- INSTALLATION AND DEPENDENCY CHANGES
- BSON BEHAVIOR CHANGES
- NON-BSON BEHAVIOR CHANGES
- REMOVED FEATURES
- COPYRIGHT AND LICENSE
MongoDB::Upgrading - Deprecations and behavior changes from v1 to v2
The v2 driver represents an evolutionary rather than revolutionary release, but with enough differences to justify a major version bump.
The most significant change in v2 is a switch away from the embedded BSON encoder/decoder to an external library, BSON and an optional optimization addon, BSON::XS. Many applications will continue to work unmodified, but some may need changes.
This document is intended to help developers update their code to take into account API changes from the v1 driver to the v2 driver.
API Changes are never something to do lightly. Changes in the v2 driver were deemed necessary to achieve certain goals, all of which echo themes of the v1 driver release:
consistency – particularly with regards to Perl <-> BSON data conversion, the v2 driver provides a complete, consistently-designed set of BSON type wrappers, and significantly improved round-trip capabilities.
server compatibility – as the MongoDB server deprecates or removes functionality, the driver must be updated to match so that users don't develop apps around features that are going away.
portability – the switch to an external library that has both pure-Perl and XS optimized versions allows the MongoDB driver to support environments without a C compiler available.
Because the v2 driver is pure-Perl capable (see below), its Perl installation directory is different. If upgrading, you need to be sure that the old version doesn't shadow the new one.
That's easy with
cpanm --uninst-shadows MongoDB
For the traditional CPAN client, you'll need to configure the
make_install_arg config argument like this:
$ perl -MCPAN -e shell cpan> o conf make_install_arg UNINST=1 cpan> o conf commit cpan> install MongoDB
The MongoDB driver now requires Perl v5.10.1 or later. This provides better pure-Perl support, several core dependencies, and many fewer bugs involving Unicode and threads. While threads are discouraged, threads under Perl v5.8 were so broken that driver tests were regularly failing.
The MongoDB driver can now be installed without needing a compiler. If a compiler is detected, additional XS-based dependencies will be added to the prerequisites list for improved performance. You can also specify
PUREPERL_ONLY=1 as a
Makefile.PL argument to disable compiler detection.
For detailed information on handling MongoDB data types in Perl, see MongoDB::DataTypes. The following sections provide an overview of major changes from past versions.
The BSON module provides a complete set of classes mapping to every BSON type. When decoding, these types will be returned for types that don't map by default to Perl types.
Code that uses
ref to check documents returned from the database for legacy types (e.g. MongoDB::BSON::Regexp) will need to be updated for the new type wrappers.
All the legacy type wrappers have been updated to be subclasses of their corresponding BSON library equivalents. For example, MongoDB::BSON::Regexp is a subclass of BSON::Regex. Most of them are empty subclasses -- the BSON-library versions provide the same API -- but some have some additional constructor argument behaviors for backwards compatibility.
The BSON library knows how to encode legacy types, so code that uses legacy types for encoding values should be able to work without modification.
The legacy type wrappers will be removed in a future major version release of the driver.
The legacy driver defaulted to decoding the BSON date type as a DateTime object. Unfortunately, that type is very heavy-weight and slow to construct; it's a poor choice as a default as it inflicts that cost whether or not users ultimately need or want objects of that type.
dt_type configuration argument has been removed from MongoDB::MongoClient and the default date type of the BSON library is BSON::Time, which is extremely lightweight and provides convenience methods to convert to various popular time classes. It also works well with Time::HiRes for recording datetimes with millisecond precision.
Depending on their history of use, non-reference Perl scalars may have both string and number representations internally and the MongoDB driver wasn't always clear on how it treated them. Moreover, this treatment could vary slightly by Perl version. The heuristics are now standardized as follows:
If the value has a valid double representation, it will be encoded to BSON as a double.
Otherwise, if the value has a valid integer interpretation, it will be encoded as either Int32 or Int64; the smallest type that the value fits will be used; a value that overflows will error.
Otherwise, the value will be encoded as a UTF-8 string.
The BSON library provides the
prefer_numeric attribute to more aggressively coerce number-like strings that don't already have a numeric representation into a numeric form.
This is essentially the same as the legacy heuristic but some edge cases have been made consistent.
To make it easy to use type wrappers (and to avoid unintentionally using a deprecated one), the BSON::Types module has a standard set of type helper functions:
use BSON::Types ':all'; $int32 = bson_int32(42); $time = bson_time(); # now $ordered = bson_doc( first => "John", last => "Doe );
The MongoDB database uses the first key of the document provided to
run_command as the name of the command. Due to Perl's hash order randomization, use of a hash reference with more than one key as an argument to
run_command is not reliable. This restriction is now enforced. The argument must be a BSON::Doc object, a Tie::IxHash object, an array reference with an even number of keys, or a hash reference with a single key.
count method is deprecated.
The reasons for this change are as follows:
The performance and correctness characteristics of the
countmethod could vary widely depending on whether or not a predicate is used.
countmethod could be incorrect on sharded clusters during document migration between shards.
Many users are unaware of these considerations in the use of
count. As any change to
count could surprise users with unexpected differences in either performance or correctness, the
count method has been replaced with two new API methods, which more directly convey performance and correctness expectations:
estimated_document_counttakes no predicate; it does not work in transactions; performance is O(1).
count_documentstakes a predicate (even if "empty", meaning count all documents); in can be used with or without transactions; performance is O(N) in the worst case.
NOTE: When upgrading from the deprecated
count method, some legacy operators are not supported and must be replaced:
+-------------+--------------------------------+ | Legacy | Modern Replacement | +=============+================================+ | $where | $expr (Requires MongoDB 3.6+) | +-------------+--------------------------------+ | $near | $geoWithin with $center | +-------------+--------------------------------+ | $nearSphere | $geoWithin with $centerSphere | +-------------+--------------------------------+
The MONGODB-CR authentication mechanism was deprecated in MongoDB server 3.6 and removed in MongoDB server 4.0. The Perl driver is deprecating MONGODB-CR, but will not remove it until it no longer supports older servers.
The following query options are deprecated:
maxScan -- deprecated in MongoDB server 4.0
modifiers -- the old
$prefixed modifiers have been replaced with explicit, equivalent options for
snapshot -- deprecated in MongoDB server 4.0
md5 field of GridFS documents is deprecated. Use of a checksum like MD5 has been redundant since MongoDB added write concern and MD5 itself is no longer considered a secure digest function. A future release will remove the use of MD5 entirely. In the meantime, users can disable MD5 digesting with the
disable_md5 option in MongoDB::GridFSBucket.
Users who wish to continue storing a digest are encouraged to compute their own digest using a function of their choice and store it under a user-defined key in the
metadata field of the file document.
These classes are superseded by type wrappers from BSON, as described earlier.
Features deprecated in the v1 release have now been removed. Additionally,
MongoDB::BSON has been removed in favor of BSON, as described earlier.
David Golden <firstname.lastname@example.org>
Mike Friedman <email@example.com>
Kristina Chodorow <firstname.lastname@example.org>
Florian Ragwitz <email@example.com>
This software is Copyright (c) 2020 by MongoDB, Inc.
This is free software, licensed under:
The Apache License, Version 2.0, January 2004