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Improve web data extraction by reusing stored responses while they are fresh, validating stale responses with HTTP validators, and pacing requests to match each site’s tolerance. Caching avoids repeated downloads and parsing; concurrency and delay control how quickly requests are sent. They solve different problems, and both need to fit the freshness your job requires.
How caching reduces repeated work
An HTTP cache stores a response associated with a request and can reuse it while it is fresh. For an extraction pipeline, a useful cache policy balances fewer transfers and less parsing against how current the extracted data needs to be. A cache that serves old pages beyond the job’s acceptable age may save work but deliver the wrong result. MDN’s HTTP caching guide explains the relevant cache behavior.
Choose directives deliberately
max-agesets a freshness lifetime.no-cachepermits storage, but requires validation before reuse.no-storeprevents storage.- For personalized responses, consider whether a shared cache could expose or reuse data incorrectly; MDN describes
privateas a way to avoid shared-cache reuse.
Do not apply headers as blanket rules without understanding how your cache implements them. The right policy depends on the data’s update frequency, whether responses vary by user or request, and how stale the extraction can be.
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When a cached response is stale, retain its validators and ask the server whether the resource has changed. Send If-None-Match with the stored ETag when one is available. A stored Last-Modified value can instead be sent with If-Modified-Since. These conditional requests let the server indicate whether the stored representation can still be used. See MDN’s conditional requests guide and its ETag reference.
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What a 304 means to an extractor
304 Not Modified means the representation has not changed according to the validator. The response does not retransmit the representation body, so the extractor can reuse the body it already has and refresh the cache’s validity. If the resource changed, the server returns a new representation for the client to store and process. A 304 is useful only if the client retained the prior body and can associate it with the validation request.
Separate production caching from development replay
Scrapy provides HTTP cache middleware, storage backends, and policies. Its documentation describes filesystem and DBM storage and two policies with different purposes. Configure HTTPCACHE_STORAGE and HTTPCACHE_POLICY for the intended workflow, and check the documentation matching your installed Scrapy version because documentation versions can differ. Scrapy’s downloader middleware documentation covers the options.
RFC2616 policy
Scrapy’s RFC2616 policy is HTTP-cache-aware. It is the relevant choice when the crawler should use HTTP cache-control behavior and validators rather than treating every recorded response as freely reusable.
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Dummy policy
The Dummy policy is useful for deterministic replay and development, but it treats requests as cached without HTTP cache-control awareness. That makes it handy for repeating a known crawl locally; it is not a substitute for a production freshness policy.
Set request pace based on the target site
Concurrency and delay determine how quickly a crawler sends requests; they do not determine whether stored response bytes are fresh. Increasing concurrency is not automatically faster. Scrapy warns that exceeding a site’s tolerance can lead to throttling, errors, or bans, which can slow the overall crawl. Tune settings against observed behavior and the target’s tolerance rather than choosing a universal request rate. See Scrapy’s optimization guide.
Scrapy settings to tune
CONCURRENT_REQUESTSlimits overall concurrent requests.CONCURRENT_REQUESTS_PER_DOMAINcontrols concurrency for a domain.DOWNLOAD_DELAYadds a delay between downloads.
Scrapy’s cited optimization guide says it does not act on robots.txt Crawl-delay and Request-rate directives. Where applicable, translate those directives into crawler settings, and verify the behavior of the Scrapy version you deploy.
A practical tuning loop
- Start with conservative per-domain concurrency and a delay suitable for the target.
- Observe response latency, errors, throttling signals, and whether the extracted data remains fresh enough.
- Change concurrency or delay incrementally, then compare the same workload under the same freshness requirement.
- Back off if errors or throttling increase; a higher request rate that causes failed work is not a performance improvement.
There is no universally fastest configuration established by the cited material. Site behavior, response times, and freshness needs vary, so measure your own workload rather than relying on a generic speed-up claim.
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Robots rules are operational input to a crawler and should not be treated like arbitrary page content. RFC 9309 says a crawler should not use a cached robots.txt copy for more than 24 hours unless the file is unreachable. The standard distinguishes an unavailable file from an unreachable one; for an unreachable robots.txt caused by server or network errors, it specifies that crawlers must assume complete disallow. Consult the RFC for the exact response handling rather than treating every fetch failure the same way.
Best Value
Measure whether the pipeline actually improved
Collect workload metrics before and after changing cache or pacing settings. Compare like with like: the same targets and the same acceptable data age. These are measurements to collect, not published benchmarks or guaranteed outcomes.
- Cache hit rate and bytes transferred
- Response latency and extraction or parsing time
- Error and throttle rates
- Age of the data when the extraction completes
A higher cache hit rate alone is not success if it means serving data that is too old. Likewise, fewer requests are not an improvement if a crawl misses required updates.
Or skip the browser setup
For screenshot-based extraction, ScreenshotNeo is a website screenshot API and MCP server: one GET request can return a PNG, JPEG, WebP, or PDF. For the DIY pipeline above, its response headers also report whether a page was billed and its verdict, so a cache hit or failed capture is not silently treated like an ordinary successful shot. Its parameter names match those used by other screenshot APIs, which can make switching easier.
Quick Recap
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpSee the ScreenshotNeo documentation for API details. Cookie banners are accepted like a visitor and removed before the shot, along with supported newsletter popups and chat widgets. Bot checks, blank pages, failed loads, timeouts, and cache hits are not billed. An MCP server lets AI agents use screenshot tools, and 1,000 screenshots a month are free with no card; paid plans start at $5 for 3,000. Sign up for the free plan.
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