Resharding

This guide follows on from the Get Started guides. Please make sure that you have an Operator or local installation ready. It also assumes that the MoveTables user guide has been followed (which take you through steps 101-205 and more).

Preparation

Sharding enables you to both initially shard or partition your data as well as reshard your tables — as your data set size grows over time — so that your keyspace is partitioned across several underlying tablets. A sharded keyspace has some additional restrictions on both query syntax and features such as auto_increment, so it is helpful to plan out a reshard operation diligently. However, you can always reshard again later if your sharding scheme turns out to be suboptimal.

Using our example commerce and customer keyspaces, lets work through the two most common issues.

Sequences

The first issue to address is the fact that customer and corder have auto-increment columns. This scheme does not work well in a sharded setup. Instead, Vitess provides an equivalent feature called sequences.

The sequence table is an unsharded single row table that Vitess can use to generate monotonically increasing IDs. The syntax to generate IDs is: select next value from customer_seq or select next N values from customer_seq. The vttablet that exposes this table is capable of serving a very large number of such IDs because values are reserved in chunks, then cached and served from memory. The chunk/cache size is configurable via the cache value.

The VSchema allows you to associate the column of a table with the sequence table. Once this is done, an INSERT on that table transparently fetches an ID from the sequence table, fills in the value in the new row, and then routes the row to the appropriate shard. This makes the construct backward compatible to how MySQL’s auto_increment works.

Since sequences table must be unsharded, they will be stored in the unsharded commerce keyspace. Here is the schema used:

CREATE TABLE customer_seq (id int, next_id bigint, cache bigint, primary key(id)) comment 'vitess_sequence';
INSERT INTO customer_seq (id, next_id, cache) VALUES (0, 1000, 100);
CREATE TABLE order_seq (id int, next_id bigint, cache bigint, primary key(id)) comment 'vitess_sequence';
INSERT INTO order_seq (id, next_id, cache) VALUES (0, 1000, 100);

Note the vitess_sequence comment in the create table statement. VTTablet will use this metadata to treat this table as a sequence. About the values we specified above:

• id is always 0
• next_id is set to 1000: the value should be comfortably greater than the max auto_increment used so far.
• cache specifies the number of values to reserve and cache before the primary vttablet for the commerce keyspace updates next_id to reserve and cache the next chunk of IDs.

Larger cache values perform better but can exhaust the available values more quickly since e.g. during reparent operations the new PRIMARY vttablet will start off at the next_id value and any unused values from the previously reserved chunk are lost.

The VTGate servers also need to know about the sequence tables. This is done by updating the VSchema for the commerce keyspace as follows:

{
  "tables": {
    "customer_seq": {
      "type": "sequence"
    },
    "order_seq": {
      "type": "sequence"
    },
    "product": {}
  }
}

Vindexes

The next decision is about the sharding keys or Primary Vindexes. This is a complex decision that involves the following considerations:

• What are the highest QPS queries, and what are the WHERE clauses for them?
• Cardinality of the column — it must be high.
• Do we want some rows to live together to support in-shard joins (data locality)?
• Do we want certain rows that will be in the same transaction to live together (data locality)?

Using the above considerations, in our use case, we can determine the following:

• For the customer table, the most common WHERE clause uses customer_id. So customer_id is declared as the Primary Vindex for that table.
• Given that the customer table has a lot of rows, its cardinality is also high.
• For the corder table, we have a choice between customer_id and order_id. Given that our app joins customer with corder quite often on the customer_id column, it will be beneficial to choose customer_id as the Primary Vindex for the corder table as well so that we have data locality for those joins and can avoid costly cross-shard operations.
• Coincidentally, transactions also update corder tables with their corresponding customer rows. This further reinforces the decision to use customer_id as Primary Vindex.

There are a couple of other considerations which are out of scope for now, but worth mentioning:

• It may also be worth creating a secondary lookup Vindex on corder.order_id.
• Sometimes the customer_id is really a tenant_id. For example, if your application is a SaaS, which serves tenants that themselves have customers. One key consideration here is that the sharding by the tenant_id can lead to unbalanced shards. You may also need to consider sharding by the tenant’s customer_id.

Putting it all together, we have a VSchema similar to the following for the customer keyspace:

{
  "sharded": true,
  "vindexes": {
    "hash": {
      "type": "hash"
    }
  },
  "tables": {
    "customer": {
      "column_vindexes": [
        {
          "column": "customer_id",
          "name": "hash"
        }
      ],
      "auto_increment": {
        "column": "customer_id",
        "sequence": "customer_seq"
      }
    },
    "corder": {
      "column_vindexes": [
        {
          "column": "customer_id",
          "name": "hash"
        }
      ],
      "auto_increment": {
        "column": "order_id",
        "sequence": "order_seq"
      }
    }
  }
}

Since the primary vindex columns here use the BIGINT MySQL integer type, we choose hash as the primary vindex type, which is a pseudo-random way of distributing rows into various shards. For other data types we would typically use a different vindex type:

• For VARCHAR columns, use unicode_loose_md5 or unicode_loose_xxhash.
• For VARBINARY, use binary_md5 or xxhash.
• Vitess uses a plugin system to define vindexes. If none of the predefined vindexes suit your needs, you can develop your own custom vindex.

Apply VSchema

Applying the new VSchema instructs Vitess that the keyspace is sharded, which may prevent some complex queries. It is a good idea to validate this before proceeding with this step. If you do notice that certain queries start failing, you can always revert temporarily by restoring the old VSchema. Make sure you fix all of the queries before proceeding to the Reshard process.

Using Operator

vtctldclient ApplySchema --sql="$(cat create_commerce_seq.sql)" commerce
vtctldclient ApplyVSchema --vschema="$(cat vschema_commerce_seq.json)" commerce
vtctldclient ApplyVSchema --vschema="$(cat vschema_customer_sharded.json)" customer
vtctldclient ApplySchema --sql="$(cat create_customer_sharded.sql)" customer

Using a Local Deployment

vtctldclient ApplySchema --sql-file create_commerce_seq.sql commerce
vtctldclient ApplyVSchema --vschema-file vschema_commerce_seq.json commerce
vtctldclient ApplyVSchema --vschema-file vschema_customer_sharded.json customer
vtctldclient ApplySchema --sql-file create_customer_sharded.sql customer

Create New Shards

At this point, you have finalized your sharded VSchema and vetted all the queries to make sure they still work. Now, it’s time to reshard.

The resharding process works by splitting existing shards into smaller shards. This type of resharding is the most appropriate for Vitess. There are some use cases where you may want to bring up a new shard and add new rows in the most recently created shard. This can be achieved in Vitess by splitting a shard in such a way that no existing rows end up in the new shard. However, it’s not natural for Vitess. We now have to create the new target shards:

Using Operator

kubectl apply -f 302_new_shards.yaml

Make sure that you restart the port-forward after you have verified with kubectl get pods that this operation has completed:

killall kubectl
./pf.sh &

Using a Local Deployment

./302_new_shards.sh

Start the Reshard

Now we can start the Reshard operation. It occurs online, and will not block any read or write operations to your database:

vtctldclient Reshard --target-keyspace customer --workflow cust2cust create --source-shards '0' --target-shards '-80,80-'

All of the command options and parameters for Reshard are listed in our reference page for Reshard.

Validate Correctness

After the reshard is complete, we can use VDiff to check data integrity and ensure our source and target shards are consistent:

$ vtctldclient VDiff --target-keyspace customer --workflow cust2cust create
VDiff 60fa5738-9bad-11ed-b6de-920702940ee0 scheduled on target shards, use show to view progress
$ vtctldclient VDiff --target-keyspace customer --workflow cust2cust show last
{
    "Workflow": "cust2cust",
    "Keyspace": "customer",
    "State": "completed",
    "UUID": "60fa5738-9bad-11ed-b6de-920702940ee0",
    "RowsCompared": 10,
    "HasMismatch": false,
    "Shards": "-80,80-",
    "StartedAt": "2023-01-24 06:07:27",
    "CompletedAt": "2023-01-24 06:07:28"
}

Switch Non-Primary Reads

After validating for correctness, the next step is to switch REPLICA and RDONLY targeted read operations to occur at the new location. By switching targeted read operations first, we are able to verify that the new shard’s tablets are healthy and able to respond to requests:

vtctldclient Reshard --target-keyspace customer --workflow cust2cust SwitchTraffic --tablet-types=rdonly,replica

Switch Writes and Primary Reads

After the REPLICA and RDONLY targeted reads have been switched, and the health of the system has been verified, it’s time to switch writes and all default traffic:

vtctldclient Reshard --target-keyspace customer --workflow cust2cust SwitchTraffic

Note

While we have switched tablet type targeted reads and writes separately in this example, you can also switch all traffic at the same time. This is done by default as if you don’t specify the --tablet_types parameter then SwitchTraffic will start serving all traffic from the target for all tablet types.

You should now be able to see the data that has been copied over to the new shards (assuming you previously loaded this data in the MoveTable user-guide):

$ mysql --table < ../common/select_customer-80_data.sql
Using customer/-80
Customer
+-------------+--------------------+
| customer_id | email              |
+-------------+--------------------+
|           1 | alice@domain.com   |
|           2 | bob@domain.com     |
|           3 | charlie@domain.com |
|           5 | eve@domain.com     |
+-------------+--------------------+
COrder
+----------+-------------+----------+-------+
| order_id | customer_id | sku      | price |
+----------+-------------+----------+-------+
|        1 |           1 | SKU-1001 |   100 |
|        2 |           2 | SKU-1002 |    30 |
|        3 |           3 | SKU-1002 |    30 |
|        5 |           5 | SKU-1002 |    30 |
+----------+-------------+----------+-------+
$ mysql --table < ../common/select_customer80-_data.sql
Using customer/80-
Customer
+-------------+----------------+
| customer_id | email          |
+-------------+----------------+
|           4 | dan@domain.com |
+-------------+----------------+
COrder
+----------+-------------+----------+-------+
| order_id | customer_id | sku      | price |
+----------+-------------+----------+-------+
|        4 |           4 | SKU-1002 |    30 |
+----------+-------------+----------+-------+

Finalize and Cleanup

After celebrating your second successful resharding, you are now ready to clean up the leftover artifacts:

Using Operator

vtctldclient Reshard --target-keyspace customer --workflow cust2cust complete

After the workflow has completed, you can go ahead and remove the shard that is no longer required -

kubectl apply -f 306_down_shard_0.yaml

Using a Local Deployment

vtctldclient Reshard --target-keyspace customer --workflow cust2cust complete
for i in 200 201 202; do
 CELL=zone1 TABLET_UID=$i ./scripts/vttablet-down.sh
 CELL=zone1 TABLET_UID=$i ./scripts/mysqlctl-down.sh
done
vtctldclient DeleteShards --recursive customer/0

These are the steps taken in the 306_down_shard_0.sh and 307_delete_shard_0.sh scripts. In the first script (306) we stop all tablet instances for shard 0. This will cause all those vttablet and mysqld processes to be stopped. In the second script (307) we delete the shard records from our Vitess cluster topology. Beyond this, you will also want to manually delete the on-disk directories associated with this shard. With the local examples that would be:

rm -rf ${VTDATAROOT}/vt_000000020{0,1,2}/