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Validate producer data quality

Conduktor Gateway offers a data quality producer policy that uses a SQL-like language to assert data quality before it’s being produced, based on a simple SQL statement. Records in the topic from the FROM clause have to match the WHERE clause for the statement in order to be considered valid. This is particularly useful if your data is plain JSON with no schema but it can also be applied to AVRO, Protobuf data.

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Example

You have a topic for orders with records in this form: 
{
  "id" : "B2EE6886-7FFF-4CAB-9B2A-CF0A06C9E648",
  "amount_cents": 12499,
  "currency": "EUR",
  "order_date": "2024-20-12T15:45:33Z"
}
You may want to ensure that the:
  • id is a valid UUID format
  • amount_cents is a positive integer and not too large
  • currency is one of your accepted currencies
  • order_date is in ISO 8601 format
This can be asserted with: 
SELECT
    *       -- ignored in this policy
FROM
    orders  -- topic to enforce the rule on
WHERE
        id REGEXP '^[0-9A-F-]{36}$'    -- 36 char UUID in hex with dash separators
    AND amount_cents REGEXP '[0-9]+'   -- amount must be an integer
    AND amount_cents > 0               -- ... greater than zero
    AND amount_cents < 1000000         -- ... and less than 1,000,000 cents
    AND currency REGEXP 'EUR|GBP|USD'  -- currency must be one of these three
    AND order_date REGEXP '^20[2-9][0-9]-[0-1][0-9]-[0-3][0-9]T[0-2][0-9]:[0-5][0-9]:[0-5][0-9]Z$'
                                       -- valid date, and after 2020
In the statement, the list of selected fields is actually ignored - the important parts are the FROM clause (specifying the topic that the policy is applied to) and the WHERE clause, which specifies the condition data must meet in order to be considered valid. i.e. If the select returns something, the record is valid. If it returns no results, the record is considered invalid. SELECT [ignored!] FROM [topic name] WHERE [field filter criteria] Only one topic can be specified in the FROM clause (joins will be ignored), and the topic name is matched explicitly (no regexp support). If a record does not match the WHERE clause, it will be rejected. There are a variety of options for this described in the actions below. Fields are assumed to be from the value of the record. The Interceptor currently supports values in JSON, AVRO and Protobuf formats.
Topic names with dash - characters in them must be double quoted, as the dash is not a valid character for a SQL name. E.g. for a topic our-orders you would need to use:SELECT * FROM "our-orders" WHERE ...Nested fields can be accessed as expected with dot notation in the WHERE clause, e.g.:address.street = 'Electric Avenue'

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WHERE clause

If you specify a field name in the WHERE clause that doesn’t exist in the record, the condition will always fail and the record will always be considered invalid. Fields in the WHERE clause have to exist in a record for it to be considered valid. The WHERE clause supports a subset of SQL operations:
  • The operators =, >, >=, <, <=, <> and REGEXP (RegExp MySQL Operator)
  • When providing more than one condition in the WHERE clause, only the AND
  • The IN clause is not supported, but can be approximated with a RegExp
  • By default, the fields in the WHERE clause are looked up from the value in the record. You can also filter by other parts of the record using the syntax below:
    • Record key (it also supports encoded keys which require a schema registry lookup):
      • Record key as string: - .. WHERE record.key = 'some thing'
      • Record key as schema: .. WHERE record.key.someValue.someChildValue = 'some thing'
    • Partition: .. WHERE record.partition = 1
    • Timestamp: .. WHERE record.timestamp = 98717823712
    • Header: .. WHERE record.header.someHeaderKey = 'some thing'
    • Offset: .. WHERE record.offset = 1

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Actions for invalid data

The policy acts on produce requests from Kafka clients which means it will often deal with a batch of multiple records spread over multiple topics and partitions. The policy can apply different effects to each request batch based on its configuration.
ActionDescription
BLOCK_WHOLE_BATCHIf any records in the batch are invalid, then block the whole batch. The produce request will fail for the client in this case.
BLOCK_ONLY_INVALID_RECORDSAny records that are invalid are removed from the batch, and all the valid records in the batch are saved in Kafka. If any records are written, a success response for that is returned to the client. When every message in the batch is invalid, an error is returned to the client.
AUDIT_LOG_ONLYFor any records in the produce request which are invalid, record this in the audit log only. All records still are saved in Kafka
THROTTLEIf any records in the produce request are invalid, throttle the producer for a certain amount of time (throttleTimeMs). All records are still saved in Kafka.

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Dead letter topic

If a dead letter topic service is configured for Gateway, you can optionally supply a topic name for this policy to use for any records which are considered invalid. This topic will be created with the default config for your Kafka setup. Any record that the policy considers invalid, is written to the dead letter topic and has some headers added for audit purposes. Please note that this is also done in the AUDIT_LOG_ONLY mode, even though the records in this mode are still written to the “real” topic.
HeaderMessage
X-ERROR-MSGMessage does not match the statement [ …]
X-TOPICThe topic that the message was intended to be written to
X-PARTITIONThe partition of the topic the message was intended to be written to
The generation of these headers can be disabled with the addErrorHeader configuration parameter (defaults to true). If no deadLetterTopic is configured for the policy, no messages will be written out in this manner.

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Audit log

Any policy violation is logged in the configured Gateway audit log. This is currently logged at the batch level for each topic in the produce request. There’s no per record audit - it identifies that a policy breach occurred for the produce request and identifies the tenant, username and client IP for the request.

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Configuration

KeyTypeDescription
statementStringSQL statement
schemaRegistryConfigSchema registrySchema registry config
actionActionData quality producer action
deadLetterTopicStringDead letter topic
addErrorHeaderboolean (default true)Adds the error information headers into dead letter topic
throttleTimeMsint (default: 100)Value to throttle with (only applicable when action is set to THROTTLE).
Schema registry
KeyTypeDefaultDescription
typestringCONFLUENTThe type of schema registry to use: choose CONFLUENT (for Confluent-like schema registries including OSS Kafka) or AWS for AWS Glue schema registries.
additionalConfigsmapAdditional properties to map to specific security-related parameters. For enhanced security, you can hide the sensitive values using environment variables as secrets.​
Confluent LikeConfiguration for Confluent-like schema registries
hoststringURL of your schema registry.
cacheSizestring50Number of schemas that can be cached locally by this Interceptor so that it doesn’t have to query the schema registry every time.
AWS GlueConfiguration for AWS Glue schema registries
regionstringThe AWS region for the schema registry, e.g. us-east-1
registryNamestringThe name of the schema registry in AWS (leave blank for the AWS default of default-registry)
basicCredentialsstringAccess credentials for AWS (see below section for structure)
AWS credentialsAWS credentials configuration
accessKeystringThe access key for the connection to the schema registry.
secretKeystringThe secret key for the connection to the schema registry.
validateCredentialsbooltruetrue / false flag to determine whether the credentials provided should be validated when set.
accountIdstringThe Id for the AWS account to use.
If you don’t supply a basicCredentials section for the AWS Glue schema registry, the client we use to connect will instead attempt to find the connection information it needs from the environment and the credentials required can be passed this way to Gateway as part of its core configuration. Find out more from AWS documentation. Read our blog about schema registry.

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Action

ActionDescription
BLOCK_WHOLE_BATCHIf one message is invalid, block the whole batch
BLOCK_ONLY_INVALID_RECORDSIf one message is invalid, block only the invalid message (all other messages in the batch are saved in Kafka)
AUDIT_LOG_ONLYIf messages are invalid, audit log only (all messages still are saved in Kafka)
THROTTLEIf messages are invalid, throttle the producer for a certain amount of time (throttleTimeMs)

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Example

{
  "name": "myDataQualityProducerPlugin",
  "pluginClass": "io.conduktor.gateway.interceptor.safeguard.DataQualityProducerPlugin",
  "priority": 100,
  "config": {
    "statement": "SELECT x FROM orders WHERE amount_cents > 0 AND amount_cents < 1000000",
    "schemaRegistryConfig": {
       "host": "http://schema-registry:8081"
    },
    "action": "BLOCK_WHOLE_BATCH",
    "deadLetterTopic": "dead-letter-topic",
    "addErrorHeader": false
  }
}

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Validate schema payload

To enable your Kafka consumers to confidently and independently access data, ensure that all records sent through your Kafka system conform to an agreed structure. Records with missing or invalid schemas can cause application outages, as consumers may be unable to process the unexpected record format. Moreover, the use of schemas can broadly only assert structural correctness of data and a level of compatibility for those structures. When it comes to the values in a record often all you can assert is a basic data type (integer, string, double etc.). This means that using a valid schema solves some concerns around data quality, there are other concerns to be dealt with in a bespoke or distributed manner across all the clients of a given data type. Finally, while correct structure can be enforced in a Kafka ecosystem at a client level - each client needs to ensure that it knows and follows the expectations for the data. You cannot prevent one client correctly writing AVRO to a topic, while another one writes plain JSON to the same topic. If one client doesn’t know the rules, it can’t follow them.

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Enforce centralized policies

The schema validation Interceptor provides functionality that can be configured once in your Kafka system on the source for data (a topic), to ensure that:
  • All records produced to Kafka have a schema set
  • The record contents adhere to that schema
  • The fields (values) in any given record comply to business validation rules you have set in the schema
This policy provides a centralized enforcement of the validation of these rules at the point of write to Kafka. This enforcement cannot be bypassed or ignored by a client, so provides a strong guarantee that data actually written in to Kafka does match your rule set. Read our blog about schema registry.
How does the Policy Work?
The policy operates on Produce Requests made to Kafka, and will inspect the entire batch of records in a request. Based on its setup, it performs various checks and then will take an action if it finds any problems. The first important thing to note is that the Policy will do nothing if there is no Audit Log configured for Gateway (as it does not want to silently reject any data). So for the policy to work at all, you must have the Audit Log configured. Next point of note is that the policy will only check the value for a Kafka record, and does not currently support checking the key or headers for a record. The core config values for the policy itself are:
  • topic : the topic/s to apply the rule to
  • schemaIdRequired : whether records must/must not have a schema assigned to them
  • validateSchema : whether the policy should check if the data for the record matches the schema found for the record.
  • action : what to do if a problem is found
There are three levels of check you can apply:
SetupEffect
schemaIdRequired = falseEnsures that no records have a schema!
schemaIdRequired = true, validateSchema = falseEnsures that records have a valid schema set, and that schema exists in the schema registry. Does not check whether the value actually matches the schema though.
schemaIdRequired = true, validateSchema = trueEnsures that records have a valid schema set, that schema exists in the schema registry and that the value in the record matches the schema. This includes any data validation rules in the schema (see below) as well as a structural check.

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Action

If any problems are found, the policy will take an action as configured. The action can be set to one of:
  • BLOCK → If any records in the batch fail the policy checks, record the problems in the audit log and then return an error to the client failing the entire batch. No records are written to Kafka at all if at least one of the records in the batch is considered invalid.
  • INFO → In this mode the data is always written to Kafka whether it passes the checks or not - but any problems found recorded in the audit log.
  • THROTTLE → If any records in the batch fail the policy checks, the data is still written to Kafka but the request will be throttled with time = throttleTimeMs, forcing the client to back off. Any problems found are recorded in the audit log.

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Dead letter topic

If a dead letter topic service is configured for Gateway, then you can optionally supply a topic name for this policy to use for any records which are considered invalid. This topic will be created with the default config for your Kafka setup. Any record which the policy considers invalid is written to the dead letter topic, and has some headers added for audit purposes. Please note that this is done in the AUDIT_LOG_ONLY mode also, even though the records in this mode are still written to the “real” topic.
HeaderMessage
X-ERROR-MSGDescription of the reason for the policy failure
X-TOPICThe topic the message was intended to be written to
X-PARTITIONThe partition of that topic the message was intended to be written to
The generation of these headers can be disabled if required, through the addErrorHeader configuration parameter (defaults to true). If no deadLetterTopic is configured for the policy, then no messages will be written out in this manner.

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Configuration

The full configuration topics for the policy are as below.
NameTypeDefaultDescription
topicString.*Topics that match this regex will have the Interceptor applied
schemaIdRequiredBooleanfalseRecords must/must not have schemaId
validateSchemaBooleanfalseIf true, deserialize the record, validate the record structure and fields within the data itself.
actionBLOCK, INFO, THROTTLEBLOCKAction to take if the value is outside the specified range.
schemaRegistryConfigSchema registryN/ASchema registry Config
celCacheSizeint100In memory cache size for cel expressions, balancing speed and resource use, optimize performance.
deadLetterTopicStringDead letter topic. Not used if this parameter is not set.
addErrorHeaderBooleantrueAdd or not add the error information headers into dead letter topic
throttleTimeMsint100Value to throttle with (only applicable when action is set to THROTTLE).

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Schema registry

KeyTypeDefaultDescription
typestringCONFLUENTThe type of schema registry to use: choose CONFLUENT (for Confluent-like schema registries including OSS Kafka) or AWS for AWS Glue schema registries.
additionalConfigsmapAdditional properties maps to specific security-related parameters. For enhanced security, you can hide the sensitive values using environment variables as secrets.​
Confluent LikeConfiguration for Confluent-like schema registries
hoststringURL of your schema registry.
cacheSizestring50Number of schemas that can be cached locally by this Interceptor so that it doesn’t have to query the schema registry every time.
AWS GlueConfiguration for AWS Glue schema registries
regionstringThe AWS region for the schema registry, e.g. us-east-1
registryNamestringThe name of the schema registry in AWS (leave blank for the AWS default of default-registry)
basicCredentialsstringAccess credentials for AWS (see below section for structure)
AWS CredentialsAWS Credentials Configuration
accessKeystringThe access key for the connection to the schema registry.
secretKeystringThe secret key for the connection to the schema registry.
validateCredentialsbooltruetrue / false flag to determine whether the credentials provided should be validated when set.
accountIdstringThe Id for the AWS account to use.
If you do not supply a basicCredentials section for the AWS Glue schema registry, the client we use to connect will instead attempt to find the connection information is needs from the environment, and the credentials required can be passed this way to Gateway as part of its core configuration. More information on the setup for this is found in the AWS documentation. Read our blog about schema registry.

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Example

{
  "name": "mySchemaIdValidationInterceptor",
  "pluginClass": "io.conduktor.gateway.interceptor.safeguard.SchemaPayloadValidationPolicyPlugin",
  "priority": 100,
  "config": {
    "topic": "topic_1.*",
    "schemaIdRequired": true,
    "validateSchema": true,
    "schemaRegistryConfig": {
      "host": "http://schema-registry:8081"
    },
    "action": "BLOCK",
    "celCacheSize": 100
  }
}
Schema registry with secured template
{
  "name": "mySchemaIdValidationInterceptor",
  "pluginClass": "io.conduktor.gateway.interceptor.safeguard.SchemaPayloadValidationPolicyPlugin",
  "priority": 100,
  "config": {
    "topic": "topic_1.*",
    "schemaIdRequired": true,
    "validateSchema": true,
    "schemaRegistryConfig": {
      "host": "http://schema-registry:8081",
      "additionalConfigs": {
        "schema.registry.url": "${SR_URL}",
        "basic.auth.credentials.source": "${SR_BASIC_AUTH_CRED_SRC}",
        "basic.auth.user.info": "${SR_BASIC_AUTH_USER_INFO}"
      }
    },
    "action": "BLOCK",
    "celCacheSize": 100
  }
}

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Schema payload validations

When configured to do so, the schema validation Interceptor supports validating the value in a Kafka record against a specific set custom constraints for AvroSchema records. This is similar to the validations provided by JsonSchema, such as: For fields in an Avro schema, you can specify specific constraints on what is considered a correct value. These rules operate on the specific fields value only.
  • INT, LONG, FLOAT, DOUBLE: minimum, maximum, exclusiveMinimum, exclusiveMaximum, multipleOf
  • STRING: minLength, maxLength, pattern, format
  • ARRAY: maxItems, minItems
Current supported String format values:
  • byte, date, time, date-time, duration, uri, uri-reference, uri-template, uri, email, hostname, ipv4, ipv6, regex, uuid, json-pointer, json-pointer-uri-fragment, relative-json-pointer
This Interceptor also supports further validating elements from the whole payload against specific custom constraints - or Metadata Rules - using an expression based on the CEL Common Expression Language format. This provides a means to define more advanced rules dependent on multiple values in a record.

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Metadata rule

KeyTypeDescription
namestringRule name
expressionstringCEL expression for validation, must return BOOLEAN
messagestringError message if payload not matches the expression with namespace message. represents for produced message

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JSON schema example

In JSON schema, constraints and rules are defined directly in the schema. Here’s an example that includes various validations: 
{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "type": "object",
  "properties": {
    "name": {
      "type": "string",
      "minLength": 3,
      "maxLength": 50,
      "expression": "size(name) >= 3"
    },
    "age": {
      "type": "integer",
      "minimum": 0,
      "maximum": 120,
      "expression": "age >= 0 && age <= 120"
    },
    "email": {
      "type": "string",
      "format": "email",
      "expression": "email.contains('foo')"
    },
    "address": {
      "type": "object",
      "properties": {
        "street": {
          "type": "string",
          "minLength": 5,
          "maxLength": 10,
          "expression": "size(street) >= 5 && size(street) <= 10"
        },
        "city": {
          "type": "string",
          "minLength": 2,
          "maxLength": 50
        }
      },
      "expression": "size(address.street) > 1 && address.street.contains('paris') || address.city == 'paris'"
    },
    "hobbies": {
      "type": "array",
      "items": {
        "type": "string"
      },
      "minItems": 3,
      "expression": "size(hobbies) >= 3"
    }
  },
  "metadata": {
    "rules": [
      {
        "name": "check hobbies size and name",
        "expression": "size(message.hobbies) == 3 && size(message.name) > 3",
        "message": "hobbies must have 3 items"
      },
      {
        "name": "checkAge",
        "expression": "message.age >= 18",
        "message": "age must be greater than or equal to 18"
      },
      {
        "name": "check email",
        "expression": "message.email.endsWith('yahoo.com')",
        "message": "email should end with 'yahoo.com'"
      },
      {
        "name": "check street",
        "expression": "size(message.address.street) >= 3",
        "message": "address.street length must be greater than equal to 3"
      }
    ]
  }
}

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Avro schema example

In Avro, constraints and rules are defined directly in the schema. Here’s an example that includes various validations: 
{
  "type": "record",
  "name": "User",
  "fields": [
    {"name": "name", "type": "string", "minLength": 3, "maxLength": 50, "expression": "size(name) >= 3 && size(name) <= 50"},
    {"name": "age", "type": "int", "minimum": 0, "maximum": 120, "expression": "age >= 0 && age <= 120"},
    {"name": "email", "type": "string", "format": "email", "expression": "email.contains('foo')"},
    {
      "name": "address",
      "type": {
        "type": "record",
        "name": "AddressRecord",
        "fields": [
          {"name": "street", "type": "string", "minLength": 5, "maxLength": 100, "expression": "size(street) >= 5 && size(street) <= 10"},
          {"name": "city", "type": "string", "minLength": 2, "maxLength": 50}
        ]
      },
      "expression": "size(address.street) >= 5 && address.street.contains('paris') || address.city == 'paris'"
    },
    {"name": "hobbies", "type": {"type": "array", "items": "string"}, "minItems": 3, "expression": "size(hobbies) >= 3"},
    {
      "name": "friends",
      "type": {
        "type": "array",
        "items": {
          "type": "record",
          "name": "Friend",
          "fields": [
            {"name": "name", "type": "string", "expression": "size(name) < 3"},
            {"name": "age", "type": "int", "minimum": 2, "maximum": 10}
          ]
        }
      }
    }
  ],
  "metadata": {
    "rules": [
      {
        "name": "check hobbies size and name",
        "expression": "size(message.hobbies) == 3 && size(message.name) > 3",
        "message": "hobbies must have 3 items"
      },
      {
        "name": "checkAge",
        "expression": "message.age >= 18",
        "message": "age must be greater than or equal to 18"
      },
      {
        "name": "check email",
        "expression": "message.email.endsWith('yahoo.com')",
        "message": "email should end with 'yahoo.com'"
      },
      {
        "name": "check street",
        "expression": "size(message.address.street) >= 3",
        "message": "address.street length must be greater than equal to 3"
      }
    ]
  }
}