Apache HTTP Server Version 2.0
Apache HTTP Server Version 2.0
Shortcut to one letter codes used in status format and update field requests.
Updating physical machine statics or the administrative status of farms and physicals is done through GET queries to the engine. If the request produces output, either data or error message, it is delivered as content type plain text.
If you desire to, or the web agent you are using does so automatically, you can inform the engine that query string containing all the argumetns is URL escaped. This is done by setting the first char of the query string to be "E". In general, this a pretty good idea because the format arguments are printf style, and ought to be escaped.
eg. /ath/status/phys/?Eh=app01,f="%h,%c" [expects everything after E to be url escaped]
These are listed in order of construction, with the "+" indicating what must come next, which can be another access level in the engine are the end of the path and beginning of arguments.
The location of the server running the load balancer engine. Proxy rules must be such that requests coming in on desired host:port interface are not passed to mod_proxy. You want to interact with the load balancer engine, not be load balanced on this request.
The path on the apache instance that triggers the engine to take over the request. This is configurable via directive.
Once the engine takes over the request, it will finish it, valid or not. It looks for the following paths to determine what type of request it's been sent.
A status request is read only to the shared memory that contains the state of all farms, physicals, and engine internal stats. It is used to monitor or gather information about the runtime engine. It requires a subsystem to differentiate what type of data is desired. As arguments it takes desired entity and output format arguments. The include script "ath_monitor.sh" uses it to monitor and report on the health of your servers from the engine's perspective.
An update request is a privileged write request that updates the shared memory representing the state of the load balancer. It is most used to update physical system performance metrics and farm and physical administrative statuses. It requires a subsystem and then arguments clarifying which entity and what field of that entity to update. The included "ath_agent.sh" script uses it to update cpu and load average. You can flag farms and physical systems online or offline, or report physical systems as sick (this can also be done automatically, to some extent, with the AthExpectUpdate logic, which will time out and flag down systems who are failing to report in their status).
A balance quest is used when a balancing decision is desired outside of
reverse proxy request. For example, if your web application fires off
backgrounded heavy load processes, it is likely the the balancing decision
made to initiate the UI driven request is not the right one for the
background thread.
You may even want it to go to a different, non-web-enabled farm, using some
queueing system. You can query the engine for a decision on one or many farms,
and even ask it to use a different algorithm than the preconfigured one for that
farm. When more than one farm is requested, the result is returned new-line
separated in the same order the query requested them. If all members of a
farm are unavailable, it will say
WARNING: FARM: NO MEMBER AVAILABLE.
If the farm is invalid, it will say WARNING: FARM NOT CONFIGURED.
: joins an algorithm substitution to the farm requested, joins multiple farm(:algo) requestseg. /ath/balance/?FARM1
eg. /ath/balance/?FARM1,FARM2,FARM3
eg. /ath/balance/?FARM1:dynamic-cpu-mem
This is used to mirror the state of one load balancer to another identically configured (in farms and physicals (farm members)) load balancer. The mirror request is sent to update the system that is slave to the primary. It updates more fields than the "/update" type allows. This request type can be completely disabled by directive. It may prove useful in some installations. The authors installation used to use this, but it requires the primary to be healthy in most use cases, so he switched to running 2 completely independent systems, and just sends updates to both from each member.
It requires a subsystem request, "/farm" or "/phys", followed by one argument, a serialized representation of one entity of that subsystem. The serialized representation of an entity can be retrieved using a status request with output format specified as serialized. You can mirror a farm or a phys. Mirroring a farm does NOT mirror the state of the members of the farm, just all the data unique to the farm itself. To completely mirror an engine, you need to mirror all farms and all physicals.
eg. /ath/mirror/farm/?SERIALIZED_FARM_STRING
Simply returns "hello". Use it verify that the engine is loaded and enabled.
Used for interacting with the farm subsystem. In status context, you specify which entities you want to get data from, then the output format of that data. A farm entity is handled by its "name". Out put for more than one is new line separated.
Status format AND update parameters use the same one letter code to indicate the relevant field. Format letter code is always preceded by "%".
eg. /ath/status/farm?n=FARM [outputs default readable detail]
eg. /ath/status/farm?n=FARM-A,FARM-B [outputs default readable detail for 2 farms, new line separated]
eg. /ath/status/farm?n=FARM,f=%m [outputs just its member physical handles]
eg. /ath/update/farm/?n=FARM,o=1 [sets FARM to online]
Used for interacting with the physical (farm members) subsystem.
In status context, you specify which entities you want to get data from,
then the output format of that data.
A phys entity is handled by its "hostname". Out put for more than one is new
line separated. In reality a "hostname" can actually be the concatenation of
a hostname and a port, eg. app01:8080.
Status format AND update parameters use the same one letter code to indicate the relevant field. Format letter code is always preceded by "%".
eg. /status/phys/?h=app01,f=%c [outputs cpu load of app01]
eg. /status/phys/?h=app01 [outputs default data, which is everything in readable format]