CommitLog, MemTable, and SSTable are 3 core components of cassandra, they work in tandom to gurantee durability aspect of cassandra, this is more of less similar to RDBMS databases where in commit logs are used to replay a transaction in case of of db server crash.
Here is a very basic high level life cycle of the data from the time it is written from a cassandra client to the time it is persisted to SSTables.
The complexity is slightly higher in the cluster setup, however, for starters this is good enough to understand the internals of Casandra write and update flow.
Cassandra tool "nodetool" provides an option to explicitly flush the data in commit log or Memtable into SSTables,
This tool is supposed to be used for maintenance, however it is a nice utility that can be used during maintenance to ensure all pending transactions are
flushed out to the SSTables before a node shutdown.
Let us put cassandra's durability to test with a real world ecommerce use case.
Let us assume we have a keyspace that manages user cart or orders and given a scenario of node failure let us put cassandra's durability to test.
We will do following.
Load some sample data into cassandra and shutdown the databased before cassandra performs a flush to SSTables.
Make sure cassandra server is running, start by using following command in the foreground ./bin/cassandra -f
Once the data is loaded you can exit cqlsh and check your data directory, the location of data directory is defined in ./config/cassandra.yaml files "data_file_directories" key property.
E..g if mapped to your home directory, go to ~/cassandra/data/ecommerce/orders and you should not notice any files in this directory, usually you will find a couple of files related to SSTables in this location once the flush operation is completed.
We can terminate cassandra at this point to replicate a situation where in cassandra data is not yet flused to SSTables and is only available with in the commit log and Memtables in memory data store.
Now you can bring up cassandra and you should notice few interesting log messages indicating a replay of pending records from commit log to SSTables.
Once this operation is complete check the ~/cassandra/data/ecommerce/orders folders and you should not see the data inserted before the server crash.
or you can also check the data in sstables using the ./bin/sstables utility
Further Reading
https://wiki.apache.org/cassandra/MemtableSSTable
Here is a very basic high level life cycle of the data from the time it is written from a cassandra client to the time it is persisted to SSTables.
The complexity is slightly higher in the cluster setup, however, for starters this is good enough to understand the internals of Casandra write and update flow.
Step 1: Request is received by a random node in the cluster
Step 2: Node Writes data into the local commit log file in a sequential manner.
Step 3: Memtable gets updated in asynchronous mode.
Step 4: Memtable flushes the data to SSTables periodically, SStables is really the final persistance store for the data.
Step 5: Once data makes it way to SSTables the corresponding reference of the record in commit log and Memtable is flushed out.
Step 2: Node Writes data into the local commit log file in a sequential manner.
Step 3: Memtable gets updated in asynchronous mode.
Step 4: Memtable flushes the data to SSTables periodically, SStables is really the final persistance store for the data.
Step 5: Once data makes it way to SSTables the corresponding reference of the record in commit log and Memtable is flushed out.
Cassandra tool "nodetool" provides an option to explicitly flush the data in commit log or Memtable into SSTables,
This tool is supposed to be used for maintenance, however it is a nice utility that can be used during maintenance to ensure all pending transactions are
flushed out to the SSTables before a node shutdown.
Let us put cassandra's durability to test with a real world ecommerce use case.
Let us assume we have a keyspace that manages user cart or orders and given a scenario of node failure let us put cassandra's durability to test.
We will do following.
Load some sample data into cassandra and shutdown the databased before cassandra performs a flush to SSTables.
Make sure cassandra server is running, start by using following command in the foreground ./bin/cassandra -f
Once the data is loaded you can exit cqlsh and check your data directory, the location of data directory is defined in ./config/cassandra.yaml files "data_file_directories" key property.
E..g if mapped to your home directory, go to ~/cassandra/data/ecommerce/orders and you should not notice any files in this directory, usually you will find a couple of files related to SSTables in this location once the flush operation is completed.
We can terminate cassandra at this point to replicate a situation where in cassandra data is not yet flused to SSTables and is only available with in the commit log and Memtables in memory data store.
Now you can bring up cassandra and you should notice few interesting log messages indicating a replay of pending records from commit log to SSTables.
Once this operation is complete check the ~/cassandra/data/ecommerce/orders folders and you should not see the data inserted before the server crash.
Completed flushing /home/search/cassandra/data/system/compaction_history/system-compaction_history-jb-1-Data.db (237 bytes) for commitlog position ReplayPosition(segmentId=1395543674692, position=271)
You should be able to query the same from cqlsh as well.
cqlsh> select * from ecommerce.orders;
orders_id | users_id | emails | first_name | last_name | order_comments | order_log | order_status | order_total | promotions_total | shipping_total | tax_total
-----------+----------+--------------------------------+------------+-----------+------------------------------------------------------------------------------------+----------------------------------------------------------------------------------------+--------------+-------------+------------------+----------------+-----------
4321 | 1234 | {'a@gmail.com', 'b@gmail.com'} | hariharan | vadivelu | {'comment_1': '2013-06-13 11:42:12-0500', 'comment_2': '2013-06-13 11:42:12-0500'} | {'created_on': '2013-06-13 11:42:12-0500', 'last_updated': '2013-06-13 11:42:12-0500'} | Pending | 20.3 | 5 | 2 | 1
321 | 123 | {'a@gmail.com', 'b@gmail.com'} | hariharan | vadivelu | {'comment_1': '2013-06-13 11:42:12-0500', 'comment_2': '2013-06-13 11:42:12-0500'} | {'created_on': '2013-06-13 11:42:12-0500', 'last_updated': '2013-06-13 11:42:12-0500'} | Pending | 20.3 | 5 | 2 | 1
(2 rows)
or you can also check the data in sstables using the ./bin/sstables utility
./sstable2json /home/search/cassandra/data/ecommerce/orders/ecommerce-orders-jb-1-Data.db
[
{"key": "000010e1","columns": [["1234:","",1395543482883000], ["1234:emails","1234:emails:!",1395543482882999,"t",1395543482], ["1234:emails:6140676d61696c2e636f6d","",1395543482883000], ["1234:emails:6240676d61696c2e636f6d","",1395543482883000], ["1234:first_name","hariharan",1395543482883000], ["1234:last_name","vadivelu",1395543482883000], ["1234:order_comments","1234:order_comments:!",1395543482882999,"t",1395543482], ["1234:order_comments:636f6d6d656e745f31","0000013f3e6a76a0",1395543482883000], ["1234:order_comments:636f6d6d656e745f32","0000013f3e6a76a0",1395543482883000], ["1234:order_log","1234:order_log:!",1395543482882999,"t",1395543482], ["1234:order_log:637265617465645f6f6e","0000013f3e6a76a0",1395543482883000], ["1234:order_log:6c6173745f75706461746564","0000013f3e6a76a0",1395543482883000], ["1234:order_status","Pending",1395543482883000], ["1234:order_total","20.3",1395543482883000], ["1234:promotions_total","5.0",1395543482883000], ["1234:shipping_total","2.0",1395543482883000], ["1234:tax_total","1.0",1395543482883000]]},
{"key": "00000141","columns": [["123:","",1395543482804000], ["123:emails","123:emails:!",1395543482803999,"t",1395543482], ["123:emails:6140676d61696c2e636f6d","",1395543482804000], ["123:emails:6240676d61696c2e636f6d","",1395543482804000], ["123:first_name","hariharan",1395543482804000], ["123:last_name","vadivelu",1395543482804000], ["123:order_comments","123:order_comments:!",1395543482803999,"t",1395543482], ["123:order_comments:636f6d6d656e745f31","0000013f3e6a76a0",1395543482804000], ["123:order_comments:636f6d6d656e745f32","0000013f3e6a76a0",1395543482804000], ["123:order_log","123:order_log:!",1395543482803999,"t",1395543482], ["123:order_log:637265617465645f6f6e","0000013f3e6a76a0",1395543482804000], ["123:order_log:6c6173745f75706461746564","0000013f3e6a76a0",1395543482804000], ["123:order_status","Pending",1395543482804000], ["123:order_total","20.3",1395543482804000], ["123:promotions_total","5.0",1395543482804000], ["123:shipping_total","2.0",1395543482804000], ["123:tax_total","1.0",1395543482804000]]}
]
https://wiki.apache.org/cassandra/MemtableSSTable
