Messageid is an identifier which identifies a message. How that id is
constructed, or what it contains should not matter to an application, and
an application should not assume anything about the implementation of that
id.
>What about the partition index? We have a `TopicMetadata` interface that
returns the number of partitions.
Partitioning is a first class concept, and is designed to be used by
application. How a partition is implemented should not be used by the
application .
[ People violate this all the time, and I regret that Pulsar did not
provide get_Nth_topicpartion(), which led to people hardcoding it as
topicname-N. and using that directly. Now we are stuck with it.]
Similarly batch index and batch size. Those are all logical concepts
exposed to the user. For eg: batch size is something the app is allowed to
tune
>Even for ledger id and entry id, this pair represents a logic storage
position like the offset concept in Kafka
These are not equivalent. In Pulsar these are implementation details,
while in Kafka those are logical concepts.
One might think that these are logical concepts in Pulsar, because if you
reverse engineer the current msgid implementation, you observe some
"properties".
Ledger id/entry id are logical concepts in __Bookkeeper__ , not in Pulsar.
There is the Managed Ledger abstraction on top of BK, and then there is
Pulsar on top of ML. You will break two levels of abstraction to expose
ledger/entryid to an application
An application should only care about the operations that can be done
with a messageId
- getmsgid() to return the message id as an opaque object
[Operators using one messageId ]
-serde, like tostring(). for storage/retrieval of message identifier
-getter/setter on logical properties of the message (partition id etc...)
-increment/decrement
[Operators that take multiple messageIds]
-comparator
-range
Those are the kind of operators Pulsar should provide to a user.
Applications should not implement these operators on their own by reverse
engineering the msgId. No application should be directly using ledgerid or
entryid for doing anything (math or logic),
As long as Pulsar provides these operations with msgid to the
application, it should not care whether it's represented as "0:1:-1:-1"
or "a:b:-b-b", or "#xba4231!haxcy1826923f" or as a serialized binary
object or..whatever it may be.
>>But it would be harder to know a tuple like "0:1:-1:-1" means.
A user shouldn't have to know what this means. That's the point.
Pulsar itself changed the messageId multiple times as it added
partitioning, batching and so on, and it might do so again. And bookkeeper
could change its representation of ledgers, (for eg, to uuids and byte
offsets) ML could replace BK with something else (for eg. a table in a
db.) Anything is possible - Pulsar would then just have to change the
implementation of the operator functions, and no application needs to be
rewritten.
-j
On Tue, Nov 8, 2022 at 6:05 PM Yunze Xu <y...@streamnative.io.invalid>
wrote:
> Hi Joe,
>
> Then what would we expect users to do with the MessageId? It should only
> be passed to Consumer#seek or ReaderBuilder#startMessageId?
>
> What about the partition index? We have a `TopicMetadata` interface that
> returns
> the number of partitions. If the partition is also "implementation
> details", should we expose
> this interface? Or should we support customizing a MessageRouter because it
> returns the partition index?
>
> What about the batch index and batch size? For example, we have an
> enableBatchIndexAcknowledgment method to enable batch index ACK. If batch
> index is also "implementation details", how could users know what does
> "batch
> index ack" mean?
>
> Even for ledger id and entry id, this pair represents a logic storage
> position like the offset
> concept in Kafka (though each offset represents a message while each
> entry represents
> a batch). If you see the Message API, it also exposes many attributes.
> IMO, for the
> MessageIdData, only the ack_set (a long array serialized from the
> BitSet) is the implementation
> detail.
>
> The MessageId API should be flexible, not an abstract one. If not, why
> do we still implement
> the toString() method? We should not encourage users to print the
> MessageId. It would
> be easy to know what "ledger is 0, entry id is 1" means, users only
> need to know the concepts
> of ledger id and entry id. But it would be harder to know a tuple like
> "0:1:-1:-1" means.
>
> Thanks,
> Yunze
>
> On Tue, Nov 8, 2022 at 11:16 PM Joe F <joefranc...@gmail.com> wrote:
> >
> > >Maybe this design is to hidden some details, but if
> > users don't know the details like ledger id and entry id, how could
> > you know what does "0:0:-1:0" mean?
> >
> > Abstractions exist for a reason. Ledgerid and entryid are
> implementation
> > details, and an application should not be interpreting that at all.
> > -j
> >
> >
> > On Tue, Nov 8, 2022 at 3:43 AM Yunze Xu <y...@streamnative.io.invalid>
> > wrote:
> >
> > > I didn't look into these two methods at the moment. But I think it's
> > > possible to
> > > retain only the `fromByteArray`.
> > >
> > > Thanks,
> > > Yunze
> > >
> > > On Tue, Nov 8, 2022 at 7:02 PM Enrico Olivelli <eolive...@gmail.com>
> > > wrote:
> > > >
> > > > Il giorno mar 8 nov 2022 alle ore 11:52 Yunze Xu
> > > > <y...@streamnative.io.invalid> ha scritto:
> > > > >
> > > > > Hi Enrico,
> > > > >
> > > > > > We also need a way to represent this as a String or a byte[]
> > > > >
> > > > > We already have the `toByteArray` method, right?
> > > >
> > > > Yes, correct. So we are fine. I forgot about it and I answered too
> > > quickly.
> > > >
> > > > I am not sure if this can be in the scope of this initiative, but we
> > > > should somehow get rid of
> > > > stuff like "fromByteArrayWithTopic" vs "fromByteArray".
> > > >
> > > > Thanks
> > > > Enrico
> > > >
> > > > >
> > > > > Thanks,
> > > > > Yunze
> > > > >
> > > > > On Tue, Nov 8, 2022 at 6:43 PM Enrico Olivelli <
> eolive...@gmail.com>
> > > wrote:
> > > > > >
> > > > > > Il giorno mar 8 nov 2022 alle ore 11:25 Yunze Xu
> > > > > > <y...@streamnative.io.invalid> ha scritto:
> > > > > > >
> > > > > > > Hi all,
> > > > > > >
> > > > > > > Currently we have the following 5 implementations of MessageId:
> > > > > > >
> > > > > > > - MessageIdImpl: (ledger id, entry id, partition index)
> > > > > > > - BatchMessageIdImpl: adds (batch index, batch size, acker),
> > > where
> > > > > > > acker is a wrapper of a BitSet.
> > > > > > > - ChunkMessageIdImpl: adds another MessageIdImpl that
> represents
> > > > > > > the first MessageIdImpl of a BitSet.
> > > > > > > - MultiMessageIdImpl: adds a map that maps the topic name to
> the
> > > > > > > MessageId.
> > > > > > > - TopicMessageIdImpl: adds the topic name and the partition
> name
> > > > > > >
> > > > > > > These implementations are such a mess. For example, when users
> get
> > > a
> > > > > > > MessageId from `Producer#send`:
> > > > > > >
> > > > > > > ```java
> > > > > > > var id = producer.send("msg");
> > > > > > > ```
> > > > > > >
> > > > > > > There is no getter to get some specific fields like ledger id.
> You
> > > can
> > > > > > > only see a representation from `toString` method and got some
> > > output
> > > > > > > like "0:0:-1:0". Maybe this design is to hidden some details,
> but
> > > if
> > > > > > > users don't know the details like ledger id and entry id, how
> could
> > > > > > > you know what does "0:0:-1:0" mean? What if
> `MessageId#toString`'s
> > > > > > > implementation changed? Should it be treated as a breaking
> change?
> > > > > > >
> > > > > > > The original definition of the underlying MessageIdData is much
> > > more
> > > > > > > clear:
> > > > > > >
> > > > > > > ```proto
> > > > > > > message MessageIdData {
> > > > > > > required uint64 ledgerId = 1;
> > > > > > > required uint64 entryId = 2;
> > > > > > > optional int32 partition = 3 [default = -1];
> > > > > > > optional int32 batch_index = 4 [default = -1];
> > > > > > > repeated int64 ack_set = 5;
> > > > > > > optional int32 batch_size = 6;
> > > > > > >
> > > > > > > // For the chunk message id, we need to specify the first
> > > chunk message id.
> > > > > > > optional MessageIdData first_chunk_message_id = 7;
> > > > > > > }
> > > > > > > ```
> > > > > > >
> > > > > > > IMO, MessageId should be a wrapper of MessageIdData. It's more
> > > natural
> > > > > > > to have an interface like:
> > > > > > >
> > > > > > > ```java
> > > > > > > interface MessageId {
> > > > > > > long ledgerId();
> > > > > > > long entryId();
> > > > > > > Optional<Integer> partition();
> > > > > > > Optional<Integer> batchIndex();
> > > > > > > // ...
> > > > > > > ```
> > > > > >
> > > > > > This is very good for client applications.
> > > > > > We also need a way to represent this as a String or a byte[],
> this
> > > way
> > > > > > client applications have a standard way to store
> > > > > > message offsets into an external system (for instance when you
> want
> > > to
> > > > > > user the Reader API and keep track of the position by yourself)
> > > > > >
> > > > > > Enrico
> > > > > >
> > > > > > >
> > > > > > > Additionally, there are many places that use only the triple of
> > > > > > > (ledger id, entry id, batch index) as the key to represent the
> > > position.
> > > > > > > Currently, they are done by adding a conversion from
> > > > > > > BatchMessageIdImpl to MessageIdImpl. However, it's more
> intuitive
> > > to
> > > > > > > write something like:
> > > > > > >
> > > > > > > ```java
> > > > > > > class MessageIdPosition implements
> Comparable<MessageIdPosition> {
> > > > > > > private final MessageId messageId;
> > > > > > > // TODO: compare only the triple (ledger, entry, batch)
> > > > > > > ```
> > > > > > >
> > > > > > > Therefore, I'm going to write a proposal to redesign the
> MessageId
> > > > > > > interface only by adding some getters. Regarding the 5 existing
> > > > > > > implementations, I think we can drop them because they are a
> part
> > > > > > > of `pulsar-client`, not `pulsar-client-api`.
> > > > > > >
> > > > > > > Please feel free to share your points.
> > > > > > >
> > > > > > > Thanks,
> > > > > > > Yunze
> > >
>
