Resource

Here’s a very high level definition of the term:

A computer resource is any physical or virtual component of limited availability within a computer or information management system.

FLoM provides only two operations:

• lock(resource)
• unlock(resource)

FLoM creates and destroy the resources automatically, when they are necessary.
Some options allow you to change the default behavior, but basically you don’t have to care about creation, initialization and clean-up of the resources.

FLoM resources are not mapped to system resources like files, sockets, etc… You define your own resources inside your own synchronization domain.

FLoM provides different types of resources that allows you to implement different use cases without developing & testing your own logic. FLoM tries to give you the highest abstraction level.

FLoM infers the type of a resource from its name: there’s a naming convention that you must use.

FLoM implements the same Lock Modes proposed by OpenVMS DLM (Distributed Lock Manager).

The term locker is used to express the concept of a process that wants to get a lock.

Resource types

Here’s the complete list:

• simple
• numeric
• set
• hierarchical
• sequence
• timestamp

FLoM by examples provides a lot of information related the usage of every type of resource.
Below there’s a brief summary to get oriented and to figure out the features at a glance.

Simple

It’s simple, it’s like a semaphore, it’s the cornerstone of process synchronization.

Syntax (regular expression):

([[:alpha:]][[:alpha:][:digit:]]*)

Examples:

foo
bar
foo123bar
fOoBaR

Lock modes:

All: Null, Concurrent Read, Concurrent Write, Protected Read, Protected Write, Exclusive (default).

Numeric

It allows to model a synchronization scenario with producers and consumers typical of Petri Net modeling.
A producer produces 1 or more units, a consumer consumes 1 or more units

Syntax (regular expression):

([[:alpha:]][[:alpha:][:digit:]]*)\\[([[:digit:]]+)\\]

Examples:

foo[4]
bar[34]
foo123bar[99]
fOoBaR[6]

Lock modes:

Exclusive only, locked quantity can be specified (default 1).

Set

A predefined set of resources is available, the resources inside the set are distinguishable: every locker get the first available one (using a round robin algorithm).

Syntax (regular expression):

([[:alpha:]][[:alpha:][:digit:]]*)(\\.[[:alpha:]][[:alpha:][:digit:]]*)+

Examples:

foo.bar
red.green.blue
Apple.Pear.Strawberry.Cherry

Lock modes:

Exclusive only. The locked resource name, one in the set, is returned to the client that can use with some custom logic.

Hierarchical

Useful to model a hierarchy, typical use case: filesystem.
A lock applied to some level of the hierarchy, propagates to all the below levels.

Syntax (regular expression):

\\/[^\\/]+(\\/[^\\/]+)*

Examples:

/foo/bar
/red/green/blue
/Apple/Pear/Strawberry/Cherry

Lock modes:

All: Null, Concurrent Read, Concurrent Write, Protected Read, Protected Write, Exclusive (default).

Sequence

Useful to synchronize processes and get a valid unique ID with a single atomic operation.
Sequence resources can be transactional and non transactional: transactional sequence resources solves the annoying problem “holes/gaps in numbers generated by a sequence”.

Syntax (regular expression):

_[sS]_([[:alpha:]][[:alpha:][:digit:]]*)\\[([[:digit:]]+)\\]

Examples:

_s_foo[1]
_S_bar[23]

Lock modes:

Exclusive only. The locked resource name, a unique sequence value (1, 2, 3, …), is returned to the client that can use it with some custom logic.

Timestamp

Useful to synchronize processes and get a valid unique timestamp with a single atomic operation.
Timestamp resources solves the annoying problem of the “how can I be sure that 2 processes get different timestamp in a concurrent distributed environment”.

Syntax (regular expression):

_[t]_([%#[:alpha:]][%#\\.\\:[:alpha:][:digit:]]*)\\[([[:digit:]]+)\\]

Examples:

_t_foo.%D.%T[4]
_t_bar:%S#fff[1]

Lock modes:

Exclusive only. The locked resource name, a unique timestamp (07/05/16.11:30:08, 07/05/16.11:30:09, 07/05/16.11:30:10, …), is returned to the client that can use it with some custom logic.