You have to first understand the rules of
1. Sstables are immutable so Color-1-Data.db will not be modified and only
deleted once compacted
2. Memtables are flushed when reaching a limit so if Blue:{hex} is modified,
it is done in the in-memory memtable that is eventually flushed
3. Once flushed, it is an SSTable on disk and you have two values for "hex"
both with two timestamps so we know which one is the current value
When it finally compacts, the old value can go away.
Dean
From: java8964 java8964 <[email protected]<mailto:[email protected]>>
Reply-To: "[email protected]<mailto:[email protected]>"
<[email protected]<mailto:[email protected]>>
Date: Tuesday, September 17, 2013 7:32 AM
To: "[email protected]<mailto:[email protected]>"
<[email protected]<mailto:[email protected]>>
Subject: RE: questions related to the SSTable file
Hi, Takenori:
Thanks for your quick reply. Your explain is clear for me understanding what
compaction mean, and I also can understand now same row key will exist in multi
SSTable file.
But beyond that, I want to know what happen if one row data is too large to put
in one SSTable file. In your example, the same row exist in multi SSTable files
as it is keeping changing and flushing into the disk at runtime. That's fine,
in this case, in every SSTable file of the 4, there is no single file contains
whole data of that row, but each one does contain full picture of individual
unit ( I don't know what I should call this unit, but it will be larger than
one column, right?). Just in your example, there is no way in any time, we
could have SSTable files like following, right:
- Color-1-Data.db: [{Lavender: {hex: #E6E6FA}}, {Blue: {hex: #0000}}]
- Color-1-Data_1.db: [{Blue: {hex:FF}}]
- Color-2-Data.db: [{Green: {hex: #008000}}, {Blue: {hex2: #2c86ff}}]
- Color-3-Data.db: [{Aqua: {hex: #00FFFF}}, {Green: {hex2: #32CD32}}, {Blue:
{}}]
- Color-4-Data.db: [{Magenta: {hex: #FF00FF}}, {Gold: {hex: #FFD700}}]
I don't see any reason Cassandra will ever do that, but just want to confirm,
as your 'no' answer to my 2 question is confusion.
Another question from my originally email, even though I may get the answer
already from your example, but just want to confirm it.
Just use your example, let's say after the first 2 steps:
- Color-1-Data.db: [{Lavender: {hex: #E6E6FA}}, {Blue: {hex: #0000FF}}]
- Color-2-Data.db: [{Green: {hex: #008000}}, {Blue: {hex2: #2c86ff}}]
There is a incremental backup. After that, there is following changes coming:
- Add a column of (key, column, column_value = Green, hex2, #32CD32)
- Add a row of (key, column, column_value = Aqua, hex, #00FFFF)
- Delete a row of (key = Blue)
---- memtable is flushed => Color-3-Data.db ----
Another incremental backup right now.
Now in this case, my assumption is only Color-3-Data.db will be in this backup,
right? Even though Color-1-Data.db and Color-2-Data.db contains the data of the
same row key as Color-3-Data.db, but from a incremental backup point of view,
only Color-3-Data.db will be stored.
The reason I asked those question is that I am thinking to use MapReduce jobs
to parse the incremental backup files, and rebuild the snapshot in Hadoop side.
Of course, the column families I am doing is pure Fact data. So there is
delete/update in Cassandra for these kind of data, just appending. But it is
still important for me to understand the SSTable file's content.
Thanks
Yong
________________________________
Date: Tue, 17 Sep 2013 11:12:01 +0900
From: [email protected]<mailto:[email protected]>
To: [email protected]<mailto:[email protected]>
Subject: Re: questions related to the SSTable file
Hi,
> 1) I will expect same row key could show up in both sstable2json output, as
> this one row exists in both SSTable files, right?
Yes.
> 2) If so, what is the boundary? Will Cassandra guarantee the column level as
> the boundary? What I mean is that for one column's data, it will be
> guaranteed to be either in the first file, or 2nd file, right? There is no
> chance that Cassandra will cut the data of one column into 2 part, and one
> part stored in first SSTable file, and the other part stored in second
> SSTable file. Is my understanding correct?
No.
> 3) If what we are talking about are only the SSTable files in snapshot,
> incremental backup SSTable files, exclude the runtime SSTable files, will
> anything change? For snapshot or incremental backup SSTable files, first can
> one row data still may exist in more than one SSTable file? And any boundary
> change in this case?
> 4) If I want to use incremental backup SSTable files as the way to catch data
> being changed, is it a good way to do what I try to archive? In this case,
> what happen in the following example:
I don't fully understand, but snapshot will do. It will create hard links to
all the SSTable files present at snapshot.
Let me explain how SSTable and compaction works.
Suppose we have 4 files being compacted(the last one has bee just flushed, then
which triggered compaction). Note that file names are simplified.
- Color-1-Data.db: [{Lavender: {hex: #E6E6FA}}, {Blue: {hex: #0000FF}}]
- Color-2-Data.db: [{Green: {hex: #008000}}, {Blue: {hex2: #2c86ff}}]
- Color-3-Data.db: [{Aqua: {hex: #00FFFF}}, {Green: {hex2: #32CD32}}, {Blue:
{}}]
- Color-4-Data.db: [{Magenta: {hex: #FF00FF}}, {Gold: {hex: #FFD700}}]
They are created by the following operations.
- Add a row of (key, column, column_value = Blue, hex, #0000FF)
- Add a row of (key, column, column_value = Lavender, hex, #E6E6FA)
---- memtable is flushed => Color-1-Data.db ----
- Add a row of (key, column, column_value = Green, hex, #008000)
- Add a column of (key, column, column_value = Blue, hex2, #2c86ff)
---- memtable is flushed => Color-2-Data.db ----
- Add a column of (key, column, column_value = Green, hex2, #32CD32)
- Add a row of (key, column, column_value = Aqua, hex, #00FFFF)
- Delete a row of (key = Blue)
---- memtable is flushed => Color-3-Data.db ----
- Add a row of (key, column, column_value = Magenta, hex, #FF00FF)
- Add a row of (key, column, column_value = Gold, hex, #FFD700)
---- memtable is flushed => Color-4-Data.db ----
Then, a compaction will merge all those fragments together into the latest ones
as follows.
- Color-5-Data.db: [{Lavender: {hex: #E6E6FA}, {Aqua: {hex: #00FFFF}, {Green:
{hex: #008000, hex2: #32CD32}}, {Magenta: {hex: #FF00FF}}, {Gold: {hex:
#FFD700}}]
* assuming RandomPartitioner is used
Hope they would help.
- Takenori
(2013/09/17 10:51), java8964 java8964 wrote:
Hi, I have some questions related to the SSTable in the Cassandra, as I am
doing a project to use it and hope someone in this list can share some thoughts.
My understand is the SSTable is per column family. But each column family could
have multi SSTable files. During the runtime, one row COULD split into more
than one SSTable file, even this is not good for performance, but it does
happen, and Cassandra will try to merge and store one row data into one SSTable
file during compassion.
The question is when one row is split in multi SSTable files, what is the
boundary? Or let me ask this way, if one row exists in 2 SSTable files, if I
run sstable2json tool to run on both SSTable files individually:
1) I will expect same row key could show up in both sstable2json output, as
this one row exists in both SSTable files, right?
2) If so, what is the boundary? Will Cassandra guarantee the column level as
the boundary? What I mean is that for one column's data, it will be guaranteed
to be either in the first file, or 2nd file, right? There is no chance that
Cassandra will cut the data of one column into 2 part, and one part stored in
first SSTable file, and the other part stored in second SSTable file. Is my
understanding correct?
3) If what we are talking about are only the SSTable files in snapshot,
incremental backup SSTable files, exclude the runtime SSTable files, will
anything change? For snapshot or incremental backup SSTable files, first can
one row data still may exist in more than one SSTable file? And any boundary
change in this case?
4) If I want to use incremental backup SSTable files as the way to catch data
being changed, is it a good way to do what I try to archive? In this case, what
happen in the following example:
For column family A:
at Time 0, one row key (key1) has some data. It is being stored and back up in
SSTable file 1.
at Time 1, if any column for key1 has any change (a new column insert, a column
updated/deleted, or even whole row being deleted), I will expect this whole row
exists in the any incremental backup SSTable files after time 1, right?
What happen if the above row just happen to store in more than one SSTable file?
at Time 0, one row key (key1) has some data, and it just is stored in SSTable
file1 and file2, and being backup.
at Time 1, if one column is added in row key1, and the change in fact will
happen in SSTable file2 only in this case, and if we do a incremental backup
after that, what SSTable files should I expect in this backup? Both SSTable
files? Or Just SSTable file 2?
I was thinking incremental backup SSTable files are good candidate for catching
data being changed, but as one row data could exist in multi SSTable file makes
thing complex now. Did anyone have any experience to use SSTable file in this
way? What are the lessons?
Thanks
Yong
