The term FAST Search may not be entirely new to you if you managed SharePoint sites anytime during the last decade. With Microsoft Exchange Server 2016, the FAST search architecture is introduced to Exchange. With FAST search, various new features are made available to Exchange 2016 mailbox users, including Search suggestions and People suggestions. These suggestions are dynamically computed based on your previous searches and the names of people that you frequently communicate with. The new feature set is readily available when using Outlook on the Web to connect to a mailbox on Exchange Server 2016 or Exchange Online.* Additionally, the newer Outlook 2016 desktop client also takes advantage of the new FAST search when connecting to these same mailboxes.
* Note Exchange Online on Microsoft 365 is migrating search features from FAST to Microsoft Search. However, on-premises Exchange Server will continue to use FAST search. For more information about the migration of Exchange Online search features to Microsoft Search, see How Outlook for Windows connected to Exchange Online utilizes Microsoft Search.
As far back as in Outlook 2007, Outlook has utilized Windows Desktop Search (WDS) to index Outlook Data (.pst) and Offline Outlook Data (.ost) files. These indexes are created on the local disk. At the same time, Outlook uses the same WDS architecture to search these local indexes when you initiate a search. The use of WDS in Outlook is sometimes referred to as Instant Search.
Let us focus strictly on searching Exchange mailbox content... Exchange content is indexed by WDS when Outlook is configured to connect to the Exchange mailbox using Cached Exchange Mode. When an Exchange email account is in Cached Exchange Mode, the mailbox contents are synchronized to the local .ost file. It is this local file that is indexed by WDS. WDS does not index the mailbox store directly. Why am I still talking about WDS? Because it is still relevant, even when using the latest and greatest versions of Outlook and Exchange.
In Cached Exchange Mode, Outlook 2016, Outlook 2019, and Outlook for Office 365 still query WDS desktop search indexes... and they query Exchange 2016 and Exchange 2019 using FAST Search.
Assuming FAST Search is not disabled by policy, Outlook initially submits a query to Exchange on-premises using FAST Search. If FAST Search results are returned in a timely manner (about 5 seconds), these are displayed. But, if the FAST Search timeout is reached before a server response, then a message indicating that We're having trouble fetching results from the server..., along with the link "Let's look on your computer instead." If you click this link, then WDS search results are displayed.
Note Outlook also assesses the performance of the network connection and if it determines there is latency, it may default to displaying WDS search results.
But wait, there's more! Some search scopes set search criteria that cannot be processed by FAST. If you set the scope to All Outlook Items, Subfolders, or All Mailboxes Outlook defaults to WDS results. The search scope is set next to the Search text box, as shown here:
Additionally, if you type search criteria that is not supported by FAST, Outlook returns WDS results (of course, assuming WDS supports the criteria). For example, Outlook supports the use of natural language queries, such as received:this year. However, Outlook FAST search does not. Therefore, if you submit received:this year, Outlook automatically displays WDS search results. To use FAST search for a similar query, you would need to submit the following criteria: received>12/31/2016.
To recap, Outlook uses FAST search and/or returns results using FAST, except if any one of the following conditions are true:
To determine which results Outlook is displaying, you can focus on some small user interfaces cues.
If the results displayed are those generated by FAST, the bottom of your message list will display the following cue:
If there are more than 75 results from FAST, the bottom of your message list may display the following message until reaching the service or client limit, whichever is smaller:
As mentioned earlier, if the service times out or is unavailable, the message list will display the link “Let’s look on your computer instead.”
If you click Let’s look on your computer instead, the results displayed are those generated by WDS. Of course, this is assuming that your local copy of the .ost cache contains items that match your serach criteria. The items that you are searching for may not be locally cached if a limited time window is specified for the Mail to keep offline setting. When this occurs, no local WDS results are returned, but you are given the option to check the Exchange mailbox by clicking Find more on the server:
Just as earlier Outlook 2016 client versions did, clicking More sends a non-FAST query to Exchange. This is because the client has already determined that FAST search to the service is not available… no need to go back and try that again.
And yes, Exchange Server 2016 supports both FAST and legacy Exchange Search functionality. The latter is still required for backward compatibility with Outlook 2013 and earlier versions.
When Outlook 2016 released, the plan was to only support FAST search on your primary mailbox. However, starting in the Fall of 2017, Microsoft 365 Apps (formerly known as Office 365 ProPlus) in Current Channel and Enterprise Channels, and the latest Office 2019 version, now fully support FAST Search for any Exchange email accounts added through File | Account Settings (also known as MultiEx or multiple Exchange accounts). This does not apply to Outlook 2016 installed with the Office 2016 suite that uses Windows Installer (MSI) installation technology.
If you are limited to using an older build of Outlook 2016 or one installed using the MSI installation technology, searching in a MultiEx mailbox may submit the FAST query in the context of the logged on user (you). This causes the search results to be pulled from your own mailbox, not the secondary mailbox that currently has the focus in your Outlook client. If you are unable to upgrade your Outlook 2016 client to the November 2017 Monthly Channel Build, you can temporarily work around this limitation by using one of the following options:
Note Other types of secondary stores, such as those added via Open these additional mailboxes or those that are automatically configured via AutoMapping, are still limited to WDS.
Currently, the service limits the results to 1000 results. However, Outlook can further limit the results to 250 if the File, Options, Search group, Improve search speed by limiting the number of results shown checkbox is enabled.
To disable FAST search (server assisted search) and revert back to using WDS, an administrator can apply a group policy or implement the DisableServerAssistedSearch and DisableServerAssistedSuggestions user registry values. These are documented in the following table, along with other registry values to allow you to control additional FAST search functionality.
Note The 16.0 registry subkey name is valid for both Outlook 2016 and Outlook 2019.
Disable Server Assisted Search |
Group Policy registry path:
|
Disables Outlook from requesting and using Search results from Exchange for cached and non-cached mailbox items. Instead, it will use search results from Windows search service. Set to 1 to enforce. |
Disable Server Assisted Suggestions |
Group Policy registry path:
|
Disables Outlook from requesting search suggestions from Exchange. Set to 1 to enforce. |
Default Server Latency Limit |
Group Policy registry path:
|
If the connection to Exchange is slower than DefaultServerLatencyLimit in milliseconds, then do not issue an Exchange Server Assisted search (will only use local search). |
Server Assisted Search Timeout |
Group Policy registry path:
|
Timeout in milliseconds that Outlook should wait for Exchange to provide search results before falling back to use local search. |
Search Suggestion Keyword Max Display |
Group Policy registry path:
|
Maximum number of keyword suggestions displayed in Search textbox. |
Search Suggestion People Max Display |
Group Policy registry path:
|
Maximum number of people suggestions displayed in Search textbox. |
Search Suggestion Total Max Display |
Group Policy registry path:
|
Maximum total number of suggestions displayed in Search textbox. |
Wordwheel Time Out Exchange Search |
Group Policy registry path:
|
The number of milliseconds that Outlook will wait after text changes in the Search textbox before executing a Search using the Instant Search (FAST) API. |
Search Suggestion Time Out |
Group Policy registry path:
|
The number of milliseconds that Outlook will wait after text changes in the Search textbox before issuing a query to FAST for suggestions. |
Special thanks to Paul Slaathaug for reminding me to get this blog wrapped up and for providing additional topics to cover.
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