Release Notes for MongoDB 1.8

Preparation

Read through all release notes before upgrading and ensure that no changes will affect your deployment.

Upgrading a Standalone mongod

1. Download the v1.8.x binaries from the MongoDB Download Page.
2. Shutdown your mongod instance.
3. Replace the existing binary with the 1.8.x mongod binary.
4. Restart MongoDB.

Upgrading a Replica Set

1.8.x secondaries can replicate from 1.6.x primaries.

1.6.x secondaries cannot replicate from 1.8.x primaries.

Thus, to upgrade a replica set you must replace all of your secondaries first, then the primary.

For example, suppose you have a replica set with a primary, an arbiter and several secondaries. To upgrade the set, do the following:

1. For the arbiter:

   1. Shut down the arbiter.
   2. Restart it with the 1.8.x binary from the MongoDB Download Page.

2. Change your config (optional) to prevent election of a new primary.

   It is possible that, when you start shutting down members of the set, a new primary will be elected. To prevent this, you can give all of the secondaries a priority of 0 before upgrading, and then change them back afterwards. To do so:

   1. Record your current config. Run rs.config() and paste the results into a text file.

   2. Update your config so that all secondaries have priority 0. For example:

      config = rs.conf()
      {
           "_id" : "foo",
           "version" : 3,
           "members" : [
                   {
                           "_id" : 0,
                           "host" : "ubuntu:27017"
                   },
                   {
                           "_id" : 1,
                           "host" : "ubuntu:27018"
                   },
                   {
                           "_id" : 2,
                           "host" : "ubuntu:27019",
                           "arbiterOnly" : true
                   }
                   {
                           "_id" : 3,
                           "host" : "ubuntu:27020"
                   },
                   {
                           "_id" : 4,
                           "host" : "ubuntu:27021"
                   },
           ]
      }
      config.version++
      3
      rs.isMaster()
      {
           "setName" : "foo",
           "ismaster" : false,
           "secondary" : true,
           "hosts" : [
                   "ubuntu:27017",
                   "ubuntu:27018"
           ],
           "arbiters" : [
                   "ubuntu:27019"
           ],
           "primary" : "ubuntu:27018",
           "ok" : 1
      }
      // for each secondary
      config.members[0].priority = 0
      config.members[3].priority = 0
      config.members[4].priority = 0
      rs.reconfig(config)

3. For each secondary:

   1. Shut down the secondary.
   2. Restart it with the 1.8.x binary from the MongoDB Download Page.

4. If you changed the config, change it back to its original state:

   config = rs.conf()
   config.version++
   config.members[0].priority = 1
   config.members[3].priority = 1
   config.members[4].priority = 1
   rs.reconfig(config)

5. Shut down the primary (the final 1.6 server), and then restart it with the 1.8.x binary from the MongoDB Download Page.

Upgrading a Sharded Cluster

1. Turn off the balancer:

   mongo <a_mongos_hostname>
   use config
   db.settings.update({_id:"balancer"},{$set : {stopped:true}}, true)

2. For each shard:

   • If the shard is a replica set, follow the directions above for Upgrading a Replica Set.
   • If the shard is a single mongod process, shut it down and then restart it with the 1.8.x binary from the MongoDB Download Page.

3. For each mongos:

   1. Shut down the mongos process.
   2. Restart it with the 1.8.x binary from the MongoDB Download Page.

4. For each config server:

   1. Shut down the config server process.
   2. Restart it with the 1.8.x binary from the MongoDB Download Page.

5. Turn on the balancer:

   use config
   db.settings.update({_id:"balancer"},{$set : {stopped:false}})

Returning to 1.6

If for any reason you must move back to 1.6, follow the steps above in reverse. Please be careful that you have not inserted any documents larger than 4MB while running on 1.8 (where the max size has increased to 16MB). If you have you will get errors when the server tries to read those documents.

Journaling

MongoDB now supports write-ahead Journaling to facilitate fast crash recovery and durability in the storage engine. With journaling enabled, a mongod can be quickly restarted following a crash without needing to repair the collections. An aggregation pipeline makes it possible to do aggregation.

Sparse and Covered Indexes

Sparse Indexes are indexes that only include documents that contain the fields specified in the index. Documents missing the field will not appear in the index at all. This can significantly reduce index size for indexes of fields that contain only a subset of documents within a collection.

Covered Indexes enable MongoDB to answer queries entirely from the index when the query only selects fields that the index contains.

Incremental MapReduce Support

The mapReduce command supports new options that enable incrementally updating existing collections. Previously, a MapReduce job could output either to a temporary collection or to a named permanent collection, which it would overwrite with new data.

You now have several options for the output of your MapReduce jobs:

• You can merge MapReduce output into an existing collection. Output from the Reduce phase will replace existing keys in the output collection if it already exists. Other keys will remain in the collection.
• You can now re-reduce your output with the contents of an existing collection. Each key output by the reduce phase will be reduced with the existing document in the output collection.
• You can replace the existing output collection with the new results of the MapReduce job (equivalent to setting a permanent output collection in previous releases)
• You can compute MapReduce inline and return results to the caller without persisting the results of the job. This is similar to the temporary collections generated in previous releases, except results are limited to 8MB.

For more information, see the out field options in the mapReduce document.

Additional Changes and Enhancements

Release Announcement Forum Pages

• 1.8.1, 1.8.0
• 1.7.6, 1.7.5, 1.7.4, 1.7.3, 1.7.2, 1.7.1, 1.7.0