Auto generated _sync:dcp_ck documents

glenn · December 20, 2017, 10:56am

Hi,

As it was suggested, we updated sync gateway & couchbase server to the latest stable versions(1.5.1 & 5.0.0). Right now, when I look at the bucket, it includes lots of documents which is related with dcp.

For instance, in my dev bucket the following document was added somehow:

{
“seqStart”: 0,
“seqEnd”: 0,
“snapStart”: 0,
“snapEnd”: 1,
“failOverLog”: [
[
104807047453991,
0
]
]
}

And secondly, id of these documents are incremental but for some numbers there is no document generated. _sync:dcp_ck:644 exists but not _sync:dcp_ck:645

Is it a normal behavior or any error occured behind the scene? Actually I don’t want to fill my bucket with that kind of stuff, if possible.

Thanks.

traun · December 21, 2017, 6:28pm

Hey @glenn,

Yes, those are “dcp checkpoint” documents and are expected behavior. Here is the code that is associated with those documents, in case you want to dig in any further:

github.com

couchbase/sync_gateway/blob/e076bc97123a6ce80d18095928e5689c856e14f5/base/dcp_feed.go#L43


// used in MCRequest.DataType
const (
	MemcachedDataTypeJSON = 1 << iota
	MemcachedDataTypeSnappy
	MemcachedDataTypeXattr
)


// Memcached datatype for raw (binary) document (non-flag)
const MemcachedDataTypeRaw = 0


const DCPCheckpointPrefix = "_sync:dcp_ck:" // Prefix used for DCP checkpoint persistence (is appended with vbno)


// Number of non-checkpoint updates per vbucket required to trigger metadata persistence.  Must be greater than zero to avoid
// retriggering persistence solely based on checkpoint doc echo.
// Based on ad-hoc testing w/ travel-sample bucket, increasing this value doesn't significantly improve performance, since under load
// DCP will already be sending more documents per snapshot.
const kCheckpointThreshold = 1


type SimpleFeed struct {
	eventFeed  chan sgbucket.FeedEvent
	terminator chan bool

Since there are typically only 1024 vbuckets, there will be a fixed number of 1024 dcp checkpoint documents.

Muhammet_Ali_Asan · February 27, 2020, 10:02am

We faced this very recently. We have not updated anything. In one day, those 1024 documents have appeared. We understand this is expected behavior but why it suddenly happened ?