Description #

VDiff does a row by row comparison of all tables associated with the workflow, diffing the source keyspace and the target keyspace and reporting counts of missing/extra/unmatched rows.

It is highly recommended that you do this before you finalize a workflow with SwitchTraffic and complete.

Command #

VDiff takes different sub-commands or actions similar to how the MoveTables/Reshard commands work. Please see the command's reference docs for additional info. The following sub-commands or actions are supported:

Start a New VDiff #

The create action schedules a VDiff to run on the primary tablet of each target shard to verify the subset of data that will live on the given shard. If you do not pass a specific UUID then one will be generated.

Each scheduled VDiff has an associated UUID which is returned by the create action. You can use it to monitor progress. Example:

$ vtctldclient --server=localhost:15999 VDiff --target-keyspace customer --workflow commerce2customer create
VDiff a35b0006-e6d9-416e-bea9-917795dc5bf3 scheduled on target shards, use show to view progress

Resume a Previous VDiff #

The resume action allows you to resume a previously completed VDiff, picking up where it left off and comparing the records where the Primary Key column(s) are greater than the last record processed — with the progress and other status information saved when the run ends. This allows you to do approximate rolling or differential VDiffs (e.g. done after MoveTables finishes the initial copy phase and then again just before SwitchTraffic).

Example:

$ vtctldclient --server=localhost:15999 VDiff --target-keyspace customer --workflow commerce2customer resume 4c664dc2-eba9-11ec-9ef7-920702940ee0
VDiff 4c664dc2-eba9-11ec-9ef7-920702940ee0 resumed on target shards, use show to view progress

Show Progress/Status of a VDiff #

Using the show action you can either show a specific UUID or use the last convenience shorthand to look at the most recently created VDiff. Example:

$ vtctldclient --server=localhost:15999 VDiff --target-keyspace customer --workflow commerce2customer show last

VDiff Summary for customer.commerce2customer (4c664dc2-eba9-11ec-9ef7-920702940ee0)
State:        completed
RowsCompared: 196
HasMismatch:  false
StartedAt:    2022-06-26 22:44:29
CompletedAt:  2022-06-26 22:44:31

Use "--format=json" for more detailed output.

$ vtctldclient --server=localhost:15999 VDiff --format=json --target-keyspace customer --workflow commerce2customer show last
{
	"Workflow": "commerce2customer",
	"Keyspace": "customer",
	"State": "completed",
	"UUID": "4c664dc2-eba9-11ec-9ef7-920702940ee0",
	"RowsCompared": 196,
	"HasMismatch": false,
	"Shards": "0",
	"StartedAt": "2022-06-26 22:44:29",
	"CompletedAt": "2022-06-26 22:44:31"
}

$ vtctldclient --server=localhost:15999 VDiff --format=json --target-keyspace customer --workflow p1c2 show daf1f03a-03ed-11ed-9ab8-920702940ee0
{
	"Workflow": "p1c2",
	"Keyspace": "customer",
	"State": "started",
	"UUID": "daf1f03a-03ed-11ed-9ab8-920702940ee0",
	"RowsCompared": 51,
	"HasMismatch": false,
	"Shards": "-80,80-",
	"StartedAt": "2022-07-15 03:26:03",
	"Progress": {
		"Percentage": 48.57,
		"ETA": "2022-07-15 03:26:10"
	}
}

show all lists all VDiffs created for the specified keyspace and workflow.

Stopping a VDiff #

The stop action allows you to stop a running VDiff for any reason — for example, the load on the system(s) may be too high at the moment and you want to postpone the work until off hours. You can then later use the resume action to start the VDiff again from where it left off. Example:

$ vtctldclient --server=localhost:15999 VDiff --format=json --target-keyspace customer --workflow commerce2customer stop ad9bd40e-0c92-11ed-b568-920702940ee0
{
	"UUID": "ad9bd40e-0c92-11ed-b568-920702940ee0",
	"Action": "stop",
	"Status": "completed"
}

Delete VDiff Results #

You use the delete action to either delete a specific UUID or use the all shorthand to delete all VDiffs created for the specified keyspace and workflow. Example:

$ vtctldclient --server=localhost:15999 VDiff --target-keyspace customer --workflow commerce2customer delete all
VDiff delete completed

$ vtctldclient --server=localhost:15999 VDiff --format=json --target-keyspace customer --workflow commerce2customer delete all
{
	"Action": "delete",
	"Status": "completed"
}

Metrics #

VDiff exports several metrics using the expvars interface. These are available at the /debug/vars endpoint of vttablet's http status pages. More details can be found here. All of these are exported on the target primary vttablet processes where the VDiff controllers run.

VDiffCount #

The number of vdiffs that exist on the vttablet — meaning ones that were created and have not yet been deleted so they could still be stopped, resumed, or deleted. This count would match the number of vdiffs you would see in the vdiff show all client command output.

VDiffErrorCountTotal #

The number of errors encountered across all vdiff actions on the vttablet.

VDiffErrors #

The count of specific errors seen during the lifetime of a vdiff.

VDiffPhaseTimings #

This metric reflects the amount of time spent (in nanoseconds) in the main phases that are run during the lifetime of a vdiff.

VDiffRowsComparedTotal #

The number of rows compared across all vdiffs on the vttablet.

VDiffRowsCompared #

The live number of rows compared per vdiff by table.

VDiffRestartedTableDiffsCount #

The numnber of time that table diffs were restarted due to --max-diff-duration by table.