Query.prototype.read()

Parameters

  • pref «String» one of the listed preference options or aliases

  • [tags] «Array» optional tags for this query

Returns:
  • «Query» this

Determines the MongoDB nodes from which to read.

Preferences:

  1. primary - (default) Read from primary only. Operations will produce an error if primary is unavailable. Cannot be combined with tags.
  2. secondary            Read from secondary if available, otherwise error.
  3. primaryPreferred     Read from primary if available, otherwise a secondary.
  4. secondaryPreferred   Read from a secondary if available, otherwise read from the primary.
  5. nearest              All operations read from among the nearest candidates, but unlike other modes, this option will include both the primary and all secondaries in the random selection.

Aliases

  1. p   primary
  2. pp  primaryPreferred
  3. s   secondary
  4. sp  secondaryPreferred
  5. n   nearest

Example:

  1. new Query().read('primary')
  2. new Query().read('p')  // same as primary
  4. new Query().read('primaryPreferred')
  5. new Query().read('pp') // same as primaryPreferred
  7. new Query().read('secondary')
  8. new Query().read('s')  // same as secondary
  10. new Query().read('secondaryPreferred')
  11. new Query().read('sp') // same as secondaryPreferred
  13. new Query().read('nearest')
  14. new Query().read('n')  // same as nearest
  16. // read from secondaries with matching tags
  17. new Query().read('s', [{ dc:'sf', s: 1 },{ dc:'ma', s: 2 }])

Read more about how to use read preferrences here and here.