Triggers, also known as callbacks, are functions which the server executes when certain events happen.
To associate an event with a callback,
one should pass the callback to the corresponding
- box.session.on_connect() or box.session.on_disconnect(),
- space_object:on_replace() or space_object:before_replace(),
- box.on_commit() or box.on_rollback(),
- net.box.on_connect() or net.box.on_disconnect(),
- box.ctl.on_schema_init() or box.ctl.on_schema_init(),
Then the server will store the callback function and call it when the corresponding event happens.
All triggers have the following characteristics:
- Triggers are defined only by the ‘admin’ user.
- Triggers are stored in the Tarantool instance’s memory, not in the database. Therefore triggers disappear when the instance is shut down. To make them permanent, put function definitions and trigger settings into Tarantool’s initialization script.
- Triggers have low overhead. If a trigger is not defined, then the overhead is minimal: merely a pointer dereference and check. If a trigger is defined, then its overhead is equivalent to the overhead of calling a function.
- There can be multiple triggers for one event. In this case, triggers are executed in the reverse order that they were defined in. (Exception: member triggers are executed in the order that they appear in the member list.)
- Triggers must work within the event context. However, effects are undefined if a function contains requests which normally could not occur immediately after the event, but only before the return from the event. For example, putting os.exit() or box.rollback() in a trigger function would be bringing in requests outside the event context.
- Triggers are replaceable. The request to “redefine a trigger” implies
passing a new trigger function and an old trigger function
to one of the
on_eventfunctions all have parameters which are function pointers, and they all return function pointers. Remember that a Lua function definition such as
function f() x = x + 1 endis the same as
f = function () x = x + 1 end- in both cases
fgets a function pointer. And
trigger = box.session.on_connect(f)is the same as
trigger = box.session.on_connect(function () x = x + 1 end)- in both cases
triggergets the function pointer which was passed.
- You can call any
on_eventfunction with no arguments to get a list of its triggers. For example, use
box.session.on_connect()to return a table of all connect-trigger functions.
- Triggers can be useful in solving problems with replication. See details in Resolving replication conflicts.
Here we log connect and disconnect events into Tarantool server log.
log = require('log') function on_connect_impl() log.info("connected "..box.session.peer()..", sid "..box.session.id()) end function on_disconnect_impl() log.info("disconnected, sid "..box.session.id()) end function on_auth_impl(user) log.info("authenticated sid "..box.session.id().." as "..user) end function on_connect() pcall(on_connect_impl) end function on_disconnect() pcall(on_disconnect_impl) end function on_auth(user) pcall(on_auth_impl, user) end box.session.on_connect(on_connect) box.session.on_disconnect(on_disconnect) box.session.on_auth(on_auth)