Growing number of couchbase binary documents in XDCR destination bucket

I am running couchbase enterprise edition version 6.6.2 on Windows server 2016 standard edition. I have two buckets called A and B. Bucket A is configured to run with enable_shared_bucket_access = true, my sync gateway creates new documents in bucket A, a bunch of services change and delete these documents.

XDCR replicates documents from bucket A to bucket B. All changes to documents in bucket A are replicated to bucket B, except deletions in bucket A are not replicated to bucket B. When documents in bucket B get older than 62 days they get deleted by an external service.

Over time I noticed that 93% of the documents in bucket B are binary documents! My own documents are in JSON, I don’t use any kind of binary documents in my solution. This leads me to the conclusion that these binary documents are some internal couchbase documents.

Here is a example of these binary documents

{
        "$1": {
            "cas": 1667520921496387584,
            "expiration": 0,
            "flags": 50331648,
            "id": "_sync:rev:00001abd-1f99-4b4e-a695-d11574ea9ed8:0:",
            "type": "base64"
        },
        "pa": "<binary (1 b)>"
    },
    {
        "$1": {
            "cas": 1667484959445614592,
            "expiration": 0,
            "flags": 50331648,
            "id": "_sync:rev:00001abd-1f99-4b4e-a695-d11574ea9ed8:34:2-d3fb2d58672f853d98ce343d3ae84c1d",
            "type": "base64"
        },
        "pa": "<binary (1129 b)>"
}

My issue with these documents is that they increase dramatically over time! and they don’t get cleaned up automatically! So they just grow and consume resources!

What are these documents used for?
Why aren’t these documents cleaned automatically?
Is it safe to simply delete these documents?
Is this a bug or a feature? :-)

Regards, Siraf

Solution

The issue was solved by adding this AND NOT REGEXP_CONTAINS(META().id,"^_sync:rev") to the XDCR replication filter expression. This stopped binary documents do be replicated from bucket A to B.