X DevAPI User Guide
X DevAPI relies on server based document ID generation, added
in MySQL version 8.0.11, which results in sequentially
increasing document IDs across all clients.
InnoDB uses the document ID as a
primary key, therefore these sequential primary keys for all
clients result in efficient page splits and tree
reorganizations.
This section describes the properties and format of the automatically generated document IDs.
The _id field of a document behaves in the
same way as any other field of the document during queries,
except that its value cannot change once inserted to the
collection. The _id field is used as the
primary key of the collection (using stored generated
columns). It is possible to override the automatic generation
of document IDs by manually including an ID in an inserted
document.
If you are using manual document IDs, you must ensure that IDs from the server's automatically generated document ID sequence are never used. X Plugin is not aware of the data inserted into the collection, including any IDs you use. Thus in future inserts, if the document ID which you assigned manually when inserting a document uses an ID which the server was going to use, the insert operation fails with an error due to primary key duplication.
Whenever an _id field value is not present
in an inserted document, the server generates an
_id value. The generated
_id value used for a document is returned
to the client as part of the document insert
Result message. If you are using X DevAPI
on an InnoDB Cluster, the automatically generated
_id must be unique within the cluster. Use
the
mysqlx_document_id_unique_prefix
option to ensure that the unique_prefix
part of the document ID is unique to the cluster.
The _id field must be sequential (always
incrementing) for optimal InnoDB insertion performance (at
least within a single server). The sequential nature of
_id values is maintained across server
restarts.
In a multi-primary Group Replication or InnoDB Cluster
environment, the generated _id values of a
table are unique across instances to avoid primary key
conflicts and minimize transaction certification.
This section describes how document IDs are formatted. The
general structure of the collection table remains unchanged,
except for the type of the generated _id
column, which changes from
VARCHAR(32) to
VARBINARY(32).
The format of automatically generated document ID is:
| unique_prefix | start_timestamp | serial |
|---|---|---|
| 4 bytes | 8 bytes | 16 bytes |
Where:
serial is a per-instance automatically
incremented integer serial number value, which is hex
encoded and has a range of 0 to 2**64-1. The initial value
of serial is set to the
auto_increment_offset
system variable, and the increment of the value is set by
the
auto_increment_increment
system variable.
start_timestamp is the time stamp of
the startup time of the server instance, which is hex
encoded. In the unlikely event that the value of
serial overflows, the
start_timestamp is incremented by 1 and
the serial value then restarts at 0.
unique_prefix is a value assigned by
InnoDB Cluster to the instance, which is used to make
the document ID unique across all instances from the same
cluster. The range of unique_prefix is
from 0 to 2**16-1, which is hex encoded, and defaults to 0
if not set by InnoDB Cluster or the
mysqlx_document_id_unique_prefix
system variable has not been configured.
This document ID format ensures that:
The primary key value monotonically increments for inserts originating from a single server instance, although the interval between values is not uniform within a table.
When using multi-primary Group Replication or
InnoDB Cluster, inserts to the same table from different
instances do not have conflicting primary key values;
assuming that the instances have the
auto_increment_* system variables
configured properly.