đź‘Ť Best Regards.
Jia Zhai Beijing, China Mobile: +86 15810491983 On Mon, Nov 9, 2020 at 2:24 PM Aloys Zhang <lofterzh...@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. >
