WASD VMS Web Services - Features and Facilities

8 - Instances and Environments

8.1 - Server Instances 8.1.1 - VMS Clustering Comparison 8.1.2 - Considerations 8.1.3 - Configuration 8.2 - Server Environments 8.2.1 - Ad Hoc Server Wrapper 8.2.2 - Formal Environments 8.2.3 - Considerations
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WASD instances and environments are two distinct mechanisms for supporting multiple WASD server processes on a single system.

Server instances are multiple, cooperating server processes providing the same set of configured resources.

Server environments are multiple, independent server processes providing differently configured resources.

8.1 - Server Instances

The term instance is used by WASD to describe an autonomous server process. WASD will support multiple server processes running on a single system, alone or in combination with multiple server processes running across a cluster. This is not the same as supporting multiple virtual servers (see "WASD VMS Web Services - Install and Config"; 4.3 - Virtual Services WASD Web Services - Install and Config ) When multiple instances are configured on a single system they cooperate to distribute the request load between themselves and share certain essential resources such as accounting and authorization information.

WARNING

Versions earlier than Compaq TCP/IP Services v5.3 and some TCPware v5.n (at least) have a problem with socket listen queuing that can cause services to "hang" (should this happen just disable instances and restart the server). Ensure you have the requisite version/ECO/patch installed before activating multiple instances on production systems!


8.1.1 - VMS Clustering Comparison

The approach WASD has used in providing multiple instance serving may be compared in many ways to VMS clustering.

A cluster is often described as a loosely-coupled, distributed operating environment where autonomous processors can join, process and leave (even fail) independently, participating in a single management domain and communicating with one another for the purposes of resource sharing and high availability.

Similarly WASD instances run in autonomous, detached processes (across one or more systems in a cluster) using a common configuration and management interface, aware of the presence and activity of other instances (via the Distributed Lock Manager and shared memory), sharing processing load and providing rolling restart and automatic "fail-through" as required.

Load Sharing

On a multi-CPU system there are performance advantages to having processing available for scheduling on each. WASD employs AST (I/O) based processing and was not originally designed to support VMS kernel threading. Benchmarking has shown this to be quite fast and efficient even when compared to a kernel-threaded server (OSU) across 2 CPUs. The advantage of multiple CPUs for a single multi-threaded server also diminishes where a site frequently activates scripts for processing. These of course (potentially) require a CPU each for processing. Where a system has many CPUs (and to a lesser extent with only two and few script activations) WASD's single-process, AST-driven design would scale more poorly. Running multiple WASD instances addresses this.

Of course load sharing is not the only advantage to multiple instances ...

Restart

When multiple WASD instances are executing on a node and a restart is initiated only one process shuts down at a time. Others remain available for requests until the one restarting is again fully ready to process them itself, at which point the next commences restart. This has been termed a rolling restart. Such behaviour allows server reconfiguration on a busy site without even a small loss of availability.

Fail-Through

When multiple instances are executing on a node and one of these exits for some reason (resource exhaustion, bugcheck, etc.) the other(s) will continue to process requests. Of course requests in-progress by the particular instance at the time of instance failure are disconnected (this contrasts with the rolling restart behaviour described above). If the former process has actually exited (in contrast to just the image) a new server process will automatically be created after a few seconds.

The term fail-through is used rather than failover because one server does not commence processing as another ceases. All servers are constantly active with those remaining immediately and automatically taking all requests in the absence any one (or more) of them.

8.1.2 - Considerations

Of course "there is no such thing as a free lunch" and supporting multiple instances is no exception to this rule. To coordinate activity between and access to shared resources, multiple instances use low-level mutexes and the VMS Distributed Lock Manager (DLM). This does add some system overhead and a little latency to request processing, however as the benchmarks indicate increases in overall request throughput on a multi-CPU system easily offset these costs. On single CPU systems the advantages of rolling restart and fail-through need to be assessed against the small cost on a per-site basis. It is to be expected many low activity sites will not require multiple instances to be active at all.

When managing multiple instances on a single node it is important to consider each process will receive a request in round-robin distribution and that this needs to be considered when debugging scripts, using the Server Administration page and the likes of WATCH, etc. (see 10.1 - Server Instances).

8.1.3 - Configuration

If not explicitly configured only one instance is created. The configuration directive [InstanceMax] allows multiple instances to be specified (see "WASD VMS Web Services - Install and Config"; 8 - Global Configuration WASD Web Services - Install and Config ) When this is set to an integer that many instances are created and maintained. If set to "CPU" then one instance per system CPU is created. If set to "CPU-integer" then one instance for all but one CPU is created, etc. The current limit on instances is eight, although this is somewhat arbitrary. As with all requests, Server Administration page access is automatically shared between instances. There are occasions when consistent access to a single instance is desirable. This is provided via an admin service (see "WASD VMS Web Services - Install and Config"; 9 - Service Configuration WASD Web Services - Install and Config )

When executing, the server process name appends the instance number to the "WASD". Associated scripting processes are named accordingly. This example shows such a system:

  Pid    Process Name    State  Pri      I/O       CPU       Page flts  Pages
21600801 SWAPPER         HIB     16        0   0 00:06:53.65         0      0
21600807 CLUSTER_SERVER  HIB     12     1879   0 00:01:14.51        91    112
21600808 CONFIGURE       HIB     10       30   0 00:00:01.46        47     23
...
21600816 ACME_SERVER     HIB     10    71525   0 00:01:28.08       508    713 M
21600818 SMISERVER       HIB      9    11197   0 00:00:02.29       158    231
21600819 TP_SERVER       HIB      9  1337711   0 00:05:55.78        80    105
...
216421F1 WASD1:80        HIB      5  5365731   0 00:23:12.86     37182   7912
2164523F WASD2:80        HIB      5  5347938   0 00:23:31.41     38983   7831
2162BA5D WASD_WOTSUP     HIB      3     2111   0 00:00:00.47       735    518
2164ABCF WASD1:80-651    LEF      6    57884   0 00:00:16.71      3562   3417
2164CBDB WASD2:80-612    LEF      4    19249   0 00:00:04.16      3153   3116
21631BDC WASD2:80-613    LEF      5    18663   0 00:00:07.19      3745   3636
2164BBE6 WASD1:80-658    LEF      5     3009   0 00:00:00.94      2359   2263
...

8.2 - Server Environments

WASD server environments allow multiple, distinctly configured environments to execute on a single system. Generally, WASD's unlimited virtual servers and multiple account scripting eliminates the need for multiple execution environments to kludge these requirements. However there may be circumstances that make this desirable; regression and forward-compatibility testing comes to mind.

First some general comments on the design of WASD.

All of these mechanisms support multiple, independent environments on a single system. Due to design and implementation considerations there are fifteen such environments available per system. The primary (default) is one. Environments two to fifteen are available for site usage. (Demonstration mode, /DEMO uses environment zero.) Server instances (8.1 - Server Instances) share a single environment.

There are two approaches to provisioning such multiple, independent environments.

8.2.1 - Ad Hoc Server Wrapper

This is a DCL procedure that allows virtually any WASD release HTTP server to be executed in a detached process, either by itself or concurrently with a full release or other ad hoc detached server. The server image and associated configuration files used by this process can be specified within the procedure allowing completely independent versions and environments to be fully supported.

Full usage instructions may be found in the example procedure(s) in WASD_ROOT:[EXAMPLE]*ADHOC*.COM

Two versions are provided, one for pre-v10 and one for post-v10 (due to changes in logical naming schema).

8.2.2 - Formal Environments

Although the basic infrastructure for supporting multiple environments (i.e. the 0..15 environment number) has been in place since version 8, formal support in server CLI qualifiers and DCL procedures has only been available since version 10. To support version 9 or earlier environments the 8.2.1 - Ad Hoc Server Wrapper must be used.

WASD version 10 startup and other run-time procedures have been modified to support running multiple WASD environments simply from independent WASD file-system trees. The standard STARTUP.COM procedure accepts the WASD_ENV parameter to specify which environment (1..15) the server should execute within (primary/default is 1). The procedure then derives the WASD_ROOT logical name from the location of the startup procedure.

For example:

$! start current release
$ WASD_STARTUP = "/SYSUAF=(ID,SSL)/PERSONA"
$ @DKA0:[WASD_ROOT.STARTUP]STARTUP.COM
$! start previous release in environment 2
$ WASD_ENV = 2
$ @DKA0:[WASD_ROOT_MINUS1.STARTUP]STARTUP.COM

8.2.3 - Considerations

WASD environments each fully support all WASD features and facilities (including multiple server instances) with the exception of DECnet scripting where because of DECnet objects' global (per-system) definition only the one must be shared between environments.

Per-environment configuration must be done in its own WASD_ROOT part of the file-system and logical names must be defined in the environment's associated logical name table. The site administrator must keep track of which environment requires to be accessed from the command-line and set the process logical name search list using the appropriate

$ @WASD_FILE_DEV[n]
where n can be a non-primary environment number (see "WASD VMS Web Services - Install and Config"; 3.5 - Logical Names WASD Web Services - Install and Config ).

It is not possible to have multiple environments bind their services to the same IP address and port (for fundamental networking reasons). Unless the network interface is specifically multi-homed for the purpose, services provided by separate environments must be configured to use unique IP ports.

Non-primary environments (2...15) prefix the environment as a (hex) digit before the "WASD" in the process name. The above example when executing, each with a single scripting process, would appear in the system as (second environment providing a service on port 2280):

  Pid    Process Name    State  Pri      I/O       CPU       Page flts  Pages
00000101 SWAPPER         HIB     16        0   0 00:00:11.98         0      0
...
00000111 ACME_SERVER     HIB     10     6247   0 00:00:12.63       540    611 M
00000112 QUEUE_MANAGER   HIB     10      328   0 00:00:00.18       136    175
00000122 TCPIP$INETACP   HIB     10  1249419   0 00:07:33.95       401    326
00000123 TCPIP$ROUTED    LEF      6  3495839   0 00:01:15.49       166    165 S
...
00000468 WASD:80         HIB      6   132924   0 00:01:29.26     17868   2856
0000046D 2WASD:2280      HIB      6   129344   0 00:01:29.26     17712   2840
0000049D WASD:80-8       LEF      4     4449   0 00:00:00.67       934    194
00000503 2WASD:2280-2    LEF      4      565   0 00:00:00.28       732    102
...

Cleaning Up

As described earlier each environment creates and maintains logical name table(s) and system-level name(s), detached scripting processes, lock resources and permananent global sections. Lock resources disappear with the server processes. Logical names, global sections, rights identifiers and occasionally detached scripting processes may require some cleaning up when a non-primary environment's use is concluded.


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