Why do auto-increment columns not work in sharded Vitess? #

Auto-increment columns do not work very well for sharded tables. Vitess sequences solve this problem. Sequence tables must be specified in the VSchema and then tied to table columns. At the time of insert, if no value is specified for such a column, VTGate will generate a number for it using the sequence table.

Vitess also supports sequence generators that can be used to generate new ids that work like MySQL auto increment columns. The VSchema allows you to associate table columns to sequence tables.

What is resharding? How does it work? #

Vitess supports resharding, in which the number of shards is changed on a live cluster. This can be either splitting one or more shards into smaller pieces, or merging neighboring shards into bigger pieces.

During resharding, the data in the source shards is copied into the destination shards, allowed to catch up on replication, and then compared against the original to ensure data integrity. Then the live serving infrastructure is shifted to the destination shards, and the source shards are deleted.

How are shards named? #

Shard names have the following characteristics:

• They represent a range, where the left number is included, but the right is not.
• Their notation is hexadecimal.
• They are left justified.
• A - prefix means: anything less than the RHS value.
• A - postfix means: anything greater than or equal to the LHS value.
• A plain - denotes the full keyrange.

An example of a shard name is -80 and following the rules above this means: -80 == 00-80 == 0000-8000 == 000000-800000

Similarly 80- is not the same as 80-FF because 80-FF == 8000-FF00. Therefore FFFF will be out of the 80-FF range as 80- means: ‘anything greater than or equal to 0x80

A hash vindex produces an 8-byte number. This means that all numbers less than 0x8000000000000000 will fall in shard -80. Any number with the highest bit set will be >= 0x8000000000000000, and will therefore belong to shard 80-.

What does “/0” or “/-”mean? #

0 or - indicates that the keyspace in question is unsharded. This means that a single shard covers the entire keyrange. Note that while both 0 and - are supported for legacy reasons, you can’t merge into shard 0 only -. For this and other reasons, it is recommended to use - instead of 0.

On the other hand a sharded cluster will have multiple keyranges, for example -80 and 80- if you have two shards. Note, that you can still manually target a single shard from your sharded cluster. You can read more about that here.