ANY, ONE, TWO, THREE, QUORUM, ALL, LOCAL_QUORUM, EACH_QUORUM, and LOCAL_ONE Semantics

Learning outcomes

AtlasMart has one team using ONE everywhere for speed, another using ALL everywhere for “safety,” and a third copying EACH_QUORUM from a multi-DC article into a single-DC service. The result is inconsistent business guarantees and avoidable outages. This lesson replaces names with response arithmetic.

docker network inspect atlasmart-cassandra >/dev/null 2>&1 || docker network create atlasmart-cassandradocker volume create atlasmart-cass-1-datadocker volume create atlasmart-cass-2-datadocker volume create atlasmart-cass-3-datadocker inspect atlasmart-cass-1 >/dev/null 2>&1 || docker run -d --name atlasmart-cass-1 --hostname atlasmart-cass-1 --network atlasmart-cassandra -e CASSANDRA_CLUSTER_NAME=atlasmart-course -e CASSANDRA_DC=dc1 -e CASSANDRA_RACK=rack1 -e CASSANDRA_ENDPOINT_SNITCH=GossipingPropertyFileSnitch -e CASSANDRA_NUM_TOKENS=16 -v atlasmart-cass-1-data:/var/lib/cassandra cassandra:5.0.9# Wait until node 1 reports UN before starting peers.docker exec atlasmart-cass-1 nodetool statusdocker inspect atlasmart-cass-2 >/dev/null 2>&1 || docker run -d --name atlasmart-cass-2 --hostname atlasmart-cass-2 --network atlasmart-cassandra -e CASSANDRA_CLUSTER_NAME=atlasmart-course -e CASSANDRA_DC=dc1 -e CASSANDRA_RACK=rack2 -e CASSANDRA_ENDPOINT_SNITCH=GossipingPropertyFileSnitch -e CASSANDRA_NUM_TOKENS=16 -e CASSANDRA_SEEDS=atlasmart-cass-1 -v atlasmart-cass-2-data:/var/lib/cassandra cassandra:5.0.9docker inspect atlasmart-cass-3 >/dev/null 2>&1 || docker run -d --name atlasmart-cass-3 --hostname atlasmart-cass-3 --network atlasmart-cassandra -e CASSANDRA_CLUSTER_NAME=atlasmart-course -e CASSANDRA_DC=dc1 -e CASSANDRA_RACK=rack3 -e CASSANDRA_ENDPOINT_SNITCH=GossipingPropertyFileSnitch -e CASSANDRA_NUM_TOKENS=16 -e CASSANDRA_SEEDS=atlasmart-cass-1 -v atlasmart-cass-3-data:/var/lib/cassandra cassandra:5.0.9# Continue only after all three nodes are UN.docker exec atlasmart-cass-1 nodetool statusdocker exec atlasmart-cass-1 nodetool versiondocker exec atlasmart-cass-1 java -versiondocker exec atlasmart-cass-1 cqlsh -e "SHOW VERSION"

CREATE KEYSPACE IF NOT EXISTS atlasmart_consistencyWITH replication = {'class':'NetworkTopologyStrategy','dc1':3};CREATE TABLE IF NOT EXISTS atlasmart_consistency.order_status_by_id (    order_id text PRIMARY KEY,    status text,    version int,    updated_at timestamp) WITH compaction = {'class':'UnifiedCompactionStrategy'};CREATE TABLE IF NOT EXISTS atlasmart_consistency.inventory_reservation_by_product (    product_id text,    reservation_id text,    customer_id text,    state text,    created_at timestamp,    PRIMARY KEY (product_id, reservation_id)) WITH compaction = {'class':'UnifiedCompactionStrategy'};CONSISTENCY LOCAL_QUORUM;INSERT INTO atlasmart_consistency.order_status_by_id(order_id,status,version,updated_at)VALUES ('order-42','PAID',1,toTimestamp(now()));DESCRIBE KEYSPACE atlasmart_consistency;

Core terms used throughout this chapter

Replication factor (RF) is the number of replicas Cassandra is configured to keep for each partition in a datacenter. A replica stores a copy of a partition; the coordinator is the node handling one client request and is not a permanent leader. A consistency level (CL) is the per-request rule that says how many and which replicas must acknowledge a write or satisfy a read before the coordinator can return success. A datacenter (DC) is Cassandra's logical locality/failure-domain grouping; a rack is a smaller placement grouping within a DC. A quorum is a majority, calculated as floor(RF/2)+1 for the relevant replica scope. CQL means Cassandra Query Language; cqlsh is its shell. A Lightweight Transaction (LWT) is a conditional CQL operation using Paxos consensus; it has a serial phase and a regular learn/data phase. Availability here means whether enough replicas are alive/responding for the requested CL—not whether the cluster has any node alive. Staleness means a read can return an older replica version because the requested read CL did not necessarily intersect the replicas that acknowledged a prior write.

1. The consistency menu is arithmetic over a replica scope

For RF=3 in one DC, ONE/LOCAL_ONE need one replica response, TWO needs two, and THREE/QUORUM/LOCAL_QUORUM/ALL all require three, two, two, and three respectively according to their definitions. The names only become different when RF or topology changes. Cassandra still sends ordinary writes to all natural replicas; CL controls how many acknowledgments the coordinator must wait for before returning success.

2. Controlled one-replica failure

Pause one disposable replica. With RF=3, LOCAL_QUORUM and QUORUM still have two live replicas and should succeed. ALL and THREE cannot satisfy their response requirement and should fail with an unavailable-style error once failure detection knows the node is down. Exact error text/timing depends on failure detection and cqlsh/server versions.

docker pause atlasmart-cass-3# Give gossip/failure detection time to mark the endpoint down, then inspect.docker exec atlasmart-cass-1 nodetool status

CONSISTENCY LOCAL_QUORUM;UPDATE atlasmart_consistency.order_status_by_id SET status='PACKING', version=2, updated_at=toTimestamp(now()) WHERE order_id='order-42';SELECT * FROM atlasmart_consistency.order_status_by_id WHERE order_id='order-42';CONSISTENCY ALL;SELECT * FROM atlasmart_consistency.order_status_by_id WHERE order_id='order-42';CONSISTENCY ONE;SELECT * FROM atlasmart_consistency.order_status_by_id WHERE order_id='order-42';

docker unpause atlasmart-cass-3# Wait until all three nodes are UN again.docker exec atlasmart-cass-1 nodetool status

3. ANY is a write-specific escape hatch, not “consistency zero”

ANY can acknowledge a mutation when either a replica responds or the coordinator durably stores a hint for a temporarily unavailable target replica. That is why it can offer very high write availability but cannot serve a read: a hint is not queryable application data. cqlsh/native protocol should reject using ANY for a SELECT. To observe hint-only success requires a live coordinator that is not one of the unavailable target replicas; the three-node RF=3 baseline has every node as a replica, so the safer deterministic lab below verifies the write-only boundary and defers a hint-only topology to Chapter 15.

CONSISTENCY ANY;UPDATE atlasmart_consistency.order_status_by_id SET status='SHIPPED', version=3, updated_at=toTimestamp(now()) WHERE order_id='order-42';-- Expected: SELECT at ANY is rejected because ANY is accepted only for writes.SELECT * FROM atlasmart_consistency.order_status_by_id WHERE order_id='order-42';CONSISTENCY LOCAL_QUORUM;SELECT * FROM atlasmart_consistency.order_status_by_id WHERE order_id='order-42';

4. Verification and reset

• All three nodes return UN.
• The keyspace shows RF=3 in dc1.
• The learner can compute quorum as floor(3/2)+1=2.
• ANY is described and observed as write-only.
• Unavailable and timeout are not conflated.

CONSISTENCY LOCAL_QUORUM;UPDATE atlasmart_consistency.order_status_by_id SET status='PAID', version=10, updated_at=toTimestamp(now()) WHERE order_id='order-42';SELECT * FROM atlasmart_consistency.order_status_by_id WHERE order_id='order-42';

Production judgment

Choose consistency from a business invariant, topology, RF, and failure budget—not from a cluster-wide slogan. Record which reads must observe which writes, whether the invariant is local to one DC or global, whether conditional uniqueness/compare-and-set is required, and what latency/availability degradation is acceptable when replicas or a whole DC are unavailable. Then test that contract with the real driver, routing policy, request timeout, retry/speculative-execution policy, idempotency classification, and representative network latency.

Consistency level does not replace durable commit-log/storage design, repair, backup/restore, security isolation, schema/partition design, compaction/tombstone management, or application-level idempotency. ALL is not “permanent durability”; LOCAL_ONE is not tenant isolation; a timeout is not proof that a write failed; and a successful weak write does not promise a subsequent weak read will be fresh. Storage-Attached Indexing (SAI) or vector search can add read work but do not redefine RF/CL arithmetic. Managed Cassandra services may restrict topology visibility or CL choices; verify provider semantics rather than assuming Apache Cassandra behavior is exposed unchanged. Lesson 2 extends the arithmetic to two datacenters and shows why LOCAL_QUORUM is usually the latency/availability-aware choice for DC-local application traffic.

Summary and next bridge

A consistency level is a per-request replica-response rule. Its meaning comes from RF, DC scope, operation type, and current replica availability. Next, derive the same rules in a real multi-DC layout where global QUORUM and EACH_QUORUM behave differently from LOCAL_QUORUM.

Authoritative references

Re-check these version-sensitive sources when regenerating this course.

• Apache Cassandra 5.0 downloads and Java Driver releases
• Dynamo architecture and tunable consistency semantics
• cqlsh CONSISTENCY and SERIAL CONSISTENCY commands
• Native protocol consistency-level codes
• Cassandra guarantees and LWT linearizable consistency
• Hints and eventual convergence