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.\" ========================================================================
.\"
.IX Title "MongoDB::ChangeStream 3pm"
.TH MongoDB::ChangeStream 3pm "2020-08-15" "perl v5.30.3" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
MongoDB::ChangeStream \- A stream providing update information for collections.
.SH "VERSION"
.IX Header "VERSION"
version v2.2.2
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 2
\&    $stream = $collection\->watch( $pipeline, $options );
\&    while(1) {
\&
\&        # This inner loop will only iterate until there are no more
\&        # changes available.
\&        while (my $change = $stream\->next) {
\&            ...
\&        }
\&    }
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
This class models change stream results as returned by the
\&\*(L"watch\*(R" in MongoDB::Collection method.
.SH "STREAM METHODS"
.IX Header "STREAM METHODS"
.SS "next"
.IX Subsection "next"
.Vb 2
\&    $change_stream = $collection\->watch(...);
\&    $change = $change_stream\->next;
.Ve
.PP
Waits for the next change in the collection and returns it.
.PP
\&\fBNote\fR: This method will wait for the amount of milliseconds passed
as \f(CW\*(C`maxAwaitTimeMS\*(C'\fR to \*(L"watch\*(R" in MongoDB::Collection or the server's
default wait-time. It will not wait indefinitely.
.SS "get_resume_token"
.IX Subsection "get_resume_token"
Users can inspect the \f(CW\*(C`_id\*(C'\fR on each \f(CW\*(C`ChangeDocument\*(C'\fR to use as a
resume token. But since MongoDB 4.2, \f(CW\*(C`aggregate\*(C'\fR and \f(CW\*(C`getMore\*(C'\fR responses
also include a \f(CW\*(C`postBatchResumeToken\*(C'\fR. Drivers use one or the other
when automatically resuming.
.PP
This method retrieves the same resume token that would be used to
automatically resume. Users intending to store the resume token
should use this method to get the most up to date resume token.
.PP
For instance:
.PP
.Vb 3
\&    if ($local_change) {
\&        process_change($local_change);
\&    }
\&
\&    eval {
\&        my $change_stream = $coll\->watch([], { resumeAfter => $local_resume_token });
\&        while ( my $change = $change_stream\->next) {
\&            $local_resume_token = $change_stream\->get_resume_token;
\&            $local_change = $change;
\&            process_change($local_change);
\&        }
\&    };
\&    if (my $err = $@) {
\&        $log\->error($err);
\&    }
.Ve
.PP
In this case the current change is always persisted locally,
including the resume token, such that on restart the application
can still process the change while ensuring that the change stream
continues from the right logical time in the oplog. It is the
application's responsibility to ensure that \f(CW\*(C`process_change\*(C'\fR is
idempotent, this design merely makes a reasonable effort to process
each change at least once.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
The Change Streams manual section <https://docs.mongodb.com/manual/changeStreams/>.
.PP
The Change Streams specification <https://github.com/mongodb/specifications/blob/master/source/change-streams.rst>.
.SH "AUTHORS"
.IX Header "AUTHORS"
.IP "\(bu" 4
David Golden <david@mongodb.com>
.IP "\(bu" 4
Rassi <rassi@mongodb.com>
.IP "\(bu" 4
Mike Friedman <friedo@friedo.com>
.IP "\(bu" 4
Kristina Chodorow <k.chodorow@gmail.com>
.IP "\(bu" 4
Florian Ragwitz <rafl@debian.org>
.SH "COPYRIGHT AND LICENSE"
.IX Header "COPYRIGHT AND LICENSE"
This software is Copyright (c) 2020 by MongoDB, Inc.
.PP
This is free software, licensed under:
.PP
.Vb 1
\&  The Apache License, Version 2.0, January 2004
.Ve