I think protobuf has the ability to check if a field is enabled. i.e. RAW_METADATA_MAGIC_NUMBER and RAW_METADATA_SIZE are included in the protobuf-ed struct. In Kafka, a magic number represents the version of protocol not if the feature is enabled. If we need a *real* magic number, we must make it clear.
On 2020/11/09 06:24:18, Aloys Zhang <l...@gmail.com> wrote: > Hi all,> > > We have drafted a proposal for supporting lightweight raw Message metadata> > which can be found at> > https://github.com/apache/pulsar/wiki/PIP-70%3A-Introduce-lightweight-raw-Message-metadata> > > and> > https://docs.google.com/document/d/1IgnF9AJzL6JG6G4EL_xcoQxvOpd7bUXcgxFApBiPOFY> > > > Also， I copy it to the email thread for easier viewing.> > > Any suggestions or ideas are welcomed to join the discussion.> > > > > ## PIP-70： Introduce lightweight raw Message metadata> > > ### 1. Motivation> > > For messages in Pulsar, If we want to add new property, we always change> > the `MessageMetadata` in protocol(PulsarApi.proto), this kind of property> > could be understood by both the broker side and client side by> > deserializing the `MessageMetadata` . But in some different cases,, the> > property needs to be added from the broker side, Or need to be understood> > by the broker side in a low cost way. When the broker side gets the message> > produced from the client, we could add the property at a new area, which> > does not combine with `MessageMetadata`, and no need deserializing original> > `MessageMetadata` when gets it out ; and when the broker sends the message> > to client, we could choose to filter out this part of property(or not as> > the client needs). We call this kind of property “raw Message metadata”. By> > this way, the “raw Message metadata” consumption is independent, and not> > related with the original `MessageMetadata`.> > > The benefit for this kind of “raw Message metadata” is that the broker does> > not need to serialize/deserialize for the protobuf-ed `MessageMetadata`,> > this will provide a better performance. And also could provide a lot of> > features that are not supported yet.> > > Here are some of the use cases for raw Message metadata:> > 1) Provide ordered messages by time(broker side) sequence to make message> > seek by time more accurate.> > Currently, each message has a `publish_time`, it uses client side time, but> > for different producers in different clients, the time may not align> > between clients, and cause the message order and the message time> > (`publish_time`) order may be different. But each topic-partition only has> > one owner broker, if we append broker side time in the “raw Message> > metadata”, we could make sure the message order is aligned with broker side> > time. With this feature, we could handle the message seek by time more> > accurately.> > > 2) Provide continuous message sequence-Id for messages in one> > topic-partition.> > MessageId is a combination of ledgerId+entryId+batchIndex; for a partition> > that contains more than one ledger, the Ids inside is not continuous. By> > this solution, we could append a sequence-Id at the end of each Message.> > This will make the message sequence management earlier.> > > In this proposal, we will take count in the first feature “provide ordered> > message by time(broker side) sequence” mentioned above, this will be easier> > to go through the proposal.> > > ### 2. Message and “raw Message metadata” structure changes.> > > As mentioned above, there are 2 main benefits in this proposal:> > > 1. Most of all the change happened on the Broker side.> > 2. Avoid to serialize/deserialize for the protobuf-ed `MessageMetadata`.> > > #### 2.1 Raw Message metadata structure in Protobuf> > > Protobuf used a lot in Pulsar, we could use Protobuf to do the raw Message> > metadata serialize/deserialize.> > In this example, we will save the broker side timestamp when each message> > is sent from the broker to BookKeeper. So the definition is very simple.> > > ```protobuf> > message RawMessageMetadata {> > optional uint64 broker_timestamp = 1;> > }> > ```> > > #### 2.2 Message and “raw Message metadata” structure details> > > Each message is send from producer client to broker in this frame format:> > > ```> > [TOTAL_SIZE] [CMD_SIZE][CMD] [MAGIC_NUMBER] [CHECKSUM] [METADATA_SIZE]> > [METADATA] [PAYLOAD]> > ```> > > The first 3 fields “[TOTAL_SIZE] [CMD_SIZE ] [CMD]” will be read in> > `LengthFieldBasedFrameDecoder` and `PulsarDecoder`, and left the rest part> > handled in method> > `org.apache.pulsar.broker.service.Producer.publishMessage`. The left part> > “[MAGIC_NUMBER] [CHECKSUM] [METADATA_SIZE] [METADATA] [PAYLOAD]” is usually> > treated as “headersAndPayload” in the code. As described above, we do not> > want this part to be changed at all, so we could take this part as a whole> > package.> > > ```> > [MAGIC_NUMBER] [CHECKSUM] [METADATA_SIZE] [METADATA] [PAYLOAD] ==>> > [HEADERS_AND_PAYLOAD]> > ```> > > We need to add some fields to make ”Raw Message metadata” work well.> > > ```> > [RAW_METADATA_MAGIC_NUMBER] [RAW_METADATA_SIZE] [RAW_METADATA]> > [HEADERS_AND_PAYLOAD]> > ```> > > RAW_METADATA_MAGIC_NUMBER is used to identify whether this feature is on,> > and the message is handled by this feature.> > > RAW_METADATA_SIZE is added for this feature, and records the size of the> > serialised raw Message metadata content.> > > RAW_METADATA is the serialised raw Message metadata content.> > > “HEADERS_AND_PAYLOAD” is the original ByteBuf data that contains metadata> > and payload.> > > We put the new added fields before instead of after "HEADERS_AND_PAYLOAD"> > here, this is because> > > - Firstly , the “raw Message metadata” is an addition for origin protocol> > and it can support this new feature without much modification for origin> > wire protocol, especially it does not need to serialize/deserialize for the> > protobuf-ed `MessageMetadata` build from producer.> > - Secondly, if we put new fields after "HEADERS_AND_PAYLOAD" , we can't> > know where the offset for new added fields since we don't know the the> > length of "PAYLOAD", and also, fields after "PAYLOAD" will change the CRC32> > checksum which means we need to recalculate the checksum one more time.> > > #### 2.3 Message and “raw Message metadata” lifecycle> > > The message process logic would be like this:> > > 1. Producer client send the original message to broker, and parsed by> > “PulsarDecoder”, only “[HEADERS_AND_PAYLOAD]” left;> > 2. originally, “[HEADERS_AND_PAYLOAD]” will be write into BookKeeper by> > method `asyncAddEntry` in `ManagedLedger`; but now we first add above> > “[RAW_METADATA]” and related parts along with “[HEADERS_AND_PAYLOAD]”, then> > write the whole ByteBuf into BookKeeper by `asyncAddEntry` ; so the "raw> > Message metadata" is kept together with original message in BookKeeper.> > 3. When some features need to use this “raw Message metadata”, read the new> > BookKeeper “Entry”, and get the raw Message metadata information to serve> > the new feature.> > 4. In this “provide ordered message by time(broker side) sequence” feature,> > when a seek-by-time operation passes into the broker, we readout> > “broker_timestamp” from “[RAW_METADATA]”.> > 5. After brokers read BookKeeper Entry out, and before send to the consumer> > client, we need to filter out the raw Message metadata related part, and> > only return “[HEADERS_AND_PAYLOAD]” part.> > > By this way, we don't need to serialize/deserialize for the protobuf-ed> > `MessageMetadata`.> > > For more details, in these steps:> > > ##### a)) produced message handle and store> > > Producer client side message send to broker will be handled by method> > > ```> > publishMessageToTopic(ByteBuf headersAndPayload, long sequenceId, long> > batchSize, boolean isChunked)> > ```> > > in class `org.apache.pulsar.broker.service.Producer`. And it will finally> > call into the method `asyncAddEntry` of `ManagedLedger`, it will write the> > passed in serialized message as an Entry stored in BookKeeper.> > > ```> > void asyncAddEntry(ByteBuf headersAndPayload, AddEntryCallback callback,> > Object ctx)> > ```> > > For raw Message metadata content, we need to put the current broker> > timestamp in `message RawMessageMetadata` and use protobuf to serialize it> > into `ByteBuf`. Then we put the size and content of serialized `ByteBuf`> > along with original `ByteBuf headersAndPayload`.> > > Then “[HEADERS_AND_PAYLOAD]” turned into “[RAW_METADATA_MAGIC_NUMBER]> > [RAW_METADATA_SIZE] [RAW_METADATA] [HEADERS_AND_PAYLOAD]” and stored in> > BookKeeper.> > > ##### b)) seek by time happens> > > Originally, when a seek_by_time comes in, it will finally calls into class> > > `org.apache.pulsar.broker.service.persistent.PersistentMessageFinder.findMessages`> > > and the original logic is like this> > > ```java> > public void findMessages(final long timestamp,> > AsyncCallbacks.FindEntryCallback callback) {> > …> > > cursor.asyncFindNewestMatching(ManagedCursor.FindPositionConstraint.SearchAllAvailableEntries,> > > entry -> {> > MessageImpl msg = null;> > msg = MessageImpl.deserialize(entry.getDataBuffer()); < ===> > deserialize> > return msg.getPublishTime() < timestamp;> > }, this, callback);> > …> > ```> > > Find message through the topic-partition, and read out the target Entry> > from BookKeeper, deserialize Entry into a MessageImpl and read the message> > publish time that match the requirements; if not match, iterate to another> > round of read another message.> > > By the new design, after reading out the target Entry from BookKeeper, it> > does not need to do the deserialize, but only need to read out the> > “broker_timestamp” from the “raw Message metadata”.> > As mentioned above, the frame stored in BookKeeper is as :> > “[RAW_METADATA_MAGIC_NUMBER] [RAW_METADATA_SIZE] [RAW_METADATA]> > [HEADERS_AND_PAYLOAD]”,> > > 1. Still use `entry.getDataBuffer()` to get the ByteBuf out;> > 2. Read and check “[RAW_METADATA_MAGIC_NUMBER]” to see if “raw Message> > metadata”feature is enabled; if not enabled, turn into old behaviour;> > 3. If enabled, read “ [RAW_METADATA_SIZE] “, then read “[RAW_METADATA] ”.> > 4. Use protobuf to deserialize “[RAW_METADATA]”, and read> > “broker_timestamp” out.> > > ##### c)) read an entry from BookKeeper and send it to the consumer client> > > The main logic happened in method `sendMessages` in class> > `org.apache.pulsar.broker.service.Consumer`, the change is similar to above> > “seek by time”, after read BookKeeper Entry, we need to filter out> > "RAW_METADATA" related part, and only return “[HEADERS_AND_PAYLOAD]” to> > consumer client.> > > ### 3 Summary of changes that need> > > Here is a summary of above changes that related:> > > 1. Add raw Message metadata protobuf> > > ```protobuf> > message RawMessageMetadata {> > optional uint64 broker_timestamp = 1;> > }> > ```> > > 2. change how produced message is saved in bookkeeper:> > `org.apache.pulsar.broker.service.Producer`> > > 3. change how message is seek-by-time:> > > > `org.apache.pulsar.broker.service.persistent.PersistentMessageFinder.findMessages`> > > > 4. change how message send back to Consumer> > `org.apache.pulsar.broker.service.Consumer`> > > other changes:> > > 1. A broker config to enable this raw Message metadata feature. e.g.> > “useBrokerTimestampsForMessageSeek”.> > 2. besides change> > `org.apache.pulsar.broker.service.persistent.PersistentMessageFinder.findMessages`,> > > search other places,> > which called `MessageImpl.deserialize(entry.getDataBuffer())` and> > `msg.getPublishTime()`, such as `PersistentTopic.isOldestMessageExpired()`> > to read message publish-time from new “raw Message metadata” instead of> > deserialize the whole BookKeeper Entry into Message and call> > `Message.getPublishTime()` if this feature enabled.> > > > ### 4. Compatibility, Deprecation and Migration Plan> > > For the first feature, since most of the changes are internally changes in> > brokers, and it is a new feature which doesn’t change pulsar’s wire> > protocol(between broker and client), and not changed public api. There is> > no backward compatibility issue. It is a newly added feature. So there is> > nothing to deprecate or migrate.> > > In the future, we will consider the compatibility when implementing the> > secondary feature of “continuous message sequence-Id” since we should send> > messages with “sequence_id” to consumers. In this case, we will take> > different actions dependent on the data format, feature enable or not and> > consumer client version . Data is new format if it contains> > "[RAW_METADATA_MAGIC_NUMBER][RAW_METADATA_SIZE][RAW_METADATA]" and consumer> > is a new version if it can process message contains "[RAW_METADATA]".> > > ### 5. Test Plan> > > * Unit tests for each individual change: broker/client worked well for> > produce/consume.> > * Integration tests or UT for end-to-end pipeline: multi producers set> > message's time in random time order; seek-by-time could seek to the right> > message.> > * Load testing for ensuring performance: seek should be more quick.> > > > -- > > Best,> > Aloys.> >
