apache/cassandra · error · ConfigurationException

concurrent_writes must be at least 2, but was ${conf.concurr

Error message

concurrent_writes must be at least 2, but was ${conf.concurrent_writes}

What it means

concurrent_writes sets the write stage worker thread count and must be at least 2. applySimpleConfig enforces this at startup, but skips the check while the TEST_FAIL_MV_LOCKS_COUNT system property is set, which some materialized-view tests use to run with degenerate pools.

Source

Thrown at src/java/org/apache/cassandra/config/DatabaseDescriptor.java:733

                                               "(options: direct/auto) " + conf.compaction_read_disk_access_mode);
        }
        logger.info("compaction_read_disk_access_mode resolved to: {}", compactionReadDiskAccessMode);

        /* phi convict threshold for FailureDetector */
        if (conf.phi_convict_threshold < 5 || conf.phi_convict_threshold > 16)
        {
            throw new ConfigurationException("phi_convict_threshold must be between 5 and 16, but was " + conf.phi_convict_threshold, false);
        }

        /* Thread per pool */
        if (conf.concurrent_reads < 2)
        {
            throw new ConfigurationException("concurrent_reads must be at least 2, but was " + conf.concurrent_reads, false);
        }

        if (conf.concurrent_writes < 2 && TEST_FAIL_MV_LOCKS_COUNT.getString("").isEmpty())
        {
            throw new ConfigurationException("concurrent_writes must be at least 2, but was " + conf.concurrent_writes, false);
        }

        if (conf.concurrent_counter_writes < 2)
            throw new ConfigurationException("concurrent_counter_writes must be at least 2, but was " + conf.concurrent_counter_writes, false);

        if (conf.networking_cache_size == null)
            conf.networking_cache_size = new DataStorageSpec.IntMebibytesBound(Math.min(128, (int) (Runtime.getRuntime().maxMemory() / (16 * 1048576))));

        if (conf.file_cache_size == null)
            conf.file_cache_size = new DataStorageSpec.IntMebibytesBound(Math.min(512, (int) (Runtime.getRuntime().maxMemory() / (4 * 1048576))));

        // round down for SSDs and round up for spinning disks
        if (conf.file_cache_round_up == null)
            conf.file_cache_round_up = conf.disk_optimization_strategy == Config.DiskOptimizationStrategy.spinning;

        if (conf.memtable_offheap_space == null)
            conf.memtable_offheap_space = new DataStorageSpec.IntMebibytesBound((int) (Runtime.getRuntime().maxMemory() / (4 * 1048576)));
        // for the moment, we default to twice as much on-heap space as off-heap, as heap overhead is very large

View on GitHub (pinned to 88fd0f6a0e)

Solutions

1. Set `concurrent_writes` to at least 2 (shipped default is 32).
2. Remove the override to use the default.
3. If a test harness intentionally sets low values, ensure TEST_FAIL_MV_LOCKS_COUNT is only set in test environments, never in production.

Example fix

// before (cassandra.yaml)
concurrent_writes: 0
// after (cassandra.yaml)
concurrent_writes: 32

// Java, before toolInitialization()
int writes = /* configured concurrent_writes */;
boolean testFlag = !System.getProperty("cassandra.test.fail_mv_locks_count", "").isEmpty();
if (writes < 2 && !testFlag)
    throw new IllegalArgumentException("concurrent_writes must be >= 2, got " + writes);

When it happens

Trigger: cassandra.yaml (or programmatic Config) has `concurrent_writes` below 2 (e.g. 0 or 1) when DatabaseDescriptor.toolInitialization/applyAll runs, and the TEST_FAIL_MV_LOCKS_COUNT system property is unset/empty.

Common situations: Shrinking pools on memory-constrained containers; CI images with generated configs producing 0/1; mixing test-only property flags into production startup scripts that then behave differently from expected.

Understand the failure class

Background: "Invalid value" and "allowed values are" config errors: what your library rejected and how to fix it — this error's family across 41 libraries.

Related errors

• concurrent_reads must be at least 2, but was ${conf.concurre
• Missing value for commitlog_sync_group_window.
• Missing value for commitlog_sync_period.
• concurrent_counter_writes must be at least 2, but was ${conf
• memtable_flush_writers must be at least 1, but was ${conf.me

AI-assisted analysis of apache/cassandra@88fd0f6a0e (2026-09-10). Data as JSON: /api/errors/1dc89e4642718465. Report an issue: GitHub.