Recently I needed to delete some documents that I saved in MongoDB after some time. I can think of a few examples of why we would want to delete data after some time:
We could do it by running a cronjob that deletes the data, delete the data every time we insert new data or any other solution.
Luckily for me, my wife told me that MongoDB already has that mechanism built-in.
TTL (Time-To-Live) indexes are special single-field indexes that MongoDB can use to automatically remove documents from a collection after a certain amount of time.
A background thread in MongoDB reads the values in the index and removes expired documents from the collection (usually every minute).
For example, to create a TTL index on the
lastModifiedDate
field of the eventlog
collection, use the following operation in the mongo shell:db.eventlog.createIndex( { "lastModifiedDate": 1 }, { expireAfterSeconds: 3600 } )
As of MongoDB 3.2, a collection can be partially indexed using a specified filter expression,
partialFilterExpression
. TTL index can also be used with partial indexes.For example:
Delete documents that created 1 hour ago if
state
equals to TMP
db.eventlog.createIndex(
{ created_at: 1 },
{ expireAfterSeconds: 3600, partialFilterExpression: { state: 'TMP' } }
);
Delete documents that created 1 day ago if
count
is lower than 5
db.eventlog.createIndex(
{ created_at: 1 },
{ expireAfterSeconds: 86400, partialFilterExpression: { count: { $lt: 5 } } }
);
Additional info on partial indexes
Recently I finished developing BundleMon, which is a free tool that helps you monitor your apps bundle size.
One of the components in BundleMon is a service that saves historic reports in order to compare bundle size between branches. So when you open a PR, BundleMon saves a record with the current bundle size report.
No need to save the report for more than 30 days, so I just added a TTL index:
db.reports.createIndex(
{ creationDate: 1 },
{ expireAfterSeconds: 2592000, partialFilterExpression: { prNumber: { $exists: true } } }
);
Also published at https://dev.to/lironer/delete-expired-documents-automatically-with-mongodb-ttl-index-l44