MongoDB read preferences give you control over read behavior when using replicasets. Writes in every environment go to primary, but reads can be configured to read from secondary or primary based on various criteria. In most versions of mongo, the read preference defaults to primary in the client but you should check your version for the default.
primary
Read from the primary. if the primary is down, the read will fail. This is useful if you need the latest successful write and have zero tolerance for stale data. The terms “latest” and “successful” are a bit of a sliding scale depending on the read concern. For example, with read concern local to a primary, it’s possible to read writes from other clients that have not been acknowledged by the primary node. In practical terms, this just means that the data could be lost in the event of a rollback event since it may not have been replicated to a majority of nodes.
Regardless of a write’s write concern, other clients using
https://www.mongodb.com/docs/manual/core/read-isolation-consistency-recency/#read-uncommitted"local"or"available"read concern can see the result of a write operation before the write operation is acknowledged to the issuing client.
But I thought mongo maintained a WAL using an on-disk journal? Why would data be rolled back at all?
Yes, the journal is used for data recovery in the event of a node shutdown. But when a primary node goes down, a new node is elected as the primary, and then the previous primary rejoins the cluster as a secondary it may have data / writes that are ahead of the new primary. Since the new primary is the source of truth, those writes need to be rolled back to be consistent with the new primary!
If you want data that has the highest durability level, use majority read concern. When read concern is majority, it will only return data that has been successfully acknowledged by a majority of nodes. However, this does sacrifice some level of data freshness because the latest write from a client may not have been replicated to the majority of nodes yet, so you may be reading an older value even though you’re reading from the primary.
Another factor to consider when choosing this setting is your primary nodes capacity and current load – if your primary is close to capacity on cpu or memory use, switching reads to primary may impact writes.
primaryPreferred
Read from the primary, but if the primary is unavailable, the read goes to a secondary. This setting is appropriate cases you want the most recent write, but you can tolerate occasional stale reads from a secondary. Replication lag can range pretty widely from single digit ms to minutes depending on the write volume, so keep in mind that during periods of high replication lag you’re more likely to be reading stale data.
secondary
All reads go to secondaries. Not all secondaries though – just one based on a server selection process. The server selection works like this:
First, there’s the default threshhold value which is 15ms. Now this isn’t the window used for server selection though – it’s only a part of it. The other part is the lowest average network round trip time (RTT) of secondary nodes. So if the closest node has an RTT of 100ms, the total set will be nodes that fall within 100 + 15 (115ms). It’ll use this time to randomly select a node to forward the request to.
This read preference can make sense if your system is very read heavy AND you can tolerate stale reads. Scaling out with many secondary nodes is one way to spread your read load and improve read availability. At the same time, it may also add latency to both your primary (due to the increased oplog syncing), increase inconsistency and likelihood of data staleness because there are now more nodes that are likely to be out of sync with writes, and potentially reduced write latency particularly for writes that require majority acknowledgement because you’ll need N / 2 + 1 acknowledgements.
secondaryPreferred
All reads go to secondaries, but if not available, read from the primary. The same concerns for “secondary” read preference applies here. The additional factor to consider here is whether your primary can afford to take on this additional load.
nearest
Doesn’t matter which, just pick the one with lowest latency based on the server selection algorithm I mentioned in the secondary read preference section.