Paper Note: Omid, Reloaded: Scalable and Highly-Available Transaction Processing

An entirely re-designed version of Omid1.

Omid's job is to manage the transaction control plane.

The transaction metadata includes:

• A dedicated table(Commit Tabla, CT) holds a single record per committing transaction.
• Per-row metadata for items accessed by transactions.

Feature: A middleware oriented.

Snapshot isolate should provide:

• Visibility check rules.
• Write conflicts detection.

01Visibility Check Rules#

Intuitively, with SI, a transaction's reads all appear to occur the time when it begins() while its writes appear to execute when it commits().

Omid uses as the version number.

To commit a transaction, the TM writes the pair to Commit Table(CT), which makes the transaction durable, and is considered its commit point.

When running visibility checks, client get and directly from the data table:

• version is just the txid.
• cf is the corresponding time when the transaction commits.

Following a commit, the transaction updates the commit fields of its written data items with its , and then removes itself from the CT.

02Write Conflict Detection#

This is done by the TM(Transaction Manager).

When a transaction wants to commit, it sends its txid and write set to TM. TM runs a CONFLICTDETECT function which checks for conflicts using a hash table in main memory.

Here comes two problems:

• Find and replace operation needs to be atomic, but locking the whole table will severely limit concurrency.

  • The paper limits the granularity of atomicity to a single bucket.

• The table may be too big to fit in the main memory.

  • Each bucket holds a fixed array of the most recent pairs. Like Omid1, reduce the table size while increasing the possibility of false aborts.

03High Availability#

This is achieved via the primary-backup paradigm: during normal operation, a single primary TM handles client requests, while a backup TM runs in hot standby mode. Upon detecting the primary’s failure, the backup performs a failover and becomes the new primary.