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From: Apache Wiki <wikidiffs@apache.org>
To: Apache Wiki <wikidiffs@apache.org>
Date: Sun, 28 Feb 2010 20:10:26 -0000
Message-ID: <20100228201026.29618.45150@eos.apache.org>
Subject: 
 =?utf-8?q?=5BCassandra_Wiki=5D_Update_of_=22ArchitectureCommitLog=22_by_J?=
 =?utf-8?q?onathanEllis?=

Dear Wiki user,

You have subscribed to a wiki page or wiki category on "Cassandra Wiki" for=
 change notification.

The "ArchitectureCommitLog" page has been changed by JonathanEllis.
The comment on this change is: explain RowMutation and CLContext roles.
http://wiki.apache.org/cassandra/ArchitectureCommitLog?action=3Ddiff&rev1=
=3D3&rev2=3D4

--------------------------------------------------

- Each !CommitLog has a !CommitLogHeader. A !CommitLogHeader has one entry =
per !ColumnFamily. Each entry has a dirty bit and a replay offset, indicati=
ng the position in the !CommitLog file to start replaying the log for a par=
ticular !ColumnFamily.
+ The !CommitLog class manages the !CommitLogSegments, each of which corres=
ponds to a file on disk containing a fixed-size !CommitLogHeader followed b=
y serialized !RowMutation objects.
+ =

+ A !CommitLogHeader has one entry per !ColumnFamily, consisting of a dirty=
 bit and a replay offset, indicating the position in the !CommitLog file to=
 start replaying the log for a particular !ColumnFamily.
  =

  Each insertion (deletion) has to first write a log entry to the !CommitLo=
g.
  =

   * The writing of all log entries is handled by a single thread in !Commi=
tLogExecutorService.
+  * For the first insert to a given !ColumnFamily CF in each !CommitLogSeg=
ment, the !CommitLogHeader is updated: the CF's dirty bit is turned on and =
the replay offset for CF in the !CommitLogHeader is updated with the curren=
t position (represented by a !CommitLogContext object) in the CommitLog fil=
e.
+  * A !RowMutation entry is then appended to the !CommitLogSegment
-  * For each insert, the !CommitLogHeader is first updated, if necessary.
-   * For each !ColumnFamily CF to be inserted, if the dirty bit for CF in =
the !CommitLogHeader is off, the dirty bit is turned on and the replay offs=
et for CF in the !CommitLogHeader is updated with the current position in t=
he CommitLog file.
-  * A log entry is then added to the tail of the !CommitLog file.
-  * If !CommitLogSync is set to batch, the insertion further waits until t=
he !CommitLog file is sync-ed to disk before the insert is allowed to proce=
ed
+  * If !CommitLogSync is set to batch, the insertion further waits until t=
he !CommitLogSegment is sync-ed to disk before the insert is allowed to pro=
ceed
-  * Once a !CommitLog file (!CommitLogSegment) becomes too large, a new se=
gment is created and new operations are appended there instead.
+  * Once a !CommitLogSegment becomes too large, a new segment is created a=
nd new operations are appended there instead.
  =

- On the completion of a flush for a !ColumnFamily,
+ On the completion of a flush for a !ColumnFamily CF,
  =

   * For each !CommitLogSegment F generated when or before the flush is ini=
tiated,
    * If F is not the one being used when the flush was initated, the dirty=
 bit for CF in the !CommitLogHeader of F is turned off