apache/cassandra · error · UnsupportedOperationException

TXN_OPS + " is a write-only table"

Error message

TXN_OPS + " is a write-only table"

What it means

The AccordDebugKeyspace virtual table `system_views.accord_txn_ops` only accepts writes (INSERTs that enqueue debug transaction operations); it has no read path. Any SELECT against it throws UnsupportedOperationException because collect() is deliberately unimplemented for this write-only table.

Solutions

1. Do not SELECT from this table; it is write-only by design - only INSERT rows to enqueue transaction debug operations.
2. Read the effects of enqueued operations from the intended output tables (e.g. accord_debug tables) instead.
3. If a read view is needed, contribute a separate readable virtual table upstream rather than patching collect().

Example fix

// before
SELECT * FROM system_views.accord_txn_ops;
// after
-- write-only table: enqueue an op instead
INSERT INTO system_views.accord_txn_ops (txn_id, op) VALUES ('<txnId>', 'APPLY');

// Treat accord_txn_ops as insert-only
if (query.trim().toUpperCase().startsWith("SELECT") && query.contains("accord_txn_ops"))
    throw new IllegalArgumentException("accord_txn_ops is write-only; do not SELECT from it");

When it happens

Trigger: Running any SELECT query (cqlsh or driver) against the TXN_OPS virtual table, e.g. `SELECT * FROM system_views.accord_txn_ops;`, which invokes the table's collect(PartitionsCollector) method.

Common situations: A developer inspects the table with SELECT first to see what's in it before inserting a debug operation, assuming virtual tables are always readable; also scripts or monitoring that enumerate all rows of every virtual table in system_views.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• COMMAND_STORE_OPS + " is a write-only table"
• NODE_OPS + " is a write-only table"
• metadata + " only supports single partition key queries"
• Must specify 'op'
• Unknown column " + columns[i].name.toString()

AI-assisted analysis of apache/cassandra@88fd0f6a0e (2026-09-10). Data as JSON: /api/errors/14280e48ee4ebaaf. Report an issue: GitHub.

Appendix: source

Thrown at src/java/org/apache/cassandra/db/virtual/AccordDebugKeyspace.java:1773

                this.description = description;
            }
        }
        private TxnOpsTable()
        {
            super(parse(VIRTUAL_ACCORD_DEBUG, TXN_OPS,
                        "Update Accord Command State",
                        "CREATE TABLE %s (\n" +
                        "  txn_id text,\n" +
                        "  command_store_id int,\n" +
                        "  op text," +
                        "  PRIMARY KEY (txn_id, command_store_id)" +
                        ')', UTF8Type.instance), FAIL, UNSORTED);
        }

        @Override
        protected void collect(PartitionsCollector collector)
        {
            throw new UnsupportedOperationException(TXN_OPS + " is a write-only table");
        }

        @Override
        protected void applyRowUpdate(Object[] partitionKeys, Object[] clusteringKeys, ColumnMetadata[] columns, Object[] values)
        {
            TxnId txnId = TxnId.parse((String) partitionKeys[0]);
            int commandStoreId = (Integer) clusteringKeys[0];

            TxnOp op = tryParse(values[0], true, TxnOp.class, TxnOp::valueOf);

            switch (op)
            {
                default: throw new UnhandledEnum(op);
                case LOCALLY_ERASE_VESTIGIAL:
                    cleanup(txnId, commandStoreId, Cleanup.VESTIGIAL);
                    break;
                case LOCALLY_INVALIDATE:
                    cleanup(txnId, commandStoreId, Cleanup.INVALIDATE);

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