Crusty - polite && scalable broad web crawler

Introduction

Broad web crawling is an activity of going through practically boundless web by starting from a set of locations(urls) and following outgoing links

It presents a unique set of challenges one must overcome to get a stable and scalable system, Crusty is an attempt to tackle on some of those challenges to see what's out here while having fun with Rust ;)

This particular implementation could be used to quickly fetch a subset of all observable internet, discover most popular domains/links

Built on top of crusty-core which handles all low-level aspects of web crawling

Key features

• Configurability && extensibility

  see a typical config file with some explanations regarding available options

• Fast single node performance

  Crusty is written in Rust on top of green threads running on tokio, so it can achieve quite impressive single-node performance even on a moderate PC

  Additional optimizations are possible to further improve this(mostly better html parsing, there are tasks that do not require full DOM parsing, this implementation does full DOM parsing mostly for the sake of extensibility and configurability)

  Additionally, Crusty has small and predictable memory footprint and is usually cpu/network bound. There is no GC pressure and no war over memory.

• Scalability

  Each Crusty node is essentially an independent unit which we can run hundreds of in parallel(on different machines of course), the tricky part is job delegation and domain discovery which is solved by a high performance sharded queue-like structure built on top of clickhouse(huh!).

  One might think, "clickhouse? wtf?!" but this DB is so darn fast while providing rich querying capabilities, indexing, filtering.

  The idea is basically a huge sharded table where each domain belongs to some shard(crc32(domain_name) % number_of_shards), now each Crusty instance can read from a unique subset of all those shards while can write to all of them(so called domain discovery). I.e. each shard is readable by only one Crusty instance by a single thread(it's important, to avoid interference in multi-node setups). On moderate installments(~ <16 nodes) such systems is viable as this, although if someone tries to take this to a mega-scale dynamic shard manager might be required...

  There is additional challenge of domain discovery deduplication in multi-node setups, - right now we dedup locally and on clickhouse(AggregatingMergeTree) but the more nodes we add the less efficient local deduplication becomes

  In big setups a dedicated dedup layer might be required, alternatively one might try to simply push overflowing deduplication jobs to clickhouse by creating more shards

• Basic politeness

  While we can crawl thousands of domains in parallel - we should absolutely limit concurrency on per-domain level to avoid any stress to crawled sites, see job_reader.default_crawler_settings.concurrency. It's also a good practice to introduce delays between visiting pages, see job_reader.default_crawler_settings.delay.

  Additionally, there are massive sites with millions of sub-domains(usually blogs) such as tumblr.com. Special care should be taken when crawling them, as such we implement a sub-domain concurrency limitation as well, see job_reader.domain_tail_top_n setting which defaults to 3 - no more than 3 sub-domains can be crawled concurrently

  Currently, there's no robots.txt support but this can be added easily(and will be)

• Observability

  Crusty uses tracing and stores multiple metrics in clickhouse that we can observe with grafana - giving a real-time insight in crawling performance

Getting started

• before you start

install docker && docker-compose, follow instructions at

https://docs.docker.com/get-docker/

https://docs.docker.com/compose/install/

• get Crusty source code

git clone https://github.com/let4be/crusty
cd crusty

• study config file and adapt to your needs, there are sensible defaults for a 100mbit channel, if you have more/less bandwidth or poor cpu you might need to adjust concurrency_profile

• build docker-compose build

• run CRUSTY_SEEDS=https://example.com docker-compose up (abortable with ctrl+c)

• to run in background CRUSTY_SEEDS=https://example.com docker-compose up -d

• to stop background run and retain crawling data docker-compose down

• to stop background run and erase crawling data(clickhouse/grafana) docker-compose down -v

• see Crusty live at http://localhost:3000/d/crusty-dashboard/crusty?orgId=1&refresh=5s

• to see running containers docker ps(should be 3 - crusty-grafana, crusty-clickhouse and crusty)

• to see logs: docker logs crusty

if you decide to build manually via cargo build, remember - release build is a lot faster(and default is debug)

In the real world usage scenario on high bandwidth channel docker might become too expensive,so it might be a good idea either to run directly or at least in network_mode = host

• external service dependencies - clickhouse and grafana

just use docker-compose, it's the recommended way to play with Crusty

however...

to create / clean db use this script(must be executed -in context- of clickhouse docker container)

grafana dashboard is exported as json model

Contributing

I'm open to discussions/contributions, - use github issues,

pull requests are welcomed ;)