apache/cassandra · error · ConfigurationException

Multiple entries for DC

Error message

Multiple entries for DC %s

What it means

When splitting the 'dcs' option on commas, each entry is parsed into a WeightedDc and inserted into a map keyed by DC name. If the same DC name appears more than once, the duplicate insertion is rejected with a ConfigurationException instead of silently overwriting.

Solutions

1. Remove duplicate entries from the 'dcs' list so each DC appears once
2. Normalize DC casing before building the list (e.g. all lowercase)
3. Deduplicate in tooling that generates the dcs string from multiple sources

Example fix

// before
Map<String,String> opts = Map.of("dcs", "dc1,dc1=0.5");
ParameterizedFastPathStrategy.fromMap(opts); // throws
// after
Map<String,String> opts = Map.of("dcs", "dc1=0.5");
ParameterizedFastPathStrategy.fromMap(opts); // ok

Set<String> seen = new HashSet<>();
for (String dc : dcsString.split(",")) {
    if (!seen.add(dc.trim()))
        throw new IllegalArgumentException("duplicate DC entry: " + dc);
}

When it happens

Trigger: Calling fromMap() with {"dcs": "dc1,dc1"} or case/format variants that parse to the same name, e.g. "dc1:auto,dc1=0.5".

Common situations: Appending a DC to an existing list without noticing it was already present; scripted config generation concatenating overlapping DC sets; inconsistent casing across entries.

Understand the failure class

Background: Conflicting config options: "cannot be used together" — configuration validation errors across open-source libraries — this error's family across 162 libraries.

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AI-assisted analysis of apache/cassandra@88fd0f6a0e (2026-09-10). Data as JSON: /api/errors/0e92c658b586b8ce. Report an issue: GitHub.

Appendix: source

Thrown at src/java/org/apache/cassandra/service/accord/topology/ParameterizedFastPathStrategy.java:274

        if (size < 1)
            throw cfe("%s must be greater than zero", SIZE);

        ImmutableMap<String, WeightedDc> dcMap;
        if (map.containsKey(DCS) && !map.get(DCS).isEmpty())
        {
            Map<String, WeightedDc> mutableDcs = new HashMap<>();
            String dcsString = map.get(DCS);
            if (dcsString.trim().isEmpty())
                throw cfe("%s must specify at least one DC", DCS);

            int autoIdx = 0;
            boolean hasAuto = false;
            boolean hasManual = false;
            for (String dcString : COMMA_SEPARATOR.split(dcsString))
            {
                WeightedDc dc = WeightedDc.fromString(dcString, autoIdx++);
                if (mutableDcs.containsKey(dc.name))
                    throw cfe("Multiple entries for DC %s", dc.name);

                if (dc.autoWeight)
                {
                    if (hasManual) throw cfe("Cannot mix auto and manual DC weights");
                    hasAuto = true;
                }
                else
                {
                    if (hasAuto) throw cfe("Cannot mix auto and manual DC weights");
                    hasManual = true;
                }

                mutableDcs.put(dc.name, dc);
            }
            dcMap = ImmutableMap.copyOf(mutableDcs);
        }
        else
        {

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