openstack-manuals/doc/training-guides/module003-ch009-replication.xml

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    <title>Replication</title>
    <para>Because each replica in swift functions independently, and
        clients generally require only a simple majority of nodes
        responding to consider an operation successful, transient
        failures like network partitions can quickly cause replicas to
        diverge. These differences are eventually reconciled by
        asynchronous, peer-to-peer replicator processes. The
        replicator processes traverse their local filesystems,
        concurrently performing operations in a manner that balances
        load across physical disks.</para>
    <para>Replication uses a push model, with records and files
        generally only being copied from local to remote replicas.
        This is important because data on the node may not belong
        there (as in the case of handoffs and ring changes), and a
        replicator can’t know what data exists elsewhere in the
        cluster that it should pull in. It’s the duty of any node that
        contains data to ensure that data gets to where it belongs.
        Replica placement is handled by the ring.</para>
    <para>Every deleted record or file in the system is marked by a
        tombstone, so that deletions can be replicated alongside
        creations. The replication process cleans up tombstones after
        a time period known as the consistency window. The consistency
        window encompasses replication duration and how long transient
        failure can remove a node from the cluster. Tombstone cleanup
        must be tied to replication to reach replica
        convergence.</para>
    <para>If a replicator detects that a remote drive has failed, the
        replicator uses the get_more_nodes interface for the ring to
        choose an alternate node with which to synchronize. The
        replicator can maintain desired levels of replication in the
        face of disk failures, though some replicas may not be in an
        immediately usable location. Note that the replicator doesn’t
        maintain desired levels of replication when other failures,
        such as entire node failures, occur because most failure are
        transient.</para>
    <para>Replication is an area of active development, and likely
        rife with potential improvements to speed and
        correctness.</para>
    <para>There are two major classes of replicator - the db
        replicator, which replicates accounts and containers, and the
        object replicator, which replicates object data.</para>
    <para><guilabel>DB Replication</guilabel></para>
        <para>The first step performed by db replication is a low-cost
            hash comparison to determine whether two replicas already
            match. Under normal operation, this check is able to
            verify that most databases in the system are already
            synchronized very quickly. If the hashes differ, the
            replicator brings the databases in sync by sharing records
            added since the last sync point.</para>
        <para>This sync point is a high water mark noting the last
            record at which two databases were known to be in sync,
            and is stored in each database as a tuple of the remote
            database id and record id. Database ids are unique amongst
            all replicas of the database, and record ids are
            monotonically increasing integers. After all new records
            have been pushed to the remote database, the entire sync
            table of the local database is pushed, so the remote
            database can guarantee that it is in sync with everything
            with which the local database has previously
            synchronized.</para>
        <para>If a replica is found to be missing entirely, the whole
            local database file is transmitted to the peer using
            rsync(1) and vested with a new unique id.</para>
        <para>In practice, DB replication can process hundreds of
            databases per concurrency setting per second (up to the
            number of available CPUs or disks) and is bound by the
            number of DB transactions that must be performed.</para>
        <para><guilabel>Object Replication</guilabel></para>
        <para>The initial implementation of object replication simply
            performed an rsync to push data from a local partition to
            all remote servers it was expected to exist on. While this
            performed adequately at small scale, replication times
            skyrocketed once directory structures could no longer be
            held in RAM. We now use a modification of this scheme in
            which a hash of the contents for each suffix directory is
            saved to a per-partition hashes file. The hash for a
            suffix directory is invalidated when the contents of that
            suffix directory are modified.</para>
        <para>The object replication process reads in these hash
            files, calculating any invalidated hashes. It then
            transmits the hashes to each remote server that should
            hold the partition, and only suffix directories with
            differing hashes on the remote server are rsynced. After
            pushing files to the remote server, the replication
            process notifies it to recalculate hashes for the rsynced
            suffix directories.</para>
        <para>Performance of object replication is generally bound by
            the number of uncached directories it has to traverse,
            usually as a result of invalidated suffix directory
            hashes. Using write volume and partition counts from our
            running systems, it was designed so that around 2% of the
            hash space on a normal node will be invalidated per day,
            which has experimentally given us acceptable replication
            speeds.</para>
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