dvlopt.kafka
Start here.
This library aims to be clojure idiomatic while not being too smart about wrapping the java libraries, so that upgrading will not be laborious in the future.
It is organized by specific namespaces. Producers (dvlopt.kafka.out) produce records, consumers (dvlopt.kafka.in)
consume records, and administrators can alter Kafka (dvlopt.kafka.admin). For anything related to Kafka Streams, see the
dvlopt.kstreams namespace.
Records
A record is a map containing at least a ::topic. It might also hold :
::headers
List of [Key Value] where the Key is an arbitrary string and Value is an arbitrary byte array which can be missing.
Keys can appear more than once.
::offset Offset of the record in the topic-partition.
::key Key of the record, serialized or deserialized. Can be nil.
::timestamp Timestamp of the record.
::timestamp.type A timestamp can refer to when the record was created (:create) or sent (:log-append).
::value Value of the record, serializer of deserializer. Can be nil
Obviously, the offset cannot be decided by the user when sending the record. Headers allow records to have metadata clearly distinct from the value. They exerce the same role as in other protocols such as HTTP.
Connecting to nodes
Opening a resource such as a producer requires a list of nodes (ie. list of [host port]). If not provided, the library tries to reach [["localhost" 9092]]. When described, a node is a map containing ::host, ::port, ::id (numerical identification), and ::rack when one is assigned. A replica node also contains the attribute ::synced?.
Ser/de
Within Kafka, records are kept as transparent byte arrays. When sending records, the key and the value need to be serialized and when consuming some, deserialized.
A serializer is a function mapping some data to a byte array or nil. It is needed for producing records.
Ex. (fn serializer [data metadata] (some-> v nippy/freeze))
A deserializer does the opposite, it maps a byte array (or nil) to a value. It is needed for consuming records.
Ex. (fn deserializer [ba metadata] (some-> ba nippy/thaw))
Both type of functions are often used throughout the library. The provided metadata might help the user to decide on how to ser/de the data. It is a map containing the ::topic involved as well as ::headers when there are some.
Built-in deserializers and serializers are available at deserializers and serializers respectively. Because ser/de
is so common, a key word specifying one of the built-in functions can be provided :
:boolean :byte-array :byte-buffer :double :integer :keyword :long :string
If a ser/de is not provided by the user when needed, the default is :byte-array.
Time
All time intervals such as timeouts, throughout all this library, are expressed as [Duration Unit] where Duration is coerced to an integer and Unit is one of :
:nanoseconds :microseconds :milliseconds :seconds :minutes :hours :days
Ex. [5 :seconds]
Start here.
This library aims to be clojure idiomatic while not being too smart about wrapping the java libraries, so that upgrading
will not be laborious in the future.
It is organized by specific namespaces. Producers (dvlopt.kafka.out) produce records, consumers (dvlopt.kafka.in)
consume records, and administrators can alter Kafka (dvlopt.kafka.admin). For anything related to Kafka Streams, see the
`dvlopt.kstreams` namespace.
Records
=======
A record is a map containing at least a ::topic. It might also hold :
::headers
List of [Key Value] where the `Key` is an arbitrary string and `Value` is an arbitrary byte array which can be missing.
Keys can appear more than once.
::offset
Offset of the record in the topic-partition.
::key
Key of the record, serialized or deserialized. Can be nil.
::timestamp
Timestamp of the record.
::timestamp.type
A timestamp can refer to when the record was created (:create) or sent (:log-append).
::value
Value of the record, serializer of deserializer. Can be nil
Obviously, the offset cannot be decided by the user when sending the record. Headers allow records to have metadata
clearly distinct from the value. They exerce the same role as in other protocols such as HTTP.
Connecting to nodes
===================
Opening a resource such as a producer requires a list of nodes (ie. list of [host port]). If not provided, the library
tries to reach [["localhost" 9092]]. When described, a node is a map containing ::host, ::port, ::id (numerical
identification), and ::rack when one is assigned. A replica node also contains the attribute ::synced?.
Ser/de
======
Within Kafka, records are kept as transparent byte arrays. When sending records, the key and the value need to be
serialized and when consuming some, deserialized.
A serializer is a function mapping some data to a byte array or nil. It is needed for producing records.
Ex. (fn serializer [data metadata]
(some-> v
nippy/freeze))
A deserializer does the opposite, it maps a byte array (or nil) to a value. It is needed for consuming records.
Ex. (fn deserializer [ba metadata]
(some-> ba
nippy/thaw))
Both type of functions are often used throughout the library. The provided metadata might help the user to decide on
how to ser/de the data. It is a map containing the ::topic involved as well as ::headers when there are some.
Built-in deserializers and serializers are available at `deserializers` and `serializers` respectively. Because ser/de
is so common, a key word specifying one of the built-in functions can be provided :
:boolean
:byte-array
:byte-buffer
:double
:integer
:keyword
:long
:string
If a ser/de is not provided by the user when needed, the default is :byte-array.
Time
====
All time intervals such as timeouts, throughout all this library, are expressed as [Duration Unit] where
Duration is coerced to an integer and Unit is one of :
:nanoseconds
:microseconds
:milliseconds
:seconds
:minutes
:hours
:days
Ex. [5 :seconds]defaultsclj
Default values and options used throughout this library.
Default values and options used throughout this library.
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