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# Example of Allocating and Reshuffling Hosts

There are many factors that can affect the allocation of hosts among timeslots. Consider the following scenarios which illustrate how hosts are allocated among performance timeslots and how changes in the available testing hosts can affect the host allocation. You will see that Lab Management attempts to optimize the overall resource usage by reshuffling the hosts that are available among the timeslots.

Assume the following hosts belong to the host pool of the project:

Host | Properties
--- | ---
Host1 | Controller
Host2 | Controller + Load Generator
Host3 | Load Generator with the following property: Priority = 1_Lowest
Host4 | Load Generator with the following properties: Citrix. The host can run scripts based on Citrix protocols. Priority = 9_Highest

At 07:00 am, John reserves the following timeslot:

Timeslot | Reserved at | Reserved for | Requested Resources, Properties
--- | --- | --- | ---
TS1 | 07:00 | 08:00am - 10:00am | 1 Controller, Any 1 Load Generator, Any

John submits the request. The system allocates Host1 as the Controller, leaving Host2 available to serve as either load generator or Controller in other timeslots. Additionally, the system allocates Host4 as the load generator, since it has higher priority than both Host2 and Host3. The following hosts are allocated successfully:

Requested Hosts | Allocated Hosts
--- | ---
1 Controller, Any | Host1
1 Load Generator, Any | Host4

At 07:30 am, Sue submits the following timeslot reservation:

Timeslot | Reserved at | Reserved for | Requested Resources, Properties
--- | --- | --- | ---
TS2 | 07:30 | 09:00am - 11:00am Autostart | 1 Controller, Any 1 Load Generator, Citrix

Because Host4 is the only Citrix load generator, the system checks if it can allocate a different load generator to John's timeslot so that Host4 can be allocated to Sue's timeslot instead.

The necessary hosts are available, so the following reshuffle occurs:

- Host3 is allocated to John; Host4 is allocated to Sue

- John's Controller, Host1 remains allocated to John; Host2, also a Controller, (not yet allocated), is allocated to Sue

Sue successfully submits the request. The new host allocation looks as follows:

Timeslot | Requested Hosts | Allocated Hosts
--- | --- | ---
TS1 | 1 Controller, Any 1 Load Generator, Any | Host1 Host3 (replaced Host4)
TS2 | 1 Controller, Any 1 Load Generator, Citrix | Host2 Host4

Note: Host allocation works on a "first-come, first-served" basis. Since John reserved Host4 first, if there had been no other host to allocate to his timeslot, Host4 would have remained allocated to John, and Sue would not have been able to reserve her timeslot successfully.

At 07:45am, Peter reserves the following timeslot:

Timeslot | Reserved at | Reserved for | Requested Resources, Properties
--- | --- | --- | ---
TS3 | 07:45 | 10:00am - 12:00pm | 1 Controller, Host2 1 Load Generator, Any

Peter is specifically requesting Host2 for his Controller, and any load generator. The system checks if the requested hosts can be allocated, taking into consideration requests and timing of the other timeslots:

- To allocate Host2 to Peter's timeslot, it must be available by 10:00.

- Sue is willing to have any Controller.

- John's timeslot will end at 10:00 and free resources.

The following reshuffle occurs so that all the timeslots can be accommodated:

- Sue's timeslot gives up Host2, and is allocated Host1 instead; Host4 (Citrix) remains allocated to Sue's timeslot

- Host2 is allocated to John's timeslot; Host3 remains allocated to John's timeslot

- Host2 and Host3 can then be allocated to Peter's timeslot because John's timeslot will be finished by 10:00 when Peter's starts.

Peter successfully submits his request. The new host allocation looks as follows:

Timeslot | Requested Hosts | Allocated Hosts
--- | --- | ---
TS1 | 1 Controller, Any 1 Load Generator, Any | Host2 Host3
TS2 | 1 Controller, Any 1 Load Generator, Citrix | Host1 Host4
TS3 | 1 Controller, Host2 1 Load Generator, Any | Host2 Host3

Note: If John and Peter's timeslots had overlapped, Host2 would not have been available for part of Peter's timeslot. In this case, the reshuffle would not have been possible and Peter would not have been able to reserve his timeslot successfully.

Now let's say at 07:55 Host2 becomes non-operational. As a result, TS1 takes back Host1, and starts at 08:00. It follows from the information above, that TS2 and TS3 both become invalid as their resources have become partially allocated.

Then, at 09:05, Host2 becomes operational again. It is allocated to TS2, and TS2 starts, though five minutes late, but still during the specified retries period. (For details about configuring retries, see Lab Management project settings.)

At 11:00, TS3 remains invalid (partially allocated) and is unable to start because Host2 is still being used by TS2.

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See also:

- Host Allocation

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