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Module expirationd
For a commercial-grade example of a Lua `rock <https://rocks.tarantool.org/>`_ that works with Tarantool, let us look at the source code of ``expirationd``, which Tarantool supplies on `GitHub <https://github.com/tarantool/expirationd/blob/master/expirationd.lua>`_ with an Artistic license. The ``expirationd.lua`` program is lengthy (about 500 lines), so here we will only highlight the matters that will be enhanced by studying the full source later.
task.worker_fiber = fiber.create(worker_loop, task)
log.info("expiration: task %q restarted", task.name)
...
fiber.sleep(expirationd.constants.check_interval)
...
Whenever one hears "daemon" in Tarantool, one should suspect it's being done with a :doc:`fiber<../reference_lua/fiber>`. The program is making a fiber and turning control over to it so it runs occasionally, goes to sleep, then comes back for more.
for _, tuple in scan_space.index[0]:pairs(nil, {iterator = box.index.ALL}) do
...
expiration_process(task, tuple)
...
/* expiration_process() contains:
if task.is_tuple_expired(task.args, tuple) then
task.expired_tuples_count = task.expired_tuples_count + 1
task.process_expired_tuple(task.space_id, task.args, tuple) */
The "for" instruction can be translated as "iterate through the index of the space that is being scanned", and within it, if the tuple is "expired" (for example, if the tuple has a timestamp field which is less than the current time), process the tuple as an expired tuple.
-- default process_expired_tuple function
local function default_tuple_drop(space_id, args, tuple)
box.space[space_id]:delete(construct_key(space_id, tuple))
end
/* construct_key() contains:
local function construct_key(space_id, tuple)
return fun.map(
function(x) return tuple[x.fieldno] end,
box.space[space_id].index[0].parts
):totable()
end */
Ultimately the tuple-expiry process leads to ``default_tuple_drop()`` which does a "delete" of a tuple from its original space. First the fun :ref:`fun <fun-module>` module is used, specifically `fun.map <https://luafun.github.io/transformations.html#fun.map>`_. Remembering that :codenormal:`index[0]` is always the space's primary key, and :codenormal:`index[0].parts[`:codeitalic:`N`:codenormal:`].fieldno` is always the field number for key part :codeitalic:`N`, fun.map() is creating a table from the primary-key values of the tuple. The result of fun.map() is passed to :ref:`space_object:delete() <box_space-delete>`.
local function expirationd_run_task(name, space_id, is_tuple_expired, options)
...
At this point, if the above explanation is worthwhile, it is clear that ``expirationd.lua`` starts a background routine (fiber) which iterates through all the tuples in a space, sleeps cooperatively so that other fibers can operate at the same time, and -- whenever it finds a tuple that has expired -- deletes it from this space. Now the "``expirationd_run_task()``" function can be used in a test which creates sample data, lets the daemon run for a while, and prints results.
For those who like to see things run, here are the exact steps to get ``expirationd`` through the test.
Get ``expirationd.lua``. There are standard ways -- it is after all part of a `standard rock <https://luarocks.org/modules/rtsisyk/expirationd>`_ -- but for this purpose just copy the contents of ``expirationd.lua`` to a directory on the Lua path (type ``print(package.path)`` to see the Lua path).
Start the Tarantool server as described before.
Execute these requests:
fiber = require('fiber')
expd = require('expirationd')
box.cfg{}
e = box.schema.space.create('expirationd_test')
e:create_index('primary', {type = 'hash', parts = {1, 'unsigned'}})
e:replace{1, fiber.time() + 3}
e:replace{2, fiber.time() + 30}
function is_tuple_expired(args, tuple)
if (tuple[2] < fiber.time()) then return true end
return false
end
expd.run_task('expirationd_test', e.id, is_tuple_expired)
retval = {}
fiber.sleep(2)
expd.task_stats()
fiber.sleep(2)
expd.task_stats()
expd.kill_task('expirationd_test')
e:drop()
os.exit()
The database-specific requests (``cfg``, :doc:`space.create </reference/reference_lua/box_schema/space_create>`, :ref:`create_index <box_space-create_index>`) should already be familiar.
The function which will be supplied to ``expirationd`` is :codenormal:`is_tuple_expired`, which is saying "if the second field of the tuple is less than the :ref:`current time <fiber-time>` , then return true, otherwise return false".
The key for getting the rock rolling is ``expd = require('expirationd')``. The `require <https://www.lua.org/pil/8.1.html#require>`_ function is what reads in the program; it will appear in many later examples in this manual, when it's necessary to get a module that's not part of the Tarantool kernel, but is on the Lua path (``package.path``) or the C path (``package.cpath``). After the Lua variable expd has been assigned the value of the ``expirationd`` module, it's possible to invoke the module's ``run_task()`` function.

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testing_project / Documentation: reference/reference_rock/expirationd — English

task.worker_fiber = fiber.create(worker_loop, task)
log.info("expiration: task %q restarted", task.name)
...
fiber.sleep(expirationd.constants.check_interval)
...
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