The root cause was a broken check for duplicates in the DB.

This solution favors creating new ID, based on a composition of partitionId and position.
See comments in PR camunda-community-hub#280 and Issue camunda-community-hub#278 for more details.
Tests are added to show implementation works properly
Unfortunately, because of JPA requires an @id field, a new field was introduced
and there's an UPGRADE instruction documented, so people out there would be able to migrate their installations.

README.md

@@ -16,7 +16,11 @@ The application imports the data from Zeebe using the [Hazelcast exporter](https
 
 ![how-it-works](docs/how-it-works.png)
 
-## Install
+## Upgrading from prior version
+
+See [upgrade instructions](./UPGRADE.md)
+
+## Fresh install
 
 ### Docker
 

UPGRADE.md

@@ -0,0 +1,36 @@
+
+Upgrade documentation for Zeebe Simple Monitor 
+==================================================
+
+## Upgrading from v2.0.0
+
+Due to some issues with storing variables and element instances, the database structure changed.
+Zeebe Simple Monitor will not alter existing database tables automatically, but if you have a PostgreSQL
+or other DB running, you need to alter the table structures manually, in order to keep your data.
+
+Of course, if you use an in-memory DB or do not need to keep prior data, then simply drop all tables and sequences,
+and let Zeebe Simple Monitor create them again for you (automatic creation works).
+
+### Upgrade procedure
+
+1. stop Zeebe Simple Monitor (v2.0.0)
+2. run the SQL script below against your PostgreSQL Database
+3. start up Zeebe Simple Monitor (new version)
+
+```sql
+-- part 1, element_instance table changes
+ALTER TABLE element_instance ADD COLUMN ID varchar(255);
+UPDATE element_instance SET id = (partition_id_::varchar || '-' || position_::varchar) where true;
+ALTER TABLE element_instance DROP CONSTRAINT element_instance_pkey;
+ALTER TABLE element_instance ADD PRIMARY KEY (id);
+CREATE INDEX element_instance_processInstanceKeyIndex ON element_instance (process_instance_key_);
+-- part 2, variable table changes
+ALTER TABLE variable ADD COLUMN ID varchar(255);
+ALTER TABLE variable ADD COLUMN PARTITION_ID_ integer DEFAULT 1;
+UPDATE variable SET id = (partition_id_::varchar || '-' || position_::varchar) where true;
+ALTER TABLE variable DROP CONSTRAINT variable_pkey;
+ALTER TABLE variable ADD PRIMARY KEY (id);
+CREATE INDEX variable_processInstanceKeyIndex ON variable (process_instance_key_);
+```
+(This SQL was developed and tested using a recent PostgreSQL instance.
+ You might need to adopt that if you're using another Database)

src/main/java/io/zeebe/monitor/entity/ElementInstanceEntity.java

@@ -15,14 +15,19 @@
  */
 package io.zeebe.monitor.entity;
 
-import javax.persistence.Column;
-import javax.persistence.Entity;
-import javax.persistence.Id;
+import javax.persistence.*;
 
 @Entity(name = "ELEMENT_INSTANCE")
+@Table(indexes = {
+    // performance reason, because we use it in the ElementInstanceRepository.findByProcessInstanceKey()
+    @Index(name = "element_instance_processInstanceKeyIndex", columnList = "PROCESS_INSTANCE_KEY_"),
+})
 public class ElementInstanceEntity {
 
   @Id
+  @Column(name = "ID")
+  private String id;
+
   @Column(name = "POSITION_")
   private Long position;
 
@@ -53,6 +58,24 @@ public class ElementInstanceEntity {
   @Column(name = "TIMESTAMP_")
   private long timestamp;
 
+  public String getId() {
+    return id;
+  }
+
+  private void setId(final String id) {
+    // made private, to avoid accidental changes
+    this.id = id;
+  }
+
+  public final String getGeneratedIdentifier() {
+    return this.partitionId + "-" + this.position;
+  }
+
+  @PrePersist
+  private void prePersistDeriveIdField() {
+    setId(getGeneratedIdentifier());
+  }
+
   public long getKey() {
     return key;
   }

src/main/java/io/zeebe/monitor/entity/VariableEntity.java

@@ -15,18 +15,25 @@
  */
 package io.zeebe.monitor.entity;
 
-import javax.persistence.Column;
-import javax.persistence.Entity;
-import javax.persistence.Id;
-import javax.persistence.Lob;
+import javax.persistence.*;
 
 @Entity(name = "VARIABLE")
+@Table(indexes = {
+    // performance reason, because we use it in the VariableRepository.findByProcessInstanceKey()
+    @Index(name = "variable_processInstanceKeyIndex", columnList = "PROCESS_INSTANCE_KEY_"),
+})
 public class VariableEntity {
 
   @Id
+  @Column(name = "ID")
+  private String id;
+
   @Column(name = "POSITION_")
   private Long position;
 
+  @Column(name = "PARTITION_ID_")
+  private int partitionId;
+
   @Column(name = "NAME_")
   private String name;
 
@@ -46,6 +53,24 @@ public class VariableEntity {
   @Column(name = "TIMESTAMP_")
   private long timestamp;
 
+  public String getId() {
+    return id;
+  }
+
+  private void setId(final String id) {
+    // made private, to avoid accidental changes
+    this.id = id;
+  }
+
+  public final String getGeneratedIdentifier() {
+    return this.partitionId + "-" + this.position;
+  }
+
+  @PrePersist
+  private void prePersistDeriveIdField(){
+    setId(getGeneratedIdentifier());
+  }
+
   public String getState() {
     return state;
   }
@@ -101,4 +126,12 @@ public Long getPosition() {
   public void setPosition(final Long position) {
     this.position = position;
   }
+
+  public int getPartitionId() {
+    return partitionId;
+  }
+
+  public void setPartitionId(final int partitionId) {
+    this.partitionId = partitionId;
+  }
 }

src/main/java/io/zeebe/monitor/repository/ElementInstanceRepository.java

@@ -18,7 +18,7 @@
 import io.zeebe.monitor.entity.ElementInstanceEntity;
 import org.springframework.data.repository.CrudRepository;
 
-public interface ElementInstanceRepository extends CrudRepository<ElementInstanceEntity, Long> {
+public interface ElementInstanceRepository extends CrudRepository<ElementInstanceEntity, String> {
 
   Iterable<ElementInstanceEntity> findByProcessInstanceKey(long processInstanceKey);
 }

src/main/java/io/zeebe/monitor/repository/VariableRepository.java

@@ -20,7 +20,7 @@
 
 import java.util.List;
 
-public interface VariableRepository extends PagingAndSortingRepository<VariableEntity, Long> {
+public interface VariableRepository extends PagingAndSortingRepository<VariableEntity, String> {
 
   List<VariableEntity> findByProcessInstanceKey(long processInstanceKey);
 }

src/main/java/io/zeebe/monitor/zeebe/importers/ProcessAndElementImporter.java

@@ -17,11 +17,15 @@
 @Component
 public class ProcessAndElementImporter {
 
-  @Autowired private ProcessRepository processRepository;
-  @Autowired private ProcessInstanceRepository processInstanceRepository;
-  @Autowired private ElementInstanceRepository elementInstanceRepository;
+  @Autowired
+  private ProcessRepository processRepository;
+  @Autowired
+  private ProcessInstanceRepository processInstanceRepository;
+  @Autowired
+  private ElementInstanceRepository elementInstanceRepository;
 
-  @Autowired private ZeebeNotificationService notificationService;
+  @Autowired
+  private ZeebeNotificationService notificationService;
 
   public void importProcess(final Schema.ProcessRecord record) {
     final int partitionId = record.getMetadata().getPartitionId();
@@ -44,7 +48,6 @@ public void importProcessInstance(final Schema.ProcessInstanceRecord record) {
     if (record.getProcessInstanceKey() == record.getMetadata().getKey()) {
       addOrUpdateProcessInstance(record);
     }
-
     addElementInstance(record);
   }
 
@@ -97,13 +100,10 @@ private void addOrUpdateProcessInstance(final Schema.ProcessInstanceRecord recor
   }
 
   private void addElementInstance(final Schema.ProcessInstanceRecord record) {
-
-    final long position = record.getMetadata().getPosition();
-    if (!elementInstanceRepository.existsById(position)) {
-
-      final ElementInstanceEntity entity = new ElementInstanceEntity();
-      entity.setPosition(position);
-      entity.setPartitionId(record.getMetadata().getPartitionId());
+    final ElementInstanceEntity entity = new ElementInstanceEntity();
+    entity.setPartitionId(record.getMetadata().getPartitionId());
+    entity.setPosition(record.getMetadata().getPosition());
+    if (!elementInstanceRepository.existsById(entity.getGeneratedIdentifier())) {
       entity.setKey(record.getMetadata().getKey());
       entity.setIntent(record.getMetadata().getIntent());
       entity.setTimestamp(record.getMetadata().getTimestamp());
@@ -112,11 +112,8 @@ private void addElementInstance(final Schema.ProcessInstanceRecord record) {
       entity.setFlowScopeKey(record.getFlowScopeKey());
       entity.setProcessDefinitionKey(record.getProcessDefinitionKey());
       entity.setBpmnElementType(record.getBpmnElementType());
-
       elementInstanceRepository.save(entity);
-
-      notificationService.sendProcessInstanceUpdated(
-          record.getProcessInstanceKey(), record.getProcessDefinitionKey());
+      notificationService.sendProcessInstanceUpdated(record.getProcessInstanceKey(), record.getProcessDefinitionKey());
     }
   }
 

src/main/java/io/zeebe/monitor/zeebe/importers/VariableImporter.java

@@ -9,22 +9,21 @@
 @Component
 public class VariableImporter {
 
-  @Autowired private VariableRepository variableRepository;
+  @Autowired
+  private VariableRepository variableRepository;
 
   public void importVariable(final Schema.VariableRecord record) {
-
-    final long position = record.getMetadata().getPosition();
-    if (!variableRepository.existsById(position)) {
-
-      final VariableEntity entity = new VariableEntity();
-      entity.setPosition(position);
-      entity.setTimestamp(record.getMetadata().getTimestamp());
-      entity.setProcessInstanceKey(record.getProcessInstanceKey());
-      entity.setName(record.getName());
-      entity.setValue(record.getValue());
-      entity.setScopeKey(record.getScopeKey());
-      entity.setState(record.getMetadata().getIntent().toLowerCase());
-      variableRepository.save(entity);
+    final VariableEntity newVariable = new VariableEntity();
+    newVariable.setPosition(record.getMetadata().getPosition());
+    newVariable.setPartitionId(record.getMetadata().getPartitionId());
+    if (!variableRepository.existsById(newVariable.getGeneratedIdentifier())) {
+      newVariable.setTimestamp(record.getMetadata().getTimestamp());
+      newVariable.setProcessInstanceKey(record.getProcessInstanceKey());
+      newVariable.setName(record.getName());
+      newVariable.setValue(record.getValue());
+      newVariable.setScopeKey(record.getScopeKey());
+      newVariable.setState(record.getMetadata().getIntent().toLowerCase());
+      variableRepository.save(newVariable);
     }
   }
 

src/test/java/io/zeebe/monitor/repository/ElementInstanceRepositoryTest.java

@@ -0,0 +1,48 @@
+package io.zeebe.monitor.repository;
+
+import io.zeebe.monitor.entity.ElementInstanceEntity;
+import org.junit.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+
+import java.util.Optional;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class ElementInstanceRepositoryTest extends ZeebeRepositoryTest {
+
+  @Autowired
+  private ElementInstanceRepository elementInstanceRepository;
+
+  @Test
+  public void JPA_will_automatically_update_the_ID_attribute() {
+    // given
+    ElementInstanceEntity elementInstance = createElementInstance();
+
+    // when
+    elementInstanceRepository.save(elementInstance);
+
+    // then
+    assertThat(elementInstance.getId()).isEqualTo("123-456");
+  }
+
+  @Test
+  public void variable_can_be_retrieved_by_transient_ID() {
+    // given
+    ElementInstanceEntity elementInstance = createElementInstance();
+
+    // when
+    elementInstanceRepository.save(elementInstance);
+
+    // then
+    Optional<ElementInstanceEntity> entity = elementInstanceRepository.findById("123-456");
+    assertThat(entity).isPresent();
+  }
+
+  private ElementInstanceEntity createElementInstance() {
+    ElementInstanceEntity elementInstance = new ElementInstanceEntity();
+    elementInstance.setPartitionId(123);
+    elementInstance.setPosition(456L);
+    return elementInstance;
+  }
+
+}

src/test/java/io/zeebe/monitor/repository/TestContextJpaConfiguration.java

@@ -0,0 +1,60 @@
+package io.zeebe.monitor.repository;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.data.jpa.repository.config.EnableJpaRepositories;
+import org.springframework.jdbc.datasource.DriverManagerDataSource;
+import org.springframework.orm.jpa.JpaTransactionManager;
+import org.springframework.orm.jpa.JpaVendorAdapter;
+import org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean;
+import org.springframework.orm.jpa.vendor.HibernateJpaVendorAdapter;
+import org.springframework.transaction.PlatformTransactionManager;
+import org.springframework.transaction.annotation.EnableTransactionManagement;
+
+import javax.sql.DataSource;
+import java.util.Properties;
+
+@Configuration
+@EnableJpaRepositories(basePackages = "io.zeebe.monitor.repository")
+@EnableTransactionManagement
+public class TestContextJpaConfiguration {
+
+  @Bean
+  public DataSource dataSource() {
+    DriverManagerDataSource dataSource = new DriverManagerDataSource();
+    dataSource.setDriverClassName("org.h2.Driver");
+    dataSource.setUrl("jdbc:h2:mem:zeebe-monitor-test;DB_CLOSE_DELAY=-1");
+    dataSource.setUsername("sa");
+    dataSource.setPassword("");
+    return dataSource;
+  }
+
+  @Bean
+  public LocalContainerEntityManagerFactoryBean entityManagerFactory(@Autowired DataSource dataSource) {
+    LocalContainerEntityManagerFactoryBean em = new LocalContainerEntityManagerFactoryBean();
+    em.setDataSource(dataSource);
+    em.setPackagesToScan("io.zeebe.monitor.entity");
+
+    JpaVendorAdapter vendorAdapter = new HibernateJpaVendorAdapter();
+    em.setJpaVendorAdapter(vendorAdapter);
+    em.setJpaProperties(getAdditionalJpaProperties());
+
+    return em;
+  }
+
+  @Bean
+  public PlatformTransactionManager transactionManager(@Autowired LocalContainerEntityManagerFactoryBean entityManagerFactory) {
+    JpaTransactionManager transactionManager = new JpaTransactionManager();
+    transactionManager.setEntityManagerFactory(entityManagerFactory.getObject());
+    return transactionManager;
+  }
+
+  private Properties getAdditionalJpaProperties() {
+    Properties p = new Properties();
+    p.setProperty("database-platform", "org.hibernate.dialect.H2Dialect");
+    p.setProperty("hibernate.hbm2ddl.auto", "update");
+    return p;
+  }
+
+}