unlock <key>\r\n

The response will be:

u unlocked\r\n

Errors

A few errors are possible, like unlocking a lock you do not hold, re-locking a lock you already hold, etc. Errors are always of the format:

e error message\r\n

Your client should raise an exception and provide the error message as a descriptive string for the error.

A protocol syntax error will cause the connection to be rudely dropped on your client (and dreadlock will print a warning to stderr).

Building

To build dreadlock, you need Russ Cox's libtask library installed. For running high concurrency dreadlock servers on linux, it's recommended to use this fork which supports epoll() on linux. The standard libtask has a fixed limit of 1024 sockets; the epoll()-enabled fork has no arbitrary limits (other than ulimit).

make and make install will do it; make all will attempt to use ragel to rebuild the parsing state machine C file, but you must have a recent version of ragel installed on your system to do so.

Running

Just run dreadlock and it will listen on port 6001. Optionally, if you specify a command line argument, it will listen on that port instead:

dreadlock

or

dreadlock 11066

Status

Consider dreadlock beta software for now.

Author

Jamie Turner [email protected]

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High-Speed Locking Daemon

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