swift/doc/source/overview_replication.rst

Replication

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.

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.

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.

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.

Replication is an area of active development, and likely rife with potential improvements to speed and correctness.

There are two major classes of replicator - the db replicator, which replicates accounts and containers, and the object replicator, which replicates object data.

DB Replication

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.

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.

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.

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.

Object Replication

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.

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.

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.

Work continues with a new ssync method where rsync is not used at all and instead all-Swift code is used to transfer the objects. At first, this ssync will just strive to emulate the rsync behavior. Once deemed stable it will open the way for future improvements in replication since we'll be able to easily add code in the replication path instead of trying to alter the rsync code base and distributing such modifications.

One of the first improvements planned is an "index.db" that will replace the hashes.pkl. This will allow quicker updates to that data as well as more streamlined queries. Quite likely we'll implement a better scheme than the current one hashes.pkl uses (hash-trees, that sort of thing).

Another improvement planned all along the way is separating the local disk structure from the protocol path structure. This separation will allow ring resizing at some point, or at least ring-doubling.

Note that for objects being stored with an Erasure Code policy, the replicator daemon is not involved. Instead, the reconstructor is used by Erasure Code policies and is analogous to the replicator for Replication type policies. See overview_erasure_code for complete information on both Erasure Code support as well as the reconstructor.

Hashes.pkl

The hashes.pkl file is a key element for both replication and reconstruction (for Erasure Coding). Both daemons use this file to determine if any kind of action is required between nodes that are participating in the durability scheme. The file itself is a pickled dictionary with slightly different formats depending on whether the policy is Replication or Erasure Code. In either case, however, the same basic information is provided between the nodes. The dictionary contains a dictionary where the key is a suffix directory name and the value is the MD5 hash of the directory listing for that suffix. In this manner, the daemon can quickly identify differences between local and remote suffix directories on a per partition basis as the scope of any one hashes.pkl file is a partition directory.

For Erasure Code policies, there is a little more information required. An object's hash directory may contain multiple fragments of a single object in the event that the node is acting as a handoff or perhaps if a rebalance is underway. Each fragment of an object is stored with a fragment index, so the hashes.pkl for an Erasure Code partition will still be a dictionary keyed on the suffix directory name, however, the value is another dictionary keyed on the fragment index with subsequent MD5 hashes for each one as values. Some files within an object hash directory don't require a fragment index so None is used to represent those. Below are examples of what these dictionaries might look like.

Replication hashes.pkl:

{'a43': '72018c5fbfae934e1f56069ad4425627',
 'b23': '12348c5fbfae934e1f56069ad4421234'}

Erasure Code hashes.pkl:

{'a43': {None: '72018c5fbfae934e1f56069ad4425627',
         2: 'b6dd6db937cb8748f50a5b6e4bc3b808'},
 'b23': {None: '12348c5fbfae934e1f56069ad4421234',
         1: '45676db937cb8748f50a5b6e4bc34567'}}

Dedicated replication network

Swift has support for using dedicated network for replication traffic. For more information see Overview of dedicated replication network <Dedicated-replication-network>.