Reading Data Records from a CLFS Stream
There are two types of records in a Common Log File System (CLFS) stream: data records and restart records. This topic explains how to read a sequence of data records from a stream. For information about how to read restart records, see Reading Restart Records from a CLFS Stream.
There are several variations on reading a sequence of data records from a stream. You can read forward in the stream from a specified record or you can read backward along a chain of linked records.
For all variations on reading a sequence of data records, complete the following steps.
Call ClfsReadLogRecord to obtain a read context and the first data record in the sequence.
Pass the read context you obtained in step 1 to ClfsReadNextLogRecord repeatedly to obtain the remaining data records in the sequence.
Caution Read contexts are not thread-safe. Clients are responsible for serializing access to read contexts.
The following subtopics discuss the details of reading the different types of record sequences and chains.
Reading forward from a specified data record
To read forward in a CLSF stream (starting at the data record of your choice), you must create a read context that has its mode set to ClfsContextForward. To create a read context and read the first record (in the set that you have chosen to read), call ClfsReadLogRecord as shown in the following table.
| Parameter name | Value |
|---|---|
pvMarshalContext |
Supply a pointer to a marshalling area. |
plsnFirst |
Supply the LSN of the first record you want to read. This must be the LSN of a data record, not a restart record. |
peContextMode |
Supply the value ClfsContextForward. |
ppvReadBuffer |
Receive your record data. |
pcbReadBuffer |
Receive the size of your record data. |
peRecordType |
Receive the record type. This value is a set of flags that indicate various features of the record. The record is a data record, so the value you receive should have the ClfsDataRecord flag set and the ClfsRestartRecord flag clear. |
plsnUndoNext |
Receive the undo-next LSN of the data record. You do not need this value to continue reading the chain, so you can ignore it. |
plsnPrevious |
Receive the previous LSN of the data record. You do not need this value to continue reading the chain, so you can ignore it. |
ppvReadContext |
Receive a pointer to an opaque read context. Use the read context to read subsequent records. |
After you have obtained the read context and the first record, you can obtain subsequent records in the stream by calling ClfsReadNextLogRecord repeatedly. When there are no more data records in the stream, ClfsReadNextLogRecord returns STATUS_END_OF_FILE. The following table shows how to set and interpret the parameters.
| Parameter name | Value |
|---|---|
pvReadContext |
Supply a pointer to the read context you received from ClfsReadLogRecord. |
ppvBuffer |
Receive your record data. |
pcbBuffer |
Receive the size of your record data. |
peRecordType |
Supply the value of ClfsDataRecord. |
plsnUndoNext |
Receive the undo-next LSN field of the data record. You do not need this value to continue reading the chain, so you can ignore it. |
plsnPrevious |
Receive the previous-LSN field of the data record. You do not need this value to continue reading the chain, so you can ignore it. |
plsnRecord |
Receive the LSN of the data record that was read. |
Reading a chain of data records linked by the previous LSN
When you write a data record to a CLFS stream, you can set the previous LSN of the data record to the LSN of any record that you previously wrote to the stream. By setting the previous LSN, you can create a chain of related records that can later be traversed in reverse order. For example, suppose you are performing a database transaction and you must write several CLFS log records to describe the updates made by the transaction. Each time you write a log record that describes a transaction update, you could set the previous LSN of the record to the LSN of the previous log record that describes an update made by the same transaction.
Suppose you have written a chain of data records that are linked by their previous LSNs. To read the chain of records, you must create a read context that has its mode set to ClfsContextPrevious. To create a read context and read the first record in the chain, call ClfsReadLogRecord as shown in the following table.
| Parameter name | Value |
|---|---|
pvMarshalContext |
Supply a pointer to a marshalling area. |
plsnFirst |
Supply the LSN of the first record in the chain. This must be the LSN of a data record, not a restart record. |
peContextMode |
Supply the value of ClfsContextPrevious. |
ppvReadBuffer |
Receive your record data. |
pcbReadBuffer |
Receive the size of your record data. |
peRecordType |
Receive the record type. This value is a set of flags that indicate various features of the record. The record is a data record, so the value you receive should have the ClfsDataRecord flag set and the ClfsRestartRecord flag clear. |
plsnUndoNext |
Receive the undo-next LSN of the data record. You do not need this value to continue reading the chain, so you can ignore it. |
plsnPrevious |
Receive the previous LSN of the data record. You do not need this value to continue reading the chain, so you can ignore it. |
ppvReadContext |
Receive a pointer to an opaque read context. Use the read context to read the previous records in the chain. |
After you have the read context and the first record, you can read the remaining records in the chain by calling ClfsReadNextLogRecord repeatedly. The following table shows how to set and interpret the parameters.
| Parameter name | Value |
|---|---|
pvReadContext |
Supply a pointer to the read context you received from ClfsReadLogRecord. |
ppvBuffer |
Receive your record data. |
pcbBuffer |
Receive the size of your record data. |
peRecordType |
Supply the value of ClfsDataRecord. |
plsnUndoNext |
Receive the undo-next LSN of the data record. You do not need this value to continue reading the chain, so you can ignore it. |
plsnPrevious |
Receive the previous LSN of the data record. You do not need this value to continue reading the chain, so you can ignore it. |
plsnRecord |
Receive the LSN of the data record that was read. |
As you make repeated calls to ClfsReadNextLogRecord, your sequence of calls will end in one of the following ways.
Eventually you will read a data record that has its previous LSN set to CLFS_LSN_INVALID. The next time you call ClfsReadNextLogRecord, it will return STATUS_END_OF_FILE.
Eventually you will read a data record that has a previous LSN that is less than both the base LSN of the stream and the archive tail of the stream. The next time you call ClfsReadNextLogRecord, it will return STATUS_LOG_START_OF_LOG.
Reading a chain of data records linked by the undo-next LSN
When you write a data record to a CLFS stream, you can set the undo-next LSN of the data record to the LSN of any record that you previously wrote to the stream. By setting the undo-next LSN, you can create a chain of related records that can be traversed in reverse order. For more information about creating and interpreting undo-next chains, see CLFS Log Sequence Numbers.
Suppose you have written a chain of data records that are linked by their undo-next LSNs. To read the chain of records, you must call ClfsReadLogRecord to create a read context that has its mode set to ClfsContextUndoNext. After that, the process is identical to reading a chain linked by previous LSNs (described previously in this topic).
Reading a chain of data records linked by the user LSN
In addition to chains linked by previous LSNs and undo-next LSNs, you can create chains linked by your own LSNs that you embed in your record data.
Suppose you have written a chain of data records that are linked by LSNs you have stored in the record data itself. To read the chain of records, you must create a read context that has its mode set to either ClfsContextPrevious or ClfsContextUndoNext. Create your read context and obtain the most recently written record in the chain by calling ClfsReadLogRecord. Then call ClfsReadNextLogRecord repeatedly to obtain the previous records in the chain. Each time you call ClfsReadNextLogRecord, set the plsnUser parameter to the LSN of the previous record in your chain. The LSN you supply in plsnUser overrides any values stored in the current record's previous-LSN or undo-next LSN fields.
Note that you can only move backward in the stream when you call ClfsReadNextLogRecord to read a record chain. The LSN you supply in plsnUser must be less than the LSN of the current record in the chain.