Why don't support startAfter field ？

[xyxsns]

Bug Report

What reason is this?

Environment

php -i | grep -E 'mongodb|libmongoc|libbson'

mongodb
libbson bundled version => 1.16.2
libmongoc bundled version => 1.16.2
libmongoc SSL => enabled
libmongoc SSL library => Secure Transport
libmongoc crypto => enabled
libmongoc crypto library => Common Crypto
libmongoc crypto system profile => disabled
libmongoc SASL => enabled
libmongoc ICU => disabled
libmongoc compression => enabled
libmongoc compression snappy => disabled
libmongoc compression zlib => enabled
libmongocrypt bundled version => 1.0.3
libmongocrypt crypto => enabled
libmongocrypt crypto library => Common Crypto
mongodb.debug => no value => no value

composer show mongodb/mongodb

name : mongodb/mongodb
descrip. : MongoDB driver library
keywords : database, driver, mongodb, persistence
versions : * 1.6.1
type : library
license : Apache License 2.0 (Apache-2.0) (OSI approved) https://spdx.org/licenses/Apache-2.0.html#licenseText
homepage : https://jira.mongodb.org/browse/PHPLIB
source : [git] https://github.com/mongodb/mongo-php-library.git 4bb040c
dist : [zip] https://api.github.com/repos/mongodb/mongo-php-library/zipball/4bb040c620ab95c487d05d5ff50cbc92fa68e920 4bb040c
path : /private/var/www/zfy/soa/vendor/mongodb/mongodb
names : mongodb/mongodb

autoload
psr-4
MongoDB\ => src/
files

requires
ext-hash *
ext-json *
ext-mongodb ^1.7
php ^5.6 || ^7.0

requires (dev)
phpunit/phpunit ^5.7.27 || ^6.4 || ^8.3
sebastian/comparator ^1.0 || ^2.0 || ^3.0
squizlabs/php_codesniffer ^3.4
symfony/phpunit-bridge ^4.4@dev

Test Script

<?php 

    include_once __DIR__. '/vendor/autoload.php';

    $db = (new MongoDB\Client("mongodb://192.168.1.49:27019"))->selectDatabase("zfy");

    $changeStream = $db -> selectCollection("demo") ->watch([], ['fullDocument' => MongoDB\Operation\Watch::FULL_DOCUMENT_UPDATE_LOOKUP]);
    $changeStream = $db -> selectCollection("demo")
        ->watch([],
            [
                'startAfter'=>(object)["_id"=>["_data"=>"82605DCA8B000000012B022C0100296E5A100401BA8F24F40446978F73B0856BF3FC6D46645F696400645F68549CB505DF3AA77DD50A0004"]],
                'fullDocument' => MongoDB\Operation\Watch::FULL_DOCUMENT_UPDATE_LOOKUP]);

    while (true) {

        $changeStream->next();

        if ($changeStream->valid()) {
            $nextChange = $changeStream->current();

            echo json_encode($nextChange);


        }
    }

Expected and Actual Behavior

Nothing

Debug Log

Fatal error: Uncaught MongoDB\Driver\Exception\CommandException: BSON field '$changeStream.startAfter' is an unknown field. in /private/var/www/mongo-changestream-php-master/vendor/mongodb/mongodb/src/Operation/Aggregate.php:295
Stack trace:
#0 /private/var/www/mongo-changestream-php-master/vendor/mongodb/mongodb/src/Operation/Aggregate.php(295): MongoDB\Driver\Server->executeReadCommand('zfy', Object(MongoDB\Driver\Command), Array)
#1 /private/var/www/mongo-changestream-php-master/vendor/mongodb/mongodb/src/Operation/Watch.php(337): MongoDB\Operation\Aggregate->execute(Object(MongoDB\Driver\Server))
#2 /private/var/www/mongo-changestream-php-master/vendor/mongodb/mongodb/src/Operation/Watch.php(316): MongoDB\Operation\Watch->executeAggregate(Object(MongoDB\Driver\Server))
#3 /private/var/www/mongo-changestream-php-master/vendor/mongodb/mongodb/src/Operation/Watch.php(285): MongoDB\Operation\Watch->createChangeStreamIterator(Object(MongoDB\Driver\Server))
#4 /private/var/www/mongo-changestream-php-master/vendor/mongodb/mo in /private/var/www/mongo-changestream-php-master/vendor/mongodb/mongodb/src/Operation/Aggregate.php on line 295

[jmikola]

What version of MongoDB are you connecting to? I'll note that the bug report template specifically asks for this information:

What version of MongoDB are you using? Include details about the topology (e.g. replica set, sharded cluster) and how you are connecting to the database (e.g. TLS connections, authentication). If possible, share your connection string and additional URI options (be sure to redact any auth credentials). Please specify whether you are self-hosting the database or using a cloud provider, such as MongoDB Atlas.

The startAfter option requires MongoDB 4.2+ and the PHP library's Collection::watch() documentation for startAfter also states:

This is not supported for server versions prior to 4.2 and will result in an exception at execution time if used.

The CommandException message "BSON field '$changeStream.startAfter' is an unknown field" originates from the server's own IDL parsing, which is also expected if you're using a server version prior to 4.2.0.