Details
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New Feature
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Status: Closed
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Minor
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Resolution: Fixed
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None
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None
Description
We need to be able to perform a series of reads from and writes to a single row without any potential interference from other clients.
Unfortunately this is a bit involved because normal reads currently do not acquire row locks, so it requires adding additional get/getRow calls that obtain and release a row lock.
In addition, there will be two additional client calls, lockRow/unlockRow, which actually acquire and release the locks. Though each lock is associated with an HRegion, this will be tracked within the HRegionServer. When a lock is acquired from the client, it is handled much like a Scanner. We obtain the row lock from the HRegion, store the region name and lock identifier in a synchronized Map, and also obtain a lease to ensure that the lock will eventually be released even if the client dies.
This also required adding a RowLockListener (implements LeaseListener) private class in HRS to handle row lock lease expiration.
HRS.lockRow will return a long lockId (as openScanner does) that will be used in subsequent client calls to reuse this existing row lock. These calls will check that the lock is valid and perform the operations without any locking (wrappers around get*, new versions of batchUpdate, openScanner, etc).
This is going to really add some noise to the list of available HTable/client methods so I'm not sure if it's something people would want to commit into a normal release. Regardless this does provide some very convenient functionality that may be useful to others.
We are also looking into Clint Morgan's HBASE-669, but one major downside is that there is a significant amount of overhead involved. This row locking is already built in and this will only extend the API to allow clients to work with them directly. There is little to no overhead at all. The only (obvious) performance consideration is that this should only used where necessary as rows will not be locked and unlocked as quickly with round-trip client calls. In our design, we will have specific notes in our schema about which tables (or even which families or columns) must be accessed with row locks at all times and which do not.
This is my first attempt at adding any additional functionality, so comments, criticism, code reviews are encouraged.
I should have a patch up tomorrow.
