Triggers, also known as callbacks, are functions which the server executes when certain events happen.
There are three types of triggers in Tarantool:
- connection triggers, which are executed when a session begins or ends,
- authentication triggers, which are executed during authentication, and
- replace triggers, which are for database events.
All triggers have the following characteristics:
- Triggers associate a function with an event. The request to “define a trigger” implies passing the trigger’s function to one of the “on_event()” functions: box.session.on_connect(), box.session.on_auth(), box.session.on_disconnect(), or space_object:on_replace().
- 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.
- 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_event()” functions.
- The “on_event()” functions 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 end” is 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.
To get a list of triggers, you can use:
- on_connect() – with no arguments – to return a table of all connect-trigger functions;
- on_auth() to return all authentication-trigger functions;
- on_disconnect() to return all disconnect-trigger functions;
- on_replace() to return all replace-trigger functions.
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)