V4 High Availability

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Contents

Background

High availability versions of SARK have been available since SARK V1. They use Heartbeat and our own in-house written handlers for file replication and fail-over. These Heartbeat clusters are very reliable but limited in their ability to quickly fail-back after a protracted fail-over event, usually involving some level of limited manual intervention. The new SARK V4 high availability release uses an openAIS stack with DRBD to create a true cluster. Whereas HA1/HA2 ran as a fixed primary/secondary pair, V4 is not particularly concerned which node is currently active so there is no real concept of a primary or secondary node. The three principle software components used by V4 HA are Pacemaker, Corosync and DRBD. In broad terms, Pacemaker is the cluster manager (think of it as init.d for clusters), Corosync is the cluster transport layer (it replaces Heartbeat) and DRBD (Distributed Replicated Block Device) is the cluster data manager. DRBD is effectively a RAID 1 device which spans two or more cluster nodes. SARK, and indeed Asterisk, know little about the cluster although both components are aware that it is running.





Production V4 HA nodes use a minimum of 2 NICs. Eth0 attaches to the regular LAN while Eth1 provides a dedicated point-to-point communication link between the 2 nodes. DRBD and Corosync use this dedicated "cluster LAN" to communicate and move data around the cluster. Ideally the dedicated link should run between Gigabit NICs with a directly attached Cat5e or Cat6 cable (no switches and DEFINITELY no routing). Older NICs may require a crossover cable but most modern equipment will auto switch so you may not need one.

ASHA - a cluster helper

A scratch openAIS stack install is not for the feint-hearted. If you are unfamiliar with the components, you can spend a long frustrating time playing "whack-a-mole" with recalcitrant cluster nodes that simply refuse to do as they're asked. If that doesn't deter you then also be aware that you must define a strategy for managing the cluster application data. This must now reside in the DRBD partition so you'll have to figure out a way to get it there and address it without changing the existing applications which use it. All in all, it's not exactly a walk in the park.

For V4 HA, we built a little helper utility called ASHA (Asterisk SARK High Availability) which reduces the entire install to 2 commands. ASHA is a work in progress but we published it early in the hope that it may be of use. It is not a panacea and it is by no means perfect (the idempotency is suspect in a few areas) but it will get you a vanilla SARK/Asterisk cluster up and running in just a few minutes... which brings us to platform choices;

Platform

V4 HA runs on Debian Wheezy.

HA V3 vs HA V4

  • No concept of Primary or Secondary nodes in HA V4, just nodes
  • No clean-up required after a V4 fail-over/fail-back
  • V4 nodes run full Shorewall firewall (V3 nodes require an upstream firewall)
  • V4 nodes run Fail2ban intrusion detection (V3 runs ossec)
  • V4 fail-over is slightly slower than V3 (around 30 seconds vs 12 seconds in V3 - blame MySQL start-up delay)
  • V4 Node removal/replacement is much easier – nodes can be pre-prepared off-site and sync is fully automated
  • V4 clusters can detect and resolve Asterisk hangs and freezes (V3 could only detect asterisk failures)
  • V4 clusters share all of their asterisk data (including the CDR's and AstDB) in the DRBD
  • V4 clusters need to be pre-planned during initial Linux install
  • V3-HA upgrade to V4-HA requires a re-install
  • V4 upgrade to V4-HA requires either a re-install or repartitioning (or an extra drive)
  • V4 clusters can run at dynamic Eth0 IP addresses
  • Rhino card support is much simplified in V4
  • V4 Nodes require a minimum of 2 NICs (at least for production)
  • V4 dedicated ethernet cluster link is much faster and more robust than V3 serial link
  • V4 nodes can be up to 90Mtrs apart (V3 is 15Mtrs)
  • V4-HA requires a steep learning curve to fully understand
  • V4-HA installation has been simplified with the ASHA helper utility

What does the V4 cluster manage?

  • Apache
  • Asterisk and ALL of its data
  • MySQL and ALL of its data
  • SARK and its SQLite database
  • DRBD

None of these components will be available on the passive node when the cluster is flying (DRBD is running on the passive node but you can't access its data; at least not without a fight). Even if you were to manually start any of these components on the passive node (don't!), they would not see the live data.

Because all of the Asterisk volatile data is being managed by DRBD you don't need any special programming in areas like your billing or interactive systems to take account of failover or failback. As far as external programs are concerned there is only one Asterisk and one set of Asterisk data. Simply ensure that any remote routines always communicate through the Cluster Virtual IP and the data they require will be available.

Installation Prerequisites

  • A minimal Debian Wheezy install (When tasksel runs you should only select openssh and nothing else)
  • An empty partition on each cluster node.
    • The partition MUST be exactly the same size on each node
    • It should be marked as "Do not Use" when you define it.
    • The partition needs to be large enough to hold all of your MySQL and Asterisk data (including room for call recordings if you plan to make them).
    • By convention we define the first logical partition (/dev/sda5) as the empty partition
  • A minimum of 2 NICs on each node
  • The SARK V4 PBX workbench already installed onto the system - see HERE for installation notes

How big should the DRBD partition be?

DRBD partition size is a trade off between making it big enough to handle any eventuality and the time it takes to synchronize when you bring a new or repaired node into the cluster. As a rough guide, on a single dedicated Gigabit link, the cluster will sync at around 30 MB/sec (depending upon what the drives can actually handle). On our own SARK systems we use 64GB SSD drives and we allocate 20GB to the DRBD partition, 38GB to root and 4GB swap. We do quite a lot of call-centre type installs and this gives ample room for call recording with periodic offload and it syncs a new node in about 15 minutes. However, for a regular office phone system you'll probably get away with 10GB, or even less, for DRBD. If you wish, you can increase the resilience, and maybe also the sync speed, by using extra cluster LAN NICs (with NIC bonding) but the basic install we discuss here does not provide it. The DRBD documentation gives further details.

Installation using the ASHA utility

Unlike most Pacemaker install guides which describe installing the nodes in tandem, the V4 HA install proceeds one node at a time. Thus your first cluster will, somewhat paradoxically, consist of only a single node. This is perfectly fine. It's a lot less confusing to deal with one node at a time and it actually makes the creation of the second node considerably easier because Pacemaker and DRBD will do most of the heavy lifting for you automatically.

Once you have Debian installed, choose a candidate node and install SARK V4 using the notes HERE. You may also wish to install your favourite source code editor at this point and any ancillary tools you feel you may need.

Now, before you install asha, check where your DRBD partition is. You can do this with fdisk or lsblk

fdisk -l

Disk /dev/sda: 8589 MB, 8589934592 bytes
255 heads, 63 sectors/track, 1044 cylinders, total 16777216 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000f00bc

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *        2048     7813119     3905536   83  Linux
/dev/sda2         7813120     9766911      976896   82  Linux swap / Solaris
/dev/sda3         9768958    15626239     2928641    5  Extended
/dev/sda5         9768960    15626239     2928640   83  Linux
lsblk
NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sr0     11:0    1   249M  0 rom  
sda      8:0    0     8G  0 disk 
├─sda1   8:1    0   3.7G  0 part /
├─sda2   8:2    0   954M  0 part [SWAP]
├─sda3   8:3    0     1K  0 part 
└─sda5   8:5    0   2.8G  0 part 

Good, our unused partition is at /dev/sda5 (asha will need to know). As you can see, we only used a 3GB partition on this test system.

Now we can go ahead and install ASHA.

apt-get install asha

ASHA will drag the openAIS stack as part of its dependencies so you shouldn't need to install anything else, unless there's some other stuff you feel you can't live without. ASHA will ask you a series of questions and use the answers you provide to generate a set of scripts to define your cluster and fetch it on-line.

What does asha need to know?

ASHA will ask the following

  • Whether this is the first node to be defined for this cluster (or the second). The generated scripts will be different for the first and subsequent nodes.
  • The virtual IP of your cluster (specify a free static IP address on the local LAN)
  • Whether you wish to deploy with 1 or 2 NICs. You should NEVER deploy a cluster with a single NIC except for learning and play.
  • The IP address of the cluster communication NIC (Eth1) on this node. This should be on a separate subnet to the LAN (See the Schematic)
  • The IP address of the cluster communication NIC (Eth1) on the other node. This should be the same subnet as the previous line
  • The node name (uname -n) of the other node (which may not actually exist yet)
  • An administrator email address for cluster alerts
  • A unique identifier string which will be placed into every email alert subject line for this cluster

You may notice from the above that ASHA doesn't ask for the regular Eth0 addresses of either of the nodes. This is because it doesn't care; they take no part in cluster management in a production system. If you elect to build a single NIC system (for recreational purposes only) then ASHA will ask for the Eth0 IP address of the other node.

Completing the installation

After the ASHA install completes, if you are feeling brave, you can simply run the installer

sh /opt/asha/install

For the more prudent, you may wish to examine and understand the asha-generated scripts before you run them. You will find them in /opt/asha/scripts. Their number and content will vary depending upon whether this is the first node you have defined for your cluster or the second. Here are the scripts for a first node install.

root@crm1:~# ls /opt/asha/scripts/
10-insert_pacemaker_drbd_sark_rules.sh	50-make_corosync_live.sh
20-bring_up_drbd_first_time.sh		70-pacemaker_crm.sh
30-initial_copy_to_drbd.sh

the scripts are run in order, lets see what they do

10-insert_pacemaker_drbd_sark_rules.sh

This script sets up the cluster communication NIC, brings it up and sets the shorewall firewall rules to allow it to communicate with its peer.

20-bring_up_drbd_first_time.sh

This will bring up DRBD. If this is the first node defined it will also build the DRBD filesystem.

30-initial_copy_to_drbd.sh

If this is the first node, the source data from Asterisk, SARK and MySQL will be initially copied into the DRBD partition.

50-make_corosync_live.sh

Does what it says. It will start the Corosync cluster communication layer

70-pacemaker_crm.sh

This script will build the Pacemaker CIB (Cluster Information Base) using the Pacemaker CRM (Cluster Resource Manager). The CRM provides an abstracted view of the underlying XML which Pacemaker uses to do its stuff. It is complex, arcane and easily broken. In general you should not touch the CIB until you absolutely know what you are doing and why. You should NEVER touch the underlying XML unless you want to wind up crying in the corner of a darkened room. There is copious CIB documentation available on-line for the curious and the brave.

The Aftermath

Once you've run the scripts, your cluster will be up and running. However, nothing should look much different to before. The SAIL browser app will be running and you can use it just as you always have. Asterisk, Apache and MySQL will all be doing their stuff.
So how do we poke it with a stick to see what it does? Well, the first place to look is the Cluster Virtual Address. You should be able to browse to SARK at that address. Indeed you should make it your practice to always log-in using the cluster virtual address because you may not always be logging into the same cluster node, depending upon the current condition of the cluster (i.e which node is currently the active one).
The next place to look is the cluster status page which ASHA has installed for you. You will find it at

http://your.cluster.virtual.ip/crm_mon.htm 

It will give you near-realtime information on the cluster status (similar to the Apache status page) . Below is a screen shot of a single node SARK cluster which has just been brought up for the first time. The node is called crm1 (guess what the other node will be called) and the virtual IP is 192.168.1.90.



The important bits are the Node List (which tells you the names of the nodes in the cluster and their overall status) and the Resource List, which shows each of the managed resources and their individual status. The screen auto-refreshes every few seconds so you can use it to visually see what your cluster is doing.

You will also see a few subtle, but important changes in the SARK browser app. If you go to the Globals panel you will notice some new information



In the information box, to the right, where there used to be a single LAN address you will now see three addresses; the LAN IP, the Virtual Cluster IP and the dedicated comms link IP.

The red Asterisk STOP button still exists but you will never see a START button if you do stop Asterisk. That's because it isn't needed. Either Pacemaker will automatically restart Asterisk for you, or it will fail-over to the other node, depending upon what mood it is in (only kidding, it will simply restart it).

The only other thing the eagle eyed among you may spot, is that SARK no longer gives you the option to change the system Hostname in the network tab. This is because you can't change hostnames without changing the DRBD config so we don't let you do it. If you ever find you need to then you will need to read the DRBD documentation and do it yourself. It isn't particularly difficult but you have to change the config on both nodes so it isn't easy to automate.

Now reboot your single node cluster and check that everything comes back up as it should.

The next step is to bring up the second cluster. The procedure is identical to bringing up the first, you just answer ASHAs questions slightly differently. ASHA will generate fewer scripts because most of the hard work has already been done. Here are the scripts from the second install, there are only three.

root@crm2:~# ls /opt/asha/scripts/
10-insert_pacemaker_drbd_sark_rules.sh	50-make_corosync_live.sh
20-bring_up_drbd_first_time.sh

After running the scripts, the second node will come up and begin to sync automatically. It is important that you DO NOT dash ahead and practice power down failovers during this phase. Be patient, drink some tea and let it finish. For the terminally curious, you can watch the progress of the sync on either node by doing

watch cat /proc/drbd

Here's one syncing

 cat /proc/drbd
version: 8.3.11 (api:88/proto:86-96)
srcversion: F937DCB2E5D83C6CCE4A6C9 
 0: cs:SyncTarget ro:Secondary/Primary ds:Inconsistent/UpToDate C r-----
    ns:0 nr:459920 dw:459664 dr:0 al:0 bm:28 lo:3 pe:1279 ua:4 ap:0 ep:1 wo:f oos:2469104
	[==>.................] sync'ed: 15.9% (2469104/2928512)K
	finish: 0:01:47 speed: 22,896 (21,876) want: 30,720 K/sec

As you can see, it is just about 16% through the sync and it is syncing at just under 23M/Sec. Above we said to expect 30M/Sec but this example is just 2 test nodes running on a VM with virtual NICs so they sync a little slower. Make me a most excellent promise right now that you will NEVER run two production cluster nodes on the same VM.
Done that?
Good.

So now your sync has completed and the cluster monitor should look like this - 2 nodes and almost ready to go.



Both the nodes are up and all of our resources are currently running on crm1. There is just one more thing to do before we are ready to start experimenting. Reboot the new (second) node. This will give the SARK system startup routines a chance to get everything into the right frame of mind for the coming foolishness.

root@crm2:~# reboot

OK, it's back up so lets log in and do some failovers.

If you want to watch the failover in real-time, as opposed to the near-time of the browser monitor you can run a live monitor in the CLI.

crm_mon

It looks exactly the same as the browser version but it reacts immediately to change so it's more fun to watch (Note to self; I really should try to get out more).

So, the easiest way to force a fail-over is to log-in to the active node and take it offline (log-in to it's real LAN address - because the Virtual IP will stop when you fail it over). You can fire up crm_mon on the second node (crm2 in our case) to watch the proceedings. To take the node offline execute the following command

root@crm1:~# crm node standby

If you are watching the crm monitor in real-time you will see it become very agitated. It will refresh the display a lot, shut down all of the resources on the active node and then bring them all back up on other node (notice how long it takes MySQL to come up - it's a bit of slug that one).

Once its all back up your cluster should be fully available, making and taking phone calls as normal (after all, this is what it's actually about).

To fetch the node back on-line do

root@crm1:~# crm node online

The node will come back on-line almost immediately but no fail-back will take place. Why not? Under normal circumstances, Pacemaker will calculate the least expensive outcome in terms of time and effort. In the absence of any other constraints Pacemaker sees no more value in one node than the other so it is quite happy to leave things as they are. If you want to fail it back you'll either need to take the now-active node off-line and back or reboot it.

You will also, in your extensive research, no doubt come across cluster resource moves and migrates. Do be careful with these commands when you use them and do be fully aware of what they "really" do. I'll say no more here except to conclude that Pacemaker can sometimes be a bit sneaky!

As an aside, Pacemaker CIB also understands concepts of location, collocation and preference (stickiness). These allow resources to "prefer" certain nodes (and all manner of clever stuff). But for our simple little 2-node cluster these seem, at least up to now, unnecessary. As far as V4 HA is concerned, any node will do...

Except perhaps where real copper (or fibre) telephone lines are involved.. But then again, perhaps not.

Handling PRI and other PSTN lines

There are several schools of thought on the handling of fail-over with PRI (or any other physical) telephone lines. You can do what Xorcom and Digium have done, which is to place an appliance between the HA Cluster and the PSTN. This allows the cluster nodes to each talk to the appliance which in turn talks to the PSTN. Incidentally, from what we've read, both Digium and Xorcom use USB to connect the nodes to the appliance. Now, the maximum cable length of high speed USB is 5 meters so the two nodes and the appliance are all going to have to be snuggled up together in the same place.

Whatever the HA implications of node collocation, a cautious observer might conclude that such an appliance is just another Linux box sitting in the path introducing another single point of failure.

A similar approach would be to connect each of the cluster nodes (via SIP) to any of the widely available SIP/PSTN gateway devices from Sangoma, Mediatrix, Cisco et al, but again these would seem, on the face of it, to present yet another complex single point of failure, albeit one that can be pretty much any distance from the nodes while allowing the nodes themselves to be physically remote to one another. For many, this may be a reasonable compromise.

SARK V4 HA clusters can run with any SIP/PSTN gateway (we ourselves tend to prefer Sangoma's Vega gateways, simply because we have more experience with those devices and we find them reliable and reasonably priced). It is less certain that V4 would run with either Digium or Xorcom's USB connected appliances. In any event it has never been tested with either of them.

What also seems interesting to us is the idea of a simple, rugged, low-cost flip-flop switch which has just enough intelligence to be fired under software control and which will fail-over under power-loss (so that it will still pass a TDM signal, even when the power is removed). The Rhino 1SPF card is just such a device. It is relatively dumb, relatively cheap and it just works. The card is powered from a USB socket on one of the servers - this means that it needs to be collocated with one (and only one) of the nodes. In V4HA, it doesn't actually matter which node powers the card, but by convention we use the first node defined. The other node can be up to 100 meters away, or perhaps further if you risk using an intermediate switch but YMMV.



With "Power-on" the card feeds the TDM signal to the node which is powering it (via the NO port), "Power-off" it feeds TDM signal to the other node (via the NC port). So that's all simple enough.

Additionally, Rhino Corp. provides a C library which allows us to switch the card under program control. In practice it's dead simple. We run a little state machine on the node which the card gets its power from. It simply queries Asterisk every few seconds and waits for a response. If it doesn't get one, it switches the card signal to the other node, changes state and waits for Asterisk to reappear, at which point it flips everything back again. It's only a few lines of code but it does what we need and we get simple, effective PRI fail-over/fail-back at a fraction of the price of a big complex appliance. We can also sleep a little better knowing the nodes are in different racks in different fire zones. You could of course argue that the little Rhino card itself represents a single point of failure, and you'd likely be right, but the fact that even when we remove the power it still flows TDM suggests that it represents a good low-cost compromise.

To install the card simply download and build the library from the Rhino website (there's a guide in the manual). There's no deb so you'll have to compile it from source. The Make fails when it tries to install the man pages (it tries to put them in the wrong place) but you can either fix it or simply comment out the man install (what do you want Man pages for anyway?)

Once it's installed plug the rhino up to a USB port on your chosen node. Use dmesg to see which USB port its on (likely /dev/ttyUSB0).

You can poke it directly from the CLI (it's all in the manual). Flip it from A to B like this

rhinofailover /dev/ttyUSB0 RELAYOFF 0

You should see the port indicator lamp move from one output port to the other on the card itself. Flip it back with

rhinofailover /dev/ttyUSB0 RELAYON 0

OK, if that all went well you are now ready to start the little watchdog daemon. First of all you must modify the asha.conf file to tell ASHA about the Rhino card. You'll find a conf file at /opt/asha/asha.conf

PRIMARY=true
USENIC1=true
CLUSTERIP=192.168.1.90
CLUSTERALERT=admin@aelintra.com
SITEID=AEL TEST CLUSTER
ANODEIP=10.0.3.251
BNODEIP=10.0.3.252
ANODENAME=crm1
BNODENAME=crm2
DRBDDEV=/dev/sda5
CLUSTERNET=10.0.3.0
IP=10.0.3.251
RHINOSPF=YES
RHINOUSB=/dev/ttyUSB0

Set RHINOSPF=YES and set the correct device address for the card. Save the config and exit. ONLY make this change on the node which powers the card. Now you can start the daemon.

cd /etc/service
ln -s /opt/asha/service/shadow
rm /opt/asha/service/shadow/down
chmod +x /opt/asha/service/shadow/run
sv u shadow

That's it, you're done. You should be able to fail-over and fail-back and the PRI circuit will follow Asterisk back and forth (with a little delay while the state machine catches up).

The service will auto start when the system reboots. You can turn it off with

sv d shadow

You can disable it so that it won't restart upon a reboot with

touch /opt/asha/service/shadow/down

Next steps - and a word of caution

Pacemaker, Corosync and DRBD are extraordinarily beautiful, powerful and complex pieces of software. You need to understand them and, at least in outline, be familiar with how they work together with one another. The ASHA Utility, makes V4 installation very simple and straightforward and hides a lot of the complexity but you do need to be aware of how to recover from some of the possible failures that can occur when running a cluster. Getting it wrong, or changing parameters without rehearsal can result in data loss so you must have a clear data backup and recovery strategy for your cluster before you make changes.

Now go read lots about Pacemaker and its companions.

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