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From "Hiller, Dean" <Dean.Hil...@nrel.gov>
Subject Re: questions related to the SSTable file
Date Tue, 17 Sep 2013 13:39:48 GMT
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 <java8964@hotmail.com<mailto:java8964@hotmail.com>>
Reply-To: "user@cassandra.apache.org<mailto:user@cassandra.apache.org>" <user@cassandra.apache.org<mailto:user@cassandra.apache.org>>
Date: Tuesday, September 17, 2013 7:32 AM
To: "user@cassandra.apache.org<mailto:user@cassandra.apache.org>" <user@cassandra.apache.org<mailto:user@cassandra.apache.org>>
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: tsato@cloudian.com<mailto:tsato@cloudian.com>
To: user@cassandra.apache.org<mailto:user@cassandra.apache.org>
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


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