discollect

discollect

dis' collector.

distributed web scraper inspired by scrapy

Architecture

discollect is designed to power known-entrypoint, structured scrapes, unlike a "web crawler". You must have an entrypoint - an initial URL(s) that you're going to submit as a Task. From there, every URL you submit is able to be matched to a Handler, much like you would match routes in a web server and each handler returns more Tasks and datums, or an error.

By changing the entrypoints for a given plugin, we're able to change the nature of a scrape - collecting small amounts of priority information from a site using the same code that drives a full site scrape. i.e. for themoviedb.org, instead of entering at the the published list of all movie IDs on the site, we could enter at an individual movie level.

This architecture allows for large scale, distributed, structured scraping of a variety of websites.

These tasks are stored in the Task Queue, while information and statistics about individual jobs are stored in the Metastore. Also in the Metastore are detailed error reports from when Handlers return errors during execution and fail to retry.

Getting Started (Library Use)

discollect is fundamentally a very flexible library - 99% of the time users will want to implement their own plugins, writers, metastores and task queues to better fit into their environment. Natively, discollect comes with many building blocks to make this as easy as possible. A basic setup may be as easy as

package main

import (
    "log"

    "github.com/fortytw2/discollect"
    "github.com/fortytw2/discollect/metastore/sqlite"
    "github.com/your-org/plugins"
)

func main() {
    d, err := discollect.New(
        discollect.WithWriter(discollect.NewFileWriter("/output")),
        discollect.WithTaskQueue(discollect.NewInMemTaskQueue()),
        discollect.WithMetastore(sqlite.NewMetastore("discollect.db")),
        discollect.WithErrorReporter(discollect.)
    )

    err := d.RegisterPlugins(
        plugins.NewTMDBPlugin("api-key-here"),
        plugins.NewBlogCrawler(),
    )

    // parse all CLI options
    d.ParseFlags()

    // handle SIGTERM/SIGKILL gracefully
    go d.SignalHandler()

    // serve HTTP
    log.Fatal(d.ListenAndServe())
}

However, discollect also ships with a binary discollectd, which includes all task queues, metastores, and writers that are part of the core library. If you build plugins entirely in skylark or lua and do not need to implement any custom queues, metastores, or writers, this may be a good route to go.

Plugins

Plugins are written in either go, skylark, or lua. Go plugins require a recompilation of discollect to be deployed, whereas plugins built in skylark or lua simply require a reload.

discollect includes a large number of test helper functions to aid you in writing effective, fast tests for your network-based plugins. Read the docs to learn more. (DOC LINK HERE)

discollect also supports HTTP-based remote plugins, which can be scaled out easily and implemented in any language that can run an HTTP server.

Metastores

A metastore is the key to a discollect deployment, collecting task and scrape information as well as handling leader election (in a distributed, HA environment)

As always, discollect ships with a variety of metastore implementations for you to choose from

• SQLite3 (good for single node deployments, can work for worker deployments)
• PostgreSQL 9.4+ (for single master, many worker deployments)

Task Queues

A reliable task queue is the heart of any distributed work system. For discollect, there are many options to choose from

• Disk-backed, durable queue (only for single node deployments)
• Redis 3+ (BRPOPLPUSH based)
• Beanstalkd
• Amazon SQS
• Google Cloud Pub/Sub

Writers

When a datum is returned from a Handler, discollect can perform several actions natively

• POST an endpoint over HTTP with the datum encoded as JSON
• write the datum to a file per scrape run

However, it is very easy to extend discollect to perform any action with a datum by implementing the simple Writer interface -

// A Writer is able to process and output datums retrieved by Discollect plugins
type Writer interface {
	Write(ctx context.Context, datum interface{}) error
	io.Closer
}

Writers can also be composed using the discollect.NewMultiWriter(w ...Writer) Writer helper function.

Scheduled Scrapes

discollect packs a powerful, reliable, cron-based scheduler into every deployment that tightly integrates with plugin authorship and the built-in alerting capabilites.

Alerting and Monitoring

discollect is able to monitor internal scrape stability (i.e. number of tasks and datums per run of the same plugin and config over time) and alert to several different providers

• Slack
• IRC
• Pagerduty

Region-Aware Plugins & Proxy Rotation

For many scraping tasks, it's neccessary to only make requests from certain IPs.

AJAX Rendering

discollect can utilize headless Chrome to render the DOM of AJAX sites. 99% of the time this is wholly unneccessary, as you can emulate XHR requests from within Handlers. To enable, use discollect.RenderHeadless(r *http.Request) before making a request with client.Do

License

MIT