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© Rakhesh Sasidharan

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[Aside] Various Exchange 2013 links

I am reading up on Exchange 2013 nowadays (yes, I know, bit late in the day to be doing that considering it is going out of support :) and these are some links I want to put here as a bookmark to myself. Some excellent blog posts and videos that detail the changes in Exchange 2013.

(By way of background: I am not an Exchange admin. I am Exchange 2010 certified as I have a huge interest in Exchange and as part of preparing for the certification I had attended a course and setup a lab on my laptop and even begun this blog to start posting about my adventures with it. I never got to work with Exchange 2010 at work – except as a helpdesk administrator one could say – but I am familiar with the concepts even though I have forgotten more than I remember. I have dabbled with Exchange 2000 before that. Going through these links and videos is like a trip down memory line – seeing concepts that I was once familiar with but have since changed for the better. Hopefully this time around I get to do more Exchange 2013 work! Fingers crossed).

If you don’t like reading, start with this video.

Alternatively, start with these links but I’d strongly recommend watching the above video once you finish reading.

Preferred Architecture

From the preferred architecture link I like to highlight this point about DAG design as I wasn’t aware of it (PA == Preferred Architecture; this is also discussed in the video):

Data resiliency is achieved by deploying multiple database copies. In the PA, database copies are distributed across the site resilient datacenter pair, thereby ensuring that mailbox data is protected from software, hardware and even datacenter failures.

Each database has four copies, with two copies in each datacenter, which means at a minimum, the PA requires four servers. Out of these four copies, three of them are configured as highly available. The fourth copy (the copy with the highest Activation Preference) is configured as a lagged database copy. Due to the server design, each copy of a database is isolated from its other copies, thereby reducing failure domains and increasing the overall availability of the solution as discussed in DAG: Beyond the “A”.

The purpose of the lagged database copy is to provide a recovery mechanism for the rare event of system-wide, catastrophic logical corruption. It is not intended for individual mailbox recovery or mailbox item recovery.

The lagged database copy is configured with a seven day ReplayLagTime. In addition, the Replay Lag Manager is also enabled to provide dynamic log file play down for lagged copies. This feature ensures that the lagged database copy can be automatically played down and made highly available in the following scenarios:

  • When a low disk space threshold is reached
  • When the lagged copy has physical corruption and needs to be page patched
  • When there are fewer than three available healthy copies (active or passive) for more than 24 hours

By using the lagged database copy in this manner, it is important to understand that the lagged database copy is not a guaranteed point-in-time backup. The lagged database copy will have an availability threshold, typically around 90%, due to periods where the disk containing a lagged copy is lost due to disk failure, the lagged copy becoming an HA copy (due to automatic play down), as well as, the periods where the lagged database copy is re-building the replay queue.

With all of these technologies in play, traditional backups are unnecessary; as a result, the PA leverages Exchange Native Data Protection.

The last line made me smile. Never thought I’d read someplace that backups for Exchange are unnecessary! :) If you have a lagged copy database, then you can enable circular logging on the database (this only affects the non-lagged copies) and skip taking backups – or at least not worry about the database dismounting because your backups are failing and logs are filling up disk space!

So what’s a lagged database copy? Basically it’s a copy of the database (in a DAG) that lags behind other members by a specified duration (maximum is 14 days). So if the other servers in your DAG have some issue, rather than restore the database from backup you can simply “play down” the lagged database copy (i.e. tell that copy to process all the transaction logs it already has an thus become up-to-date) and activate it. Neat, huh. I want to delve a bit more into this, so check out this “Lagged copy enhancements” section from the Exchange 2013 HA improvements page.

First there’s Safety Net (it’s not related to lagged copies, but it plays along well with it in a cool way so worth pointing out):

Safety Net is a feature of transport that replaces the Exchange 2010 feature known as transport dumpster. Safety Net is similar to transport dumpster, in that it’s a delivery queue that’s associated with the Transport service on a Mailbox server. This queue stores copies of messages that were successfully delivered to the active mailbox database on the Mailbox server. Each active mailbox database on the Mailbox server has its own queue that stores copies of the delivered messages. You can specify how long Safety Net stores copies of the successfully delivered messages before they expire and are automatically deleted.

Ok – so each mailbox server has a queue for each of its active database (remember lagged copies are active too, just that they have a higher number and hence not preferred). This queue contains messages that were delivered. Even after a message is delivered to a user, Safety Net can keep it around. You get to specify how long a message is kept for. Cool! Next up is this cool integration:

With the introduction of Safety Net, activating a lagged database copy becomes significantly easier. For example, consider a lagged copy that has a 2-day replay lag. In that case, you would configure Safety Net for a period of 2 days. If you encounter a situation in which you need to use your lagged copy, you can suspend replication to it, and copy it twice (to preserve the lagged nature of the database and to create an extra copy in case you need it). Then, take a copy and discard all the log files, except for those in the required range. Mount the copy, which triggers an automatic request to Safety Net to redeliver the last two days of mail. With Safety Net, you don’t need to hunt for where the point of corruption was introduced. You get the last two days mail, minus the data ordinarily lost on a lossy failover.

Whoa! So when a lagged copy is mounted, it asks Safety Net to redeliver all messages in the specified period – so as long as your Safety Net and lagged database copy have the same period, if you mount the lagged copy from the specified period ago, Safety Net will deliver all the messages since then. (It’s cool, but yeah I can imagine users complaining about a whole bunch of unread messages now, and missing Sent Items etc. – but it’s cool, I like it for the geek factor). :)

To re-emphasize something that was mentioned earlier:

Lagged copies can now care for themselves by invoking automatic log replay to play down the log files in certain scenarios:

  • When a low disk space threshold is reached
  • When the lagged copy has physical corruption and needs to be page patched
  • When there are fewer than three available healthy copies (active or passive only; lagged database copies are not counted) for more than 24 hours

Lagged copy play down behavior is disabled by default, and can be enabled by running the following command.

After being enabled, play down occurs when there are fewer than three copies. You can change the default value of 3, by modifying the following DWORD registry value.

HKLM\Software\Microsoft\ExchangeServer\v15\Replay\Parameters\ReplayLagManagerNumAvailableCopies

To enable play down for low disk space thresholds, you must configure the following registry entry.

HKLM\Software\Microsoft\ExchangeServer\v15\Replay\Parameters\ReplayLagLowSpacePlaydownThresholdInMB

After configuring either of these registry settings, restart the Microsoft Exchange DAG Management service for the changes to take effect.

As an example, consider an environment where a given database has 4 copies (3 highly available copies and 1 lagged copy), and the default setting is used for ReplayLagManagerNumAvailableCopies. If a non-lagged copy is out-of-service for any reason (for example, it is suspended, etc.) then the lagged copy will automatically play down its log files in 24 hours.

For future reference this doc has steps on how to mount a lagged database copy – i.e. if you are not doing the automatic play down behavior. You can manually play down via the Move-ActiveMailboxDatabase cmdlet with the -SkipLagChecks switch.

However, it is recommended you first suspend the copy (i.e. make it not “active”) and make a copy of the database and logs just in case.

Optionally, if you want to recover to a specific point in time you’d 1) suspend the database, 2) make a copy just in case, 3) move elsewhere all log files after the time you want to recover, 4) delete the checkpoint file, 5) run eseutil to recover the database – this is what replays the remaining logs and brings the database up to the point in time you want, and 6) move the database elsewhere to use as a recovery database for a restore. After this you move back the logs file previously moved away, and resume the database copy. This blog post has a bit more details but it is more or less same as the Microsoft doc. Note: I’ve never ever done this, so all this more of info for future me. :)

Lastly, that doc also has info on how activate a logged copy using Safety Net. Step 4 of the instructions made no sense to me.

Moving on … (but pointing to this HA TechNet link again coz it has a lot of other info that I skipped here).

Outlook Anywhere & OWA behind a WAP server

Some links around publishing Exchange namespaces such as OWA and Outlook Anywhere externally via a WAP server:

The easiest thing to do is pass-through everything via the WAP to the internal URL. But if you want, you can setup OWA authentication via ADFS claims. A step-by-step official guide is here, but the two links above cover the same stuff.

Healthcheck.htm

Exchange 2013 has a new monitoring architecture. When monitoring via a load balancer one can use a “healthcheck.htm” URL to test the health of each virtual directory (corresponding to each of the user consumed services). This URL is per virtual directoy, here’s an example from Citrix on how to add monitors for each service in NetScaler:

If the service is up the URL returns an HTTP 200 OK.

Virtual Directory cmdlets

Speaking of virtual directories, if any of the PowerShell Get- cmdlets for virtual directories are slow this blog post tells you why and what to do about it. These are the cmdlets, and the workaround is to add a switch -ADPropertiesOnly (this makes the cmdlet query AD Config partition for the same info rather than query each server’s IIS Metabase, which is slower):

  • Get-WebServicesVirtualDirectory
  • Get-OwaVirtualDirectory
  • Get-ActiveSyncVirtualDirectory
  • Get-AutodiscoverVirtualDirectory
  • Get-EcpVirtualDirectory
  • Get-PowerShellVirtualDirectory
  • Get-OABvirtualDirectory
  • Get-OutlookAnywhere

Update: Thought I’d add more videos and links to this post than make separate posts.

Transport Architecture

Check out this talk: https://channel9.msdn.com/events/TechEd/2013/OUC-B319. Slides available online here. I wanted to put a screenshot of the transport components as a quick reference to myself in this post:

So the CAS has stateless SMTP service. Affectionately called FET, or Front-End Transport.

The MBX has a stateful and stateless SMTP service. Called Transport and Mailbox Transport respectively. (Transport replaces the Hub Transport role of Exchange 2010).

There’s no longer a Hub Transport role. Previously the Hub Transport role on one server could directly talk to the store of another server – thus there were no clear layers. Everything was messy and tied up using RPC. Now there are clear layers as below and communication between servers happen at the protocol layer. Within a server communication goes up & down the layer; across servers it is using protocols like SMTP, EWS, and MRS proxy. Everything is clean.

Some slides on all three components:

Outbound email from the transport component on a MBX server can go out directly to an external SMTP server, or it can be delivered to the FET on any CAS server in the same site. This delivery happens on port 717 and needs to be specifically enabled.

Transport component listens on port 25 if MBX and CAS are on separate servers. Else it listens on port 2525 as CAS is already listening on 25. These ports are for accepting messages from the FET. For accepting messages from the Mailbox Transport component, it listens on port 465.

Remember that Transport is stateful.

Destination can be a CAS server or another transport component (on another MBX server). The Transport component is what does the lookup of the mailbox database.

Last component: Mailbox Transport. This is the component that actually talks to the next layer in the mailbox server. This talks MAPI and receives emails from the Transport component. This is also the component that does the message conversion (TNF to MIME and vice versa). No extensibility at this component as all that is at the Transport component. Once a message reaches Mailbox Transport there’s no changes happening to it!

[Aside] Various Exchange 2013 links by rakhesh is licensed under a Creative Commons Attribution 4.0 International License.