Description

sh.status() counts failed balancer rounds by checking details.errorOccured on the last 5 balancer.round entries in config.actionlog. SERVER-56276 (server 5.1.0, commit dd6a29fcd0) fixed the spelling, and the server now writes details.errorOccurred. That commit also updated the legacy shell (src/mongo/shell/utils_sh.js), but mongosh never got the same change.

So with any server from 5.1 on, including all current LTS releases (6.0, 7.0, 8.0), sh.status() always shows 'Failed balancer rounds in last 5 attempts': 0 and never shows Last reported error or Time of Reported error. This happens even when every recent balancer round has failed.

The check was added in commit 0beb56c8 (MONGOSH-638, released in v0.5.0) and is unchanged on main, in packages/shell-api/src/helpers.ts:

if (r.details.errorOccured) {

Affected versions

• mongosh: every version from 0.5.0 on. Confirmed on 2.8.x, 2.9.x and 2.10.0.
• Server: confirmed on Percona Server for MongoDB 8.0.34-15, whose mongod binary only contains the string errorOccurred.

Steps to reproduce

1. Deploy a sharded cluster on 6.0+ with 3 shards.
2. Shard a collection, e.g. sh.shardCollection("percona.percona", {"user.name": 1}), and load data (we used 5M docs). Lower chunksize so the balancer has work to do.
3. Make one shard's primary unavailable (we stopped the only node of shard sh03).
4. Wait for a few balancer rounds (about 2 minutes).
5. Check the actionlog:{{{}db.getSiblingDB("config").actionlog.find(
   {what: "balancer.round"}, {_id: 0, time: 1, details: 1}
   ).sort({time: -1}).limit(5){}}}All 5 entries have errorOccurred: true and errmsg: 'Could not find host matching read preference { mode: "primary" } for set sh03'.
6. Run sh.status().

Expected vs actual

Expected

{{balancer: {
'Failed balancer rounds in last 5 attempts': 5,
'Last reported error': 'Could not find host ...',
'Time of Reported error': ISODate(...),
...
}}}

Actual

{{balancer: {
'Currently enabled': 'yes',
'Failed balancer rounds in last 5 attempts': 0,
...
}}}
Counting the same 5 documents both ways:

{{const a = db.getSiblingDB("config").actionlog
.find({what: "balancer.round"}).sort({time: -1}).limit(5).toArray();
({ errorOccured: a.filter(d => d.details.errorOccured).length, // 0 <- what mongosh checks
errorOccurred: a.filter(d => d.details.errorOccurred).length }) // 5 <- what the server writes}}

Suggested fix

Accept both spellings so the check keeps working against 5.0 and older servers:

if (r.details.errorOccurred || r.details.errorOccured) {

References

• SERVER-56276: correct typo in config.actionlog document field (commit dd6a29fcd0, first released in r5.1.0)
• MONGOSH-638: Enable sharding methods (commit 0beb56c8)
• mongosh helpers.ts on main

Let me know if you need more information