Sessionstorage is constantly growing

[xshadow]

The session storage seems to be constantly growing When running etherpad we see a constantly growing number of session storage values in the database.

Is there a way to clean them up? We already looked into the script, which didn't help. https://github.com/ether/etherpad-lite/blob/develop/src/bin/deleteAllGroupSessions.js . So it seems that this sessions are no group sessions.

A standard session storage entry looks like:

| sessionstorage:----KdezcUojWiGfuzBgo1L-OnF4SbbZ | {"cookie":{"path":"/","_expires":null,"originalMaxAge":null,"httpOnly":true,"sameSite":"Lax","secure":false}} |
| sessionstorage:---0sPTO0IIvtWBpbYpccywxSImVqSf_ | {"cookie":{"path":"/","_expires":null,"originalMaxAge":null,"httpOnly":true,"sameSite":"Lax","secure":false}} |
| sessionstorage:---2pUJfjIE00yXpIkrzvHvUEOCLKI2X | {"cookie":{"path":"/","_expires":null,"originalMaxAge":null,"httpOnly":true,"sameSite":"Lax","secure":false}} |

Server (please complete the following information):

• Etherpad version: 1.8.13
• OS: Debian Buster
• Node.js version (node --version): v14.16.1
• npm version (npm --version): 6.14.12

Additional context

All session storage keys values from mariadb:

MariaDB [etherpad]> SELECT COUNT(*) FROM store WHERE `key` LIKE 'sessionstorage%';
+----------+
| COUNT(*) |
+----------+
| 16306299 |
+----------+
1 row in set (14.974 sec)

None session storage keys values from mariadb:

MariaDB [etherpad]> SELECT COUNT(*) FROM store WHERE `key` NOT LIKE 'sessionstorage%';
+----------+
| COUNT(*) |
+----------+
| 54156416 |
+----------+
1 row in set (36.645 sec)

[webzwo0i]

My first guess is that you can delete them after stopping Etherpad in case you run the default configuration (no special auth plugins and settings.json with requireAuthentication:false, requireAuthorization:false), but I'll take a deeper look when I have more time. Afaik sessions would just be re-generated as soon as users reconnect - but I need to be sure about this. They are different from group sessions (which you'd use, if you run an instance that can't be used without a valid session and generate the sessions via API), and thus as you noticed the script won't work.

They are also not used for author information (user names/author attribution across pads).

What they are used for is accessing /admin endpoints and other auth stuff depending on your configuration/plugins used.

https://github.com/ether/etherpad-lite/issues/4898 has more discussion on the underlying issue.

[xshadow]

Thanks for etherpad as software and the fast reply. Thanks for pointing me to this issue.

If I calculated this correct, those 16 million session won't require more space than 300MB, so this is fine. But I think it would be great if they could expire automatically or by or a session garbage collector job :)

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[webzwo0i]

Closing this, as https://github.com/ether/etherpad-lite/issues/4898 has more info and in the course of fixing #4898 we will need a way to purge old sessions anyway (manually or even automatically).

[webzwo0i]

Reopening again, as #4898 does not necessarly require cleaning up old sessions...

[rhansen]

Design idea:

• express-session config changes:
  • set maxAge, value TBD (maybe 7 days so the user doesn't have to keep logging in?)
  • set rolling=true so that a session stays alive as long as the user stays connected
  • set saveUninitialized=false to lower the load on the db (though it might not help in our case because I think we always modify the session object)
  • set resave=false to lower the load on the db (though it might not help due to rolling=true)
• SessionStore changes:
  • implement the touch() method
  • when creating/updating/touching a session, (re)set a timer that auto-deletes the session's db record when it expires
• call req.session.touch() each time the server receives a socket.io ping or message from the client

A problem with the above scheme: The constant .touch() calls might increase the load on the db considerably. We could reduce db load by skipping a db write unless it would extend the saved lifetime by more than some threshold. (In other words: Trade accuracy for reduced load.) For example, we could set maxAge to 14 days and only update the db record if it would extend the lifetime by 7 days or more. That should reduce db load to one write per week per session, which is trivial.

Another problem: We would need to clean up old session records after a dirty shutdown, or after upgrading from a version of Etherpad that doesn't expire sessions. ueberdb doesn't have cursor support so we can't just iterate over all records that match sessionstorage:*. We could add cursor support to ueberdb, but that would take a lot of effort. Alternatively, with some clever key prefixing we can iterate over old sessions in small batches. Here's one approach:

• Add two new numeric db records:
  • instanceId: A counter that is incremented each time Etherpad starts up.
  • caughtUpToInstanceId: Used to clean up session state from previous Etherpad runs. Always less than or equal to instanceId.
• When saving a session, use key `sessionstore:${instanceId}:${sid}` (instead of the current `sessionstorage:${sid}`) and delete records `sessionstore:${caughtUpToInstanceId}:${sid}` through `sessionstore:${instanceId - 1}:${sid}`.
• When looking up a session ID, fetch all keys `sessionstore:${caughtUpToInstanceId}:${sid}` through `sessionstore:${instanceId}:${sid}` (inclusive) in parallel. Of the non-null results, use the one with the highest instance ID.
• On startup, launch a worker thread to clean up or migrate session state from previous runs:

  for (; caughtUpToInstanceId < instanceId; ++caughtUpToInstanceId) {
    // The following query should return a small number of keys (the number of
    // sessions that were active when instance caughtUpToInstanceId exited).
    for (const key of getAllDbKeysWithPrefix(`sessionstore:${caughtUpToInstanceId}:`)) {
      const sess = getDbValue(key);
      deleteDbRecord(key);
      // If the session is still valid, save under the current instanceId and start a cleanup timer.
      if (!isSessionExpired(sess)) saveSession(sess);
    }
  }
  

To clean up legacy `sessionstorage:${sid}` records, we could do something like this:

const alphabet = 'abcdefghijklmnopqrstuvwxyz';
const sidCharset = `_-0123456789${alphabet}${alphabet.toUpperCase()}`;
for (const chars of cartesianProductGenerator(Array(4).fill(sidCharset))) {
  for (const key of getAllDbKeysWithPrefix(`sessionstorage:${chars.join('')}`) {
    deleteDbRecord(key);
  }
}      

But it would probably be better to just have the user issue a native DB query to delete all of the legacy records.