com.twitter.distributedlog

Class LogRecordWithDLSN

java.lang.Object

com.twitter.distributedlog.LogRecord

com.twitter.distributedlog.LogRecordWithDLSN

public class LogRecordWithDLSN
extends LogRecord

Log record with DLSN and SequenceId.

Sequence Numbers

A log record will be assigned with an unique system generated sequence number DLSN when it is written to a log. At the mean time, a 64-bits long number is assigned to the record indicates its position within a log, which is called SequenceId. Besides DLSN and SequenceID, application can assign its own sequence number (called TransactionID) to the log record while writing it.

Transaction ID

Transaction ID is a positive 64-bits long number that is assigned by the application. It is a very helpful field when application wants to organize the records and position the readers using their own sequencing method. A typical use case of TransactionID is DistributedLog Write Proxy. It assigns the non-decreasing timestamps to log records, which the timestamps could be used as `physical time` to implement `TTL` in a strong consistent database to achieve consistent `TTL` over replicas.

DLSN

DistributedLog Sequence Number (DLSN) is the sequence number generated during written time. It is comparable and could be used to figure out the order between records. The DLSN is comprised with 3 components. They are Log Segment Sequence Number, Entry Id and Slot Id. (See DLSN for more details). The DLSN is usually used for comparison, positioning and truncation.

Sequence ID

Sequence ID is introduced to address the drawback of DLSN, in favor of answering questions like `how many records written between two DLSNs`. It is a 64-bits monotonic increasing number (starting from zero). Sequence ids are only accessible by readers. That means writers don't know the sequence ids of records after they wrote them.

Nested Class Summary

Nested classes/interfaces inherited from class com.twitter.distributedlog.LogRecord

LogRecord.Reader, LogRecord.Writer

Field Summary

Fields inherited from class com.twitter.distributedlog.LogRecord

MAX_LOGRECORD_SIZE, MAX_LOGRECORDSET_SIZE

Constructor Summary

Constructors

Constructor and Description
LogRecordWithDLSN(DLSN dlsn, long txid, byte[] data, long startSequenceIdOfCurrentSegment)

Method Summary

Methods

Modifier and Type	Method and Description
DLSN	getDlsn() Get the DLSN of the record in the log.
long	getSequenceId() Get the sequence id of the record in the log.
String	toString()

Methods inherited from class com.twitter.distributedlog.LogRecord

getMetadata, getPayload, getPayLoadInputStream, getPositionWithinLogSegment, getTransactionId, isControl, isControl, isRecordSet, isRecordSet, readPayload, setControl, setMetadata, setRecordSet, setTransactionId

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Constructor Detail

LogRecordWithDLSN

public LogRecordWithDLSN(DLSN dlsn,
                 long txid,
                 byte[] data,
                 long startSequenceIdOfCurrentSegment)

Method Detail

getSequenceId

public long getSequenceId()

Get the sequence id of the record in the log.

Returns:

sequence id of the record in the log.

getDlsn

public DLSN getDlsn()

Get the DLSN of the record in the log.

Returns:

DLSN of the record in the log.

toString

public String toString()

Overrides:

toString in class Object