# Configuration

Deimos supports a succinct, readable syntax which uses
pure Ruby to allow flexible configuration.

You can access any configuration value via a simple `Deimos.config.whatever`.

Nested configuration is denoted in simple dot notation:
`kafka.ssl.enabled`. Headings below will follow the nested
configurations.

## Base Configuration
Config name|Default|Description
-----------|-------|-----------
logger|`Logger.new(STDOUT)`|The logger that Deimos will use.
phobos_logger|`Deimos.config.logger`|The logger passed to Phobos.
metrics|`Deimos::Metrics::Mock.new`|The metrics backend use for reporting.
tracer|`Deimos::Tracer::Mock.new`|The tracer backend used for debugging.

## Defining Producers

You can define a new producer thusly:
```ruby
Deimos.configure do
  producer do
    class_name 'MyProducer'
    topic 'MyTopic'
    schema 'MyTopicSchema'
    namespace 'my.namespace'
    key_config field: :id

  # If config.schema.path is app/schemas, assumes there is a file in
  # app/schemas/my/namespace/MyTopicSchema.avsc
  end
end
```

You can have as many `producer` blocks as you like to define more producers.
|Config name|Default|Description
-----------|-------|-----------
class_name|nil|Class name of the producer class (subclass of `Deimos::Producer`.)
topic|nil|Topic to produce to.
schema|nil|Name of the schema to use to encode data before producing.
namespace|nil|Namespace of the schema to use when finding it locally.
key_config|nil|Configuration hash for message keys. See [Kafka Message Keys](../README.md#installation)

## Defining Consumers

Consumers are defined almost identically to producers:

```ruby
Deimos.configure do
  consumer do
    class_name 'MyConsumer'
    topic 'MyTopic'
    schema 'MyTopicSchema'
    namespace 'my.namespace'
    key_config field: :id

  # If config.schema.path is app/schemas, assumes there is a file in
  # app/schemas/my/namespace/MyTopicSchema.avsc
  end
end
```

In addition to the producer configs, you can define a number of overrides
to the basic consumer configuration for each consumer. This is analogous to
the `listener` config in `phobos.yml`.
|Config name|Default|Description
-----------|-------|-----------
class_name|nil|Class name of the consumer class (subclass of `Deimos::Consumer`.)
topic|nil|Topic to produce to.
schema|nil|This is optional but strongly recommended for testing purposes; this will validate against a local schema file used as the reader schema, as well as being able to write tests against this schema. This is recommended since it ensures you are always getting the values  you expect.
namespace|nil|Namespace of the schema to use when finding it locally.
key_config|nil|Configuration hash for message keys. See [Kafka Message Keys](../README.md#installation)
group_id|nil|ID of the consumer group.
max_concurrency|1|Number of threads created for this listener. Each thread will behave as an independent consumer. They don't share any state.
start_from_beginning|true|Once the consumer group has checkpointed its progress in the topic's partitions, the consumers will always start from the checkpointed offsets, regardless of config. As such, this setting only applies when the consumer initially starts consuming from a topic
max_bytes_per_partition|512.kilobytes|Maximum amount of data fetched from a single partition at a time.
min_bytes|1|Minimum number of bytes to read before returning messages from the server; if `max_wait_time` is reached, this is ignored.
max_wait_time|5|Maximum duration of time to wait before returning messages from the server, in seconds.
force_encoding|nil|Apply this encoding to the message payload. If blank it uses the original encoding. This property accepts values defined by the ruby Encoding class (https://ruby-doc.org/core-2.3.0/Encoding.html). Ex: UTF_8, ASCII_8BIT, etc.
delivery|`:batch`|The delivery mode for the consumer. Possible values: `:message, :batch, :inline_batch`. See Phobos documentation for more details.
session_timeout|300|Number of seconds after which, if a client hasn't contacted the Kafka cluster, it will be kicked out of the group.
offset_commit_interval|10|Interval between offset commits, in seconds.
offset_commit_threshold|0|Number of messages that can be processed before their offsets are committed. If zero, offset commits are not triggered by message processing
heartbeat_interval|10|Interval between heartbeats; must be less than the session window.
backoff|`(1000..60_000)`|Range representing the minimum and maximum number of milliseconds to back off after a consumer error.

## Kafka Configuration
Config name|Default|Description
-----------|-------|-----------
kafka.logger|`Deimos.config.logger`|Logger passed to RubyKafka.
kafka.seed_brokers|`['localhost:9092']`|URL for the Kafka brokers.
kafka.client_id|`phobos`|Identifier for this application.
kafka.connect_timeout|15|The socket timeout for connecting to the broker, in seconds.
kafka.socket_timeout|15|The socket timeout for reading and writing to the broker, in seconds.
kafka.ssl.enabled|false|Whether SSL is enabled on the brokers.
kafka.ssl.ca_cert|nil| A PEM encoded CA cert, a file path to the cert, or an Array of certs to use with an SSL connection.
kafka.ssl.client_cert|nil|A PEM encoded client cert to use with an SSL connection, or a file path to the cert.
kafka.ssl.client_cert_key|nil|A PEM encoded client cert key to use with an SSL connection.

## Consumer Configuration

These are top-level configuration settings, but they can be overridden
by individual consumers.

|Config name|Default|Description
-----------|-------|-----------
consumers.session_timeout|300|Number of seconds after which, if a client hasn't contacted the Kafka cluster, it will be kicked out of the group.
consumers.offset_commit_interval|10|Interval between offset commits, in seconds.
consumers.offset_commit_threshold|0|Number of messages that can be processed before their offsets are committed. If zero, offset commits are not triggered by message processing
consumers.heartbeat_interval|10|Interval between heartbeats; must be less than the session window.
consumers.backoff|`(1000..60_000)`|Range representing the minimum and maximum number of milliseconds to back off after a consumer error.
consumers.reraise_errors|false|Default behavior is to swallow uncaught exceptions and log to the metrics provider. Set this to true to instead raise all errors. Note that raising an error will ensure that the message cannot be processed - if there is a bad message which will always raise that error, your consumer will not be able to proceed past it and will be stuck forever until you fix your code. See also the `fatal_error` configuration. This is automatically set to true when using the `TestHelpers` module in RSpec.
consumers.report_lag|false|Whether to send the `consumer_lag` metric. This requires an extra thread per consumer.
consumers.fatal_error|`proc { false }`|Block taking an exception, payload and metadata and returning true if this should be considered a fatal error and false otherwise. E.g. you can use this to always fail if the database is available. Not needed if reraise_errors is set to true.

## Producer Configuration
|Config name|Default|Description
-----------|-------|-----------
producers.ack_timeout|5|Number of seconds a broker can wait for replicas to acknowledge a write before responding with a timeout.
producers.required_acks|1|Number of replicas that must acknowledge a write, or `:all` if all in-sync replicas must acknowledge.
producers.max_retries|2|Number of retries that should be attempted before giving up sending messages to the cluster. Does not include the original attempt.
producers.retry_backoff|1|Number of seconds to wait between retries.
producers.max_buffer_size|10_000|Number of messages allowed in the buffer before new writes will raise `BufferOverflow` exceptions.
producers.max_buffer_bytesize|10_000_000|Maximum size of the buffer in bytes. Attempting to produce messages when the buffer reaches this size will result in `BufferOverflow` being raised.
producers.compression_codec|nil|Name of the compression codec to use, or nil if no compression should be performed. Valid codecs: `:snappy` and `:gzip`
producers.compression_threshold|1|Number of messages that needs to be in a message set before it should be compressed. Note that message sets are per-partition rather than per-topic or per-producer.
producers.max_queue_size|10_000|Maximum number of messages allowed in the queue. Only used for async_producer.
producers.delivery_threshold|0|If greater than zero, the number of buffered messages that will automatically trigger a delivery. Only used for async_producer.
producers.delivery_interval|0|if greater than zero, the number of seconds between automatic message deliveries. Only used for async_producer.
producers.persistent_connections|false|Set this to true to keep the producer connection between publish calls. This can speed up subsequent messages by around 30%, but it does mean that you need to manually call sync_producer_shutdown before exiting, similar to async_producer_shutdown.
producers.schema_namespace|nil|Default namespace for all producers. Can remain nil. Individual producers can override.
producers.topic_prefix|nil|Add a prefix to all topic names. This can be useful if you're using the same Kafka broker for different environments that are producing the same topics.
producers.disabled|false|Disable all actual message producing. Generally more useful to use the `disable_producers` method instead.
producers.backend|`:kafka_async`|Currently can be set to `:db`, `:kafka`, or `:kafka_async`. If using Kafka directly, a good pattern is to set to async in your user-facing app, and sync in your consumers or delayed workers.

## Schema Configuration
|Config name|Default|Description
-----------|-------|-----------
schema.registry_url|`http://localhost:8081`|URL of the Confluent schema registry.
schema.path|nil|Local path to find your schemas.

## Database Producer Configuration
|Config name|Default|Description
-----------|-------|-----------
db_producer.logger|`Deimos.config.logger`|Logger to use inside the DB producer.
db_producer.log_topics|`[]`|List of topics to print full messages for, or `:all` to print all topics. This can introduce slowdown since it needs to decode each message using the schema registry. 
db_producer.compact_topics|`[]`|List of topics to compact before sending, i.e. only send the last message with any given key in a batch. This is an optimization which mirrors what Kafka itself will do with compaction turned on but only within a single batch.  You can also specify `:all` to compact all topics.

## Configuration Syntax

Sample: 

```ruby
Deimos.configure do
  logger Logger.new(STDOUT)
  # Nested config field
  kafka.seed_brokers ['my.kafka.broker:9092']

  # Multiple nested config fields via block
  consumers do
    session_timeout 30
    offset_commit_interval 10
  end

  # Define a new producer
  producer do
    class_name 'MyProducer'
    topic 'MyTopic'
    schema 'MyTopicSchema'
    key_config field: :id
  end

  # Define another new producer
  producer do
    class_name 'AnotherProducer'
    topic 'AnotherTopic'
    schema 'AnotherSchema'
    key_config plain: true
  end

  # Define a consumer
  consumer do
    class_name 'MyConsumer'
    topic 'TopicToConsume'
    schema 'ConsumerSchema'
    key_config plain: true
    # include Phobos / RubyKafka configs
    start_from_beginning true
    heartbeat_interval 10 
  end

end
```

Note that all blocks are evaluated in the context of the configuration object.
If you're calling this inside another class or method, you'll need to save
things you need to reference into local variables before calling `configure`.