Run modes

A diagram showing architecture of running modes:

../_images/high-level-arc.png
Mode Parent class Components needed Available backends
Single process Backend single process running the crawler Memory, SQLAlchemy
Distributed spiders Backend spiders and single db worker Memory, SQLAlchemy
Distributed backends DistributedBackend spiders, strategy worker (s) and db worker(s). SQLAlchemy, HBase

Single process

Frontera is instantiated in the same process as fetcher (for example in Scrapy). To achieve that use BACKEND setting set to storage backend subclass of Backend. This run mode is suitable for small number of documents and time non-critical applications.

Distributed spiders

Spiders are distributed and backend isn’t. Backend is running in db worker and it’s communicating with spiders using message bus.

  1. Use BACKEND in spider processes set to MessageBusBackend
  2. In DB worker BACKEND should point to Backend subclass.
  3. Every spider process should have it’s own SPIDER_PARTITION_ID, starting from 0 to SPIDER_FEED_PARTITIONS.
  4. Both spiders and workers should have it’s MESSAGE_BUS setting set to the message bus class of your choice, and other implementation depending settings.

This mode is suitable for applications where it’s critical to fetch documents fast, at the same time amount of them is relatively small.

Distributed spiders and backend

Spiders and backend are distributed. Backend is divided on two parts: strategy worker and db worker. Strategy worker instances are assigned to their own part of spider log.

  1. Use BACKEND in spider processes set to MessageBusBackend
  2. In DB and SW workers BACKEND should point to DistributedBackend subclasses. And selected backend have to be configured.
  3. Every spider process should have it’s own SPIDER_PARTITION_ID, starting from 0 to SPIDER_FEED_PARTITIONS. Last must be accessible also to all DB worker instances.
  4. Every SW worker process should have it’s own SCORING_PARTITION_ID, starting from 0 to SPIDER_LOG_PARTITIONS. Last must be accessible to all SW worker instances.
  5. Both spiders and workers should have it’s MESSAGE_BUS setting set to the message bus class of your choice and selected message bus have to be configured.

Only Kafka message bus can be used in this mode out of the box and SQLAlchemy and HBase distributed backends.

This mode is suitable for broad crawling and large amount of pages.