Experimental: Multi-tenant import support in Vitess

[rohit-nayak-ps]

Description

Please read the design RFC first at #15403.

This proposal outlines support for importing data from a multi-tenant database cluster into a single Vitess cluster

Motivation

The user has a multi-tenant database architecture. They have several separate MySQL databases, with common schemas, one per tenant. Each database will be imported using one MoveTables workflow, using an external keyspace pointing to the
source, with a common target keyspace. All tables, both in the source and target, will include a tenant_id column to uniquely identify the tenant.

We can run multiple such workflows in parallel, until all tenants are imported. Just like normal imports, we would also
like reverse workflows to be running: to allow for rollback.

Ideally, routing rules should be established. This ensures that queries targeting a specific tenant ID are routed correctly to the target if the tenant has already been imported. Otherwise, they should route to the source if the import workflow is in progress but not yet switched to the target.

Many setups with multi-tenancy will use a multi-schema approach, where each tenant has their own MySQL database on a
single server. In this case the queries will use a database qualifier, which matches their database name. So when they
are running the migrations into Vitess, they will continue to use the same database qualifier.

Assumptions

• The target keyspace has its schema already initialized.
• Each table in the target keyspace has a tenant id column with the same name and type on both source and target, and it
  should be part of the primary key of each table.
• If the target keyspace is sharded, the vindex should be a multi-column vindex which includes the tenant id column.

Notes

• The current implementation requires that all traffic, across all tablet types, be switched simultaneously for a tenant.
• Although reverse replication will be operational, initiating ReverseTraffic is not feasible. This is because halting writes on the target is necessary for ReverseTraffic, which conflicts with the ongoing writes from previously migrated tenants and concurrent migrations for other tenants.
• Only vtctldclient is supported

Related Issue(s)

#15403

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

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• Is this flag really necessary?
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Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

Attention: Patch coverage is 35.54084% with 292 lines in your changes are missing coverage. Please review.

Project coverage is 68.01%. Comparing base (e5eb981) to head (36027f3).
Report is 3 commits behind head on main.

Files	Patch %	Lines
go/vt/vtctl/workflow/server.go	43.26%	59 Missing ⚠️
go/vt/vtctl/workflow/traffic_switcher.go	10.76%	58 Missing ⚠️
go/vt/vtctl/workflow/utils.go	14.92%	57 Missing ⚠️
...cmd/vtctldclient/command/keyspace_routing_rules.go	12.90%	54 Missing ⚠️
go/vt/vtctl/grpcvtctldserver/server.go	0.00%	22 Missing ⚠️
go/vt/vtctl/workflow/materializer.go	59.37%	13 Missing ⚠️
go/vt/vtctl/grpcvtctldclient/client_gen.go	0.00%	8 Missing ⚠️
go/vt/topo/vschema.go	71.42%	6 Missing ⚠️
go/vt/vtctl/localvtctldclient/client_gen.go	0.00%	4 Missing ⚠️
go/vt/vtctl/workflow/switcher_dry_run.go	0.00%	4 Missing ⚠️
... and 4 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15503      +/-   ##
==========================================
+ Coverage   65.78%   68.01%   +2.23%     
==========================================
  Files        1561     1562       +1     
  Lines      194838   195606     +768     
==========================================
+ Hits       128171   133051    +4880     
+ Misses      66667    62555    -4112     

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[mattlord]

Looks good! Nice work on this. I see that there's a few FIXMEs left so I'll come back to this and review the new code. At this point I only had minor comments and notes.

[rohit-nayak-ps]

Looks good! Nice work on this. I see that there's a few FIXMEs left so I'll come back to this and review the new code. At this point I only had minor comments and notes.

No more FIXMEs remaining. @mattlord please review when you get a chance...

[mattlord]

Nice work on this, @rohit-nayak-ps ! Very clean. I only had a few very minor comments that I'll leave to your discretion. Thanks!

[mattlord]

Do we mean a valid JSON document as a string or something else? This is where examples in the command definition can be helpful for the user. For example, see:

vitess/go/cmd/vtctldclient/command/vreplication/materialize/create.go

Lines 44 to 71 in fabd746

	Example: `vtctldclient --server localhost:15999 materialize --workflow product_sales --target-keyspace commerce create --source-keyspace commerce --table-settings '[{"target_table": "sales_by_sku", "create_ddl": "create table sales_by_sku (sku varbinary(128) not null primary key, orders bigint, revenue bigint)", "source_expression": "select sku, count(*) as orders, sum(price) as revenue from corder group by sku"}]' --cells zone1 --cells zone2 --tablet-types replica`,
	Long: `Materialize is a lower level VReplication command that allows for generalized materialization
	of tables. The target tables can be copies, aggregations, or views. The target tables are kept
	in sync in near-realtime. The primary flag used to define the materializations (you can have
	multiple per workflow) is table-settings which is a JSON array where each value must contain
	two key/value pairs. The first required key is 'target_table' and it is the name of the table
	in the target-keyspace to store the results in. The second required key is 'source_expression'
	and its value is the select query to run against the source table. An optional key/value pair
	can also be specified for 'create_ddl' which provides the DDL to create the target table if it
	does not exist -- you can alternatively specify a value of 'copy' if the target table schema
	should be copied as-is from the source keyspace. Here's an example value for table-settings:
	[
	{
	"target_table": "customer_one_email",
	"source_expression": "select email from customer where customer_id = 1"
	},
	{
	"target_table": "states",
	"source_expression": "select * from states",
	"create_ddl": "copy"
	},
	{
	"target_table": "sales_by_sku",
	"source_expression": "select sku, count(*) as orders, sum(price) as revenue from corder group by sku",
	"create_ddl": "create table sales_by_sku (sku varbinary(128) not null primary key, orders bigint, revenue bigint)"
	}
	]
	`,