Read concern "snapshot" and the corresponding clusterTime

Tomasz_Lenarcik · June 26, 2022, 9:38pm

I have already asked this question on StackOverflow but then I thought this forum would be more appropriate.

There’s been a really handy addition in MongoDB 5.x that allows enforcing snapshot read concern in some read operations outside transactions and specify the timestamp at which the snapshot is taken via atClusterTime . If the timestamp is not provided, mongo will select it automatically and kindly return its value to the user for future reference. See:

Since MongoDB 4.x also supports read concern snapshot - but only inside transactions with write concern majority - I am wondering if it is also possible to obtain some information about when exactly the snapshot was taken, similarly to what 5.x is doing. I understand that I cannot explicitly specify atOperationTime but somehow mongo needs to select the timestamp on its own, so it seems reasonable to expect that this information should be available somewhere.

Tomasz_Lenarcik · June 27, 2022, 11:25am

After reading a little bit of code here:

github.com

mongodb/node-mongodb-native/blob/main/src/sessions.ts

import { Binary, Document, Long, Timestamp } from './bson';
import type { CommandOptions, Connection } from './cmap/connection';
import { ConnectionPoolMetrics } from './cmap/metrics';
import { isSharded } from './cmap/wire_protocol/shared';
import { PINNED, UNPINNED } from './constants';
import type { AbstractCursor } from './cursor/abstract_cursor';
import {
  AnyError,
  MongoAPIError,
  MongoCompatibilityError,
  MONGODB_ERROR_CODES,
  MongoDriverError,
  MongoError,
  MongoErrorLabel,
  MongoExpiredSessionError,
  MongoInvalidArgumentError,
  MongoRuntimeError,
  MongoServerError,
  MongoTransactionError,
  MongoWriteConcernError

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it seems to be that it would be fair to assume that inside a read-only transaction within a causally consistent session, the timestamp I am looking for is simply session.operationTime after the read was performed. This is of course under the assumption that transaction’s read concern was set to snapshot and write concern to majority (even though there was no actual write).

I would appreciate if someone with deep knowledge of MongoDB internals could confirm if my assumptions are correct. I also feel like this information is somehow missing from documentation as I wasn’t able to find it anywhere.