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Server Selection Algorithm

MongoDB drivers use a Server Selection algorithm to choose which replica set member to use or, when connected to multiple mongos instances, which mongos instance to use.

Server selection occurs once per operation.

Note

Transactions that contain read operations must use read preference primary. All operations in a given transaction must route to the same member.

Read Preference for Replica Sets

Server selection occurs once per operation and is governed by the read preference and localThresholdMS settings to determine member eligibility for reads. The read preference is re-evaluated for each operation.

Read Preference Mode	Selection Process
`primary` (Default)	The driver selects the primary.
`secondary`	The driver assembles a list of eligible secondary members. maxStalenessSeconds and tag sets specified in the read preference can further restrict the eligibility of the members. If the list of eligible members is not empty, the driver determines which eligible member is the "closest" (i.e. the member with the lowest average network round-trip-time) and calculates a latency window by adding the average round-trip-time of this "closest" server and the `localThresholdMS`. The driver uses this latency window to pare down the list of eligible members to those members that fall within this window. From this list of eligible members that fall within the latency window, the driver randomly chooses an eligible member.
`nearest`	The driver assembles a list of eligible members (primary and secondaries). maxStalenessSeconds and tag sets specified in the read preference can further limit the eligibility of the members. If the list of eligible members is not empty, the driver determines which eligible member is the "closest" (i.e. the member with the lowest average network round-trip-time) and calculates a latency window by adding the average round-trip-time of this "closest" server and the `localThresholdMS` [1]. The driver uses this latency window to pare down the list of eligible members to those members that fall within this window. From this list of eligible members that fall within the latency window, the driver randomly chooses an eligible member.
`primaryPreferred`	If the primary is available, driver selects the primary. If the primary is unavailable, server selection follows the process for the read preference `secondary` to select an eligible secondary member.
`secondaryPreferred`	Following the server selection process for the read preference `secondary`, if a list of eligible secondary members is non-empty, driver chooses an eligible secondary member. Otherwise, if the list is empty, driver selects the primary.

Read Preference for Sharded Clusters

Load Balancing

If there is more than one mongos instance in the connection seed list, the driver determines which mongos is the "closest" (that is, the member with the lowest average network round-trip-time) and calculates the latency window by adding the average round-trip-time of this "closest" mongos instance and the localThresholdMS. The driver load balances randomly across the mongos instances that fall within the latency window.

Note

localThresholdMS biases server selection toward lower-latency local targets. The setting can't remove the cross-region network topology that a single private endpoint creates or guarantee all connections remain in the local region. In this configuration, a client can still connect through a mongos in another region.

Read Preference and Shards

For sharded clusters that have replica set shards, mongos applies the read preference when reading from the shards. Server selection is governed by the read preference and replication.localPingThresholdMs settings. The read preference is re-evaluated for each operation.

Read Preference Mode	Selection Process
`primary` (Default)	The `mongos` selects the primary.
`secondary`	The `mongos` assembles a list of eligible secondary members. maxStalenessSeconds and tag sets specified in the read preference can further restrict the eligibility of the members. If the list of eligible members is not empty, the `mongos` determines which eligible member is the "closest" (i.e. the member with the lowest average network round-trip-time) and calculates a latency window by adding the average round-trip-time of this "closest" server and the `replication.localPingThresholdMs` (or `--localThreshold` command line option). The `mongos` uses this latency window to pare down the list of eligible members to those members that fall within this window. From this list of eligible members that fall within the latency window, the `mongos` randomly chooses an eligible member.
`nearest`	The `mongos` assembles a list of eligible members (primary and secondaries). maxStalenessSeconds and tag sets specified in the read preference can further limit the eligibility of the members. If the list of eligible members is not empty, the `mongos` determines which eligible member is the "closest" (i.e. the member with the lowest average network round-trip-time) and calculates a latency window by adding the average round-trip-time of this "closest" server and the `replication.localPingThresholdMs` (or `--localThreshold` command line option) [1]. The `mongos` uses this latency window to pare down the list of eligible members to those members that fall within this window. From this list of eligible members that fall within the latency window, the `mongos` randomly chooses an eligible member.
`primaryPreferred`	If the primary is available, `mongos` selects the primary. If the primary is unavailable, server selection follows the process for the read preference `secondary`.
`secondaryPreferred`	Following the server selection process for the read preference `secondary`, if a list of eligible secondary members is non-empty, `mongos` chooses an eligible secondary. If the list of eligible secondary members is empty, `mongos` selects the primary.

[1]	(1, 2) The default threshold value is 15 milliseconds.

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Hedged Reads

Replication Reference