MySQL - Prevent Insertion of Record if ID and Timestamp Unique Combo Constraint Within Timeframe

Question

I have a scenario in which 3 standalone agents are reporting uptime statuses for various hosts. If the hosts go down and are offline, a downtime record should be created. Unfortunately, since the agents report exactly at the same time with the same information, I've seen duplicate entries that are 1-2 seconds apart.

I have a unique constraint that was created on the table for both the datetime and the host ID. Thus, they cannot be the same. But if the requests from the agents come in at the same time or a second apart, a duplicate might be created despite code checks looking for an existing entry (in this case, an entry hasn't been created yet in all three instances if the agents report at the same time). The unique constraint won't prevent the duplicates either, since the datetime might be 1 second ahead or behind when the PHP / MySQL call finishes getting processed...

So, what is the best way to handle this situation? Is there a way in MySQL to specify that if a unique constraint (which includes a datetime field) is within a certain time frame of another record with the unique constraint, it shouldn't be allowed to insert?

Do I need to run a job that removes entries within a few seconds of each other, or is there a way to get MySQL to do this for me somehow?

Table structure looks like this

entry_id host_id datetime

Entries might be

1 121 01/17/2019 02:38:04 AM

1 121 01/17/2019 02:38:05 AM

1 121 01/17/2019 02:36:04 AM

I want to prevent the insertion of the bold entry since it's within 1 second from the last entry that was inserted. Code checks won't work because no entries may exist at the time it checks for one. I already have a code check looking for an existing entry, and since it doesn't find one and the code can be run at the same time for each request, the check fails and says a new entry should be created.

There are more datetime columns in my table, but they aren't needed to understand this situation. Any help is appreciated.

Answer 1

If you're using MySQL 5.7 or newer, you can define a generated column that contains the timestamp rounded down to a 5 or 10 second range. Then you can define a unique index on that column, so you'll get a constraint violation if you try to create two records with timestamps in that same period.

ALTER TABLE yourTable 
ADD datetime_10sec INT AS (10 * ROUND(UNIX_TIMESTAMP(datetime)/10)) PERSISTENT, 
ADD UNIQUE INDEX (host_id, datetime_10sec);

Change both 10 to whatever time granularity you want to use.