Need help setting up a logic behind a Facebook notification system style - php

Question

I want build a notification system to my website, similar to Facebook. The notifications don't need to be in real time.
This is what I have in mind:
User create an event (upload a new photo, add a new comment, like a photo, or even an administration alert to all users)
Run a cronjob every 5 minutes to add the notifications into the notifications table: id|id_content|type_of_content_enum|affected_user_id|date|seen_bool
The cronjob, will run several functions for each type of notification, for example:
add_photos_notification() // This function will check all photos added in the past 5 minutes, and insert a row in the notification table, for each user following this person. The function will group all photos added into the past 5 minutes, so the follower don't get many notifications for the same type of content. Resulting in a notification like: User X added Y photos in his profile.
add_admin_notification() // This function will check all news added by the administration of the site in the past 5 minutes, and insert a row in the notification table, for each user on the system ...
Is this a correct approach to build a notification system?
Is it possible to miss an event, running a cron every 5 minutes where functions retrieve the past 5 minutes events?
To be safe, do you think an alternative could be checking all events where a field 'notified' is not true? The same function that will grab the events where 'notified' = false, update it to true after adding that notification into the notifications table.
Thanks,

Answer 1

I went with the cronjob route and working good so far. Since our system got so many users to be notified, I found it the most appropriate way to do for two reasons.
I don't need to edit my current scripts code, inserting functions to add notifications for every event I want notify.
Since there gonna be some actions where many users are affected, adding notifications in real time could result in long script delays and time outs.
I built a class called notifications and inside this class, there are functions to add notification for every event I want notify, for example: user_added_a_new_photo(); user_commented_on_a_photo();
For every notification generate, Im adding 1 entry per user to be notified. This is how my notifications db looks like:
id
affected_user_id //user being notified
user_generating_the_notification_id
content_type // enum containing all type of notifications my system has (example: photo, video, comment) ...
content_json // a json containing the notification content. Based on the content type, on the display view file, I call helpers that will format the notification row using the json info.
date // the date the notification was added
seen_on // the date the user saw the notification
clicked_on // if user clicked on the notification, store the date he clicked on it
display // true or false
For this purpose, I added the display field cause for every new notification I create, I check the database if the same user, has another not seen notification, from the same generating user. If this condition is true, I set the old notification to display = false, and group the two new notifications resulting in something like: User X added X new photos in his gallery.
The clicked_on field, stores the date the item was clicked so I can generate reports based on this info if I need to. When displaying the content, if this item is not null, I highlight the notification to mark those not checked yet.
I created a text field to store the notification content in json, cause different notifications has different styles to present the users. For example, a new comment notification, has only texts, but a new photo notification, has a preview thumb.
So far, no issue running it and working for my needs.
The only downside, since the cronjobs can only be run only every 1 minute, the notifications may have 1 minute delay. But since I don't need it in real time, I set the cronjob to run every 5 minutes.

Answer 2

I've been looking into something like this as well and i just found this question on stackoverflow
Building a notification system
Have a look at the answer, the user does shed a fair bit of light on the theory of how to implement this sort of system.
As far as i can see you would need to create a record or object for each notification, if 100 people are subscribe to that event then 100 records will be generated. When a page is loaded your system will find all the notifications that correspond to the user logged in (Maybe by record ID) and then you would notify the user of how many notifications they have.
Now another way of doing this (Note i haven't implemented any of these, they are just ideas) would be to make a notification in a a table, then we'd have a second table that will hold a users ID and a notification ID, a user will be added to this table when they are subscribed to a notification, for example, confirmation of a friend request.
Please note again, these aren't proven methods, they're a result of some of my research of the matter, i'd read the post i gave you before doing anything.