Key Generation

Every record in Hudi is uniquely identified by a primary key, which is a pair of record key and partition path where the record belongs to. Using primary keys, Hudi can impose a) partition level uniqueness integrity constraint b) enable fast updates and deletes on records. One should choose the partitioning scheme wisely as it could be a determining factor for your ingestion and query latency.

In general, Hudi supports both partitioned and global indexes. For a dataset with partitioned index(which is most commonly used), each record is uniquely identified by a pair of record key and partition path. But for a dataset with global index, each record is uniquely identified by just the record key. There won't be any duplicate record keys across partitions.

Key Generators

Hudi provides several key generators out of the box that users can use based on their need, while having a pluggable implementation for users to implement and use their own KeyGenerator. This page goes over all different types of key generators that are readily available to use.

Here is the interface for KeyGenerator in Hudi for your reference.

Before diving into different types of key generators, let’s go over some of the common configs required to be set for key generators.

Config	Meaning/purpose
hoodie.datasource.write.recordkey.field	Refers to record key field. This is a mandatory field.
hoodie.datasource.write.partitionpath.field	Refers to partition path field. This is a mandatory field.
hoodie.datasource.write.keygenerator.class	Refers to Key generator class(including full path). Could refer to any of the available ones or user defined one. This is a mandatory field.
hoodie.datasource.write.partitionpath.urlencode	When set to true, partition path will be url encoded. Default value is false.
hoodie.datasource.write.hive_style_partitioning	When set to true, uses hive style partitioning. Partition field name will be prefixed to the value. Format: “<partition_path_field_name>=<partition_path_value>”. Default value is false.

There are few more configs involved if you are looking for TimestampBasedKeyGenerator. Will cover those in the respective section.

Lets go over different key generators available to be used with Hudi.

SimpleKeyGenerator

Record key refers to one field(column in dataframe) by name and partition path refers to one field (single column in dataframe) by name. This is one of the most commonly used one. Values are interpreted as is from dataframe and converted to string.

ComplexKeyGenerator

Both record key and partition paths comprise one or more than one field by name(combination of multiple fields). Fields are expected to be comma separated in the config value. For example "Hoodie.datasource.write.recordkey.field" : “col1,col4”

GlobalDeleteKeyGenerator

Global index deletes do not require partition value. So this key generator avoids using partition value for generating HoodieKey.

NOTE: The "GlobalDeleteKeyGenerator" has to be used with a global index to delete records solely based on the record key. It works for a batch with deletes only. The key generator can be used for both partitioned and non-partitioned table. Note that when using this key generator, the config hoodie.[bloom|simple|hbase].index.update.partition.path should be set to false in order to avoid redundant data written to the storage.

NonpartitionedKeyGenerator

If your hudi dataset is not partitioned, you could use this “NonpartitionedKeyGenerator” which will return an empty partition for all records. In other words, all records go to the same partition (which is empty “”)

CustomKeyGenerator

This is a generic implementation of KeyGenerator where users are able to leverage the benefits of SimpleKeyGenerator, ComplexKeyGenerator and TimestampBasedKeyGenerator all at the same time. One can configure record key and partition paths as a single field or a combination of fields.

hoodie.datasource.write.recordkey.field
hoodie.datasource.write.partitionpath.field
hoodie.datasource.write.keygenerator.class=org.apache.hudi.keygen.CustomKeyGenerator

This keyGenerator is particularly useful if you want to define complex partition paths involving regular fields and timestamp based fields. It expects value for prop "hoodie.datasource.write.partitionpath.field" in a specific format. The format should be "field1:PartitionKeyType1,field2:PartitionKeyType2..."

The complete partition path is created as <value for field1 basis PartitionKeyType1>/<value for field2 basis PartitionKeyType2> and so on. Each partition key type could either be SIMPLE or TIMESTAMP.

Example config value: “field_3:simple,field_5:timestamp”

RecordKey config value is either single field incase of SimpleKeyGenerator or a comma separate field names if referring to ComplexKeyGenerator. Example:

hoodie.datasource.write.recordkey.field=field1,field2

This will create your record key in the format field1:value1,field2:value2 and so on, otherwise you can specify only one field in case of simple record keys. CustomKeyGenerator class defines an enum PartitionKeyType for configuring partition paths. It can take two possible values - SIMPLE and TIMESTAMP. The value for hoodie.datasource.write.partitionpath.field property in case of partitioned tables needs to be provided in the format field1:PartitionKeyType1,field2:PartitionKeyType2 and so on. For example, if you want to create partition path using 2 fields country and date where the latter has timestamp based values and needs to be customised in a given format, you can specify the following

hoodie.datasource.write.partitionpath.field=country:SIMPLE,date:TIMESTAMP

This will create the partition path in the format <country_name>/<date> or country=<country_name>/date=<date> depending on whether you want hive style partitioning or not.

Bring your own implementation

You can implement your own custom key generator by extending the public API class here:

https://github.com/apache/hudi/blob/master/hudi-common/src/main/java/org/apache/hudi/keygen/KeyGenerator.java

TimestampBasedKeyGenerator

This key generator relies on timestamps for the partition field. The field values are interpreted as timestamps and not just converted to string while generating partition path value for records. Record key is same as before where it is chosen by field name. Users are expected to set few more configs to use this KeyGenerator.

Configs to be set:

Config	Meaning/purpose
hoodie.deltastreamer.keygen.timebased.timestamp.type	One of the timestamp types supported(UNIX_TIMESTAMP, DATE_STRING, MIXED, EPOCHMILLISECONDS, SCALAR)
hoodie.deltastreamer.keygen.timebased.output.dateformat	Output date format
hoodie.deltastreamer.keygen.timebased.timezone	Timezone of the data format
oodie.deltastreamer.keygen.timebased.input.dateformat	Input date format

Let's go over some example values for TimestampBasedKeyGenerator.

Timestamp is GMT

Config field	Value
hoodie.deltastreamer.keygen.timebased.timestamp.type	"EPOCHMILLISECONDS"
hoodie.deltastreamer.keygen.timebased.output.dateformat	"yyyy-MM-dd hh"
hoodie.deltastreamer.keygen.timebased.timezone	"GMT+8:00"

Input Field value: “1578283932000L”
Partition path generated from key generator: “2020-01-06 12”

If input field value is null for some rows.
Partition path generated from key generator: “1970-01-01 08”

Timestamp is DATE_STRING

Config field	Value
hoodie.deltastreamer.keygen.timebased.timestamp.type	"DATE_STRING"
hoodie.deltastreamer.keygen.timebased.output.dateformat	"yyyy-MM-dd hh"
hoodie.deltastreamer.keygen.timebased.timezone	"GMT+8:00"
hoodie.deltastreamer.keygen.timebased.input.dateformat	"yyyy-MM-dd hh:mm:ss"

Input field value: “2020-01-06 12:12:12”
Partition path generated from key generator: “2020-01-06 12”

If input field value is null for some rows.
Partition path generated from key generator: “1970-01-01 12:00:00”

Scalar examples

Config field	Value
hoodie.deltastreamer.keygen.timebased.timestamp.type	"SCALAR"
hoodie.deltastreamer.keygen.timebased.output.dateformat	"yyyy-MM-dd hh"
hoodie.deltastreamer.keygen.timebased.timezone	"GMT"
hoodie.deltastreamer.keygen.timebased.timestamp.scalar.time.unit	"days"

Input field value: “20000L”
Partition path generated from key generator: “2024-10-04 12”

If input field value is null.
Partition path generated from key generator: “1970-01-02 12”

ISO8601WithMsZ with Single Input format

Config field	Value
hoodie.deltastreamer.keygen.timebased.timestamp.type	"DATE_STRING"
hoodie.deltastreamer.keygen.timebased.input.dateformat	"yyyy-MM-dd'T'HH:mm:ss.SSSZ"
hoodie.deltastreamer.keygen.timebased.input.dateformat.list.delimiter.regex	""
hoodie.deltastreamer.keygen.timebased.input.timezone	""
hoodie.deltastreamer.keygen.timebased.output.dateformat	"yyyyMMddHH"
hoodie.deltastreamer.keygen.timebased.output.timezone	"GMT"

Input field value: "2020-04-01T13:01:33.428Z"
Partition path generated from key generator: "2020040113"

ISO8601WithMsZ with Multiple Input formats

Config field	Value
hoodie.deltastreamer.keygen.timebased.timestamp.type	"DATE_STRING"
hoodie.deltastreamer.keygen.timebased.input.dateformat	"yyyy-MM-dd'T'HH:mm:ssZ,yyyy-MM-dd'T'HH:mm:ss.SSSZ"
hoodie.deltastreamer.keygen.timebased.input.dateformat.list.delimiter.regex	""
hoodie.deltastreamer.keygen.timebased.input.timezone	""
hoodie.deltastreamer.keygen.timebased.output.dateformat	"yyyyMMddHH"
hoodie.deltastreamer.keygen.timebased.output.timezone	"UTC"

Input field value: "2020-04-01T13:01:33.428Z"
Partition path generated from key generator: "2020040113"

ISO8601NoMs with offset using multiple input formats

Config field	Value
hoodie.deltastreamer.keygen.timebased.timestamp.type	"DATE_STRING"
hoodie.deltastreamer.keygen.timebased.input.dateformat	"yyyy-MM-dd'T'HH:mm:ssZ,yyyy-MM-dd'T'HH:mm:ss.SSSZ"
hoodie.deltastreamer.keygen.timebased.input.dateformat.list.delimiter.regex	""
hoodie.deltastreamer.keygen.timebased.input.timezone	""
hoodie.deltastreamer.keygen.timebased.output.dateformat	"yyyyMMddHH"
hoodie.deltastreamer.keygen.timebased.output.timezone	"UTC"

Input field value: "2020-04-01T13:01:33-05:00"
Partition path generated from key generator: "2020040118"

Input as short date string and expect date in date format

Config field	Value
hoodie.deltastreamer.keygen.timebased.timestamp.type	"DATE_STRING"
hoodie.deltastreamer.keygen.timebased.input.dateformat	"yyyy-MM-dd'T'HH:mm:ssZ,yyyy-MM-dd'T'HH:mm:ss.SSSZ,yyyyMMdd"
hoodie.deltastreamer.keygen.timebased.input.dateformat.list.delimiter.regex	""
hoodie.deltastreamer.keygen.timebased.input.timezone	"UTC"
hoodie.deltastreamer.keygen.timebased.output.dateformat	"MM/dd/yyyy"
hoodie.deltastreamer.keygen.timebased.output.timezone	"UTC"

Input field value: "20200401"
Partition path generated from key generator: "04/01/2020"