This wait is to make sure that backup workers are already saving mutations so
that no mutations are missed. The idea is that the CLI sets a "backupStartedKey"
in the database and waits for allWorkerStarted() key of the backup to be set.
Backup workers monitor the changes to the "backupStartedKey" and start logging
mutations. Additionally, backup worker for Tag(-2,0) monitors all other workers
have started (checking their saved progress version is larger than the backup's
start version), and then sets the allWorkerStarted() key for the backup.
The monitoring loop of system key "backupStartedKey" and decides to be in one
of two modes: NOOP and backup. In the NOOP mode, the worker just pop TLogs. In
the backup mode, the worker pulls mutations from TLogs and save the mutations
into logs.
The backup worker needs to use this version for popping when running in a NOOP
mode. This option is added to GetReadVersionRequest and proxies will send back
minKnownCommittedVersion if the option is set.
Also add a couple of knobs for backup workers.
The backup worker just blindly pop tags if the "backupStartedKey" is not set.
Note the commit version from TLog cannot be used as the pop version, because
for a single region, during a recovery the log router tags are used to recover
mutations. The backup worker can potentially pop mutations that are needed for
recovery, causing consistency errors. So the solution for now is to use commit
version - 5,000,000, which is a version guaranteed to be persisted on all
replicas.
For each mutation, its version, sub-version, and size are prefixed with big
endian representation. This is required, especially for the first version
variable, because we use 0xFF for padding purpose. A little endian version
number can easily collide with 0xFF, while big endian is guaranteed to have
0x00 as the first byte.
Jingyu Zhou