Next: , Previous: Split keys, Up: B-tree Structure and Access


2.1.5 Insertion method

Insertion in a index needs to appear atomic so that the key-order invariant is maintained. Since we are using key/value-ordered index blocks, parent-update insertion has a special extra step: we insert the new key and pointer in index block B, then swap value fields with the next entry in sequence. Unfortunately, when the insertion is the end of the block, the next entry is in the next block B', so two blocks must be modified. (The advantage of value/key ordering is that insertion never modifies more than one block; however, you get screwed in the code for deletions instead.) The code checks for this case and locks both blocks before proceeding. (An alternate solution considered is to back up the split key of B so that the new key would go at the front of B', but that introduces other problems.)

While the above method preserves correctness (even under concurrency), there is unfortunately a 2nd-order screw WHICH STILL NEEDS TO BE IMPLEMENTED: how to write the 2 blocks in an order that preserves correctness in case of disk crash. If only B is written, the database will contain 2 pointers to the block whose split caused the insertion; if only B' is written, the pointer correctness will be violated. (It would seem that this kind of problem is inherent to any operation that needs to maintain consistent changes across multiple blocks.) The fix seems to be to surround the write with a notation in the root that says this special case is being exercised and indicates what blocks were being modified (B,B') and what the pointers should be:

We elect to write out block B' first for consistency with the case where B' is a new block created by a block split. The screw case will create a damaged DB, but the danger of deleting a doubly-pointed-at block is avoided. This is adequate information to recover directly. (JONATHAN?)

(Other alternatives were considered. Backing up the split key at next level allows a subsequent insertion to be atomic, but the backing-up operation has the same two-block consistency problem in keeping ITS parent index consistent. If the parent key isn't backed up, subsequent splits of the following block will result in incorrect updates at the next level.)