Checkpoint and log files
Checkpoint files contain an image of the database on disk. TimesTen uses dual
checkpoint files for additional safety, in case the system fails while a checkpoint
operation is in progress. Changes to databases are captured in transaction logs
that are written to disk periodically. If a database needs to be recovered,
TimesTen merges the database checkpoint on disk with the completed
transactions that are still in the log files. Normal disk file systems are used for
checkpoints and log files.
For maximum performance, the replication agent detects updates to a database by
monitoring the existing transaction log. It sends updates to the subscribers in
batches, if possible. Only committed transactions are replicated. On the
subscriber node, the replication agent updates the database through an efficient
low-level interface, avoiding the overhead of the SQL layer.
Cache Connect architecture
Cache groups support the following features:
• Applications can read from and write to cache groups.
• Cache groups can be refreshed from Oracle data automatically or manually.
• Updates to cache groups can be propagated to Oracle tables automatically or manually.
• Changes to either Oracle tables or the cache group can be tracked automatically.
A cache group table can contain all or a subset of the rows and columns in the related Oracle table.
Read committed isolation
Timesten的统计信息存在SYS.TBL_STATS and SYS.COL_STATS两张基表中。
TimesTen uses T-tree indexes, which are optimized for main memory access. T-tree indexes are more economical than B-trees
because they require less memory space and fewer CPU cycles.
Multiple NULL values are permitted in a unique T-tree index.
Hash indexes are created for tables with a primary key when the UNIQUE HASH INDEX clause is specified in the CREATE TABLE statement.
There can be only one hash index for each table.
In general, hash indexes are faster than T-tree indexes for exact match lookups and equijoins.
However, hash indexes cannot be used for lookups involving ranges or the prefix of a key
and can require more space than T-tree indexes.
Nested loop join
The TimesTen log is used for the following purposes:
• Redo transactions if a system failure occurs
• Undo transactions that are rolled back
• Replicate changes to other TimesTen databases
• Replicate changes to an Oracle database
• Enable applications to monitor changes to tables through the XLA interface
By default, TimesTen keeps logs in an in-memory transaction log buffer and also stores them on disk.
When are log files deleted?
Log files are kept until TimesTen declares them to be purgeable. A log file
cannot be purged until all of the following actions has been completed:
• All transactions writing log records to the log file (or a previous log file) have committed or rolled back
• All changes recorded in the log file have been written to the checkpoint files on disk
• All changes recorded in the log file have been replicated (if replication is used)
• All changes recorded in the log file have been propagated to Oracle (if Cache Connect is used)
• All changes recorded in log files have been reported to the XLA applications (if XLA is used)
When logging is disabled, transactions cannot be rolled back and only databaselevel
locking is allowed. Disable logging only for operations like bulk loads of tables and only if the following conditions are true:
• The operation can be restarted from the beginning if a failure occurs.
• The application performing the bulk load is the only application connected to the database.
Use the Logging connection attribute to disable logging.
Checkpoints are used to keep a snapshot of the database.
The checkpoint operation scans the database for blocks that
have changed since the last checkpoint. It then updates the checkpoint file with
the changes and removes any log files that are no longer needed.
TimesTen provides two kinds of checkpoints:
Blocking checkpoints (可以通过调用ttCkptBlocking存过来实现，会hold整个库，但是数据库是一致的，不需要恢复)
TimesTen creates nonblocking checkpoints automatically.
Recovery from log and checkpoint files
During recovery, the latest checkpoint file is read into memory and all durably
committed transactions are rolled forward from the appropriate log files.
Uncommitted or rolled-back transactions are not recovered.
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