flink-ml-iteration/flink-ml-iteration-common/src/main/java/org/apache/flink/iteration/Iterations.java [331:385]:
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    private static DataStreamList addCriteriaStream(
            DataStream<?> draftCriteriaStream,
            IterationID iterationId,
            StreamExecutionEnvironment env,
            DraftExecutionEnvironment draftEnv,
            DataStreamList initVariableStreams,
            DataStreamList headStreams,
            int totalInitVariableParallelism) {
        // Deals with the criteria streams
        DataStream<?> terminationCriteria = draftEnv.getActualStream(draftCriteriaStream.getId());
        // It should always has the IterationRecordTypeInfo
        checkState(
                terminationCriteria.getType().getClass().equals(IterationRecordTypeInfo.class),
                "The termination criteria should always return IterationRecord.");
        TypeInformation<?> innerType =
                ((IterationRecordTypeInfo<?>) terminationCriteria.getType()).getInnerTypeInfo();

        DataStream<?> emptyCriteriaSource =
                env.addSource(new DraftExecutionEnvironment.EmptySource())
                        .returns(innerType)
                        .name(terminationCriteria.getTransformation().getName())
                        .setParallelism(terminationCriteria.getParallelism());
        DataStreamList criteriaSources = DataStreamList.of(emptyCriteriaSource);
        DataStreamList criteriaInputs = addInputs(criteriaSources);
        DataStreamList criteriaHeaders =
                addHeads(
                        criteriaSources,
                        criteriaInputs,
                        iterationId,
                        totalInitVariableParallelism,
                        true,
                        initVariableStreams.size());

        // Merges the head and the actual criteria stream. This is required since if we have
        // no edges from the criteria head to the criteria tail, the tail might directly received
        // the MAX_EPOCH_WATERMARK without the synchronization of the head.
        DataStream<?> mergedHeadAndCriteria =
                mergeCriteriaHeadAndCriteriaStream(
                        env, criteriaHeaders.get(0), terminationCriteria, innerType);
        DataStreamList criteriaTails =
                addTails(
                        DataStreamList.of(mergedHeadAndCriteria),
                        iterationId,
                        initVariableStreams.size());

        String coLocationGroupKey = "co-" + iterationId.toHexString() + "-cri";
        criteriaHeaders.get(0).getTransformation().setCoLocationGroupKey(coLocationGroupKey);
        criteriaTails.get(0).getTransformation().setCoLocationGroupKey(coLocationGroupKey);

        // Now we notify all the head operators to count the criteria streams.
        setCriteriaParallelism(headStreams, terminationCriteria.getParallelism());
        setCriteriaParallelism(criteriaHeaders, terminationCriteria.getParallelism());

        return criteriaTails;
    }
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flink-ml-iteration/flink-ml-iteration-1.15/src/main/java/org/apache/flink/iteration/Iterations.java [331:385]:
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    private static DataStreamList addCriteriaStream(
            DataStream<?> draftCriteriaStream,
            IterationID iterationId,
            StreamExecutionEnvironment env,
            DraftExecutionEnvironment draftEnv,
            DataStreamList initVariableStreams,
            DataStreamList headStreams,
            int totalInitVariableParallelism) {
        // Deals with the criteria streams
        DataStream<?> terminationCriteria = draftEnv.getActualStream(draftCriteriaStream.getId());
        // It should always has the IterationRecordTypeInfo
        checkState(
                terminationCriteria.getType().getClass().equals(IterationRecordTypeInfo.class),
                "The termination criteria should always return IterationRecord.");
        TypeInformation<?> innerType =
                ((IterationRecordTypeInfo<?>) terminationCriteria.getType()).getInnerTypeInfo();

        DataStream<?> emptyCriteriaSource =
                env.addSource(new DraftExecutionEnvironment.EmptySource())
                        .returns(innerType)
                        .name(terminationCriteria.getTransformation().getName())
                        .setParallelism(terminationCriteria.getParallelism());
        DataStreamList criteriaSources = DataStreamList.of(emptyCriteriaSource);
        DataStreamList criteriaInputs = addInputs(criteriaSources);
        DataStreamList criteriaHeaders =
                addHeads(
                        criteriaSources,
                        criteriaInputs,
                        iterationId,
                        totalInitVariableParallelism,
                        true,
                        initVariableStreams.size());

        // Merges the head and the actual criteria stream. This is required since if we have
        // no edges from the criteria head to the criteria tail, the tail might directly received
        // the MAX_EPOCH_WATERMARK without the synchronization of the head.
        DataStream<?> mergedHeadAndCriteria =
                mergeCriteriaHeadAndCriteriaStream(
                        env, criteriaHeaders.get(0), terminationCriteria, innerType);
        DataStreamList criteriaTails =
                addTails(
                        DataStreamList.of(mergedHeadAndCriteria),
                        iterationId,
                        initVariableStreams.size());

        String coLocationGroupKey = "co-" + iterationId.toHexString() + "-cri";
        criteriaHeaders.get(0).getTransformation().setCoLocationGroupKey(coLocationGroupKey);
        criteriaTails.get(0).getTransformation().setCoLocationGroupKey(coLocationGroupKey);

        // Now we notify all the head operators to count the criteria streams.
        setCriteriaParallelism(headStreams, terminationCriteria.getParallelism());
        setCriteriaParallelism(criteriaHeaders, terminationCriteria.getParallelism());

        return criteriaTails;
    }
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