- Replication >
- Replica Set Read and Write Semantics >
- Read Preference >
- Read Preference
maxStalenessSeconds
Read Preference maxStalenessSeconds¶
New in version 3.4.
Replica set members can lag behind the primary due to network
congestion, low disk throughput, long-running operations, etc. The read
preference maxStalenessSeconds option lets you specify a maximum
replication lag, or “staleness”, for reads from secondaries. When a secondary’s estimated staleness exceeds
maxStalenessSeconds, the client stops using it for read operations.
Important
The maxStalenessSeconds read preference option is intended for
applications that read from secondaries and want to avoid reading
from a secondary that has fallen overly far behind in replicating
the primary’s writes. For example, a secondary might stop
replicating due to a network outage between itself and the primary.
In that case, the client should stop reading from the secondary
until an administrator resolves the outage and the secondary catches
up.
To use maxStalenessSeconds, all of the
MongoDB instances in your deployment must be using MongoDB 3.4 or
later. If any instances are on an earlier version of MongoDB, the
driver or mongos will raise an error.
Note
Starting in version 4.2, MongoDB introduces a flow control mechanism to control the rate at which
the primary applies its writes with the goal of keeping majority
committed lag under a
specified maximum value.
You can specify maxStalenessSeconds with the following read
preference modes:
Max staleness is not compatible with mode primary and only
applies when selecting a
secondary member of a set for a read operation.
When selecting a server for a read operation with maxStalenessSeconds, clients
estimate how stale each secondary is by comparing the secondary’s last
write to that of the primary. The client will then direct the read
operation to a secondary whose estimated lag is less than or equal to
maxStalenessSeconds.
If there is no primary, the client uses the secondary with the most recent write for the comparison.
By default, there is no maximum staleness and clients will not consider a secondary’s lag when choosing where to direct a read operation.
You must specify a maxStalenessSeconds value of 90 seconds or
longer: specifying a smaller maxStalenessSeconds value will raise
an error. Clients estimate secondaries’ staleness by periodically
checking the latest write date of each replica set member. Since these
checks are infrequent, the staleness estimate is coarse. Thus, clients
cannot enforce a maxStalenessSeconds value of less than 90 seconds.