2.0.txt

=============================
Release Notes for MongoDB 2.0
=============================

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Upgrading
---------

Although the major version number has changed, MongoDB 2.0 is a
standard, incremental production release and works as a drop-in
replacement for MongoDB 1.8.

Preparation
~~~~~~~~~~~

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

If you create new indexes in 2.0, then downgrading to 1.8 is possible
but you must reindex the new collections.

:binary:`~bin.mongoimport` and :binary:`~bin.mongoexport` now correctly adhere to the CSV spec
for handling CSV input/output. This may break existing import/export
workflows that relied on the previous behavior. For more information see
:issue:`SERVER-1097`.

:doc:`Journaling </core/journaling/>` is **enabled by default** in 2.0 for 64-bit builds.
If you still prefer to run without journaling, start :binary:`~bin.mongod`
with the :option:`--nojournal <mongod --nojournal>` run-time option.
Otherwise, MongoDB creates journal files during startup. The first time you start :binary:`~bin.mongod` with
journaling, you will see a delay as :binary:`~bin.mongod` creates new files.
In addition, you may see reduced write throughput.

2.0 :binary:`~bin.mongod` instances are interoperable with 1.8
:binary:`~bin.mongod` instances; however, for best results, upgrade your
deployments using the following procedures:

.. _2.0-upgrade-standalone:

Upgrading a Standalone ``mongod``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

1. Download the v2.0.x binaries from the `MongoDB Download Page`_.

#. Shutdown your :binary:`~bin.mongod` instance. Replace the existing
   binary with the 2.0.x :binary:`~bin.mongod` binary and restart MongoDB.

.. _`MongoDB Download Page`: http://downloads.mongodb.org/

.. _2.0-upgrade-replica-set:

Upgrading a Replica Set
~~~~~~~~~~~~~~~~~~~~~~~

1. Upgrade the :term:`secondary` members of the set one at a time by
   shutting down the :binary:`~bin.mongod` and replacing the 1.8 binary
   with the 2.0.x binary from the `MongoDB Download Page`_.

#. To avoid losing the last few updates on failover you can
   temporarily halt your application (failover should take less than 10
   seconds), or you can set :ref:`write concern <write-concern>` in your application
   code to confirm that each update reaches multiple servers.

#. Use the :method:`rs.stepDown()` to step down the primary to allow
   the normal :ref:`failover <replica-set-failover>` procedure.

   :method:`rs.stepDown()` and :dbcommand:`replSetStepDown` provide for
   shorter and more consistent failover procedures than simply
   shutting down the primary directly.

   When the primary has stepped down, shut down its instance and
   upgrade by replacing the :binary:`~bin.mongod` binary with the 2.0.x
   binary.

.. _2.0-upgrade-shard-cluster:
.. _2.0-upgrade-sharded-cluster:

Upgrading a Sharded Cluster
~~~~~~~~~~~~~~~~~~~~~~~~~~~

1. Upgrade all :term:`config server <config database>` instances
   *first*, in any order. Since config servers use two-phase commit,
   :term:`shard` configuration metadata updates will halt until all are
   up and running.

#. Upgrade :binary:`~bin.mongos` routers in any order.

Changes
-------

Compact Command
~~~~~~~~~~~~~~~

A :dbcommand:`compact` command is now available for compacting a single
collection and its indexes. Previously, the only way to compact was to
repair the entire database.

Concurrency Improvements
~~~~~~~~~~~~~~~~~~~~~~~~

When going to disk, the server will yield the write lock when writing
data that is not likely to be in memory. The initial
implementation of this feature now exists:

See :issue:`SERVER-2563` for more information.

The specific operations yield in 2.0 are:

- Updates by ``_id``

- Removes

- Long cursor iterations

Default Stack Size
~~~~~~~~~~~~~~~~~~

MongoDB 2.0 reduces the default stack size. This change can reduce total memory
usage when there are many (e.g., 1000+) client connections, as there is
a thread per connection. While portions of a thread's stack can be
swapped out if unused, some operating systems do this slowly enough that
it might be an issue. The default stack size is lesser of the
system setting or 1MB.

.. _2.0-new-index-format:

Index Performance Enhancements
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

v2.0 includes significant improvements to the
:doc:`index </tutorial/roll-back-to-v1.8-index>`.
Indexes are often 25% smaller and 25% faster (depends on the use case).
When upgrading from previous versions, the benefits of the new index
type are realized only if you create a new index or re-index an old one.

Dates are now signed, and the max index key size has increased slightly
from 819 to 1024 bytes.

All operations that create a new index will result in a 2.0 index by
default. For example:

- Reindexing results on an older-version index results in a 2.0 index.
  However, reindexing on a secondary does *not* work in versions prior
  to 2.0. Do not reindex on a secondary. For a workaround, see
  :issue:`SERVER-3866`.

- The :dbcommand:`repairDatabase` command converts indexes to a 2.0
  indexes.

To convert all indexes for a given collection to the :ref:`2.0 type
<2.0-new-index-format>`, invoke the :dbcommand:`compact` command.

Once you create new indexes, downgrading to 1.8.x will require a
re-index of any indexes created using 2.0. See
:doc:`/tutorial/roll-back-to-v1.8-index`.

Sharding Authentication
~~~~~~~~~~~~~~~~~~~~~~~

Applications can now use authentication with :term:`sharded clusters <sharded cluster>`.

Replica Sets
~~~~~~~~~~~~

Hidden Nodes in Sharded Clusters
````````````````````````````````

In 2.0, :binary:`~bin.mongos` instances can now determine when a member of
a replica set becomes "hidden" without requiring a restart. In 1.8,
:binary:`~bin.mongos` if you reconfigured a
member as hidden, you *had* to restart :binary:`~bin.mongos` to prevent
queries from reaching the hidden member.

Priorities
``````````

Each :term:`replica set` member can now have a priority value consisting
of a floating-point from 0 to 1000, inclusive. Priorities let you
control which member of the set you prefer to have as :term:`primary`
the member with the highest priority that can see a majority of the set
will be elected primary.

For example, suppose you have a replica set with three members, ``A``, ``B``, and
``C``, and suppose that their priorities are set as follows:

- ``A``'s priority is ``2``.

- ``B``'s priority is ``3``.

- ``C``'s priority is ``1``.

During normal operation, the set will always chose ``B`` as
primary. If ``B`` becomes unavailable, the set will elect ``A`` as primary.

For more information, see the
:data:`~local.system.replset.members[n].priority` documentation.

Data-Center Awareness
``````````````````````

You can now "tag" :term:`replica set` members to indicate their
location. You can use these tags to design custom :ref:`write rules <write-concern>`
across data centers, racks, specific servers, or any other architecture
choice.

For example, an administrator can define rules such as "very important write" or
``customerData`` or "audit-trail" to replicate to certain servers,
racks, data centers, etc. Then in the application code, the developer
would say:

.. code-block:: javascript

   db.foo.insert(doc, {w : "very important write"})

which would succeed if it fulfilled the conditions the DBA defined for
"very important write".

For more information, see :doc:`/data-center-awareness`.

Drivers may also support tag-aware reads. Instead of
specifying ``slaveOk``, you specify ``slaveOk`` with tags indicating
which data-centers to read from. For details, see the
:ecosystem:`Drivers </drivers>` documentation.

``w`` : ``majority``
````````````````````

You can also set ``w`` to ``majority`` to ensure that the write
propagates to a majority of nodes, effectively committing it. The
value for "majority" will automatically adjust as you add or
remove nodes from the set.

For more information, see :doc:`/core/write-concern`.

Reconfiguration with a Minority Up
``````````````````````````````````

If the majority of servers in a set has been permanently lost, you can
now force a reconfiguration of the set to bring it back online.

For more information see :doc:`/tutorial/reconfigure-replica-set-with-unavailable-members`.

Primary Checks for a Caught up Secondary before Stepping Down
`````````````````````````````````````````````````````````````

To minimize time without a :term:`primary`, the :method:`rs.stepDown()`
method will now fail if the primary does not see a :term:`secondary`
within 10 seconds of its latest optime. You can force the primary to
step down anyway, but by default it will return an error message.

See also :doc:`/tutorial/force-member-to-be-primary`.

Extended Shutdown on the Primary to Minimize Interruption
`````````````````````````````````````````````````````````

When you call the :dbcommand:`shutdown` command, the :term:`primary`
will refuse to shut down unless there is a :term:`secondary` whose
optime is within 10 seconds of the primary. If such a secondary isn't
available, the primary will step down and wait up to a minute for the
secondary to be fully caught up before shutting down.

Note that to get this behavior, you must issue the :dbcommand:`shutdown`
command explicitly; sending a signal to the process will not trigger
this behavior.

You can also force the primary to shut down, even without an up-to-date
secondary available.

Maintenance Mode
````````````````

When :dbcommand:`repair` or :dbcommand:`compact` runs on a :term:`secondary`, the
secondary will automatically drop into "recovering" mode until the
operation finishes. This prevents clients from trying to read from it
while it's busy.

Geospatial Features
~~~~~~~~~~~~~~~~~~~

Multi-Location Documents
````````````````````````

Indexing is now supported on documents which have multiple location
objects, embedded either inline or in embedded documents. Additional
command options are also supported, allowing results to return with
not only distance but the location used to generate the distance.

For more information, see :ref:`geospatial-indexes-multi-location`.

Polygon searches
````````````````

Polygonal :query:`$within` queries are also now supported for simple polygon
shapes. For details, see the :query:`$within` operator documentation.

Journaling Enhancements
~~~~~~~~~~~~~~~~~~~~~~~

- Journaling is now enabled by default for 64-bit platforms. Use the
  ``--nojournal`` command line option to disable it.

- The journal is now compressed for faster commits to disk.

- A new :option:`--journalCommitInterval <mongod --journalCommitInterval>` run-time option exists for
  specifying your own group commit interval. The default settings do
  not change.

- A new :dbcommand:`{ getLastError: { j: true } } <getLastError>` option is
  available to wait for the group commit. The group commit will happen
  sooner when a client is waiting on ``{j: true}``. If journaling is
  disabled, ``{j: true}`` is a no-op.

New ``ContinueOnError`` Option for Bulk Insert
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Set the ``continueOnError`` option for bulk inserts, in the
:doc:`driver </applications/drivers>`, so that bulk insert will
continue to insert any remaining documents even if an insert fails, as
is the case with  duplicate key exceptions or network interruptions. The :dbcommand:`getLastError`
command will report whether any inserts have failed, not just the
last one. If multiple errors occur, the client will only receive the
most recent :dbcommand:`getLastError` results.

.. include:: /includes/note-bulk-inserts-on-sharded-clusters.rst

Map Reduce
~~~~~~~~~~

Output to a Sharded Collection
``````````````````````````````

Using the new ``sharded`` flag, it is possible to send the result of a
map/reduce to a sharded collection. Combined with the ``reduce`` or
``merge`` flags, it is possible to keep adding data to very large
collections from map/reduce jobs.

For more information, see :doc:`/core/map-reduce/` and the
:dbcommand:`mapReduce` reference.

Performance Improvements
````````````````````````

Map/reduce performance will benefit from the following:

- Larger in-memory buffer sizes, reducing the amount of disk I/O needed
  during a job

- Larger javascript heap size, allowing for larger objects
  and less GC

- Supports pure JavaScript execution with the ``jsMode`` flag. See the
  :dbcommand:`mapReduce` reference.

New Querying Features
~~~~~~~~~~~~~~~~~~~~~

Additional regex options: ``s``
```````````````````````````````

Allows the dot (``.``) to match all characters including new lines. This is
in addition to the currently supported ``i``, ``m`` and ``x``. See :query:`$regex`.

$and
````

A special boolean :query:`$and` query operator is now available.

Command Output Changes
~~~~~~~~~~~~~~~~~~~~~~

The output of the :dbcommand:`validate` command and the documents in the
``system.profile`` collection have both been enhanced to return
information as BSON objects with keys for each value rather than as
free-form strings.

Shell Features
~~~~~~~~~~~~~~

Custom Prompt
`````````````

You can define a custom prompt for the :binary:`~bin.mongo` shell. You can
change the prompt at any time by setting the prompt variable to a string
or a custom JavaScript function returning a string. For examples, see
:ref:`shell-use-a-custom-prompt`.

Default Shell Init Script
`````````````````````````

On startup, the shell will check for a ``.mongorc.js`` file in the
user's home directory. The shell will execute this file after connecting
to the database and before displaying the prompt.

If you would like the shell not to run the ``.mongorc.js`` file
automatically, start the shell with :option:`--norc <mongo --norc>`.

For more information, see the :binary:`~bin.mongo` reference.

Most Commands Require Authentication
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In 2.0, when running with authentication (e.g. :setting:`~security.authorization`) *all*
database commands require authentication, *except* the following
commands.

- :dbcommand:`isMaster`

- :dbcommand:`authenticate`

- :dbcommand:`getnonce`

- :dbcommand:`buildInfo`

- :dbcommand:`ping`

- :dbcommand:`isdbgrid`

Resources
---------

- `MongoDB Downloads <http://mongodb.org/downloads>`_
- `All JIRA Issues resolved in 2.0 <https://jira.mongodb.org/secure/IssueNavigator.jspa?mode=hide&requestId=11002>`_
- `All Backward Incompatible Changes <https://jira.mongodb.org/issues/?filter=11023&jql=project%20%3D%20SERVER%20AND%20fixVersion%20in%20(10889%2C%2010886%2C%2010784%2C%2010596%2C%2010380%2C%2010261%2C%2010232)%20AND%20%22Backwards%20Compatibility%22%20in%20%20(%22Major%20Change%22%2C%20%22Minor%20Change%22)%20ORDER%20BY%20votes%20DESC%2C%20issuetype%20DESC%2C%20key%20DESC>`_