REPLICATION IN THE WILD

Ensar Basri Kahveci

REPLICATION- Putting a data set into multiple nodes.

- Each replica has a full copy.

- A few reasons for replication:- Performance- Availability and fault tolerance

- Mostly used with partitioning.

NOTHING FOR FREE!- Very easy to do when the data is immutable.- Problems start when we have multiple copies

of the data and we want to update them.- Two main difficulties

- Handling updates- Handling failures

The dangers of replication and a solution- Gray et al. [1] classify replication models by 2

parameters:- Where to make updates: primary copy or update

anywhere- When to make updates: eagerly or lazily

WHERE: PRIMARY COPY- There is a single replica managing the updates.

- Concurrency control is easy. - No conflicts and no conflict-handling logic.- Updates are executed on the primary and

secondaries with the same order.- When primary fails, a new primary is elected.

- Ensuring a single and good primary is hard.

WHERE: UPDATE ANYWHERE- Each replica can initiate a transaction to make

an update.- Complex concurrency control.- Deadlocks or conflicts are possible. - In practice, there is also multi-leader.

WHEN: EAGER REPLICATION- Synchronously updates all replicas as part of

one atomic transaction.- Provides strong consistency. - Not very flexible. Degree of availability can

degrade on node failures.- Consensus algorithms.

WHEN: LAZY REPLICATION- Updates each replica with a separate

transaction.- Updates can execute quite fast.- Degree of availability is high.- Eventual consistency. - Data copies can diverge.

- Data loss or conflicts can occur.

WHERE?

WHEN?

PRIMARY COPY UPDATE ANYWHERE

EAGER

strong consistencysimple concurrency

slowinflexible

strong consistencycomplex concurrency

slowexpensivedeadlocks

LAZY

fasteventual consistencysimple concurrency

inconsistency

fastavailable

flexibleeventual consistency

inconsistency conflicts

WHERE?

WHEN?

PRIMARY COPY UPDATE ANYWHERE

EAGER

Multi Paxos [5]etcd and Consul (RAFT) [6]

Zookeeper (Zab) [7]Kafka

Paxos [5]Hazelcast Cluster State Change [12]

LAZY

HazelcastMongoDB

ElasticSearchRedis

Dynamo [4]Cassandra

Riak

PRIMARY COPY + EAGER REPLICATION- When the primary fails, secondaries are

guaranteed to be up to date. - Raft, Kafka etc.

- Majority approach can be used. - In Kafka, in-sync-replica set is maintained. [11] - Secondaries can be used for reads.

UPDATE ANYWHERE + EAGER REPLICATION- Each replica can initiate a new transaction. - Concurrent transactions can compete with

each other.- Possibility of deadlocks.- In the basic Paxos algorithm, there is no

designated leader.

PRIMARy COPY + LAZY REPLICATION- The primary copy can execute updates fast.- Secondaries can fall behind the primary. It is

called replication lag.- It can lead to data loss during leader failover, but

still no conflicts.- Secondaries can be used for reads.

UPDATE ANYWHERE + LAZY REPLICATION- Dynamo-style [4] highly available databases.- Quorums- Concurrent updates are first-class citizens.- Possibility of conflicts

- Avoiding, discarding, detecting & resolving conflicts- Eventual convergence

- Write repair, read repair and anti-entropy

QUORUMS- W + R > N

- W = 3, R = 1, N = 3 - W = 1, R = 3, N = 3 - W = 2, R = 2, N = 3

- If W or R is not met, consistency may be broken.- Sloppy quorums and hinted handoff.- Even if W and R are met, it can be still broken.

Conflict-free replicated data types (CRDTS)- Special data types that achieve strong

eventual consistency and monotonicity [2]- No conflicts- Merge function has 3 properties:

- Commutative: A + B = B + A- Associative: A + (B + C) = (A + B) + C- Idempotent: f(f(x)) = f(x)

- Riak Data Types [3]

DISCARDING CONFLICTS: LAST WRITE WINS- When 2 updates are concurrent, define an

arbitrary order among them.- i.e., pretend that one of them is more recent.

- Attach a timestamp to each write.- Cassandra uses physical timestamps [8], [9]

DETECTING CONFLICTS: VECTOR CLOCKS - In Dynamo paper [4], each update is done

against a particular version of a data entry. - Multiple versions of a data entry can exist together.

- Vector clocks [10] are used to track causality.- The system can determine the authoritative version:

syntactic reconciliation- The system cannot reconcile multiple versions:

semantic reconciliation

Resolving conflicts and EVENTUAL CONVERGENCE- Write repair- Read repair- Anti-entropy

- Merkle trees

Recap- We apply replication to make our systems

performant and fault tolerant. - Replication suffers from core problems of

distributed systems.- We can build many replication protocols that

vary on the 2 dimensions we discussed.- No silver bullet. It is a world of trade-offs.

REFerences[1] Gray, Jim, et al. "The dangers of replication and a solution." ACM SIGMOD Record 25.2 (1996): 173-182.[2] Shapiro, Marc, et al. "Conflict-free replicated data types." Symposium on Self-Stabilizing Systems. Springer, Berlin, Heidelberg, 2011.[3] http://docs.basho.com/riak/kv/2.2.0/learn/concepts/crdts/[4] DeCandia, Giuseppe, et al. "Dynamo: amazon's highly available key-value store." ACM SIGOPS operating systems review 41.6 (2007): 205-220.[5] Lamport, Leslie. "Paxos made simple." ACM Sigact News 32.4 (2001): 18-25.[6] Ongaro, Diego, and John K. Ousterhout. "In Search of an Understandable Consensus Algorithm." USENIX Annual Technical Conference. 2014.[7] Hunt, Patrick, et al. "ZooKeeper: Wait-free Coordination for Internet-scale Systems." USENIX annual technical conference. Vol. 8. 2010.[8] http://www.datastax.com/dev/blog/why-cassandra-doesnt-need-vector-clocks[9] https://aphyr.com/posts/299-the-trouble-with-timestamps[10] Raynal, Michel, and Mukesh Singhal. "Logical time: Capturing causality in distributed systems." Computer 29.2 (1996): 49-56.[11] http://kafka.apache.org/documentation.html#replication[12] http://docs.hazelcast.org/docs/latest/manual/html-single/index.html#managing-cluster-and-member-states

THANKS!Any questions?