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https://issues.apache.org/jira/browse/KAFKA-4353?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15647999#comment-15647999
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Randall Hauch commented on KAFKA-4353:
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Logical types and semantic types are not the same thing, and they don't carry
the same weight. The point of semantic types is not so much that every
programming language has constructs for them, but rather that a *source*
accessed by a connector has this concept and wants to capture it. Whether or
not consumers choose or will do anything with this extra semantic information
is beside the point, because as soon as its available then consumers *can* do
something with it. In this way, semantic types are very different than logical
types that build into the converters the conversion logic to and from
programming language constructs.
Sure, source connector can define their own semantic type by simply creating a
schema based upon a primitive and giving it a name. Debezium is doing precisely
this for JSON, XML, UUIDs, and temporal types so that its source connectors can
include as much information as possible about the data captured in the event
messages. The problem with this is that sink connectors written by other
communities or organizations are not likely to know about Debezium's semantic
types. The bottom line is that having some standard semantic types will mean
that more connectors are developed to support them, and that people can much
more easily mix and match source and sink connectors.
JSON is an excellent example. Source connectors can capture that {{STRING}}
fields are in fact JSON documents, arrays, or scalars, and sink connectors
pushing data into systems that *do* have some notion of JSON could take the
{{STRING}} values and parse them into JSON representation before using them. I
conceded that it's maybe not useful to have lots of similar temporal semantic
types with different units, but at a minimum I do think it is useful to have
semantic types for year, days, and ISO 8601 timestamps.
Really, semantic types are just a convention of using the existing schema
system but with well-known schema names. Perhaps it's less useful for Kafka
Connect software to define the few constants and trivial utility methods, and
more useful to treat it as a protocol that multiple organizations can
collaborate on and support.
> Add semantic types to Kafka Connect
> -----------------------------------
>
> Key: KAFKA-4353
> URL: https://issues.apache.org/jira/browse/KAFKA-4353
> Project: Kafka
> Issue Type: Improvement
> Components: KafkaConnect
> Affects Versions: 0.10.0.1
> Reporter: Randall Hauch
> Assignee: Ewen Cheslack-Postava
>
> Kafka Connect's schema system defines several _core types_ that consist of:
> * STRUCT
> * ARRAY
> * MAP
> plus these _primitive types_:
> * INT8
> * INT16
> * INT32
> * INT64
> * FLOAT32
> * FLOAT64
> * BOOLEAN
> * STRING
> * BYTES
> The {{Schema}} for these core types define several attributes, but they do
> not have a name.
> Kafka Connect also defines several _logical types_ that are specializations
> of the primitive types and _do_ have schema names _and_ are automatically
> mapped to/from Java objects:
> || Schema Name || Primitive Type || Java value class || Description ||
> | o.k.c.d.Decimal | {{BYTES}} | {{java.math.BigDecimal}} | An
> arbitrary-precision signed decimal number. |
> | o.k.c.d.Date | {{INT32}} | {{java.util.Date}} | A date representing a
> calendar day with no time of day or timezone. The {{java.util.Date}} value's
> hours, minutes, seconds, milliseconds are set to 0. The underlying
> representation is an integer representing the number of standardized days
> (based on a number of milliseconds with 24 hours/day, 60 minutes/hour, 60
> seconds/minute, 1000 milliseconds/second with n) since Unix epoch. |
> | o.k.c.d.Time | {{INT32}} | {{java.util.Date}} | A time representing a
> specific point in a day, not tied to any specific date. Only the
> {{java.util.Date}} value's hours, minutes, seconds, and milliseconds can be
> non-zero. This effectively makes it a point in time during the first day
> after the Unix epoch. The underlying representation is an integer
> representing the number of milliseconds after midnight. |
> | o.k.c.d.Timestamp | {{INT32}} | {{java.util.Date}} | A timestamp
> representing an absolute time, without timezone information. The underlying
> representation is a long representing the number of milliseconds since Unix
> epoch. |
> where "o.k.c.d" is short for {{org.kafka.connect.data}}. [~ewencp] has stated
> in the past that adding more logical types is challenging and generally
> undesirable, since everyone use Kafka Connect values have to deal with all
> new logical types.
> This proposal adds standard _semantic_ types that are somewhere between the
> core types and logical types. Basically, they are just predefined schemas
> that have names and are based on other primitive types. However, there is no
> mapping to another form other than the primitive.
> The purpose of semantic types is to provide hints as to how the values _can_
> be treated. Of course, clients are free to ignore the hints of some or all of
> the built-in semantic types, and in these cases would treat the values as the
> primitive value with no extra semantics. This behavior makes it much easier
> to add new semantic types over time without risking incompatibilities.
> Really, any source connector can define custom semantic types, but there is
> tremendous value in having a library of standard, well-known semantic types,
> including:
> || Schema Name || Primitive Type || Description ||
> | o.k.c.d.Uuid | {{STRING}} | A UUID in string form.|
> | o.k.c.d.Json | {{STRING}} | A JSON document, array, or scalar in string
> form.|
> | o.k.c.d.Xml | {{STRING}} | An XML document in string form.|
> | o.k.c.d.BitSet | {{STRING}} | A string of zero or more {{0}} or {{1}}
> characters.|
> | o.k.c.d.ZonedTime | {{STRING}} | An ISO-8601 formatted representation of a
> time (with fractional seconds) with timezone or offset from UTC.|
> | o.k.c.d.ZonedTimestamp | {{STRING}} | An ISO-8601 formatted representation
> of a timestamp with timezone or offset from UTC.|
> | o.k.c.d.EpochDays | {{INT64}} | A date with no time or timezone
> information, represented as the number of days since (or before) epoch, or
> January 1, 1970, at 00:00:00UTC.|
> | o.k.c.d.Year | {{INT32}} | The year number.|
> | o.k.c.d.MilliTime | {{INT32}} | Number of milliseconds past midnight.|
> | o.k.c.d.MicroTime | {{INT64}} | Number of microseconds past midnight.|
> | o.k.c.d.NanoTime | {{INT64}} | Number of nanoseconds past midnight.|
> | o.k.c.d.MilliTimestamp | {{INT64}} | Number of milliseconds past epoch.|
> | o.k.c.d.MicroTimestamp | {{INT64}} | Number of microseconds past epoch.|
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