[ACCUMULO-2949] Write explicit "close" markers for WALs

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Invalid
• Component/s: logger, replication
• Assignee: Unassigned
• Reporter: Josh Elser
• Created: 24/Jun/14 18:59
• Updated: 16/Apr/19 19:25
• Resolved: 16/Apr/19 19:25

Description

To ensure that WALs are not left in a dangling "open" state WRT replication, the garbage collector scans the tablets and constructs a view of WALs that are currently in use. It consults that view to determine which WALs can move to a "closed" replication state.

This isn't entirely correct because a WAL can "come back" again after being removed from a Tablet. Consider the following:

1. Table has one tablet hosted on one tserver
2. Tablet gets some mutations
3. Tablet gets MinC
4. Tablet removes WAL entry as part of MinC
5. WAL is "closed" WRT replication
6. Tablet receives more mutations, starts using the same WAL

There are a couple of ways that this could present itself, each of which would result in re-replication of data we've potentially already sent once. On an active system, I don't think this is of big concern, and we already don't guarantee a "once and only once" replication contract so this isn't critical. The combiner set on the replication table will also mitigate most of the re-replication concerns as those records persist until the entire file is replicated (which should outlast the use on the local system).

ecn recommended that we could record a "closed" marker for a WAL as a part of TabletServerLogger.close() which would prevent the need to "guess" at when a WAL will no longer be used.

If we want to move to explicit "end" tracking (see ACCUMULO-2835), we will need this implemented.

Issue Links

• blocks: ACCUMULO-2835 Track explicit lengths of WALs for more accurate reporting