Difference between Continious Crawl and Incremental Scheduled Crawl?

Question

I am trying to configure my environment, but am confused about which option to use for my search, Continious Crawl or Scheduled Incremental Crawl?

What is the real difference?

Answer 1

Incremental Crawls are search crawlers that gather the changes made since the last crawl. Incremental Crawls and Full Crawls cannot be run in parallel. They take charge and hog the whole crawler resource. This also makes it possible to have very outdated search results. An incremental crawl may not pick up changes that happen while the incremental crawl is running. Continuous Crawl fixes this problem. It may spawn several sessions in 15 minute intervals, that will pick up the changes as well as the changes made during the Continuous Crawl. It will maximize the freshness of your search content.

There is an excellent write up that explains the what's how's and why's of Continuous Crawl. http://blogs.technet.com/b/tothesharepoint/archive/2012/09/14/how-can-i-achieve-the-best-freshness-of-search-results-introducing-continuous-crawls-for-sharepoint.aspx

EDIT:

To elaborate a little... Let's say you have content changes constantly. It will take a little more than 15 minutes to process those changes. For each incremental crawl run that happens, the freshness of content decreases and decreases because the crawler needs to catch-up, but the content is getting updating faster than the crawler can crawl them. Thus will eventually lead to incremental crawl #7 will be updating content that was made during incremental crawl #5, as long as the content output continues. Once the content output slows, then the incremental crawl will catch-up. The interval doesn't matter here also because the incremental crawl will take the time it needs to finish, usually causing more than the interval.

Answer 2

Continuous crawl is created to try to help keeping your search index more up to date. The problem with incremental crawl is that only one can run at a time (for a content source) and it only updates the index when it's done.

Multiple continuous crawls can run at the same time (for the same content source) and they update the index continuously.

The main difference occurs if you sometimes have a burst of updates on content which it may take a long time to crawl.

For incremental crawl then it looks like this:

1. Burst of updates
2. Incremental crawl starts
3. More updates
4. Incremental crawl can't start because one is already running
5. More updates
6. Incremental crawl can't start because one is already running
7. More updates
8. Incremental crawl can't start because one is already running ...
9. Incremental crawl ends and updates index
10. New incremental crawl starts processing updates between 2 and 6
11. Second incremental crawl ends and updates index

So all the updates after 2 are first available after 11.

For Continuous Crawl it looks like this:

1. Burst of updates
2. Continuous crawl starts
3. More updates
4. Continuous crawl processing updates after 2 starts
5. More updates
6. Continuous crawl processing updates after 4 starts
7. More updates
8. Continuous crawl processing updates after 6 starts ...
9. Continuous crawl from 2 ends

The updates from 3 will be available as the Continuous Crawl from 4 get to it and updates the index.

A short summary of differences:

• Continuous crawl only works with SharePoint content
• Multiple continuous crawls can run simultaneously which can help getting more up to date index but also creates more load on the servers
• Continuous crawl updates the Index continuously