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The tape is read through from the beginning. This process is fast, as we can skip over the actual record contents. The relevant information here are the objectIds, the tape name and the offset of the record. For each tar record, the index is updated in the same way as it would have been when the record was written.This update will overwrite any entry that already existed in the index. As the tapes are read in order of creation-time (this is encoded in the tape name) and the records in the tape are written in order, when all entries concerning a given objectId have been indexed, the index can be sure to have the information about the newest instance of the record.
When a tape have been indexed, it is marked as such in the index.
The newest tape will be marked as indexed here, even through it is not closed. Should this change?
Broken tapes
Sometimes the tapes can become broken (not observed yet). This is detected only during startup, when the tape is read for indexing. This means that if a tape have already been indexed, it will not be verified upon startup. If an IOException occurs while reading the tape, the system regards the tape as broken and attempts to fix it. The fix is rather brutal. Every record, from the beginning of the tape, is read and written to a new temp tape, until the broken tape either runs out of bytes or the IOException reoccurs. This is done in a way to ensure that the temp tape will only have complete records. When no more can be read from the broken tape, it is deleted and replaced with the temp tape. The indexing then proceeds to the next tape in the list.
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The tapes do support purging objects. When a blob is deleted, it is written as a new instance of 0 bytes to the tape. The name of the record is now is now "objectId#timestampInMillis#DELETED". When an object is deleted, it is removed from the index. Upon indexing the tape (see above), records of 0 bytes with names ending in "#DELETED" cause the objectId to be removed from the index.
Spring Configuration
The system is configured from the file "akubra-llstore.xml" which is a spring config file. It is reproduced below with comments.
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