mongo::DistributedLock Class Reference

The distributed lock is a configdb backed way of synchronizing system-wide tasks. More...

#include <distlock.h>

Classes
class	LastPings
struct	PingData

Public Member Functions
	DistributedLock (const ConnectionString &conn, const string &name, unsigned long long lockTimeout=0, bool asProcess=false)
	The constructor does not connect to the configdb yet and constructing does not mean the lock was acquired. More...
bool	lock_try (const string &why, bool reenter=false, BSONObj *other=0, double timeout=0.0)
	Attempts to acquire 'this' lock, checking if it could or should be stolen from the previous holder. More...
bool	isLockHeld (double timeout, string *errMsg)
	Returns true if we currently believe we hold this lock and it was possible to confirm that, within 'timeout' seconds, if provided, with the config servers. More...
void	unlock (BSONObj *oldLockPtr=NULL)
	Releases a previously taken lock.
Date_t	getRemoteTime ()
bool	isRemoteTimeSkewed ()
const string &	getProcessId ()
const ConnectionString &	getRemoteConnection ()

Static Public Member Functions
static bool	checkSkew (const ConnectionString &cluster, unsigned skewChecks=NUM_LOCK_SKEW_CHECKS, unsigned long long maxClockSkew=MAX_LOCK_CLOCK_SKEW, unsigned long long maxNetSkew=MAX_LOCK_NET_SKEW)
	Check the skew between a cluster of servers.
static Date_t	remoteTime (const ConnectionString &cluster, unsigned long long maxNetSkew=MAX_LOCK_NET_SKEW)
	Get the remote time from a server or cluster. More...
static bool	killPinger (DistributedLock &lock)

Public Attributes
const ConnectionString	_conn
const string	_name
const string	_processId
const unsigned long long	_lockTimeout
const unsigned long long	_maxClockSkew
const unsigned long long	_maxNetSkew
const unsigned long long	_lockPing

Static Public Attributes
static LabeledLevel	logLvl
static LastPings	lastPings
static const string	lockPingNS
	Namespace for lock pings.
static const string	locksNS
	Namespace for locks.

Detailed Description

The distributed lock is a configdb backed way of synchronizing system-wide tasks.

A task must be identified by a unique name across the system (e.g., "balancer"). A lock is taken by writing a document in the configdb's locks collection with that name.

To be maintained, each taken lock needs to be revalidated ("pinged") within a pre-established amount of time. This class does this maintenance automatically once a DistributedLock object was constructed.

Constructor & Destructor Documentation

mongo::DistributedLock::DistributedLock	(	const ConnectionString &	conn,
		const string &	name,
		unsigned long long	lockTimeout = 0,
		bool	asProcess = false
	)

The constructor does not connect to the configdb yet and constructing does not mean the lock was acquired.

Create a new distributed lock, potentially with a custom sleep and takeover time.

Construction does trigger a lock "pinging" mechanism, though.

Parameters

conn	address of config(s) server(s)
name	identifier for the lock
lockTimeout	how long can the log go "unpinged" before a new attempt to lock steals it (in minutes).
lockPing	how long to wait between lock pings
legacy	use legacy logic

If a custom sleep time is specified (time between pings)

Member Function Documentation

bool mongo::DistributedLock::isLockHeld	(	double	timeout,
		string *	errMsg
	)

Returns true if we currently believe we hold this lock and it was possible to confirm that, within 'timeout' seconds, if provided, with the config servers.

If the lock is not held or if we failed to contact the config servers within the timeout, returns false.

bool mongo::DistributedLock::lock_try	(	const string &	why,
		bool	reenter = false,
		BSONObj *	other = 0,
		double	timeout = 0.0
	)

Attempts to acquire 'this' lock, checking if it could or should be stolen from the previous holder.

Please consider using the dist_lock_try construct to acquire this lock in an exception safe way.

Parameters

why	human readable description of why the lock is being taken (used to log)
whether	this is a lock re-entry or a new lock
other	configdb's lock document that is currently holding the lock, if lock is taken, or our own lock details if not

Returns

true if it managed to grab the lock

Date_t mongo::DistributedLock::remoteTime ( const ConnectionString &  cluster, unsigned long long  maxNetSkew = MAX_LOCK_NET_SKEW  )

static

Get the remote time from a server or cluster.

Returns the remote time as reported by the cluster or server.

The maximum difference between the reported time and the actual time on the remote server (at the completion of the function) is the maxNetSkew

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The documentation for this class was generated from the following files:

• distlock.h
• distlock.cpp