VDiff2

Compare the source and target in a workflow to ensure integrity

This is the new experimental version of VDiff which runs on vttablets as compared with VDiff1 which runs on vtctld.
Even before it's marked as production-ready (feature complete and tested widely in 1+ releases), it should be safe to use and is likely to provide much better results for very large tables. It also offers the ability to resume a VDiff that may have encountered an error, which is especially useful when working with very large tables.

For additional details, please see the RFC and the README.

Command #

VDiff2 takes different sub-commands or actions similar to how the MoveTables/Reshard commands work. The first argument is the <keyspace.workflow> followed by . The following actions are supported:

Start a new VDiff #

These take the same parameters as VDiff1 and schedule VDiff to run on the primary tablet of each target shard to verify the subset of data that will live on the given shard. Please note that if you do not specify a sub-command or action then create is assumed (this eases the transition from VDiff1 to VDiff2). If you do not pass a specific UUID then one will be generated.

VDiff -- --v2 [--source_cell=<cell>] [--target_cell=<cell>] [--tablet_types=in_order:RDONLY,REPLICA,PRIMARY]
       [--limit=<max rows to diff>] [--tables=<table list>] [--format=json] [--verbose] [--max_extra_rows_to_compare=1000]
       [--filtered_replication_wait_time=30s] [--debug_query] [--only_pks] <keyspace.workflow>  create [<UUID>]

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

$ vtctlclient --server=localhost:15999 VDiff -- --v2 customer.commerce2customer
VDiff bf9dfc5f-e5e6-11ec-823d-0aa62e50dd24 scheduled on target shards, use show to view progress

Resume a previous VDiff #

Allows you to resume an existing VDiff workflow, 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:

  1. Resume a VDiff that may have encountered an ephemeral error
  2. Do approximate rolling or differential VDiffs (e.g. done after MoveTables finishes the initial copy phase and then again just before SwitchTraffic)
VDiff -- --v2 [--source_cell=<cell>] [--target_cell=<cell>] [--tablet_types=in_order:RDONLY,REPLICA,PRIMARY]
       [--limit=<max rows to diff>] [--tables=<table list>] [--format=json] [--max_extra_rows_to_compare=1000]
       [--filtered_replication_wait_time=30s] [--debug_query] [--only_pks] <keyspace.workflow> resume <UUID>

Example:

$ vtctlclient --server=localhost:15999 VDiff -- --v2 customer.commerce2customer resume 4c664dc2-eba9-11ec-9ef7-920702940ee0
VDiff 4c664dc2-eba9-11ec-9ef7-920702940ee0 resumed on target shards, use show to view progress
We cannot guarantee accurate results for resume when different collations are used for a table between the source and target keyspaces (more details can be seen here).

Show progress/status of a VDiff #

VDiff  -- --v2  <keyspace.workflow> show [<UUID> | last | all]

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

$ vtctlclient --server=localhost:15999 VDiff -- --v2 customer.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.

$ vtctlclient --server=localhost:15999 VDiff -- --v2 --format=json customer.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"
}

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

Delete VDiff results #

VDiff  -- --v2  <keyspace.workflow> delete [<UUID> | all]

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

$ vtctlclient --server=localhost:15999 VDiff -- --v2 customer.commerce2customer delete all
VDiff delete status is completed on target shards

$ vtctlclient --server=localhost:15999 VDiff -- --v2 --format=json customer.commerce2customer delete all
{
	"Action": "delete",
	"Status": "completed"
}

Deletes are idempotent, so attempting to delete VDiff data that does not exist is a no-op.

All VDiff data associated with a VReplication workflow is deleted when the workflow is deleted.

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.

The actions supported

Parameters #

--source_cell #

optional
default all

VDiff will choose a tablet from this cell to diff the source tables with the target tables

--target_cell #

optional
default all

VDiff will choose a tablet from this cell to diff the target tables with the source tables

--tablet_types #

optional
default in_order:RDONLY,REPLICA,PRIMARY

A comma separated list of tablet types that are used while picking a tablet for sourcing data. One or more from PRIMARY, REPLICA, RDONLY.

--filtered_replication_wait_time #

optional
default 30s

VDiff finds the current position of the source primary and then waits for the target replication to reach that position for `--filtered_replication_wait_time`. If the target is much behind the source or if there is a high write qps on the source then this time will need to be increased.

--limit #

optional
default 9223372036854775807

Maximum number of rows to run vdiff on (across all tables specified). This limit is usually set while diffing a large table as a quick consistency check.

--tables #

optional
default all tables in the workflow

A comma separated list of tables to run vdiff on.

--format #

optional
default text (unstructured text output)

Only other format supported is JSON

--verbose #

optional

Show verbose vdiff output in summaries

--max_extra_rows_to_compare #

optional
default 1000

Limits the number of extra rows on both the source and target that we will perform a second compare pass on to confirm that the rows are in fact different in content and not simply returned in a different order on the source and target (which can happen when there are collation differences, e.g. different MySQL versions).

--debug_query #

optional

Adds the MySQL query to the report that can be used for further debugging

--only_pks #

optional

When reporting missing rows, only show primary keys in the report.

keyspace.workflow #

mandatory

Name of target keyspace and the associated workflow to run VDiff on.