Module fiber

Create a fiber

To create a fiber, call one of the following functions:

• fiber.create() creates a fiber and runs it immediately. The initial fiber state is running.
• fiber.new() creates a fiber but does not start it. The initial fiber state is ready. You can join such fibers by calling the fiber_object:join() function and get the result returned by the fiber’s function.

Yield control

Yield is an action that occurs in a cooperative environment that transfers control of the thread from the current fiber to another fiber that is ready to execute. The fiber module provides the following functions that yield control to another fiber explicitly:

• fiber.yield() yields control to the scheduler.
• fiber.sleep() yields control to the scheduler and sleeps for the specified number of seconds.

Cancel a fiber

To cancel a fiber, use the fiber_object.cancel function. You can also call fiber.kill() to locate a fiber by its numeric ID and cancel it.

Limit execution time

If a fiber works too long without yielding control, you can use a fiber slice to limit its execution time. The fiber_slice_default compat option controls the default value of the maximum fiber slice.

There are two slice types: a warning and an error slice.

• When a warning slice is over, a warning message is logged, for example:

  fiber has not yielded for more than 0.500 seconds
  

• When an error slice is over, the fiber is cancelled and the FiberSliceIsExceeded error is thrown:

  FiberSliceIsExceeded: fiber slice is exceeded
  

  Control is passed to another fiber that is ready to execute.

The fiber slice is checked by all functions operating on spaces and indexes, such as index_object.select(), space_object.replace(), and so on. You can also use the fiber.check_slice() function in application code to check whether the slice for the current fiber is over.

The following functions override the the default value of the maximum fiber slice:

• fiber.set_max_slice(slice) sets the default maximum slice for all fibers.
• fiber_object:set_max_slice(slice) sets the maximum slice for a particular fiber.

The maximum slice is set when a fiber wakes up. This might be its first run or wake up after fiber.yield().

You can change or increase the slice for a current fiber’s execution using the following functions:

• fiber.set_slice(slice) sets the slice for a current fiber execution.
• fiber.extend_slice(slice) extends the slice for a current fiber execution.

Note that the specified values don’t affect a fiber’s execution after fiber.yield().

Information about fibers

To get information about all fibers or a specific fiber, use the following functions:

• fiber.info returns information about all fibers.
• fiber.status() gets the current fiber’s status. To get the status of the specified fiber, call fiber_object:status().
• fiber.top() shows all alive fibers and their CPU consumption.

Garbage collection

Like all Lua objects, dead fibers are garbage collected. The Lua garbage collector frees pool allocator memory owned by the fiber, resets all fiber data, and returns the fiber (now called a fiber carcass) to the fiber pool. The carcass can be reused when another fiber is created.

A fiber has all the features of a Lua coroutine and all the programming concepts that apply to Lua coroutines apply to fibers as well. However, Tarantool has made some enhancements for fibers and has used fibers internally. So, although the use of coroutines is possible and supported, the use of fibers is recommended.

API reference

fiber.create(function[, function-arguments])¶

Create and start a fiber. The fiber is created and begins to run immediately.

Parameters:	function – the function to be associated with the fiber function-arguments – arguments to be passed to the function
Return:	created fiber object
Rtype:	userdata

Example:

The script below shows how to create a fiber using fiber.create:

-- app.lua --
fiber = require('fiber')

function greet(name)
    print('Hello, '..name)
end

greet_fiber = fiber.create(greet, 'John')
print('Fiber already started')

The following output should be displayed after running app.lua:

$ tarantool app.lua
Hello, John
Fiber already started

fiber.new(function[, function-arguments])¶

Create a fiber but do not start it. The created fiber starts after the fiber creator (that is, the job that is calling fiber.new()) yields. The initial fiber state is ready.

Note

Note that fiber.status() returns the suspended state for ready fibers because the ready state is not observable using the fiber module API.

You can join fibers created using fiber.new by calling the fiber_object:join() function and get the result returned by the fiber’s function. To join the fiber, you need to make it joinable using fiber_object:set_joinable().

Parameters:	function – the function to be associated with the fiber function-arguments – arguments to be passed to the function
Return:	created fiber object
Rtype:	userdata

Example:

The script below shows how to create a fiber using fiber.new:

-- app.lua --
fiber = require('fiber')

function greet(name)
    print('Hello, '..name)
end

greet_fiber = fiber.new(greet, 'John')
print('Fiber not started yet')

The following output should be displayed after running app.lua:

$ tarantool app.lua
Fiber not started yet
Hello, John

fiber.self()¶

Return:	fiber object for the currently scheduled fiber.
Rtype:	userdata

Example:

tarantool> fiber.self()
---
- status: running
  name: interactive
  id: 101
...

fiber.find(id)¶

Parameters:	id – numeric identifier of the fiber.
Return:	fiber object for the specified fiber.
Rtype:	userdata

Example:

tarantool> fiber.find(101)
---
- status: running
  name: interactive
  id: 101
...

fiber.sleep(time)¶

Yield control to the scheduler and sleep for the specified number of seconds. Only the current fiber can be made to sleep.

Parameters:	time – number of seconds to sleep.
Exception:	see the Example of yield failure.

Example:

The increment function below contains an infinite loop that adds 1 to the counter global variable. Then, the current fiber goes to sleep for period seconds. sleep causes an implicit fiber.yield().

-- app.lua --
fiber = require('fiber')

counter = 0
function increment(period)
    while true do
        counter = counter + 1
        fiber.sleep(period)
    end
end

increment_fiber = fiber.create(increment, 2)
require('console').start()

After running the script above, print the information about the fiber: a fiber ID, its status, and the counter value.

tarantool> print('ID: ' .. increment_fiber:id() .. '\nStatus: ' .. increment_fiber:status() .. '\nCounter: ' .. counter)
ID: 104
Status: suspended
Counter: 8
---
...

Then, cancel the fiber and print the information about the fiber one more time. This time the fiber status is dead.

tarantool> increment_fiber:cancel()
---
...

tarantool> print('ID: ' .. increment_fiber:id() .. '\nStatus: ' .. increment_fiber:status() .. '\nCounter: ' .. counter)
ID: 104
Status: dead
Counter: 12
---
...

fiber.yield()¶

Yield control to the scheduler. Equivalent to fiber.sleep(0).

Exception:	see the Example of yield failure.

Example:

In the example below, two fibers are associated with the same function. Each fiber yields control after printing a greeting.

-- app.lua --
fiber = require('fiber')

function greet()
    while true do
        print('Enter a name:')
        name = io.read()
        print('Hello, '..name..'. I am fiber '..fiber.id())
        fiber.yield()
    end
end

for i = 1, 2 do
    fiber_object = fiber.create(greet)
    fiber_object:cancel()
end

The output might look as follows:

$ tarantool app.lua
Enter a name:
John
Hello, John. I am fiber 104
Enter a name:
Jane
Hello, Jane. I am fiber 105

fiber.status([fiber_object])¶

Return the status of the current fiber. If the fiber_object is passed, return the status of the specified fiber.

Parameters:	fiber_object – (optional) the fiber object
Return:	the status of fiber. One of: dead, suspended, or running.
Rtype:	string

Example:

tarantool> fiber.status()
---
- running
...

fiber.info({[backtrace/bt]})¶

Return information about all fibers.

Parameters:	backtrace (boolean) – show backtrace. Default: true. Set to false to show less information (symbol resolving can be expensive). bt (boolean) – same as backtrace, but with lower priority.
Return:	number of context switches (csw), backtrace, total memory, used memory, fiber ID (fid), fiber name. If fiber.top is enabled or Tarantool was built with ENABLE_FIBER_TOP, processor time (time) is also returned.
Rtype:	table

Return values explained

• csw – number of context switches.
• backtrace, bt – each fiber’s stack trace, showing where it originated and what functions were called.
• memory:
  • total – total memory occupied by the fiber as a C structure, its stack, etc.
  • used – actual memory used by the fiber.

• time – duplicates the “time” entry from fiber.top().cpu for each fiber.

  Only shown if fiber.top is enabled.

Example:

tarantool> fiber.info({ bt = true })
---
- 101:
    csw: 1
    backtrace:
    - C: '#0  0x5dd130 in lbox_fiber_id+96'
    - C: '#1  0x5dd13d in lbox_fiber_stall+13'
    - L: stall in =[C] at line -1
    - L: (unnamed) in @builtin/fiber.lua at line 59
    - C: '#2  0x66371b in lj_BC_FUNCC+52'
    - C: '#3  0x628f28 in lua_pcall+120'
    - C: '#4  0x5e22a8 in luaT_call+24'
    - C: '#5  0x5dd1a9 in lua_fiber_run_f+89'
    - C: '#6  0x45b011 in fiber_cxx_invoke(int (*)(__va_list_tag*), __va_list_tag*)+17'
    - C: '#7  0x5ff3c0 in fiber_loop+48'
    - C: '#8  0x81ecf4 in coro_init+68'
    memory:
    total: 516472
    used: 0
    time: 0
    name: lua
    fid: 101
  102:
    csw: 0
    backtrace:
    - C: '#0  (nil) in +63'
    - C: '#1  (nil) in +63'
    memory:
    total: 516472
    used: 0
    time: 0
    name: on_shutdown
    fid: 102

...

fiber.top()¶

Show all alive fibers and their CPU consumption.

Return:	a table with two entries: cpu and cpu_misses

cpu itself is a table whose keys are strings containing fiber ids and names. The three metrics available for each fiber are:

1. instant (in percent), which indicates the share of time the fiber was executing during the previous event loop iteration.

2. average (in percent), which is calculated as an exponential moving average of instant values over all the previous event loop iterations.

3. time (in seconds), which estimates how much CPU time each fiber spent processing during its lifetime.

   The time entry is also added to each fiber’s output in fiber.info() (it duplicates the time entry from fiber.top().cpu per fiber).

   Note that time is only counted while fiber.top() is enabled.

cpu_misses indicates the number of times the TX thread detected it was rescheduled on a different CPU core during the last event loop iteration. fiber.top() uses the CPU timestamp counter to measure each fiber’s execution time. However, each CPU core may have its own counter value (you can only rely on counter deltas if both measurements were taken on the same core, otherwise the delta may even get negative). When the TX thread is rescheduled to a different CPU core, Tarantool just assumes the CPU delta was zero for the latest measurement. This lowers the precision of our computations, so the bigger cpu misses value the lower the precision of fiber.top() results.

Note

With 2.11.0, cpu_misses is deprecated and always returns 0.

Example:

tarantool> fiber.top()
---
- cpu:
    107/lua:
      instant: 30.967324490456
      time: 0.351821993
      average: 25.582738345233
    104/lua:
      instant: 9.6473633128437
      time: 0.110869897
      average: 7.9693406131877
    101/on_shutdown:
      instant: 0
      time: 0
      average: 0
    103/lua:
      instant: 9.8026528631511
      time: 0.112641118
      average: 18.138387232255
    106/lua:
      instant: 20.071174377224
      time: 0.226901357
      average: 17.077908441831
    102/interactive:
      instant: 0
      time: 9.6858e-05
      average: 0
    105/lua:
      instant: 9.2461986412164
      time: 0.10657528
      average: 7.7068458630827
    1/sched:
      instant: 20.265286315108
      time: 0.237095335
      average: 23.141537169257
  cpu_misses: 0
...

Notice that by default new fibers created due to fiber.create are named ‘lua’ so it is better to set their names explicitly via fiber_object:name(‘name’).

There are several system fibers in fiber.top() output that might be useful:

• sched is a special system fiber. It schedules tasks to other fibers, if any, and also handles some libev events.

  It can have high instant and average values in fiber.top() output in two cases:

  • The instance has almost no load - then practically only sched is executing, and the other fibers are sleeping. So relative to the other fibers, sched may have almost 100% load.
  • sched handles a large number of system events. This should not cause performance problems.

• main fibers process requests that come over the network (iproto requests). There are several such fibers, and new ones are created if needed. When a new request comes in, a free fiber takes it and executes it. The request can be a typical select/replace/delete/insert or a function call. For example, conn:eval() or conn:call().

Note

Enabling fiber.top() slows down fiber switching by about 15%, so it is disabled by default. To enable it, use fiber.top_enable(). To disable it after you finished debugging, use fiber.top_disable().

fiber.kill(id)¶

Locate a fiber by its numeric ID and cancel it. In other words, fiber.kill() combines fiber.find() and fiber_object:cancel().

Parameters:	id – the ID of the fiber to be cancelled.
Exception:	the specified fiber does not exist or cancel is not permitted.

Example:

tarantool> fiber.kill(fiber.id()) -- kill self, may make program end
---
- error: fiber is cancelled
...

fiber.testcancel()¶

Check if the current fiber has been cancelled and throw an exception if this is the case.

Note

Even if you catch the exception, the fiber will remain cancelled. Most types of calls will check fiber.testcancel(). However, some functions (id, status, join etc.) will return no error. We recommend application developers to implement occasional checks with fiber.testcancel() and to end fiber’s execution as soon as possible in case it has been cancelled.

Example:

tarantool> fiber.testcancel()
---
- error: fiber is cancelled
...

fiber.set_max_slice(slice)¶

Set the default maximum slice for all fibers. A fiber slice limits the time period of executing a fiber without yielding control.

Parameters:	slice (number/table) – a fiber slice, which can one of the following: a time period (in seconds) that specifies the error slice. Example: fiber.set_max_slice(3). a table that specifies the warning and error slices (in seconds). Example: fiber.set_max_slice({warn = 1.5, err = 3}).

Example:

The example below shows how to use set_max_slice to limit the slice for all fibers. fiber.check_slice() is called inside a long-running operation to determine whether a slice for the current fiber is over.

-- app.lua --
fiber = require('fiber')
clock = require('clock')

fiber.set_max_slice({warn = 1.5, err = 3})
time = clock.monotonic()
function long_operation()
    while clock.monotonic() - time < 5 do
        fiber.check_slice()
        -- Long-running operation ⌛⌛⌛ --
    end
end

long_operation_fiber = fiber.create(long_operation)

The output should look as follows:

$ tarantool app.lua
fiber has not yielded for more than 1.500 seconds
FiberSliceIsExceeded: fiber slice is exceeded

fiber.set_slice(slice)¶

Set a slice for the current fiber execution. A fiber slice limits the time period of executing a fiber without yielding control.

Parameters:	slice (number/table) – a fiber slice, which can one of the following: a time period (in seconds) that specifies the error slice. Example: fiber.set_slice(3). a table that specifies the warning and error slices (in seconds). Example: fiber.set_slice({warn = 1.5, err = 3}).

Example:

The example below shows how to use set_slice to limit the slice for the current fiber execution. fiber.check_slice() is called inside a long-running operation to determine whether a slice for the current fiber is over.

-- app.lua --
fiber = require('fiber')
clock = require('clock')

time = clock.monotonic()
function long_operation()
    fiber.set_slice({warn = 1.5, err = 3})
    while clock.monotonic() - time < 5 do
        fiber.check_slice()
        -- Long-running operation ⌛⌛⌛ --
    end
end

long_operation_fiber = fiber.create(long_operation)

The output should look as follows.

$ tarantool app.lua
fiber has not yielded for more than 1.500 seconds
FiberSliceIsExceeded: fiber slice is exceeded

fiber.extend_slice(slice)¶

Extend a slice for the current fiber execution. For example, if the default error slice is set using fiber.set_max_slice() to 3 seconds, extend_slice(1) extends the error slice to 4 seconds.

Parameters:	slice (number/table) – a fiber slice, which can one of the following: a time period (in seconds) that specifies the error slice. Example: fiber.extend_slice(1). a table that specifies the warning and error slices (in seconds). Example: fiber.extend_slice({warn = 0.5, err = 1}).

Example:

The example below shows how to use extend_slice to extend the slice for the current fiber execution. The default fiber slice is set using set_max_slice.

-- app.lua --
fiber = require('fiber')
clock = require('clock')

fiber.set_max_slice({warn = 1.5, err = 3})
time = clock.monotonic()
function long_operation()
    fiber.extend_slice({warn = 0.5, err = 1})
    while clock.monotonic() - time < 5 do
        fiber.check_slice()
        -- Long-running operation ⌛⌛⌛ --
    end
end

long_operation_fiber = fiber.create(long_operation)

The output should look as follows.

$ tarantool app.lua
fiber has not yielded for more than 2.000 seconds
FiberSliceIsExceeded: fiber slice is exceeded

FiberSliceIsExceeded is thrown after 4 seconds.

fiber.check_slice()¶

Check whether a slice for the current fiber is over. A fiber slice limits the time period of executing a fiber without yielding control.

Example:

See the examples for the following functions:

• fiber.set_max_slice()
• fiber.set_slice()
• fiber.extend_slice()

fiber.time()¶

Return:	current system time (in seconds since the epoch) as a Lua number. The time is taken from the event loop clock, which makes this call very cheap, but still useful for constructing artificial tuple keys.
Rtype:	number

Example:

tarantool> fiber.time(), fiber.time()
---
- 1448466279.2415
- 1448466279.2415
...

fiber.time64()¶

Return:	current system time (in microseconds since the epoch) as a 64-bit integer. The time is taken from the event loop clock.
Rtype:	cdata (ctype<int64_t>)

Example:

tarantool> fiber.time(), fiber.time64()
---
- 1448466351.2708
- 1448466351270762
...

fiber.clock()¶

Get the monotonic time in seconds. It is better to use fiber.clock() for calculating timeouts instead of fiber.time() because fiber.time() reports real time so it is affected by system time changes.

Return:	a floating-point number of seconds, representing elapsed wall-clock time since some time in the past that is guaranteed not to change during the life of the process
Rtype:	number

Example:

tarantool> start = fiber.clock()
---
...
tarantool> print(start)
248700.58805
---
...
tarantool> print(fiber.time(), fiber.time()-start)
1600785979.8291 1600537279.241
---
...

fiber.clock64()¶

Same as fiber.clock() but in microseconds.

Return:	a number of seconds as 64-bit integer, representing elapsed wall-clock time since some time in the past that is guaranteed not to change during the life of the process
Rtype:	cdata (ctype<int64_t>)

object fiber_object¶

fiber_object:id()¶

Parameters:	fiber_object – generally this is an object referenced in the return from fiber.create or fiber.self or fiber.find
Return:	ID of the fiber.
Rtype:	number

fiber.self():id() can also be expressed as fiber.id().

Example:

tarantool> fiber_object = fiber.self()
---
...
tarantool> fiber_object:id()
---
- 101
...

fiber_object:name()¶

Parameters:	fiber_object – generally this is an object referenced in the return from fiber.create or fiber.self or fiber.find
Return:	name of the fiber.
Rtype:	string

fiber.self():name() can also be expressed as fiber.name().

Example:

tarantool> fiber.self():name()
---
- interactive
...

fiber_object:name(name[, options])

Change the fiber name. By default a Tarantool server’s interactive-mode fiber is named ‘interactive’ and new fibers created due to fiber.create are named ‘lua’. Giving fibers distinct names makes it easier to distinguish them when using fiber.info and fiber.top(). Max length is 255.

Parameters:

fiber_object – generally this is an object referenced in the return from fiber.create or fiber.self or fiber.find name (string) – the new name of the fiber. options – truncate=true – truncates the name to the max length if it is too long. If this option is false (the default), fiber.name(new_name) fails with an exception if a new name is too long. The name length limit is 255 (since version 2.4.1).

Return:

nil

Example:

tarantool> fiber.self():name('non-interactive')
---
...

fiber_object:status()¶

Return the status of the specified fiber.

Parameters:	fiber_object – generally this is an object referenced in the return from fiber.create or fiber.self or fiber.find
Return:	the status of fiber. One of: “dead”, “suspended”, or “running”.
Rtype:	string

fiber.self():status() can also be expressed as fiber.status().

Example:

tarantool> fiber.self():status()
---
- running
...

fiber_object:cancel()¶

Send a cancellation request to the fiber. Running and suspended fibers can be cancelled. After a fiber has been cancelled, attempts to operate on it cause errors, for example, fiber_object:name() causes error: the fiber is dead. But a dead fiber can still report its ID and status.

Cancellation is asynchronous. Use fiber_object:join() to wait for the cancellation to complete. After fiber_object:cancel() is called, the fiber may or may not check whether it was cancelled. If the fiber does not check it, it cannot ever be cancelled.

Parameters:	fiber_object – generally this is an object referenced in the return from fiber.create or fiber.self or fiber.find
Return:	nil

Possible errors: cancel is not permitted for the specified fiber object.

Example:

See the fiber.sleep() example.

fiber_object:set_max_slice(slice)¶

Set a fiber’s maximum slice. A fiber slice limits the time period of executing a fiber without yielding control.

Parameters:	slice (number/table) – a fiber slice, which can one of the following: a time period (in seconds) that specifies the error slice. Example: long_operation_fiber.set_max_slice(3). a table that specifies the warning and error slices (in seconds). Example: long_operation_fiber.set_max_slice({warn = 1.5, err = 3}).

Example:

The example below shows how to use set_max_slice to limit the fiber slice. fiber.check_slice() is called inside a long-running operation to determine whether a slice for the fiber is over.

-- app.lua --
fiber = require('fiber')
clock = require('clock')

time = clock.monotonic()
function long_operation()
    while clock.monotonic() - time < 5 do
        fiber.check_slice()
        -- Long-running operation ⌛⌛⌛ --
    end
end

long_operation_fiber = fiber.new(long_operation)
long_operation_fiber:set_max_slice({warn = 1.5, err = 3})

The output should look as follows.

$ tarantool app.lua
fiber has not yielded for more than 1.500 seconds
FiberSliceIsExceeded: fiber slice is exceeded

fiber_object.storage¶

A local storage within the fiber. It is a Lua table created when it is first accessed. The storage can contain any number of named values, subject to memory limitations. Naming may be done with fiber_object.storage.name or fiber_object.storage['name']. or with a number fiber_object.storage[number]. Values may be either numbers or strings.

fiber.storage is destroyed when the fiber is finished, regardless of how is it finished – via fiber_object:cancel(), or the fiber’s function did ‘return’. Moreover, the storage is cleaned up even for pooled fibers used to serve IProto requests. Pooled fibers never really die, but nonetheless their storage is cleaned up after each request. That makes possible to use fiber.storage as a full featured request-local storage. This behavior is implemented in versions 2.2.3, 2.3.2, 2.4.1, and all later versions.

This storage may be created for a fiber, no matter how the fiber itself is created – from C or from Lua. For example, a fiber can be created in C using fiber_new(), then it can insert into a space, which has Lua on_replace triggers, and one of the triggers can create fiber.storage. That storage is deleted when the fiber is stopped.

Example:

The example below shows how to save the last entered name in a fiber storage and get this value before cancelling a fiber.

-- app.lua --
fiber = require('fiber')

function greet()
    while true do
        print('Enter a name:')
        name = io.read()
        if name ~= 'bye' then
            fiber.self().storage.name = name
            print('Hello, ' .. name)
        else
            print('Goodbye, ' .. fiber.self().storage['name'])
            fiber.self():cancel()
        end
    end
end

fiber_object = fiber.create(greet)

The output might look as follows:

$ tarantool app.lua
Enter a name:
John
Hello, John
Enter a name:
Jane
Hello, Jane
Enter a name:
bye
Goodbye, Jane

See also box.session.storage.

fiber_object:set_joinable(is_joinable)¶

Make a fiber joinable. A joinable fiber can be waited for using fiber_object:join().

The best practice is to call fiber_object:set_joinable() before the fiber function begins to execute because otherwise the fiber could become dead before fiber_object:set_joinable() takes effect. The usual sequence could be:

1. Call fiber.new() instead of fiber.create() to create a new fiber_object.

   Do not yield at this point, because that will cause the fiber function to begin.

2. Call fiber_object:set_joinable(true) to make the new fiber_object joinable.

   Now it is safe to yield.

3. Call fiber_object:join().

   Usually fiber_object:join() should be called, otherwise the fiber’s status may become ‘suspended’ when the fiber function ends, instead of ‘dead’.

Parameters:	is_joinable (boolean) – the boolean value that specifies whether the fiber is joinable
Return:	nil

Example:

See the fiber_object.join() example.

fiber_object:join()¶

Join a fiber. Joining a fiber enables you to get the result returned by the fiber’s function.

Joining a fiber runs the fiber’s function and waits until the fiber’s status is dead. Normally a status becomes dead when the function execution finishes. Joining the fiber causes a yield, therefore, if the fiber is currently in the suspended state, execution of its fiber function resumes.

Note that joining a fiber works only if the fiber is created using fiber.new() and is made joinable using fiber_object:set_joinable().

Return:

The join method returns two values: The boolean value that indicates whether the join is succeeded because the fiber’s function ended normally. The return value of the fiber’s function. If the first value is false, then the join succeeded because the fiber’s function ended abnormally and the second result has the details about the error, which one can unpack in the same way that one unpacks a pcall result.

Rtype:

boolean + result type, or boolean + struct error

Example:

The example below shows how to get the result returned by the fiber’s function.

fiber = require('fiber')

function add(a, b)
    return a + b
end

add_fiber = fiber.new(add, 5, 6)
add_fiber:set_joinable(true)
is_success, result = add_fiber:join()
print('Is successful: '.. tostring(is_success))
print('Returned value: '..result)

The output should look as follows.

$ tarantool app.lua
Is successful: true
Returned value: 11

Example of yield failure

Warning: yield() and any function which implicitly yields (such as sleep()) can fail (raise an exception).

For example, this function has a loop that repeats until cancel() happens. The last thing that it will print is ‘before yield’, which demonstrates that yield() failed, the loop did not continue until testcancel() failed.

fiber = require('fiber')
function function_name()
  while true do
    print('before testcancel')
    fiber.testcancel()
    print('before yield')
    fiber.yield()
  end
end
fiber_object = fiber.create(function_name)
fiber.sleep(.1)
fiber_object:cancel()

Example

This example should give a rough idea of what some functions for fibers should look like. It’s assumed that the functions would be referenced in fiber.create().

fiber = require('fiber')
channel = fiber.channel(10)
function consumer_fiber()
    while true do
        local task = channel:get()
        ...
    end
end

function consumer2_fiber()
    while true do
        -- 10 seconds
        local task = channel:get(10)
        if task ~= nil then
            ...
        else
            -- timeout
        end
    end
end

function producer_fiber()
    while true do
        task = box.space...:select{...}
        ...
        if channel:is_empty() then
            -- channel is empty
        end

        if channel:is_full() then
            -- channel is full
        end

        ...
        if channel:has_readers() then
            -- there are some fibers
            -- that are waiting for data
        end
        ...

        if channel:has_writers() then
            -- there are some fibers
            -- that are waiting for readers
        end
        channel:put(task)
    end
end

function producer2_fiber()
    while true do
        task = box.space...select{...}
        -- 10 seconds
        if channel:put(task, 10) then
            ...
        else
            -- timeout
        end
    end
end

Example

Assume that a Tarantool instance is running and listening for connections on localhost port 3301. Assume that guest users have privileges to connect. We will use the tt utility to start two clients.

On terminal #1, say

$ tt connect localhost:3301
tarantool> fiber = require('fiber')
tarantool> cond = fiber.cond()
tarantool> cond:wait()

The job will hang because cond:wait() – without an optional timeout argument – will go to sleep until the condition variable changes.

On terminal #2, say

$ tt connect localhost:3301
tarantool> cond:signal()

Now look again at terminal #1. It will show that the waiting stopped, and the cond:wait() function returned true.

This example depended on the use of a global conditional variable with the arbitrary name cond. In real life, programmers would make sure to use different conditional variable names for different applications.